qatar highway design manual - 1997.pdf

QATAR GENERAL PETROLEUM CORPORATION

t l , jJ l9 w1 )lii) 1U-l Llh Introduction by H.E. Minister for Municipal Affairs and Agriculture

The State of Qatar is witnessing rapid development and the road construction sector is most closely connected with this development. It is highly important when designing roads to take into consideration the latest international standards and specifications which in turn conform to environmental requirements and the fbture need to link the road network with the development programme. I

I Therefore, the initiative of the Civil Engineering Department in the Ministry of Municipal Affairs and Agricultural to update the Qatar Highway Design Manual, which was published for the first time in 1989. is the best evidence of its desire to keep up with the progress that this country is witnessing and emphasises the determination of this Ministry that its achievements are proof of its work.

We ask God to guide our steps to the Z-&l & G I k ; J- ji Jk righteous path. .

&,WJl-&& ALI %IN SAEED AL KflAYAREEN

&13jl19 -1 a9@l j, MINISTER OF MUNICIPAL AFFAIRS AND AGRICULTURE

;LJljJlj dj@l l j l j j J & j / t t Y &l& Introduction by H.E. the Undersecretary of the Ministry of Municipal Affairs

and Agriculture

The Road Network represents the arteries j 4 &+ 6 9 1 u for traffic movement in the modern state. ue iL] t j u l j . SJ--MI U J ~ I

Streets are not just for pedestrian and aU)ell uPJ & 1-G traffic movement but contain electricity d 4 Q- ~3-j : .el LL-? and telephone cables, and sewerage h 3 d J u-i)

networks. (. .. ~ j - 1 ~ 64j+u1

Therefore, "the information that should be available for the road designer should not be confined to population density, the nature of land and its topography only. The designer has to coordinate with service authorities and study the development progress of the area, its environment and the effects of road construction and the movement of traffic.

The publication of the Qatar Highway S j l j~ - d l LJ$I ;JIJ! j ! h ! i)! Design Manual, in a new issue by the Civil 691 >bll) uG L I J j l l J $41 dJ@I Engineering Depanment, is undoubtedly a jy i 9 L 9 ;+dl +b j (9 2 step in the right path, and is the fastest 4 1 &9lj . e l &,& way to reach our objective. J! !IF$ &Jl t r l 9

God is behind our purpose and will guide . &dl ~ lg ! J , + - d l GI jJ (jl hl J

us on the right path.

ddl-$?& ALI BIN SAAD AL KWARI

;Ll j g l j L&13 j'd1 i j l j j J& j

UNDERSECRETARY OF THE MINISTRY OF MUNICIPAL AFFAIRS AND AGRICULTURE

-1 -1 JjM )*L. 1 4 1 ;41L. Introduction by the Director of Civil Engineering Department

* U J i ~ b 4 b L ALI BIN NASSER RL THAN1 ;LI Jl L u r l l J j l ~ l *

DIRECTOR OF"CI\IIL E N C ~ I N E E R I ~ B DEPARTMENT

-

This is the second issue of the Qatar + U S QUI 4 -.MI ,, a i r

Highway Design Manual we present to Lpu 'J> j jJ&l vd engineers working in the roads and j jJ-l J& j &Wl & construction sector in the State of Qatar. U,J

The first issue was published in December iL. w C r j G J;Y I &JJU

1989 and we have been eager that this a& 3- & L p 3, . ir\ 9 A 9 issue should contain more details of the ++LAY I &Li;ll & I+* -1 methods and ideas which have developed 2-1 OL & & G I $1 JU~I, during this period regarding the design and LL, 6 691 GU!, + construction of roads, especially those -+JAY l ; d l 9 j &A1 $1 adopted in the USA, UK and other j & d l & b*, id1 =IJ countries in the last few years. . i&l &l+l

Whilst it is the intention of this Manual to +-I LA ?+ & & ~ 4 i! &J

be used in the road construction sector, 22: Y never the less, it should not be considered @A+J JGJI the only source; it is only a guide to eyi-YIJ highway engineers. The engineer needs to ('3 :x . &+i J J ~ Glr:Y 1 research, review and be assisted by other i .-)a, J+ Ub2 Y + M I I& scientific sources. The Manual does not d k J.A'u, 10, 6 j,&l cover the area of traffic engineering and 6 a141 & L I J J , 6 L:L ~ 3 1

. LiiG 11 . L W ! -1 d J G > related matters such as planning and -2 ,

transportation studies and issues of general &Gt;P! ,i G L I J ~ I ,i o&i;- && policy. We will welcome any observations, + \ "z

,PI C. d l &L+ suggestion or additions for future issues.

The Civil Engineering Department while &d @J G d I i JI J d! working earnestly to benefit from new -14 uC+ & S J L Y I &= engineering developments to keep up with d 6 d l c-? ;r 4 . j-IJ the times, requests all those specialisiny in 6 $a5 UJJ 4 L W I 5 9 1 e- road design in the State of Qatar to p-UdlJ c j L l d l & implement the specifications and standards . +Wl I& L+& +I contained in this Manual.

May god gives us the fortune to carry out Q U J-Us &hLl G \ ~ Y QI UJ the trust we bear and to do what benefits . ~'-1 * the Country

QATAR HIGHWAY DESIGN MANUAL DOCUMENT HISTORY

DOCUMENT HISTORY

The purpose of the Document History is to record changes to the Qatar Highway Design Manual. In the event of a revision to the manual, CEO will issue the amended pages and re-issue the Document History. The Document History pages should contain a description of the change, the issue reference and the date of issue as noted below. The updated Document History should replace the superseded history and the revised pages of the manual should be placed in the appropriate position in the manual.

Description - .

Qatar Highway Design Manual

Qatar Highway Design Manual

Issue Date

Original Issue December 1989

2nd Edition (Rev 0) January 1997

January 1997 Page DHM

QATAR HIGHWAY DESIGN MANUAL ACKNOWLEDGEMENTS

'The Qatar Highway Design Manual draws on technical input and experience from a number of recognised international sources and applies these to the road system requirements for Qatar. Within, the text there are references to publications where the engineer may seek further information on a specific topic. The main reference sources are acknowledged below:

Qatar Construction Specification Qatar Traffic Manual Design Manual for Road and Bridgeworks - British Government Highway Agency Policy on Geometric Design of Highways - American Association of State Highway and Transportation Officials. Road Design Manual - National Association of Australian State Road Authorities. Designing for Deliveries - Freight Transport Association.

Section 6 Copyright Acknowledgement

Section 6 of this manual contains text and diagrams which are based on material contained within the British Government's Highways Agency publication the 'Design Manual for Road and Bridges - Volume 6 Section 2.

Crown copyright material has been adapted with the permission of the controller of Her Majesty's ~tatione4 Office and the Highways Agency who do not accept any responsibility for the accuracy or comprehensiveness of the contents this Manual.

January 1997 Page AW1

QATAR HIGHWAY DESIGN MANUAL COWENTS

CONTENTS

Page No . . . .

.......................................................... GLOSSARY G/l

................................. ROAD SYSTEM IN QATAR ............ ; R S a l The Highway Network Primary Routes Secondary Routes Tertiary Routes The Route Classification Qatar Area Zones

SECTION 1 Clause 1.1 Clause 1.2 Clause 1.3 Clause 1.4 Clause 1.5 Clause 1.6 Clause 1.7 Clause 1.8 Ciause 1.9

DESIGN SPEED General .................................................

.......................... . Design Speed Related Parameters .................................. Selection of Design Speed

........................................... Posted Speed .............................. Changeover of Design Speed ............................. Changeover to Existing Roads .............................. Selection of Parameter Values

............................... Relaxations and Departures ................................... Special Considerations

SECTION 2 SIGHT DISTANCE ................................................. Clause2.1 General 2/1

.................................. Clause 2.2 Stopping Sight Distance 2/1 ............... ............. Clause 2.3 Full overtaking sight Distance : 2/1

............................. Clause 2.4 Obstructions to Sight Distance 2/2 ................ Clause 2.5 Effect of Horizontal Curves on Sight Distance 2/2

................................... Clause 2.6 Special Considerations a 2

ECTION 3 ause 3.1 ause 3.2 ause 3.3 ause 3.4 lause 3.5 lause 3.6 lause 3.7 lause 3.8

SECTION 4 Clause 4.1 Clause 4.2 Clause 4.3 Clause 4.4 Clause 4.5 Clause 4.6 Clause 4.7

HORIZONTAL ALIGNMENT General .................................................

...................................... Minimum Curvature ........................................ Transition Curves

............................... Camber and Superelevation ...................................... Widening on Curves

................................ Harmonising the Alignment .................................... Horizontal Clearances ................................... Special Considerations

VERTICAL ALIGNMENT ......................................... General Controls

............................ Maximum and Mlnimum Grades VerticalCurves ..........................................

......................... Harmonising the Vertical Alignment ................... Phasing Horizontal and Vertical Alignment

Vertical Clearances ........................................ .................................... Special Considerations

SECTION 5 CROSS SECTIONAL ELEMENTS . Clause 5.1 Road Resewatlons ........................................ 511

Clause 5.2 Lane Width6 .............*.................. : .............. 5111 Clause 5.3 Lane Capacity ........................................... 5112 Clause 5.4 Shoulders ............................................... 5112 Clause 5.5 Edge Strips and Shy Distances ............................. 5113 Clause5.6 Medians ................................... ; ............ 5113

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Januarv 1997 Pase C/1

9TAR HIGHWAY DESIGN MANUAL . . CofwENTS . . . . . ... . .. .

ause5.7 Verges ................................................. 5/14 ause 5.8 Parking Bays and Lanes.. ................................ 5/15 ause 5.9 Side Slopes ............................................. 5M6 ause 5.10 Auxiliary Lanes .......................................... 5M7 ause 5.1 1 Service Roads ........................................... 5M7 lause 5.12 Pedestrian Facilities ...................................... 511 8 ause 5.13 Utilities ................................................. 5/19 lause5.14 UseofKerbs ............................................. 5/19 ause 5.15 Safety Fences ............................................ 5/20 lause 5.1 6 Crash Cushions .......................................... 5/26 lause 5.17 Fencing ................................................. 5/27 lause 5.1 8 Road Closure ............................................ 5/27 lause5.19 Landscaping ............................................ 5/30

Clause 6.2

Clause 6.4 Clause 6.5 Clause 6.6

SECTION 6 JUNCTIONS Clause 6.1 General ................................................. 611

6.1 .I Junction Spacing . 6.1.2 Traffic Flows 6.1.3 Design Vehicles 6.1.4 Siting of Junctions

......................................... Types of Junction 616 6.2.1 T-Junction 6.2.2 Simple Crossroads 6.2.3 Staggered Junction 6.2.4 Skew or Y-Junction 6.2.5 Roundabout 6.2.6 Grade Separated Interchange 6.2.7 Traffic Signals

....................................... Clause 6.3 Junction Selection 6ff 6.3.1 Status of Intersecting Roads 6.3.2 Continuity of Standard 6.3.3 Junction Capacity Major/Minor Junctions - General ............................ 619

............................ Safety At MajorlMinor Junctions 619 MajorNinor Junction Types ............................... 6/9 6.6.1 The Simple T-Junction 6.6.2 T-Junction with Ghost Island . 6.6.3 T-Junction with Single Lane Dualling 6.6.4 T-Junction on a Dual Carriageway with Median

Opening (Signalized) 6.6.5 T-Junction on a Dual Carriageway with Carriageway

separation 6.6.6 Crossroads 6.6.7 Staggered Junction 6.6.8 Right and Left Hand Skew Junction

Clause 6.7 MajorIMinor Junction Elements ............................ 6/14 6.7.1 General 6.7.2 Design Speed 6.7.3 Visibility 6.7.4 Corner Radii 6.7.5 Carriageway Widths 6.7.6 Central Islands - Major Road 6.7.7 Central Island Tapers 6.7.8 Turning Length in Median 6.7.9 Direct Taper Length 6.7.1 0 Left Turning Lanes 6.7.1 1 Median Openings 6.7.1 2 Traffic Islands '

6.7.1 3 Nearside Diverging Tapers and Auxiliary Lanes 6.7.1 4 Merging Tapers

January 1997 - . Page C/2

r . . :.

QATAR HIGHWAY DESIGN MANUAL CONIENTS

6.7.1 5 Stagger Distances- 6.7.1 6 Skew Junctions 6.7.17 T-Junction with Carriageway Separation 6.7.1 8 Channelizing Islands 6.7.1 9 SplitterIRight Turn Islands 6.7.20 Drainage and Crossfall 6.7.21 Traffic Signs and Road Markings 6.7.22 Road Lighting

Clause 6.8 Roundabouts - General .. : ................................ 6.8.1 General Principles 6.8.2 Types of Roundabout

Clause 6.9 Safety at Roundabouts. .................................... 6.9.1 General 6.9.2 Two Wheeled Vehicles 6.9.3 Large Goods Vehicles

.................................... Clause 6.1 0 Roundabout Elements 6.1 0.1 Definitions 6.1 0.2 Entries 6.1 0.3 Entry Width 6.10.4 Flare Design at Entry 6.1 0.5 Entry Angle 6.1 0.6 Entry Radius 6.1 0.7 Entry Kerbing 6.1 0.8 Entry Deflection 6.1 0.9 Achieving Entry Deflection 6.1 0.1 0 Visibility 6.1 0.1 1 Circulatory Carriageway 6.1 0.1 2 Inscribed Circle Diameter (ICD) 6.1 0.1 3 Exits 6.1 0.1 4 Crossfall and Longitudinal Gradient 6.1 0.1 5 Segiegated Right Turning Lanes 6.1 0.1 6 Road Markings

........................................ Clause 6.1 1 U-Turns - General

........................................ Clause 6.1 2 Safety At U-Turns ......................................... Clause 6.13 U-Turn Elements

6.1 3.1 General 6.1 3.2 Direct Taper Length 6.1 3.3 Width of Physical Islands in the Median 6.1 3.4 Left Turn Lane 6.1 3.5 Median Openings 6.1 3.6 StorageIQueuing length 6.1 3.7 Merging Length 6.1 3.8 Pavement Construction 6.1 3.9 Road Lighting 6.1 3.1 0 Traffic Signs and Road Markings 6.1 3.1 1 Drainage and Crossfall

................... Clause 6.1 4 Urban Road - Service Road DivergeMerge ................................... Clause 6.1 5 Special Considerations

6.1 5.1 Residential Areas 6.1 5.2 Older Residential Areas 6.15.3 Other Road Users

..................................... Clause 6.1 6 Signalized Junctions 6.16.1 Introduction 6.1 6.2 Basic Requirements ,

6.1 6.3 Typical Layout Features

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QATAR HIGHWAY DESIGN MANUAL c o r n . I

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SECTION, 7 INTERCHANGES I

... - - Clause 7.1 Introduction ............................................. 711

c- - Clause 7.2 Types of Interchange. .................................... 711 7.2.1 General 1 - 7.2.2 Full Interchange 7.2.3 Compact Interchange - -

Clause 7.3

Clause 7.4

Clause 7.5

SECTION 8 Clause 8.1

Clause 8.2

Clause 8.3

Clause 8.4

Clause 8.5

Selection of Junction Type. ............................... 7.3,1 General 7.3.2 Traffic Flows and Design Year 7.3.3 Junction Spacing within the Network 7.3.4 Initial Information Requirements and Decisions 7.3.5 Types of Interchange for Preliminary Design 7.3.6 Preliminary Designs Design Elemen,ts ......................................... 7.4.1 Definitions 7.4.2 Design Speed 7.4.3 Lane Provision and Capacity

: 7.4.4 Hard Shoulders and Edge Strips 7.4.5 Merges and Diverges at Interchanges 7.4.6 Slip Roads 7.4.7 Link Roads 7.4.8 Loop Roads 7.4.9 Weaving Sections

.............................. Other Design Considerations 7.5.1 Clearance and Headroom 7.5.2 Superelevation 7.5.3 Safety Fencing 7.5.4 Signing 7.5.5 Lighting 7.5.6 Utilities 7.5.7 Emergency Vehicles 7.5.8 Maintenance Provisions 7.5.9 Environmental Issues

DRAINAGE ............................................ Introduction . ;

8.1 .I Functions of Highway Drainage 8.1.2 Minor and Major Systems

........................................... Design Criteria 8.2.1 Hydrological Data 8.2.2 Design Return Period 8.2.3 Design Method Urban Drainage. ......................................... 8.3.1 Introduction 8.3.2 Urban Catchment 8.3.3 Positive Drainage 8.3.4 Drainage of the Carriageway 8.3.5 Drainage of Medians, Footways and Verges 8.3.6 Emergency Flood Area (EFA) 8.3.7 Maintenance Strategy

.......................................... Rural Drainage. 8.4.1 - .. Introduction 8.4.2 Rural Catchment 8.4.3 Drainage of the Carriageway 8.4.4 Drainage of Medians and Verges 8.4.5 Natural Surface Drainage JunctionDrainage ........................................ 8.5.1 Introduction 8.5.2 Drainage at Junctions

January 1997 . Page Cl4. -

Clause 8.6 Subsurface Drainage ...................................... 8125 8.6.1 Introduction 8.6.2 Subsurface Drainage Methods

SECTION 9 Clause 9.1

Clause 9.2

Clause 9.3

Clause 9.4

Clause 9.5

Clause 9.6

Clause 9.7 Annex 9A

PAVEMENT Introduction ....... ;. .................................... 9.1.1 General 9.1.2 Typical Pavement Structures 9.1.3 Road ~eteriordiori 9.1.4 Variability in Materials and Road Performance TrafficAssessment ....................................... 9.2.1 Introduction 9.2.2 Design Life 9.2.3 Traffic Forecasting 9.2.4 Traffic Counts 9.2.5 Standard Axles 9.2.6 Deterrrrination of Cumulative Standard Axles 9.2.7 Design Traffic Classes

...................................... . Pavement Materials 9.3.1 Qatar Construction Specification (QCS) 9.3.2 Subgrade 9.3.3 Granular Material for Sub-base and Roadbase 9.3.4 Roadbase - Asphalt Concrete 9.3.5 Cement Bound Material 9.3.6 Wearing Course 9.3.7 Concrete for Rigid Pavements 9.3.8 Precast Paving Blocks Design Charts. .......................................... 9.4.1 General 9.4.2 Asptialt Concrete Roadbase 9.4.3 Asphalt and Granular Roadbase 9.4.4 Flexible-Composite Roadbase 9.4.5 Reinforced Jointed Concrete Slabs 9.4.6 Precast Block Paving

................................ Special Pavement Sections 9.5.1 Staged Construction (Single Layer Construction)

..................................... Pavement Evaluation 9.6.1 Introduction 9.6.2 Routine Monitoring 9.6.3 Detailed survey' 9.6.4 Detailed Investigation 9.6.5 Interpretation and Design of Remedial Works References ..............................................

........... Basis of the Design Method for Asphalt Roadbase. 9A. 1 Design Methods 9A.2 Design Strategy 9A.3 Applicable Methods 9A.4 Specific Method for Qatar 9A.5 Weak Subgrades 9A.6 References

SECTION 10 ROADWAY LIGHTING Clause 10.1 Introduction .............................................. 1011

10.1.1 Reasons for Lighting 10.1.2 Justification 10.1.3 Scope 10.1.4 Complementary Standards

Clause 10.2 Performance Requirements ................................ 1011 10.2.'1 Summary of Road Classifications in Qatar 10.2.2 Lighting Performance Recommendations 10.2.3 Limitation of Glare and "Light Pollutionn

n . . . . . . . . . . . .. ..... .Z .( ; . - C ' - . . , -.

January 1997 Page C15

QATAR HIGHWAY DESIGN MANUAL : . > , . . .

c o r n .. . . . - . :

Clause 10.3 Recommended Practice ................................... 1012 1 0.3.1 Decisions Prior to Design 10.3.2 Standard Lighting Geometries for Different ~ o a d Profiles 10.3.3 Lighting Columns as Hazards 10.3.4 Typical Lighting Layouts at Junctions

Clause 10.4 Speclflcatlon of Equipment. ............................... 1 O f f Clause 10.5 Electrical Distribution ...................................... 1 Off

10.5.1 SUPP~Y 10.5.2 Feeder Pillars - 10.5.3 Cables 10.5.4 Ducts 10.5.5 Earthing Systems 10.5.6 Safety Standards

Clause 10.6 Maintenance and Operation ................................ 1018 1 0.6.1 Design Implications 10.6.2 Quality of Equipment 10.6.3 Inventory and Fault Reports . 10.6.4 Cleaning and Lamp Replacement

: 10.6.5 Frequency of Inspections 10.6.6 Hours of Operation

APPENDIX A SURVEYS Clause A1 Introduction ............................................. Clause A2 Survey in Qatar. .........................................

A2.1 Centre for GIs - Mapping and Positioning Services A2.2 Land lnformation Centre - General Survey Section (GSS) A2.3 Planning Department A2.4 CED Survey Unit

Clause A3 Survey Work Procedures .................................. A3.1 Topographical Surveys A3.2 Services Surveys A3.3 As-built Surveys

.............................. Clause A4 Approved Survey Companies ..................... Ciause A5 Specification for Topographical Survey

AS. 1 , ~eatures to be Observed A5.2 Preparation of Survey Data A5.3 Specifications A5.4 Checking and Verification

APPENDIX B GUIDANCE NOTES TO PREPARE A BRIEF FOR GEOTECHNICAL SITE INVESTIGATIONS

Clause B1 Introduction .............................................. BM Clause 8 2 Initial Considerations ..................................... BM Clause 83 Preparation of the Brief. .................................. BI2

83.1 Geotechr~ical Investigation Works 83.2 Field Tests 83.3 Laboratory Tests

................................ Clause 84 Engineering Considerations Bf f 84.1 Methods of investigation 84.1 .I Trial Pits 84.1.2 Boreholes 84.1.3 Samples 84.2 Testing 84.2.1 In Situ Testing 84.2.2 Laboratory Testing 84.3 Earthworks 84.4 Retaining Structures 84.5 Geo-synthetics

Clause 65 Sample Pro Forma for Quantifying Geotechnical Site lnvestlgatlons 8/14

January 1997 -

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QATAR HIGHWAY DESIGN MANUAL GLOSSARY

GLOSSARY OF TERMS USED

AADT(Average Annual Daily Trafflc) - Total yearly two-way traffic volume divided by the number of days in the year.

Buffer Zone (Buffer Strip) - Land adjacent to a highway acquired by the highway authority for the purpose of preventing development that would be adversely affected by traffic noise, or for erecting noise barriers. Acceleration Lane - A speed change lane to

enable a vehicle entering a roadway to increase its speed to merge with through traffic. . Business Distrlct - That portion of a

municipality or an area within the influence of a municipality in which the dominant land use is offices, banks, hotels and government buildings

Access Road - Road providing access to a local area or individual properties from a distributor road.

Callfornla Bearing Ratio (CBR) - The ratio of the force required to penetrate a soil mass with a circular piston of 5cm diameter to the force required to penetrate a mass of high quality crushed stone with the same piston. The rate of penetration in both cases is 1.27mm per minute. Refer BS 1377.

ADT (XX) (Average Daily Traffic) - The current or projected average two-way daily traffic for the year 19xx or 20xx used to define the traffic for that year in the Gregorian Calender.

At-grade intersection - An intersection where all carriageways join or cross at the same level.

Camber - (1) A slight arch designed or built into a structure to compensate for the natural deflection after loading. (2) Slope on a single carriageway road from the centre to the edges to aid drainage.

Auxiliary Lane - The portion of the carnageway adjoining the travelled way for weaving; truck climbing, speed change, or for other purposes supplementary to through traffic movement.

Axle Load - The total load transmitted by all wheels on a single axle extending across the full width of the vehicle. Tandem axles l m or less apart shall be considered as a single axle.

Capillary Break Layer - The layer of specified or selected material placed on the subgrade to break the capillary rise of water and salts.

Capplng Layer - Layer replacing existing material under the pavement. Backslope - In cuts, the slope from the bottom

of the ditch to the top of the cut. Carrlageway - The pa? of a highway, including shoulders, for vehicular use. Single carriageway or dual carriageway.

Berm - (1) A raised and elongated area of earth intended to direct a flow of water, screen headlight glare. (2) Embankment widening to provide lateral support for the roadway. Catchment - Area feeding rainfall to a specific

point. Braking Distance - 'The distance required to stop the vehicle from the instant brake application begins.

Centreline - (1) For a two-lane highway the centerline is the middle of the travelled way, and for a divided highway the centreline may be the centre of the median. For a divided highway with independent roadways, each roadway has its own centreline. (2) The defined and surveyed line shown on the plans from which the highway construction is controlled.

Braking Reaction Distance - The distance iraversed by the vehicle from the instant the driver sights an object necessitating a stop, to the instant the brakes are applied.

Bridge - Structure supporting road or pedestrian walkway over an area to be crossed. Cloverleaf Interchange - A four-leg

interchange with loops'for left turns, and other connections for right turns. A full cloverleaf has ramps for two turning movements in each quadrant.

Broken Back Curve - An arrangement of curves in which a short tangent separates two curves in the same direction.

January 1997 . . . . Page GI1

QATAR HIGHWAY DESIGN MANUAL

Commercial Area - That portion of a municipality or an area within the influence of a municipality in which the dominant land use is shops and commercial business.

Crash Barrier - See Safety Fence

Crest Vertlcal Curve - A vertical curve having a convex shape in profile.

Crossfall - (1) A pavement superelevated toward the right or left shoulder on appreciable curves. (2) On divided highways on straights or flat curves, each one-way pavement may have a unidirectional slope across the entire width of pavement, usually downward toward the outer edge.

Culvert - A closed conduit, other than a bridge, which conveys water carried in a natural channel or waterway from one side of a highway to the other side. Culverts may be prefabricated pipes of concrete, steel,'or vitrified clay; or they may be cast-in-place structures of reinforced concrete, such a box culverts or arch culverts.

Cuwe Widening - The widening of the highway travelled way on sharp curves to compensate for the fact that the rear wheels of a vehicle do not follow exactly in the tracks of the front wheels.

Deceleration Lane - A speed-change lane that enables a vehicle to slow to a safe exit speed when making an exit turn.

Desert Road - A graded track to access a farm or small group of properties.

Deslgn Hour Volume (DHV) - The future two- way hourly traffic volume for use in design, usually the 30th highest hourly volume of the design year (30 HV).

Deslgn Lane - The lane on which the greatest slumber of equivalent 8-tonne, standard axle loads is expected. Normally, this will be either lane of a two-lane highway (single carriageway) or the outside lane of a multilane highway (dual carriageway).

Deslgn Life - The number of years of intended service life of a facility before the first major rehabilitation.

Deslgn Speed - A speed selected for purposes of design and correlation of the geometric features of a highway and a measure of the quality of service offered by the highway. It is

January 1997 . .

GLOSSARY

the highest continuous speed where individual vehicles can travel with safety upon a highway when weather conditions are favourable, traffic density is low and the geometric design features of the highway are the governing conditions for safe speed.

Design Vehicles - Selected motor vehicles with the weight, dimensions, and operating characteristics used to establish highway design controls for accommodating vehicles of designated classes.

Design Year - The future year used to estimate the probable traffic volume for which a highway is designed. A time 10 to 20 years from the start of construction is usually used.

Diamond Interchange - A four-leg interchange with a single one-way ramp in each quadrant. All left turns are made directly on the minor roadway.

Distributor Road - A type of road serving two' distinct functions. It provides a traffic service between primaries, arterial-collectors, other local roads, a town, village, industrial or commercial development, or a recreational area. It also provides direct vehicular access to privately owned properties. and service is the first consideration, but traffic service may have more than incidental significance.

Ditch - A trench dug in the earth for drainage purposes.

Diverging - The dividing of a single stream of traffic into separate streams.

Dual Carriageway - A highway with separated carriageways for traffic in opposite directions.

Eighty-fifth Percentile Speed - The speed at or below which 85 percent of the vehicles are being operated.

Elevated Hlghway - A highway on fill or structure above the level of the adjacent ground.

Embankment - A raised earth structure on which the road is placed.

Emergency Vehicle - A vehicle belonging .to the armed forces, civil defence, police, fire service or ambulance service, or other designated vehicle used for answering emergency calls for assistance.

Page GI2

QATAR HIGHWAY DESIGN MANUAL *

, . . . GLOSSARY

Emergency Flood Area (EFA) - Area set aside to store flood water during heavy rainfall.

Gutter - A paved and generally shallow waterway provided for carrying surface drainage.

ESA (Equlvalent Standard Axle) - The effect on pavement performance of any combination of axle loads of varying magnitude, equated to the number of reference single-axle loads required to produce an equivalent number of repetitions of an 8-tonne single axle. . .

Headwall - A vertical or inclined wall at the end of a culvert to prevent earth from spilling into the channel.

Hlerarchy Classlflcatlon - The grouping of individual highways in a highway system, according to their purpose or function, the type of traffic they serve, and their maintenance requirements. The main functional classes are Primary, Secondary, and Tertiary, though subclasses are'also used.

Exit -The point where traffic leaves to travel to an intersecting road.

Fencing - Item placed next to the road to define the edge of reservation or restrict animal access.

Hlghway - see Road. Foreslope - The slope from the edge of the surfaced shoulaer to the top of the subgrade, or the bottom of the ditch in cuts.

Horizontal Alignment - Horizontal geometry of the highway.

Formation - Graded surface above subgrade or capping layer on which the pavement structure is laid.

Horizontal Curve - A circular curve or transition by means of which a highway can change direction to the right er left.

Formation Drain - Porous or perforated pipe, or graded aggregate installed under a roadway or shoulder to provide subsurface drainage.

Independent Alignments - Each carriageway of a dual carriageway is designed and located to take full advantage of the terrain. .The median need not be of uniform width, and the two carriageways need not b e at the same level.

Footpath - That portion of a street or highway between the kerb line or edge of the roadway, and the adjacent edge of reservation constructed specifically for pedestrians (sometimes referred to as sidewalk).

Industrial Area - 'That portion within a municipality in which the dominant land use is light or heavy industry.

Full Overtaking Sight Dlstance (FOSD) - The minimum sight distance that must be available to enable a driver of one vehicle to pass another vehicle safely without interfering with the speed of an oncoming vehicle travelling at the design speed.

Inside Lane - the first lane of a dual carnageway, commonly referred to as the slow lane or nearside lane.

Interchange - A system of interconnecting roads in conjunction with one or more grade

'separations, providing for the movement of traffic between two or more roads on different levels.

Gantry - Signal or sign support above a carriageway.

Ghost Island - Painted or hatched marking on the road surface to guide traffic. lntersectlon - The connection of two o; more

roads is called a intersection. Gradlent - The profile of the centre of the carriageway, or its rate of ascent or descent. lntervisibility - The requirement of a vehicle

driver to see approaching'vehicles and also for his vehicle to be seen by approaching vehicles. Grade - To shape or reshape earth by means of

cutting or filling. Junction -Treatment of the intersection of two roads. Grade Separation - A structure that provides

for highway traffic to pass over or under another highway. Kerb - A structure with a vertical, horizontal or

sloping face placed along the edge of a pavement or shoulder forming part of a gutter, and strengthening or protecting the edge. '

Gully - Collection and distribution point for surface water along a gutter.

January 1997 Paae GI3


Lane - A portion of the'travelled way providing for a single line of traffic in one direction.

Overpass - A grade separation where the highway passes over an intersecting highway.

Left Lane - On a two-lane, two-way road, the traffic lane that is to the left of the centreline and normally used by traffic moving in the opposite direction; or on a multilane road, the extreme left traffic lane of those available for traffic travelling in the same direction, ie: adjacent to the median.

Parking Lanes - Additional width outside the travelled way of a highway or street that is designated for the temporary storage of vehicles.

Pavement - Structure on which vehicles travel.

Pedestrian Crossing - Any portion of a road at an intersection or elsewhere distinctly indicated for pedestrian crossing by signs, lights and by lines or other markings on the road surface.

Left-Turn Lane - A traffic lane within the normal surfaced width of a roadway or an auxiliary lane adjacent to or within a median, reserved for left- turning vehicles at an intersection.

Perception Time - The time required by a driver to perceive that he must change speed or stop.

Median - 'The portion of a divided highway separating the travelled ways of traffic travelling in opposite dirdctions.

Primary Road - Principle road within the network. Medlan Barrier - A longitudinal system used to

prevent an errant vehicle from crossing the median of a dual carriageway. Profile - A longitudinal section of a highway,

drainage course, etc. Median Opening - A gap in a median provided for crossing and turning traffic. Ramp - A short carriageway, usually one way,

to accomplish transfer movements within an interchange from the arterial highway or motorway to the minor road. Commonly referred to as a slip road.

Merging - The converging of separate streams of traffic into a single stream.

Moisture content - The percentage, by weight, of water contained in soil or other material, usually based on the dryweight.

Reaction Time - The time required for a driver to apply foot pressure to the brake after he perceived that he must stop.

Motorway - A multilane, dual carriageway designed to move large volumes of traffic at high speeds under free-flow conditions. Motorways have full control of access with interchanges incorporating grade separation and junctions.

Refuge Island - An island in a wide intersection to provide refuge for pedestrians.

Residential Area - That portion of a municipality, or an area within the influence of a municipality in which the dominant land use is residential development, but where small business areas may be included.

Network - A group of roads of varled hierarchy in a defined area.

Noise Barrier - A barrier of earth, stone, concrete, or wood placed adjacent to the highway to reduce the noise level on abutting ProPe rtY-

Rest Area - A roadside area with parking facilities separated from the carnageway providing motorists with opportunities to stop and rest for short periods.

One-way Highway - A highway or roadway having one or more lanes on which all vehicular traffic must go in the same direction.

Reverse Curve - A curve consisting of two arcs of the same or different radii curving in opposite directions and having a common tangent or transition curve at their point of junction.

Outer Separator - A separator between , a service road and the carriageway of a highway or major street.

Right-Turn Lane - An auxiliary lane or designated lane provided at intersections for right-turn movements.

Outside Lane - 'The lane nearest the median on a dual carriageway, commonly referred to as the fast lane or off-side lane.

. . .- Page G/4 January 1997


Ring Road - An arterial highway for carrying traffic around an urban area or portion thereof.

Separator - An area or a device located longitudinally between two carriageways so as to separate traffic flowing in the same or opposite directions, and so designed as to discourage or prevent passage by vehicles from the traffic lanes on one side of the separator to those on the other.

Road (Highway) - A general term denoting a public way for purposes of vehicular travel including the entire area within the reservation.

Roadbase - The layer of specified or selected material placed on a sub-base or formation. Shoulder - The portion of carriageway

contiguous with the travelled way for accommodation of stopped vehicles for emergency use, and for lateral support of base and surface courses.

Road Hump .(sleeping policeman) - Raised portion of the carriageway designed to slow passing vehicles.

Shy Distance - The portion of carriageway contiguous with the travelled way which separates the face of the kerb from the travelled way.

Road Markings - A traffic control device consisting of lines, patterns, works, symbols, or colours on the pavement, or adjacent to the road. .. .

Sight Distance - The length of roadway ahead, visible to the driver.

Road Sign - A traffic control device mounted on a support above the level of the roadway that conveys a specific message by mean's of words or symbols.

'. Standard Axle - Single axle load of 8,167 kg.

Stopping Sight Distance (SSD) - The distance required by a driver of a vehicle, travelling at a given speed, to bring his vehicle to a stop after an object on the roadway becomes visible. It includes the distance travelled during the perception and reaction times, as well as the vehicle braking distance.

Road Stud - Reflective or nonreflective stud on the road surface to define road markings and traffic positioning.

Rumble Strip - A rough textured surface, constructed for the purpose of causing the tyres of a motor vehicle driven over it to vibrate audibly as a warning to the driver.

Storm Drain (sewer) - A system of catch basins and underground conduits collecting, concentrating, and conveying water to a disposal point.

Safety Fence - A protective cable, beam or wall device placed along the carriageway edge for the purpose of redirecting vehicles that have left the roadway at a point of hazard.

Street - See Road. Sag Vertical Curve - A vertical curve having a concave shape in profile. Sub-base - The layer or layers of specified or

selected material of designed thickness placed on the subgrade to support the roadbase. Screening - The use of trees, shrubs, fences,

or other materials to obscure an objectionable view or to reduce an objectionable sound. Subgrade - (1) The top 300mm layer of

embankments or excavated areas on which the pavement structure including shoulders is constructed. (2) The top of a capping layer upon which the pavement structure and shoulders areconstructed.

Secondary Road - A highway of less national significance than a Primary road, but a highway that is intended to move large volumes of traffic at high speeds. Military installations and seaports not served by a Primary road are reached via Secondary roads. Traffic movement is the primary consideration, but this type of road may also provide some land service function.

Superelevation - The elevating of the outside edge of a curve to partially offset the centrifugal force generated when a vehicle rounds the curve.

Superelevation Runoff (application) - The transition distance between normal crown and fully superelevated roadway.

. . . .

Januarv 1997 Paae 615


Tack Coat - An application of bituminous material to an existing surface to provide bond with a superimposed course.

Time of Concentration - The time required for storm runoff to flow from the most remote point of a drainage catchment area to the point under consideration. It is usually associated with the design storm.

Toe of Slope - The intersection of an embankment side slope with the original ground surface.

Topsoil (Rodah soil) - Surface soil, usually containing organic matter.

Trafflc Barriers - Roadside barriers, median barriars, cras~cushions, and bridge parapets intended to guide or protect traffic from roadside hazards, including collision with other vehicles.

Traffic Island - An island provided in the road to separate or direct streams of traffic; includes both divisional and channelizing islands.

Trafflc Lane - That portion of the travelled way for the movement of a single line of vehicles.

Traffic Signal - Lights used to direct and stop and start traffic.

Underpass - A grade separation where the ,

highway passes under an intersecting highway. Can be a pedestrian or animal underpass which crosses under the main highway.

Verge -The portion of the highway reservation that is next to the road and is unpaved.

Vertical Curve - A curve on the longitudinal profile of a road to provide a change of gradient.

. .

Visibility - The distance at which an object can be just perceived by the eye.'

Visibility Splay - The area required for driver visibility to the left and right on the approach to a junction from the minor arm.

Wearing Course - The top layer of a pavement which resists skidding, traffic abrasion and the disintegrating effects of climate.

Weaving - The crossing of traffic streams moving in the same general direction accomplished by merging and diverging.

Weavlng Sections - Highway segments where the pattern of traffic entering and leaving at contiguous points of access results in vehicle paths crossing each other.

Transition - A section of variable pavement width required when changing from one width of travelled way to a greater or lesser width.

Transition Curve (Spiral) - A curve of variable radius intended to effect a smooth transition from straight to curved alignment.

Travelled Way - The portion of the carriageway for the movement of vehicles, exclusive of .shoulders, hard strips, shy distances and auxiliary lanes.

Turning Lanes - Auxiliary lanes provided at at- 'grade intersections for right and left turning movements.

Turning Track Width - The radial distance between the turning paths of the outside of the outer front tyre and the outside of the rear tyre that is nearest the center of the turn.

Typical Cross Section - A transverse section of a proposed highway showing the lateral dimensions and functional and structural elements cf the highway. '

January 1997 . Page GI6

ROAD SYSTEM IN QATAR

The Highway Network

Roads within the State of Qatar each fulfil certain functions within the overall network. A hierarchy exists which defines their various roles. Table 1 shows the status of road types within the hierarchy.

Primary Routes

These are 'routes of strategic significance whose purpose is to act as the principal distribution routes between the City of Doha, the main regional centres and the national border. They are generally dual carriageway roads, built to high geometric standards. .. .

The present system of Primary Routes is illustrated in Figure 1 and Figure 2.

Secondary Routes

Secondary Routes serves as area distributors by linking Primary Routes either to each other or by feeding traffic into the Tertiary Route network. They are generally dual Carriageway but in rural areas may be single carriageway.

The major Secondary Routes are also shown in Figure 1 and Figure 2.

Tertiary Routes

District distributors, local distributors and access roads are classified as Tertiary Routes. District distributors are urban dual carriageway roads providing high capacity routes between districts. Local distributor roads link access roads to either the Secondary Route network or, in urban areas, the district distributors. Both local distributors and access roads are characteristically low design speed, single carriageway roads.

The Route Classification

The Route numbering system is centred on the city of Doha. As shown in Figure 2, the origin of the Primary Route network is the D-Ring Road, this being designated Route No.1. The Primary Routes Nos. 1 to 7 extend radially outwards from the D-Ring Road. With the exception of Route No. 59, linking Route N0.5 to the national border, all Primary Routes have single digit numbers.

Secondary and Tertiary Route Numbers follow a branching system based on the Primary Route Numbers.

Qatar Area Zones

For ease of communication and coordination between Government bodies Qatar has been divided into reference Zones.

Activities such as planning, street names, Road Network Plans and Hierarchy Plans are generally referenced against the area zones. These zones are illustrated in Figure 3 and Figure 4.

. . . .. . . . . . . . . . . . . . . .. . , . . . . .

January 1997 Paae RSQ/l

QATAR HIGHWAY DESIGN MANUAL ROAD SYSTEM IN QATAR I

Table 1 Route Classification and Function

Route Classification

PRIMARY RQUZE

Rural

Urban - Rural

.. - Urban

P Rural Local Road

District Distributor

Local Distributor

Access Road

Senrice Road

sPEClAL (2)

Scenic Routes

Lorry Routes (3)

Notes (1) The general (2) These can

- - January 1997 Page RSW2

(3) The main Lorry Routes include the Regional Primary Roads and the Rural Distributors.

Class Ref

P I

P2

S1

S2

TR1

TR2

TR3

TR4

TRS

SR1

SR2

road be either

Function

A major road linking towns, or a bypass

A major urban road

A rural road linking settlements to the primary networks. Significant traffic flow or use by goods vehicles

A major urban road for through traffic

A rural road linking settlements

An urban road linking districts

A road distributing traffic within a district

A road giving direct access to properties - residential major access - residential minor access - culde-sac serving a maximum of 12 properties

A road giving direct access to properties and collecting minor roads for entrylexit onto Dual Carriageway.

Roads with special functions as dignitary routes or recreational routes

Specially designated and designed for heavy vehicles

corridors are based on the MMAA's plan primary, secondary or tertiary routes.

Carriageways

Dual 2-3 lane

Dual 2-3 lane Single 2-lane

Dual 2-3 lane

Single 2-lane

Dual 2-3 lane wide single or single 2-lane

Wide single or single 2-lane (some existing routes may be dual carriageway)

Single 2-lane

Single 1-way or 2-way

varies

varies

'General guidance

General Corridor Wldth

(m)

64

64/40

64/40132

40132

64/4W32

40/32/24/ 20

24120116

varies

varies

for road

Design

Roundabouts, minor T or grade- separated junctions. Some U- turns on rural routes

T-junctions, with double U-turns on dual carriageway, staggered junctions on single carriageway

T-junctions

Roundabouts, slip-onlsllp-off or signalised junctions. No U-turns. Limited access from existing properties. New properties to provide rear access. Parallel parking in bays

Roundabouts. T-junctions or signalised junctions. Offset X- roads. Direct access from properties. Parallel parking bays.

Roundabouts or T-junctions. Offset X-roads. Direct access from properties. Parallel parking, on street.

Mergeldiverge tapers onto dual m y . Parallel allgnment to major road way. T-Junction access for Minor roads. Speed redudlon, direct access from properties, on- street parking, parallel or angle.

Varies, emphasis on integrated landscaping and architecture.

Varies, emphasis on pavement design, appropriate junction radii etc.

cross-sections and utility disjmsltions".

QATAR HIGHWAY DESIGN MANUAL ROAD SYSTEM IN QATAR

PRlNARY ROUTES

S E C O N D A R Y ROUTES

- TERTLARY ROUT E8

- LOCAL ROUTES

Figure 1 Road Hierarchy - State of Qatar

January 1997 Page RSW3

-


- PRIYARYRWTES

- SECONDARY ROUTE8

- TERTIARY ROUTES

- LOCALRourEs

Figure 2 Road Hierarchy - Greater Doha

6

January 1997 . Page RSW4 .

. . . . . ? . - . QATAR HIGHWAY DESIGN MANUAL ROAD SYSTEM IN QATAR

N

A

ZONE BOUNDARY

65 ZONE NUMBER

Figure 3 QARS Zones - State of Qatar



. -

- ZONE BOUNDARY

68 ZONE NUMBER

Figure 4 QARS Zones - Greater Doha

January 1997 Page RSW6 . ..

QATAR HIGHWAY DESIGN MANUAL - .

ROAD SYSTEM IN QATAR

January 1997 Page RSQn

QATAR HIGHWAY DESIGN MANUAL ROAD SYSTEM IN QATAR *-


95.

96.

97.

98.

At Kharrara

Abu Smra

Sawda Natheel

Khor Al Adaid


SECTION 1 DESIGN SPEED

1.1 GENERAL

The speed of vehicles depends on the capabilities of driver and vehicle and on other general conditions such as the physical characteristics of the highway and its roadsides, the weather, the presence of other vehicles and finally, the presence of speed limitations. Although any one of these may govern, often the effects are combined.

In Qatar the weather has an adverse effect on the relationship between tyre and road surface and hence design speed. The heat results in a build-up of rubber deposit on the road surface from tyres. This in turn decreases the skid resistance of the road surface. Qatar is also subject to intense rainfall at certain times of the year. The addition of rainfall to a road surface which has reduced skid resistance increases the potential for accidents. This is particularly valid on the approach to and at junctions where turning and stopping movements are high. Furthermore, water is often spilled from water tankers at roundabouts and junctions. Bearing this in mind, the selection of design speed and hence stopping distance is extremely important.

The design speed of a highway may be defined as the highest continuous speed at which any vehicle can safely travel when given favourable weather conditions and low traffic volumes, so that the design features of the highway may govern. Such design features may include structures, or frequency of junctions. The design speed is related to the posted speed which represents the 8Sn percentile of the design speed, that is the value at which 15% of vehicles are expected to exceed the design speed. Refer to Section 1.4 for posted speeds.

The road alignment shall be designed so as to ensure that standards of alignment, visibility and superelevation are consistent with the selected design speed. This choice will essentially be dependent on the provision of the highway and its location, i.e. single or dual carriageway or whether in a rural or urban area. The visibility criteria are dealt with in Section 2 Sight Distance.

The design speed for various road classifications are shown in Table 1.1. The road classifications are defined in the front of this manual, refer to Road System in Qatar.

The selection of design speed should be approved by the Director of Civil Engineering Department. Refer to Clause 1.8 and 1.9 for

. . SECTION 1

relaxations and departures and special considerations '

Table 1 .I Design Speed for Various Road Classifications.

Design speeds for Special Roads (Class Reference SR1 and SR2) require special consideration and should be agreed with the Director of Civil Engineering.

Class Reference

P I P2

S1 S2

TR1 TR2 TR3 TR4 TR5

All reference to speed in-this manual should be taken as the design speed unless noted otherwise.

1.2 DESIGN SPEED RELATED PARAMETERS

Type of Highway

Primaw Routes Rural Urban

Secondarv Routes Rural Distributors Urban Distributors

Tertiarv Routes Rural Local Road District Distributor Local Distributor Major /Minor Access Service Roads

The driver will vary his speed according to hib impression of the road alignment and layout. Table 1.2 details the main design speed related parameters which are dealt with in greater depth in their respective clauses in this manual.

Design Speed (kph)

140 120

140 100

1 00 100 70 60 60

Posted Speed Clause 1.4

Stopping Sight Clause 2.2

Table 2.2

Horizontal Clause 3.2

Vertical Curvature . Clause 4.3 Table 4.4.3

Traffic Calming 1 Clause 1.9 I Table 1.2 Design Speed Related Parameters

January 1997 Page 111

QATAR HIGHWAY DESIGN MANUAL SECTION 1

1.3 SELECTION OF DESIGN SPEED 1.6 CONNECTION TO EXISTING ROADS

The designer must select the appropriate design speed based on his knowledge of the class of highway planned, the character of terrain, development density, traffic volumes and economic considerations. Generally for Qatar the design speed is selected using Table 1.1. .

Care shall be taken where an improved section of road rejoins an existing road. The existing standard of curvature and sight distance at the interface shall be subject to the same restrictions as would be relevant for the design speed of the improvement. Careful consideration shall also be given for roads passing between rural and urban areas, posted speed step down and also dual to single carriageways, although this latter case should be limited to junction locations only.

Design speeds shall also be selected with reference to the posted speed limit envisaged or that which is already in place for the road and the Road Network Plan. An allowance shall be made for a margin of safety for vehicles which will travel in excess of the speed limit. Refer to Section 1.4 below.

In all cases it is important to emphasise the need for clear signing at any location where there is a speed reduction.

1.4 POSTED SPEED . 1.7 SELECTION OF PARAMETER VALUES

Posted Speed. is the mandatory speed limit applied to a road. The speed limit is displayed on the roadside and is enforceable. The posted speed limits to be implemented in 'relation to design speed are shown in Table 1.3 below.

Designers should normal^.^ aim to achieve the desirable minimum values for stopping sight distance, horizontal curvature and vertical crest curvature. For sag curves, designers should normally aim to achieve at least minimum values. I Desi~;~E;eed

Speed (kph) I Posted I 1.8 RELAXATIONS AND DEPARTURES i0! 1 ;

70 60' 50 or less'

Special consideration required for lower class roads, see Clause 1.9

Generally for Qatar the design speed is selected using Table 1 .I. In certain circumstances it may be uneconomic to design an alignment to the prescribed standards and consequently a reduced standard may be used. This is termed a "relaxation". In situations of extreme difficulty where application of a relaxation does not overcome the difficulty, it may be possible to overcome them by adoption of departures from standard. Any such relaxations or departures must be agreed in writing with the Director of Civil Engineering.

Table 1.3 Relationship between Design Speed and Posted Speed

The above table allows for a margin of safety appropriate to the selected design speed.

1.5 CHANGEOVER OF DESIGN SPEED Table 1.4 shows the allowable relaxation of design speed for the different classes of roads in Qatar. Transitions between roads (or sections of a

road) with different design speeds shall be carefully implemented so as not to present the driver with an abrupt change in standards. For details of signing the speed reduction refer to the Qatar Traffic Manual.

The road classifications for Qatar and Doha are described in the front of this manual. The selection of a design speed is particularly difficult for some of the roads in the older areas of the city. These areas are not so easily classified into land use and factors such as access and parking need to be assessed in determining the design speed. Other considerations are the number and spacing of junctions on a particular section 'of road. Relaxations and departures provide a means of accommodating these areas.

Where an alignment changes from a higher to the next lower design speed, relaxations below the desirable minimum radius and desirable minimum stopping sight distance shall not be used at the start of the lower design speed section.


A . ,t ,. '4. , v ., : . . . . . .. . . .

SECTION 1

L. d I - . 1


Traffic calming measures may be introduced on existing roads to reduce traffic speed. This is achievable by the use of narrow - lanes, chicanes, width or height restrictions, speed bumps or different textures or colours of pavement.

Care shall be taken to ensure that traffic calming .measures, being introduced do not impede emergency service vehicles.

Class Reference

P I P2

S1 S2

TR1 TR2 TR3 TR4 TR5

A typical speed bump may be 3.5m in length with its profile reaching a maximum of 100mm. 'They should ideally be located at 100m intervals. Much shorter intervals result in inconvenience to the residents, whereas for much longer intervals the overall speed control is lost. For safety reasons speed bumps should not be located near junctions or sharp bends. Table 1.4 Design Speeds for Various

Road Classifications.

Type of Highway

Primarv Routes Rural Urban

Secondarv Routes Rural Distributors Urban Distributors

Tertiaw Routes Rural local Road District Distributor LocalDistributor Major Access Service Roads

For further details on traffic calming measures, refer to the Qatar Traffic Manual.

Design Speed (kph)

140-1 20 120-1 00

1 40-1 20 1 00

100 100-70 80-60 60-50 60-30

Departures below minimum values may be considered when cost or environmental savings are considered to be significant, except in the following circumstances:

immediately following an overtaking section on single carriageway roads.

on the immediate approach to a junction, other. than a roundabout, where frequent turning traffic will occur.

1.9 SPECIAL CONSIDERATIONS

Special consideration is required for residential and commercial areas.

The posted speed in residential areas is 50kph for local roads and lower for access roads. Lower speeds may be posted in special circumstances such as residential cul-de-sacs or in industrial areas where the -facilities are designed to distribute vehicles to their ;firial destination.. . .

The lower design speeds applied in residential and urban areas do not require superelevation on bends or other special dynamic related considerations.

One-way roads may be used for local and access roads usually in the form of discreet loops.

One-way roads should be designed so as not to encourage speeding. This may be achieved by the use of narrow lanes and avoiding long straight sections of road, and by implementing any one or more of the traffic calming measures listed below.



SECTION 2 SIGHT DISTANCE

b) Horizontal Plane

2.1 GENERAL

Sight distance is the continuous length of road ahead, visible to the driver, assuming adequate light, visual acuity and clear atmospheric conditions. The arrangement of geometric elements is crucial to ensure adequate sight distance exists for safe and efficient operation. There are two separate sight distances to be considered:

a Stopping Sight Distance (for all roads)

a Full Overtaking Sight. Distance (for single carriageways only) Figure 2.1 Measurement of Stopping Sight

Distance (SSD) Safe stopping distance must be provided continuously on all highways. Safe overtaking distance is applicable only on two-lane highways, primarily in rural or outlying urban areas.

2.3 FULL OVERTAKING SIGHT DISTANCE

Full Overtaking Sight Distance (FOSD) is the minimum sight distance that must be available to enable the driver of one vehicle to pass another vehicle safely and comfortably, without interfering with the speed of an oncoming vehicle travelling at the design speed. In the interests of safety and service, it is important to ensure sufficient visibility for overtaking on as much of the road as possible. FOSD influences the average speed of the traffic especially when a highway is near operating capacity.

2.2 STOPPING SIGHT DISTANCE

Stopping Sight Distance (SSD) is the distance required by the driver of a vehicle travelling at a given speed to bring his vehicle to a stop after an object on the carriageway becomes visible. SSD has three components; perception time, reaction time and braking time. A combined driver perception and reaction time of two seconds has been allowed for in Table 2.1. Table 2.2 shows for each design speed the

FOSD required for overtaking vehicles using the opposing traffic lane on single carriageway roads. These are minimum values and wherever possible, larger values should be used.

Design Speed Full Overtaking (kph) I Sight Distance (rn)

Design Speed (kph)

140 120 100 80 70 60 50 40

Table 2.1 Stopping Sight Distance SSD

Stopping Sight Distance (m)

350 295 21 5 1 60 1 20 90 70 60

Stopping Sight Distance is measured from a driver's eye height of between 1.05 and 2.00m to an object height of between 0.26 and 2.00 m, above the road surface, refer Figure 2.1. It shall be checked in both the horizontal and vertical plane between two points in the centre of the lanes on the inside of the curve (for each lane in the case of dual carriageways).

Table 2.2 Full Overtaking Sight Distance FOSD

FOSD shall be measured from a driver's eye point between 1.05m and 2.00m above the centre of the carnageway (for each lane in the case of dual carriageways) as shown in Figure 2.2 and shall be checked in both the horizontal and vertical planes.

January 1997 Page 2/1

a ? ' + *


FOSD is considerably greater than SSD and can normally only be economically provided in relatively flat terrain where the combination of horizontal and vertical alignment allows the design of a flat and relatively straight road alignment.

Envelope of visibility

. . -

Figure 2.2 Measurement of Full Overtaking Sight' Distance (FOSD)

Where passible on a single carriageway it is advisable to design sectior~s of road specifically for overtaking. This will reduce the frequency of serious accidents occurring on 'roads with continuous large radius curves.

2.4 OBSTRUCTIONS TO SIGHT DISTANCE

Care shall be taken to ensure that no substantial fixed objects obstruct the sightlines including road furniture, bridge piers, buildings, signs and cut slopes. However, isolated slim objects such as lamp columns, sign supports, or other slim objects of width 550mm or under can be ignored. Similarly, the effect of short intermittent obstructions, such as bridge parapets of minor roads under, can be ignored. Lay-bys or parking lanes should, wherever possible, be sited on straights or on the outside of curves, where stationary vehicles will not obstruct sightlines.

Sigh,tlines should be checked where safety fencing is installed.

2.5 EFFECT OF HORIZONTAL CURVES ON SIGHT DISTANCE

When a road is in a cutting or at bridge crossings it will be necessary to widen verges or increase bridge clearances to ensure that the appropriate stopping sight distance is not obstructed. Figure 2.3 shows the maximum

w - SECTION 2

central-offset required with varying horizontal curvature, in order to maintain the design speed related stopping sight distances. It can be seen that extensive widening of verges and structures, or medians with safety fence or safety barriers, would be required to maintain stopping sight distances on horizontal radii below the minimum.

Figure 2.4 shows the maximum central offset required with varying horizontal curvature, in order to maintain the design speed related full overtaking sight distance. It can be seen that the higher requirements o f FOSD result in extensive widening of verges for all but relatively straight sections of road.


It is vital that drivers on an access or minor road should have adequate visibility on the approach to a junction with a major road. The driver should have sufficient visibility to judge when to join the main carriageway. Furthermore, it is important for the driver on the major road to be aware of the vehicle approaching the junction on the minor road.

The required visibility criteria for junctions is given in Section 6 Junctions.

The required visibility criteria should also be applied to private accesses and driveways leading onto access roads.

The visibility required on bends is shown in Table 2.3 below.

Table 2.3 Required Visibility on Bends for Residential Roads.

Type of Road

Local Roads Access Roads

Where there is likely to be increased pedestrian traffic, care must be taken to ensure that visibility is not impaired by pedestrians. This could occur at the following residential and commercial locations :

Pedestrian crossing points

Sikkas and alley-ways

Schools

Visibility distance (m)

January 1997

. Absolute Minimum

50 30

Page 2f2

Desirable Minimum

70 50

9 1 - QATAR HIGHWAY DESIGN MANUAL

.. . . . . SECTION 2

• Shopping areas

a Sports venues

Cinemas

a BUS stops

In existing residential or commercial developments, it is important to review the visibility on 90 degree bends. Where it is not possible to achieve the required visibility, consideration should be given to using a larger radius or even locating a junction on the bend.

In new developments where it is not possible to avoid the use of a 90 degree bend, the following should be considered:

... .

a Avoid building 6n corner plots

a Use low landscaping

a Avoid placing street furniture and , signing within the visibility splay.

Care should also be taken when locating 'parking areas as parked cars will impede visibility at tight bends, junctions and driveway locations. Refer to Section 5 for further details on parking.

. . . . . . . . -


': The values shown am maxima and apply where SSD 3 cunre length. Land for vblbmty should be checked from the plans.

. .

CENTRAL OFFSET Xm

RADIUS Rm

\ ' QATAR HIGHWAY DESIGN MANUAL SECTION 2

Figure 2.4 Verge Widening for Full Overtaking Sight Distance

January 1997 . . , . Page 215


SECTION 3 HORIZONTAL ALIGNMENT

3.1 GENERAL

The most important consideration in determining the horizontal alignment of a road is the provision of safe and continuous operation at a uniform design speed for substantial lengths of road way. The major aspects influencing the horizontal alignment are; safety, design speed, topography, costs, vertical alignment and road classification.

All of these factors must be balanced to produce an alignment that is safe, economical, and in keeping with the natural contour of the land and the adjacent land use. Poor design will result in lower speeds and a reduction in the capacity of the road and safety.

The design of a road on straight alignment requires consideration of grades, sight distance, pavement, reservation cross section, etc. When horizontal curves are introduced; additional items including radii, transition lengths, pavement widening and superelevation require special attention.

In addition to the specific guidance given in this section, there are a number of general considerations which are important in producing a safe and economic design. These practices, as outlined below, are particularly applicable to high speed situations.

a) Flatter curves for a certain design speed should be, used where possible, retaining the most conservative standards as possible for the most critical conditions.

b) Compound curves consist of two or more consecutive curve alignments. They should be used with caution and should be avoided where conditions permit the use of a simple curve. Where compound curves are used, the radius of the flatter curve should not be more than 50 percent greater than the radius of the sharper curve for rural and -

urban conditions. On this basis, a several step compound curve may be used as a form of transition to sharp curves or a spiral, transitioning from one radius to the next. This condition can be relaxed for lower speeds at junctions and roundabouts.

c) Reverse curves on high speed roads should include an intervening tangent or transition section of sufficient length to

SECTION 3

provide adequate superelevation crossover between the curves.

d) Broken-back curves consist of two curves in the same direction connected with a short straight and should not be used. This type of curve is unexpected by drivers and is not pleasing in appearance. An attempt should be made to adopt one simple curve or even a compound curve.

e) ~onzontal alignment and its associated design speed should be consistent with other design features and topography. Co-ordination with vertical alignment is discussed in Section 4.5.

f) On dual roads, consideration may be given to independent horizontal and vertical alignments for each carriageway.

3.2 MINIMUM CURVATURE

The minimum curvature without the need for adverse camber, superelevation or transitions is shown in Table 3.1 below.

Table 3.1 Minimum Radii without transitions

Design Speed (kph)

1 40 1 20 1 00 80 70 60 50

Where the radius of curvature is less than the value indicated in Table 3.1, transition curves should be used.

Minimum Radius without Adverse

Camber, Superelevation or

Transitions (m)

3800 2880 2040 1300 1020 . 720 510

3.3 TRANSITION CURVES . . .

'The adopted form of transition between a straight and a horizontal curve is a clothoid, also known as transition curve. It provides a useful and logical section of the alignment for the development of superelevation and is the most common method adopted.

Where it is not possible to adhere to the values of curvature given in Table 3.1, a transition curve should be used.


QATAR HIGHWAY DESlGN MANUAL

The length of transition depends on the radius of the circular curve and the design speed. The basic length of the transition is given by the formula:

Where: L = length of transition (m) V = design speed (kph) 9 = rate of increase of

centripetal acceleration (rn/sec3)

R = radius of curve (m)

Normally, q should not exceed 0.3 mfsec3. However, in particularly onerous cases, it may be necessary to increase the value up to 0.6 mfsec3. On bends the length of transition should normally be limited to J(24FI) metres. For quick reference some common transition lengths are given in Table 3.2.

The elements for circular and transition curves are shown in Figure 3.1 and Figure 3.2.

Superelevation or elimination of adverse camber shall generally be applied on or within the length of the transition curve from the arc end. The basic transition appropriate to the design speed however will often result in insufficient transition length to accommodate superelevation turnover, and it will therefore be necessary to provide longer transitions to match the superelevation design. .

Transitions are not necessary in urban low speed areas such as junctions and service roads. !

Radius Design Speed (kph)'

140 120 100 80 70 60 50

2400 2200 2000 1800 1600 1400 1 200 1000 800 600 400 200 -

' Refs

82 51 89 56 . 98 62 36 109 69 40 122 n 45 140 88 51 163' 103 59 30

, 196' 123' 71 37 24 154' 89 46 31

I 119 61 41 26

91 61 39 22 122 77 45

Table 3.3 for restricted use of superelevation

Table 3.2 Basic Transition Lengths (m)

. . . . . 4" ..I ' SECTION 3

3.4 CAMBER AND SUPERELEVATION

On sections of road with radii greater than that shown in Table 3.1 for the given design speed, the crossfall or camber should be 2% from the centre of single carriageways, or from the central median of dual carriageways to the outer channels. At junctions other than roundabouts, the cross-section of the major road shall be retained across the junction, and the side road graded into the channel line of the major road. On horizontal curves, adverse camber shall be replaced by favourable crossfall of 2% when the radius for the given design speed is less than that shown in Table 3.1. However, it may be necessary to eliminate adverse camber on larger radii for aesthetic or drainage reasons. Provision of camber and superelevation in low speed areas such as commercial or residential areas has a tendency to encourage drivers to drive faster and should be avoided. Refer to Clause 3.8 for special considerations relating to low speed areas.

The following superelevation and minimum curves are recommended (Table 3.3).

Desig I n

Table 3.3 Minimum Radii with Camber and Superelevation

Minimum Radius (m) for I I

Speed (kph)

140 120 100 80 70 60 50

On radii less than those shown in Table 3.1 superelevation shall be provided, such that:

Where: .

7% may be only used at special locations and must have th permission of the Director of Civil Engineering Department prior t its use.

(a) Normal Camber

3800 2880 2040 1300 1020 720 51 0

S = Superelevation (%) V= Design Speed (kph) R = .Radius of Curve (m)

January 1997

(b) Adverse Camber

EUrninated

2880 2040 1300 1020 720 51 0 360

Page 312

(c) Superelevation

7%

1020 720 510 360 255 180 127

3.5%

2040 1300 1020 720 510 360 255

5%

1300 1020 720 510 360 255 180


Superelevation shall not exceed 5%. Only in special circumstances and with prior permission 'from the Director of Civil Engineering Department will superelevation greater than 5% be considered. Table 3.4 gives examples of si~perelevation for selected design speeds and radii.

Radius ( (m) Design Speed (kph)

140 120 100 80 70 60 90

2.88 3.15 3.15 2.31 3.47 2.56 3.85 2.83 4.33 3.18 2.21 4.95 3.64 2.53 5.78' 4.24 2.95 6.93' 5.09' 3.54 2.26

6.36' 4.42 2.83 2.17 5.89' 3.77 2.89 2.12

5.66' 4.33 3.18 2.21 6.63' 4.42

Circumstances see abwe

Table 3.4 Superelevation of curves (%)

Progressive superelevation or removal of adverse camber shall be achieved over or within the length of the transition curve from the arc end. On existing roads without transitions, between % and 2h of the cant shall be introduced on the approach straight and the remainder at the beginning of the curve.

Superelevation shall not be introduced, nor adverse camber removed, so gradually as to create large, almost flat areas of carriageway, to cause driver discomfort or to kink the edges of the carriageway. A satisfactory appearance can usually be achieved by ensuring that the camageway edge profile does not vary in grade by more than about 0.5% from the line about which the carriageway is pivoted, and by ample smoothing of all changes in edge profile. It is recommended to ensure that a minimum longitudinal gradient of at least 0.5% is maintained wherever superelevation is to be applied or reversed. The distance to satisfy this constraint is given by the equation:

Where:

G= rate of change of gradient (0.5%)

S = change in channel superelevation relative to the line about which the carriageway is pivoted (m)

L = length required to accommodate the change in superelevation (m)

When expanded, this formula provides the equation for the vertical reverse curve to be used for the superelevation curve. This reverse curve is shown in Figure 3.3

In some difficult areas, even the above requirements can lead to drainage problems, eg. where the superelevation is applied against the longitudinal gradient. It may be necessary to either modify the horizontal alignment to move the superelevation area, increase the variation in grade of the edge profile, or apply a rolling crown. Areas susceptible to such drainage problems should be identified at an early stage in the design process, before the horizontal alignment is fixed.

rJ+ w 1 . 6

I L I Y = ~sX 'A ' -2 sX'A'

where Y = offset S = maximum offset X = distance from start of application L = length of application

Figure 3.3 keverse Curve Formula

Figure 3.4 shows typical methods of developing superelevation by rotating about the edges and the centre of the road. The designer should use the most appropriate method to suit the situation. For dual carriageways, greater consideration of topography, cut and fill, catchment and median drainage is required.

January 1997 - -

- - Page 313

. . QATAR HIGHWAY DESIGN MANUAL . . .. * w .' SECTlON 3

Elements: P I = Point of Tangent Intersection BCC = Beginning of Clrcuiar Curve ECC = End of Clrcular Curve AC = Deflectlon Angle of Circular Curve R = Radius of Circular Curve T =Tangent LC = Length of Curve

Clrcular Curve

1

Figure 3.1, Circular Curve Elements

Elements: P I = Polnt of Tangent Intersection BTC = Beginning of Tnnsitlon Cunre BCC = Beginning of Clrcuhr Cunre ECC = End of Circular Curve ETC = End of Transition Curve A = Total Deflectlon Angle AT = Deflection Angle of Transition Cunre bc = Deflectbn Angle of Clrcular Cunre R Radius of Circular Cunre M T = Main Tangent TK =ShortTangentofTransitionCuwe TL = Long Tangent of Tranrltion Cunre XM = Abscbsa of the Center of Radlus Polnt AR = Circular Cunre Offset X = Abscissa of BCC or ECC Y = Ordinate of B.CC or ECC L T = Length of Transition Cuwe L c = Length of Clrculer Curve

Symmetrical Form of Transition to Clrcular Cunre

Figure 3.2 Transition and Circular Curve Elements

January 1997 - . Page 314

QATAR HIG~WAY- DESIGN MANUAL. . . . - . ' SECTION 3

Figure 3.4 Development of Superelevation

O l t sge edg~of- t ra~el led way

- - 4, - - n -,,-,- ~ l n ~ d ~ e d g e ~ t ~ v f l e d way

a q Profile control 4 \

Travelled way revolved about centreline

" . . . . . . - . . . - . . * . . - . - " . L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., . . . . .


Tangent Runout

0 I- W L

o 0 I- m

Superelevation runoff L o 0 0

Slope 1:400

Normal

0 0

crown -L.-

A

m w fde edgz of-traxeiled way

- =

B

- - - -I -

C D - . Inside edge

Profile control

Travelled way revolved about inside edge

Tangent - - Superelevation runoff I

0 0 W

Runout

2 m

Normal - - - a ---- Normal c j r o f i l e grade - Outside g g e of travelled way crown --- - _____------- I -

5 - - - - -

lnslde edge of travelled way

A B C D Outside edge 1 Profile control

Travelled way revolved about outside edge

Notes: A = Normal crown B = Level hlgh side normal crown low side C = Superelevation at normal crown rate D = Full superelevation

L

0 0

L, 0 0

'Li m

QATAR HIGHWAY DESIGN MANUAL . . .. cAs,- 7- 'SECTION 3

. - . - a . . C *. ' . . - . .

3.5 WIDENING ON CURVES

The rear wheels of vehicles do not follow the front wheels exactly on horizontal curves, and it is more difficult to steer the vehicle on curves. For these reasons it is recommended to increase travelled way widths on curves.

Widening is required for carriageways of less than standard width and for low radius curves-of standard width to allow for the swept path of long vehicles.

For carriageways of standard width, (3.65m. 7.3m and 11.0m for 1, 2 and 3 lanes respectively) an increase of 0.3m shall be allowed when the radius is between 90m and 150m. Two lane roads of width greater than 7.9m require no additional widening. Widening of road widths when the radii is less than 90m is covered in Section 6 Junction Design.

For carriageways less than the standard width, widening shall be as shown in Table,3.5.

Table 3.5 Application of Additional Lane Width

rr

January. 1997 - . . Page . 316

Additional WMth(m)

0.3

None

0.6

0.5

0.3

Lane Width

Standard Width

Standard Width

Standard Width

Less than Standard Width



Less than Standard WMth

Radius

Radlw less than 90m refer to Section 6

Radius between 90m and 1SOm

Radius greater than 150m

Radius less than 90m refer to Sectlon 6

Radius between 90m and 1 SOm subject to maximum carriageway widths d 7.9m and 11.9m (for 2 and 3 lanes respectively)

Radius between 1 50m and 300m subject to maximum carriageway widths of 7.3m and 11 .Om (for 2 and 3 lanes respectively)

Radius between 300m and 400m subject to maximum carriageway widths of 7.3m and 11 .Om (for 2 and 3 lanes mpecthfely)

0

\

. .

. .

Avoid reserve curve

. -

Transition curve may be widened on inside and outside

Simple curve may be widened on inside only

, - !

--- -

, . Figure 3.5 Widening of Pavement on Curves

January1 997 Page 3/1 !

QATAR HIGHWAY DESIGN MANUAL . - . .

3.6 HARMONlSlNG THE ALIGNMENT

The choice and arrangement of the linear elements are crucial factors in ensuring that the road will look right in its surroundings and will be pleasing to the driver of the vehicle. The design shall also provide a safe route, with the necessary stopping sight distances.

The aim of flowing alignment is to combine the various components in a manner which results in the road being experienced by the road user as a free-flowing, harmonious form without visual discontinuities. Such a design results in better integration of the road into the landscape and helps to make 'the road a construction which is visually pleasing from the viewpoint both of its users and those outside the road reservation.

.. - The principles df flowing alignment are closely linked with the way in which the driver sees the road line and in particular the shape of the road edges.

It is advisable to avoid small changes in direction in a flowing alignment. These are likely to appear unsatisfactory from the vehicle. Furthermore, small transverse displacements can present a confusing prospect for the driver.

Straight ) cuwe shofl f l Straight

1

Figure 3.6 Example of Kink

In all cases, when additional width is required, the extra width should be applied uniformly along the transition curve. . Where existing alignments are to be improved the widening should take place on the inside of curves. This is shown in Figure 3.5.

When two straights are connected, the use of a short horizontal curve is likely to cause the appearance of a kink, refer Figure 3.6. In such cases the impression can be improved by employing a larger radius, but an improvement orlly results provided the views being compared are taken from the same distance from the vertex of the curve, refer Figure 3.7. If the

SECTION 3 . . . . . . . . . . tJ, . . . .

viewpoint is at the same distance from the start of the curve then an improvement is not achieved, in fact the kink will appear to be rather more pronounced.

Figure 3.7 Improved View with Larger Radius

Even with a large radius curve, it is not possible to avoid the illusion of a sharp change in direction if the approach straights are sufficiently long, refer Figure 3.8. The best results are likely to be achieved with the flowing alignment when straights can be dispensed with. This of course is not always possible or in fact desirable. For example, in roads which are not dual carriageways, the sight lines on stretches of road where overtaking is permitted must be based on passing sight distance and not stopping sight distance. Straight lengths may then be required to achieve these sight distances. Also, it should be borne in mind that such effects will not necessarily be significant in the total view for any particular case. Each design should be considered in its landscape context. This is true of many aspects of internal harmony, although the greater the design speed, the less the external features modify the internal views. This occurs because vegetation and buildings are further back from the road edge, the carriageways are wider, sight lines longer and the roadworks generally constructed to a larger scale.

Abrupt changes in direction can be unsatisfactory on access roads as well as highways. In Figure 3.9 the straights have been joined without the use of a horizontal cunre. The appearance is quite different when a horizontal curve is added, refer Figure 3.1 0.

January 1997, . . # . . . . . . . . . . . . Page 318

.* L 8 # r C V C 4 L

QATAR HIGHWAY DESIGN MANUAL ' SECTION 3

...

Figure 3.8 .Illusion of a Sharp Bend with Long Straights

Figure 3.9 Angular Geometry ~ i ~ u r e 3.10 . .

Curved Geometry



Short straight sections of road should not be interposed between horizontal curves of opposite sense since the appearance of a kink is likely to result, refer Figure 3.1 1. A possible solution is the use of a pair of transition curves refer Figure 3.12. When designing for slower speeds or in the case of very large radii it may be feasible to join the two curves directly as shown in Figure 3.1 3. This could be done with care since here also an impression of lack of flow may result. t

_ ... x .. .

_ ~ ~ r c u ~ a r ' _ - Circular 7 -

curve Straight curve

Figure 3.1 1 Short Straight Between curves

Clrcular Circular - CU we C U I W

Transltlon Transition

Figure 3.1 2 Back to Back Transitions

Figure 3.13 Back to Back Circular Curves

4 C Ircu la r

C curve

. . ? . . - SECTION 3

.Circular CUTVB

Similarly, in the case of two subsequent curves in the same direction, the use of an intermediate short straight, as shown in Figure 3.14, is likely to produce an unsatisfactory visual effect. Here there may be the possibility of replacing the two curves and the straight with one circular curve, refer Figure 3.15. Another possibility may be to interpose one transition curve between the two radii, refer Figure 3.1 6.

A series of reverse curves is likely to produce a flowing alignment which is pleasing to the eye and comfortable for the driver. This type of line is ideal for integrating a route into an undulating landscape.

Figure 3.17 summarises alignments to be avoided and those to be attained where possible.

3.7 HORIZONTAL CLEARANCES

Generally, no structures apart from roadside .

furniture, such as signs and lighting columns, are allowed to fall within the road resetvations. The positioning of signs and other street furniture should be in accordance with the Qatar Traffic Manual. If it is not possible to position structures outside the reservation, consideration should be given to providing a safety barrier or safety cushions, refer clauses 5.15 and 5.16 respectively. Setback of crash barriers is dealt with in the clause referenced previously.

Structures should not be placed within 1.2m of the edge of the hard shoulder, or 0.6m of a kerbed road.

It is important to ensure that sight distance is .

not impaired, especially at junction and driveway locations. Refer to Section 2 Sight Distance.

January 1997 Page 311 0

. .. . A ' ' . .

QATAR H ~ G ~ W A Y - DESIGN MANUAL . . SECTION 3

Clrcular Circular . .

Figure 3.14 Two'Subsequent Curves in the Same ~irectiorr

Circular curve

[-\ ~ i ~ u r e 3.15 single Circular Curve

Transition

Circular

Figure 3.16 Single Transition Curve Between Two Curves

To be avoided

1 Small change of direction . . . . . ..

2 Short horizontal curve between two straights

3 Short straights between horizontal curves of opposite sense

4 Short straight between horizontal curves of the same sense

5 Out of balance alignment .

. To be attained

1 Well-balanced alignment

2 Use of curves rather than straights where feasible

Figure 3.1 7 Summary of Alignments to be avoided and those to be attained

. . . . . -. - : . . . . .. . . . . . .




Residential roads serve or give access to private dwellings'or properties. They should be designed to serve the needs of the residents and at the same time discourage through traffic by ensuring that the roads are not used as a short cut.

Generally the design of roads in residential areas and local street systems should consider the following:

a Local streets should be designed to rninimise through traffic movements.

a Street patterns should minimise excessive vehicle travel.

a The lo&l circulation should not have to rely on extensive traffic regulations or signs in order to function properly.

a Traffic generators within residential . * areas such as schools, masques or

shopping facilities should be considered in the overall design.

a The local street system should be designed for a relatively uniform low volume of traffic.

a Local streets should be designed to discourage excessive speeds.

a Pedestrian - vehicular conflict should be rninimised.

a Parking requirements should be provided without reducing visibility requirements or the safe operation of the road.

a There should be a minimum of intersections.

a Local streets should be related to topography from the standpoint of drainage, economics and amenities.

The speeds on residential roads are considerably lower than major, secondary and primary roads. As the dynamic element is not so critical, it is not normal to implement transitions as part of the horizontal alignment, nor to apply superelevation to a curve.

Table 3.6 Possible Road Sections at Bends

Case 1

Case 2

Case 3

Table 3.6 identifies three possible road sections at bends.

Standard cross section, carriageway falls from centre-line at nominal 2%. To continue this cross-section around a cunre would introduce adverse camber.

Minimum recommended centre line radius for local roads (TR3) is 130m and for access roads (TR4) is 55rn

, Normal 2% crossfall applied across the full section, falling from the outer kerb towards the inner kerb

Minimum recommended centre line radius for local roads (TR3) is 1 OOm and for access roads (TR4) is 45m

4% crossfall applied across the full section, falling from the outer kerb towards the inner kerb

Minimum recommended centre line radius for local roads (TR3) is 90m and for access roads (TR4) is 40m

The introduction of curves to residential roads is an effective form of speed control. However bends of smaller radius than those given in Table 3.6 exaggerate this effect and with particularly sev,ere bends, induce the sharp brakingfacceleration behaviour which has been identified as undesirable.

In short cul-de-sac or loops, such as 60m or less in length, where speeds are low the desirable minimum inner kerb radius is 15m with an absolute minimum of 1 Om.

The minimum radii to be provided at junctions is discussed in Section 6 Junction Design.

The typical driveway should be designed for passenger-car operation only. For a 90 degree turn, an inside radius of 5m and an outside swept path of a 9m radius will comfortably accommodate most drivers in all passenger cars. Temporary encroachment on the wrong side of a residential street while entering a private driveway is generally considered allowable. For higher traffic volumes expected at the driveways of school or apartment car parks, increased driveway widths are recommended.

The visibility requirements for bends on residential roads is detailed-in Section 2.

January 1997 - . Page 3M2


SECTION 4 VERTICAL ALlGNMENT

4.1 GENERAL CONTROLS

Vertical alignment consists of a series of gradients connected by vertical curves. It is controlled by safety, topography, highway class, design speed, horizontal alignment, construction costs, adjacent development, drainage, vehicular characteristics and aesthetics. The vertical alignment is usually referred to as the profile.

A smooth prbfile with gradual changes, consistent with the class of highway and the character of the terrain, is preferable to a vertical alignment with numerous sharp breaks and short lengths of gradient.

A "roller coastern or "hidden dip" type of profile should be avoided. A smoothly rolling profile, rather than a straight profile can often result in economy of construction, without sacrificing operating'characteristics and aesthetics.

As the driver progresses along the profile with increasing chainage, an increasing gradient is denoted', as being positive (+ve) and a decreasing gradient is denoted as being negative (we).

A broken-back profile (two vertical curves in the same direction separated by a short section of tangent grade) is not desirable, particularly i n sags where a full view of the profile is possible.

Where an' at-grade intersection occurs on a highway with' moderate to steep grades, the gradient through the intersection shall be reduced if possible. This is beneficial for vehicles making turns and stops, and serves to reduce potential hazards.

A superelevation runoff occurring on a vertical curve requires special attention in order to ensure that the required minimum vertical curvature is maintained across the pavement. For example, the lane profile on the opposite side of the road from the superelevation control line may have sharper curvature due to the change in superelevation rate required by the superelevation runoff. It is therefore necessary to check both edges profiles and adjust where necessary in order to maintain the desired minimum vertical curvature.

In flat terrain, the elevation of the profile is often controlled by drainage. The vertical profile must be positioned such that adequate drainage structures can be constructed. In areas where the surface water is above the ground level or the groundwater table is immediately below the

SECTION 4

surface, the profile shall be established so that the low edge of the finished shoulder is at least 0.5m above the temporary water level. If the water table is permanent then the road formation level should be at least 1 .Om above the table due to the possibility of capillary action. In areas of rock, if practical, the profile should be established so that the low edge of the finished shoulder is at least 0.3m above the rock level. This should avoid unnecessary rock excavation.

For aesthetic reasons the length of vertical curves should be substantially longer than the length required for stopping sight distance.

4.2 MAXIMUM AND MINIMUM GRADES

Generally gradients should be fixed to be consistent with the topography through which the highway passes in order to minimise excessive unnecessary earthworks. The maximum gradients for design purposes shall be as shown in Table 4.1.

Table 4.1 Maximum Gradients

Route Classification

Primary Route Secondary Route Tertiary Routes

LocaVDistrict Distributor MajortMinor Access Cul de Sac

In residential areas, where properties lie adjacent to the road, the desirable maximum gradient is 3.3%. Gradients approaching "Stopn or "Give Way" junctions should be a maximum of +/- 2% for a minimum of 15m before the "Stop" or "Give Way" line. Refer Figure 4.1.

Max. Grade ("/.I 4 6

6 10 .

. 10

4 Major Road , Minor Road

1Sm min .. L tZ% max grade '

I

Figure 4.1 Vertical Alignment at T-Junction Approach

For drainage purposes, a desirable minimum longitudinal gradient of 0.5% on kerbed roads shall 'normally be adopted. The absolute minimum longitudinal gradient for kerbed roads shall be 0.3%. In flat areas careful consideration should be given to drainage requirements.

. - - January 1997 Page 411

QATAR HIGHWAY DESIGN MANUAL -. . .

The use of over edge drainage may also be considered in conjunction with surface channels or ditches in rural areas. Refer to Section 8 for further details on drainage.

4.3 VERTICAL CURVES

Vertical curves shall be provided at all changes in gradient, except at junctions and on lower classes of roads where the arithmetic change is less than 0.5%. The curvature shall be large enough to provide for comfort and where appropriate, stopping sight distances for safe stopping at the design speed. The use of the permitted vertical curve parameters will normally meet the requirements of visibility. However, stopping sight distance should always be checked because the horizontal alignment of the road, presence of crossfall, superelevation or verge treatment and features such as signs and structures adjacent to the carriageway, will affect the interaction between vertical curvature and visibility.

A vertical curve is a curve on the longitudinal profile of a road which allows for a change of gradient.

A crest (summit) curve is a vertical curve which is convex in shape. Generally the sign of the gradient as the driver travels up chainage, changes from +ve to zero to -ve.

A sag (valley) curve is a vertical curve which is concave in shape. Generally the sign of the gradient as the driver travels up chainage, changes from -ve to zero to +ve.

t 1

A K-value is a constanf'telated to the comfort of the driver.

Vertical cuMe lengths can be determined by multiplying the K-values given by the algebraic change of gradient expressed as percentage, ie +3% grade to -2% grade indicates a grade change of 5%.

For dual carriageways curvature shall be derived from the appropriate K-value in Table 4.2.

Table 4.2 K-values for Dual Carriageways

Design Speed (kph)

140 1 20 100 80 70 60 50

, . . . . , . SECTION 4 . , . , . . .

For single carriageways where the horizontal alignment has been designed to allow overtaking, full overtaking sight distance should not be obstructed by crests. Conversely there is no merit in providing an overtaking crest if the horizontal curve does not permit overtaking. K-values for vertical curvature on single carriageways are given in Table.4.3

There are two prime factors that affect the choice of crest curvature, visibility and comfort. At design speeds of 50 kph and above, a crest in the road will restrict forward visibility to the minimum stopping sight distance before minimum comfort criteria are approached, and consequently desirable minimum crest curves are based upon visibility criteria. . This is discussed further in Section 2 Sight Distance.

Desirable Minimum

K-value for Crest

230 1 82 1 00 55 30 17 10

Table 4.3 K-values for Single Carriageways

Design Speed (kph)

100 80 70 60 50

Absolute Minimum K-value

Particular attention is needed on dual carriageways to check any restriction to visibility caused by safety fences, median kerbs, bridge piers, etc. especially at combined horizontal and vertical curvature.

Crest

1 82 100 55 30 17 10 6.5

Visibility at sag curves is usually not obstructed unless overbridges, signs or other features are present. For these curves, comfort criteria apply. The maximum rate of vertical acceleration is to be taken as 0.3m/sec*. However for design speeds of 70 kph and below, in unlit areas, flatter sag curves are necessary to ensure that headlamps illuminate the road surface for at least the required stopping sight distance. Sag curves should normally be designed not less than the absolute minimum K-values in Table 4.3.

Minimum K-value for

an Overtaking

Crest

400 285 200 142 100

Sag

50 37 26 22 20 13 9

Where, at crests, the sight line is across the verge, consideration shall be given to the design of a lower verge profile in order to allow for a maximum overall height of landscaping of 0.5m

More generous sag curves may be required under bridges and through underpasses etc. in order to maintain the envelope of required sight distances.

Avoid Crest

K-values in this Range

400 - 100 285 - 55 200-30 142-17 100-10

January 1997 -

Page 412

Absolute Minimum K-value

Crest

55 30 17 10 6.5

Sag

26 20 20 13 9

QATAR HIG~WAY- DESIGN MANUAL

The choice of vertical profile is fixed mainly by the geometric standards but is also influenced by the nature of material in the cuttings and the total earthworks. Ideally a balance should be achieved between cut and fill, and the calculations should include compaction factors for shrink and swell and allowance for suitable and unsuitable material.

Due to the topography of Qatar, it is unlikely that steep gradients would be implemented which would require a climbing lane. However, if a scheme including a climbing lane was to be considered, reference should be made to the UK Department of Transport publication, Design Manual for Roads and Bridges, Volume 6, Section 1, Highway Link Design (TO 9/93).

4.4 HARMONlSlNG THE VERTICAL ALIGNMENT

This section should be read in conjunction with Clause 3.6, harrnonising the horizontal alignment and Clause 4.5, combining the horizontal and vertical alignment.

The valley curve plays an important part in achieving internal harmony in the alignment, especially since it can often be viewed along its whole length at one time. This is not normally possible in the case of .crest curves but for this reason particular attention must be paid to ensuring that visual continuity is maintained. This leads to the avoidance of short summit curves even though they may satisfy visibility requirements.

O radien t Gradient

curve

Figure 4.2

Figure 4.3 -

As is the case with horizontal curves and straights, when a valley curve is used to join two gradients, or a gradient and a level length, the

Gradient - . L

Gradient 4:

radius must be sufficiently large for the appearance of a kink to be avoided, refer Figures 4.2 and 4.3. Even large radii will give the appearance of an abrupt change in direction if the viewpoint is sufficiently far from the curve, but this is unimportant since, at great distances, it will not be found disturbing. Drivers do not tend to become aware of an approaching valley curve until they are about 500m from the start.

Valley curve

Figure 4.4

Tangents, especially short ones, between two valley curves can result in an awkward looking line, refer Figures 4.4 and 4.5.

Figure 4.5

A vertical curve is seen as a hyperbola.. Whether or not the junction of a tangent and a vertical curve presents the appearance of a kink depends on the curvature of the sharpest bend of the hyperbola and its location in relation to the end of the tangent. It is desirable that the hyperbola does not start at the position of its, smallest radius. In critical cases it is advisable to examine perspective drawings of the line. An indication of the effect of small and large radius. vertical curves on the drivers view are shown in Figures 4.6 and 4.7 respectively.

Figure 4.6

January 1997 - -

Page 413

QATAR HIGHWAY DESIGN MANUAL .-... , . - . SECTION 4

- Summit

Figure 4.7 Figure 4.1 0

A level' length of road containing a short low When a terrace is created by a sequence of summit curve can cause a visual discontinuity summit and valley curves, whether or not there since the distant length of road, diminished in are tangents between the curves, it is likely to size by perspective, can be seen over the crest, result in an unsatisfactory view if two summits refer Figure 4.8.

. . . can be seen at the same time. An example is shown in Figure 4.1 2.

Figure 4.8 - .. --

Similarly the use of a reverse curve in the longitudinal section, causing a small change in level, can result in a visual discontinuity duelo the road surface disappearing from view and then reappearing. An example of the effect when a reverse vertical curve is used in conjunction with level straight lengths is .shown in .Figure 4.9. A view of this type can occur with a double reverse curve, refer Figure 4.1 0. In the case illustrated, the line'can be improved by increasing the length of the valley curve and decreasing .those of the summit curves, refer Figure 4.1 1.

Figure 4.9

January 1997 a

Figure 4.1 1

Figure 4.12

The lower the terrace is placed and the shorter its length the more disturbing it is likely to appear, since it can be viewed from a shorter distance. All terraces tend to appear unsatisfactory when seen from the top. As with the horizontal alignment, the ideal solution for the vertical alignment. is a series of well modulated vertical curves proportioned so that they avoid the problems discussed. Such a solution can, of course, only be used when the land form and other controlling factors make it possible.

Page 414

I 1 - QATAR HIGHWAY. DESIGN MANUAL . .. SECTION 4

I Table 4.4 summarises the type of vertical L , alignment to be avoided and that to be attained.

Longitudinal section

Figure 4.1 3

1

Short summit curves between gradients

Short vailey curves between gndiants -

Short tangent between summit curves

Short tangent between valley curvas

Reverse verticai curve causing small change in level, on r level length or gradient

A level length or gradient containing r low valley curve

A level length or gradient containing a low summit curve

Terracing on which two summits can be seen at one time

Well balanced alignment

Use of curves rather than straights or gradients where feasible

1 ,

1-1

r l - ' = l I

-

If the out of phase lengths are small this is not likely to be significant. In fact it is probably advantageous to have overlap. This may be considered to contribute to the integration of the two aspects of the line. When an overlap is used it should normally be small in comparison with the len.gth of the element. Yet there are exceptions to this:' the plan and profile combination of the type shown in Figure 4.1 4 will ' probably produce awkward looking perspectives. . In this arrangement, the horizontal curve ends at ' the same point as the vertical curve begins.

X

x

X

X

X

x

X

,/

January3 997 a Page 4/5

Table 4.4 Summary of desirable/ undesirable combinations of - . vertical alignment

Figure 4.14 4.5 PHASING OF HORIZONTAL AND

VERTICAL ALlGNMENT If prevailing conditions prevent using longer,' coincident curves, it is possible to achieve

To obtain a satisfactory alignment it is important significant improvement if longer overlapping to integrate the vertical and horizontal aspects of curves are implemented as shown in Figure the line. In order to accomplish this, the engineer 4.1 5. should consider the road as a three-dimensional unit. The elements of the horizontal and vertical alignment should be in phase wherever possible. In other words, the corresponding elements in the horizontal and vertical planes should start at approximately the same points, and end at approximately the same points, refer Figure 4.1 3.

QATAR HIGHWAY DESIGN MANUAL . , . . ..., .:, .. I : . . . . i . ! ~ *j.;:a,*SECTION 4 ,..,

Figure 4.15

The best results would be obtained if co- ordinated curves of longer radius could be used, refer Figure 4.1 6. The following combinations of horizontal and vertical alignment are some additional examples of those which are likely to result in an awkward appearance. A summary of desirable/undesirable combinations of alignment is shown in Table 4.5.

Figure 4.16

Table 4.5 Summary of desirablelundesirable combinations of alignment .

January 1997 . . : Page 416

Notes

A short valley curve within a horizontal curve. This is similar to the case of a short valley curve occurring along a strdght, but the impression of discontinuity will probably be even more pronounced.

Figure

4.17

I I-/ A LOVO~ 1.

bVd -- Summll W W O

VmUoy C U N O

Summlt W N O

. . . '.. . . i I - . . . . , . . . . . , .. ,


4.18

4.19

-

4.20

4.21

- Horhontal curve - Low summit curve within a horizontal curve. Here too the discontinuous appearance is liable to be even more pronounced than with the low summit curve on a straight.

A short vertical curve connecting gradients in a long horizontal curve. This arrangement is liable to result In

the alignment. between illusion tangents of Small a pronounced are changes as undesirable in kink direction in the in

the vertical plane as they are in the horizontal plane.

. . A horizontal curve foll&king a straight

and starting on a valley curve which follows a gradient. This combination tends to give the horizontal curve the appearance of a sharp bend.

a Valley curves joined by a level length or gradient and occurring along a straight followed by a horizontal curve.

Valley undesirable curves in joined themselves by a tangent but when are combined with a horizontal curve in this way they can produce the results show-

*

- Horhonbl ounm -

xe ondbnt Vtz Ondbnt t

-

Horizontal stnlght , CUWO -

I -

Omdbnt Valley Curve I

---+==- 1e Ondknt oune Lwml cuwo Omdknt

bl

- -

Table 4.5 Summary of desirable/undesirable combinations of alignment

QATAR . . .

4.22

4.23

4.24

4.25

4.26

January 1997 . . . . . . . . Page418 . . . .

HIGHWAY DESIGN MANUAL

summn V a l v Omdtmnt C.W. CUW. nfdmnt -- - - -

4

Hokenld _ Stnight I C U N ~

I- 2 - 8

Vatmv Summl ouwm Oradbnt aunm ... c

I HoNonlal

.--

4 .

,

Horkontal

Horizontal Hortronlal eunm -1 - oun. _

- .

SECTlON 4 I . J

A summit curve followed by a valley curve occurring along a straight followed by a horizontal curve. A disjointed effect is liable to result when the beginning of a vertical curve is hidden from the driver by an intervening summit while the continuation of the curve is visible In the distance beyond.

A tangent length between a vertical curve and a compound curve. Wherever possible such a tangent a-b should not be used. Instead the vertical alignment should be so arranged that the curves can be joined directly.

A short horizontal curve within a long valley curve. This combination can result in the appearance of a kink.

A short horizontal curve occurring on a short summit curve. This can be dangerous since the driver is unable to see the continuation of the curved horizontal alignment. An even more unsatisfactory case would be if the horizontal curve started immediately over the summit.

A reverse horizontal curve with the change in cwature situated at the top of a sharp summit curve. This also is a dangerous arrangement since the driver is not able to anticipate the change in curvature.

. . QATAR HIGHWAYDESIGN MANUAL S E C ~ O N 4

Table 4.6 shows a summary of. ,the, combinations of horizontal and vertical alignment to be avoided and that which is to be attained. - - - - - 7

7 u 7

- 7 - - n - - - - dllb

w

Table 4.6 Summary of desirable/ undesirable combinations of

I

horizontal and vertical alignment

X

X

X

, X

)(

X

X

x

X

X

X

X

4.6 VERTICAL CLEARANCES )

\ B - - - - Horizontal cuwe

containing a low valley curve within its length -

Horizontal curve containing a loW summit cuwe within its length

Short vertical curve batwean gradients in a horkontal cuwe

Horizontal cuwe foilowing a straight and starting on a valley curve which follows a gradient

Valley cuwe Joined by a level length or gradlent and occurring along a

straight followed by a horlzontal cuwe

Summit curve followed by a valley cuwe occurring aiong P straight followed by a horkontal curve

A tangent length between a vertical curve and a compound curve

Short horizontal curve within a long valley curve

Short horizontal cuwe occurring on a short summit cuwe

Reverse horizontal curve with the change in curvature situated at the top of a sharp summit cu we

Out of phase alignment

Badly balanced arrangement

The minimum vertical clearances are specified to prevent vehicles or their loads from coming into contact with any structure or roadside furniture.

The minimum clearance over the carriageway is 5.5m. This is to be provided across all trafficked lanes including and shoulder or edge strips. The figure of 5.5m allows for 200mm of pavement construction which may be applied during the maintenance of the road.

J

J

J

Minimum clearance shall be provided to a.ll structures or roadside furniture that overhangs' the carriageway. These include any bridge or building structure, sign gantry, overhead cables. or suspended lighting.

Horizontal and vertical curves in phase (the visual continuity can often be improved by having the horizontal elements slightly leading the vertical ones)

Where possible use three- dimensionable curves and avo. the use of straights

Use a well balanced three dimensional alignment

Where a public utility specifies a minimum, vertical clearance to its plant then the greater of. the clearances must be provided for. Protective measures may be required at overhead cable crossings such as guardwires. Guidance may, be sought from the Ministry of Electricity and: Water when planning works in the vicinity of their installations.

Where a road passing underneath a bridge is- on a sag curve, the headroom given above. shall be increased in accordance with Table 4.7. The sag radius is measured along the- carriageway over a 25m chord. a

. .



Table 4.7 Sag Radius Compensation

Sag Radius (m)

1000 1200 1500 2000 3000 6000 >6000


Additional Clearance

(mm)

80 70 55 45 25 15 nil

Special considerations for vertical alignment are required in many areas, one of the greatest concerns to the engineer in Qatar being the flatness of many areas and subsequent surface water drainage difficulties. Section 8 details the recommended minimum gradients and comments on the importance of drainage in nearly level areas.

Below are listed a number of vertical alignment considerations specific to certain conditions that the engineer should be aware of:

Residential and Commercial

a Need to match threshold levels in areas of existing development

a Preferred maximum slope across housing plots is 1 in 30

a Valley points where water may collect should be kept away from residential accesses

a Road alignment should preferably be kept below adjacent property level

a Minimum length of vertical cutve should be 30m due to construction tolerances

a . Vertical alignment changes where abrupt or repetitive (such as in flat areas) can be disguised by being made at the horizontal bends

a Levels of existing utilities require consideration regarding the vertical alignment of new roads

a Footpath maximum longitudinal gradient to be 1 in 10. Steps may be used to overcome worse gradients but are not preferred as they limit access by wheelchairs

. . . , i - ': . ::'-- :. ' - SECnON 4 . ..

Maximum driveway gradients to properties shall be 1 in 15.

a Low retaining walls/planters may be used to assist in matching road levels to existing plot boundaries. However, they shall n i t be allowed present a hazard to vehicle or pedestrian traffic.

Industrial . . .

a Maximum' gradients to be 1 in 20 due to road usage by heavy vehicles.

a After long or steep down gradients, heavy vehicles may require additional level areas for braking distance or emergency run-off lanes.

a On long or steep up gradients, heavy vehicles may require climbing lanes to allow faster vehicles to pass.

a Change in transverse or longitudinal grade should not be significant so as to cause loss of load.



SECTION 5 CROSS SECTIONAL ELEMENTS

5.1 ROAD RESERVATIONS

In general the different road reservations are intended to provide drivers with adequate sight distances and allow the public utilities sufficient space for existing and proposed plant. Where space for utilities is limited, 'way leaves" outside the road reservation may be obtained by contacting the relevant planning authority.

Figures 5.1 to 5.9 show cross sections depicting the essential elements in typical sections for two way single carriageways and dual carriageways for urban and rural roads. Each of the different elements comprising the cross sections is discussed in detail in the following clauses. The cross sections shown are typical and the final layout of the reservation should be agreed with the Director of the Civil Engineering Department.

The recommended reservation details for rural roads are similar to those for urban roads but reflect the reduced access and drainage requirements of the rural situation. Generally for the rural situation the near side of the carriageway would not be kerbed although flush kerbing may be considered at certain locations. Raised kerbing to the median of rural dual carriageways should only be provided at specific locations eg. bridges, U-turns. In all cases an edge strip shall be provided between the kerb and lane edge. Verges shall be designed to fall away from the carriageway in the rural situation and thus water will drain to surrounding ground.

Shoulders are not normally required on rural single carriageway roads but, generally, edge strips would be included in the design. Edge strips may also be considered as an alternative to full hard shoulder construction on rural and urban dual carriageways for economic reasons.

A standard lane width of 3.65m has been used on all typical cross sections illustrated. Exceptions are permitted where it is necessary to maintain continuity with the remainder of an existing route, and in new development areas such as Salwa Industrial Area and the New District of Doha.

In order to provide adequate drainage, a standard crossfall of 2% has been applied for carriageways and medians.

Generally the centreline of the main carriageway shall be located on the reservation centreline. However, should existing or

SECTION 5

proposed land use require carriageways to be offset to one side then approval from the Director of the Civil Engineering Dept must be sought. . .

In the case of road centrelines being offset from the reservation. The utilities layout shall be revised to suit the specific road cross section proposed, the revised utility locations to be to the approval of the Utility Authorities.

If the engineer is unable to utilise recommendations from the typical cross- sections because of existing buildings, building usage or land ownership problems, for example, then advice should be sought from the Director of Civil Engineering Department before proceeding.

Certain special routes, such as abnormal or exceptional load routes or. scenic routes, may require individual reservations to satisfy their performance criteria, eg. the Corniche. In these cases, consultation should be sought with the Director of Civil Engineering Department.

In many areas of existing development, road corridor widths or alignment may be restricted by property ownerships or old planning. In many cases, 12m reservation widths were once the norm. In these situations the designer must pay particular attention to many factors including sight distances, clearance at junctions, utility location, reduced carriageway widths, restricted access and road closure.


QATAR HIGHWAY DESIGN MANUAL SECTION 5 , . . I . . , - I . . . v ' -

Figure 5.1 Typical Urban 16m Reservation

January 1997 . . .. . . . .. . Page 512

. . . , . . , - . - , . , . ,.. . ., .. 1 ., . - QATAR HIGHWAN DESIGN MANUAL . ... -. - . I..., " ... SECTION 5 :

. . , 3 Figure 5.2 Typical Urban 2Om Resenration

January 1997 " ' Page 513

QATAR HIGHWAY DESIGN MANUAL .. .. , .-. , SECTION 5 -

Figure 5.3 Typical Urban 24m Reservation

January 1997 . Page 514

. 6 4 . .- - ,; 6ibQ18k QATAR H~GHWAV DESIGN MANUAL. -. -

Figure 5.4 Typical Urban 32m Reservation (Wide Single Carriageway)

. . . .-


QATAR HIGHWAY DESIGN MANUAL .. - SECTION 5 - . .

Figure 5.5 Typical Urban 32m Reservation (Dual Carriageway)

January 1997 . . . . Page 516

. . . L QATAR HIGHWAY DESIGN MANUAL

SECTION 5

6

3 Figure 5.6 Typical Urban 40m Reservation

January 1997 Page 5l7

QATAR HIGHWAY DESIGN MANUAL SECTION5 P ,

L . .


* . . ' QATAR HIGHWAY DESIGN MANUAL . 1 , I

.SECTION 5'.

Figure 5.8 Typical Rural Single Carriageway

Januarjr 1997 " Page 519

QATAR HIGHWAY DESIGN MANUAL . SECTION 5 i . - 1 -

: 2- - . -'.. - - . - - I - . r . . - . - . .

Figure 5.9 Typical Rural Dual Carriageway

January 1997 Page 5/10 . - . -


5.2 LANE WIDTHS

Lane widths have a great influence on the safety and comfort of driving. It has been shown that undesirable conditions are generated on two-lane, two-way, rural roads, carrying moderate traffic, with road widths less than 6.5m. Furthermore, it has been shown that narrow widths severely affect the capacity of a road.

In general, the road width to be provided should be 7.3m, based on a lane width of 3.65m. This provides adequate clearance between passing commercial vehicles. In certain circumstances it may be necessary to increase the road width to 1 1.3m. This may be considered on local distributor roads to facilitate future improvements to turning movements as the traffic volume increases. This 1 1.3m width comprises two 3.65m wide lanes with an additional 4.0m to facilitate the turning lane.

'The width of turning lanes is discussed in Clause 5.1 0.

Where. the road edge is kerbed, the carriageway should be increased in accordance with Clause 5.5.

. 4 . . . . - . . . : .

SECTION 5

The use of 4.0m lane widths may be permitted in particular situations to maintain continuity with the remainder of an existing route. If the length of new road concerned is significant, consideration should be given to adopting a tie- in for economic reasons. Where an existing road with 4.0m wide lane widths is to be redesigned, the lane widths should be redesigned as 3.65m wide.

, , Generally lane markings should be allowed for as Figure 5.1 0.

Edge lines - line provided within the edge strip.

Lane lines -included within the carriageway ,

width.

Figure 5.1 0 Lane Line / Lane Width Relationship

I

i I

Edge - Lane Wdth Lone Width - Edge I

- . . January 1997 Page 511 1

- I Stdp I I

,

Stdp

Edge $++E , Edge ,

L I ~ O ~ l n e '

w I m I I

I I

'

QATAR HIGHWAY DESIGN MANUAL . . . . . . .. -. - .; . .' SECTION 5

5.3 LANE CAPACITY provision sh'ould be made for further improvements to existing sections.

In addition to strategic importance and safety, the desired characteristics of traffic flow will generally determine the class of a road. For example, high volumes of traffic are generally associated with urban Primary Routes, where as low volumes are associated with Tertiary Routes.

Table 5.1. Recommended Road Capacity

Lane Provision

Single Lane 2-Lane Dualling 3-Lane Dualling

In most urban situations, the capacity of intersections on a particular network will govern the capacity of the network as a whole. Uninterrupted flow only takes place when the influence of at-grade intersections can be neglected. This is rarely the case on most urban road systems.

Road Capacity (vehhour)

1,600 3,200 4,800

For detailed assessment of highway capacity and level of service for different roads, refer to The Kingdom of Saudi Arabia, Ministry of Communications, Highway Design Manual, Volume 2, Design of Roadways, Section 1.03.

5.4 SHOULDERS The capacity of .a highway is affected by the composition and the habits and desires of the traffic using the road system and the controls

. that the designer imparts onto the traffic. These include:

The addition-of a shoulder to the nearside edge of a road has many advantages. Shoulders provide structural support for the pavement edges, emergency parking space for stopped vehicles and also provide side clearance between moving vehicles and stationary objects. They also provide additional running lanes for diversions and road maintenance. Shoulders are not usually required on urban single and urban dual carnageways as structura.l support is provided by the kerbs and channels and stopped vehicles can find a safe place to rest in driveways and side streets. The shoulder may be paved to the same standard as the carriageway or of lesser construction such as to road base construction. The merits of using a lesser construction should be considered accordingly for each particular situation.

- Commercial vehicles

- Lane distribution

- Variations in traffic flow

- Traffic interruptions.

Under ideal conditions, vehicles can follow one another at average minimum headways of about 1.8 seconds, giving a maximum flow rate of about 2,000 vehicles per hour. A line of vehicles can start up with an average minimum headway of about 2 seconds giving a maximum starting-up rate of approximately 1,800 vehicles per hour. These maximum rates are reduced by many prevailing road and traffic conditions.

Where there is a high traffic volume, narrow shoulders give very poor service. There is a greater number of accidents and they incur more frequent and costly maintenance. When two or more lanes are available for traffic

in a single direction, the distribution in lane-use will vary widely. 'The lane distribution will depend on traffic regulations, traffic composition, speed and volume, number and location of access points, origin-destination patterns of drivers, development, environment, and local driver habits.

In deciding whether to include a shoulder, the engineer should consider the following:

a) Additional width provides a place for safe stopping because of mechanical difficulty, flat tyre or any other emergency. This also minimises disruption to traffic flow. Due to the above factors, there are no typical

lane distributions. The recommendation for 1,600 vehicles per lane per hour recognises that flow in some individual lanes will be higher and in others lower. Refer Table 5.1. .

b) Additional width provides space for increased mobility to escape potential accidents or reduce their severity.

C) Stormwater drainage is improved as the water can be discharged further from the running carriageway.

At the planning stage, major routes should be planned and designed as multi-lane, divided, controlled access facilities even though they may be developed by staged construction. In the plans for each stage of development,

. - Page 511 2 January 1997


d) Additional width 'increases sight distance in cuttings and improves lateral clearance to signs and safety fences.

e) The apparent openness of the inside lane reduces driver stress.

f) A cost benefit analysis should be carried out at the initial scheme assessment phase.

Where shoulders are provided a width of 3.0m should be used at a standard crossfall of 2 percent or as an extension of the crossfall of the carriageway.

5.5 EDGE STRIPS AND SHY DISTANCES

Edge Strip - Edge strips provide a safer carriageway, with improved drainage and more space to move in case of an emergency. Edge strips keep roadside debris away from the running width of an outside lane and prevent edge loss on the running lane.

Edge strips are to be provided on all roads which are not kerbed.

A width of 0.5m is deemed sufficient for an edge strip width for a median edge on a dual carriageway. The edge strip width shall be allowed for within the standard median width and shall not reduce the lane width. Refer Table 5.2.

Kerb + 0.5

'Whilst awaiting services and kerbs to be installed, a

w Single Dual 2 Dual 3

temporary edge strip 0.35m shall be added to give a carriageway width of 8.0m.

Table 5.2 Edge Strips & Shy Distances

0.35 Shoulder Shoulder

Shy Distance Where a kerb is provided there is a tendency for drivers to steer a distance away from the kerb, this is termed "shy distance'. At slower speeds the requirement for shy distance is reduced and conversely, at higher speeds, an increased shy distance is required. Where there is an edge

0.5 0.5

strip there is no need to provide a shy distance.

It is recommended that a shy distance of 0.5m should be added to the road width for each kerbed road edge on roads with a design speed greater than 80kph. On kerbed dual carriageway roads of design speed less than or equal to 80kph, a shy distance of 0.35m shall be added to the outside edge as a gutter. Refer Figures 5.1 - 5.7. The shy distance is an additional pavement width and the lane width shall not be reduced. Shy distance at junctions is discussed further in Section 6.

5.6 MEDIANS

Medians are used to separate opposing traffic lanes on dual highways. They provide protection from interference by opposing traffic, minimise headlight glare, provide space for utilities and future lane width, provide additional space for crossing and turning vehicles at at- grade junctions, and a,llow pedestrian refuge in urban areas.

A median may vary in composition from say a 1.2m width with a pedestrian barrier to a 20m wide median with street lighting, drainage and landscaped areas. Medians are dependant on the width of reservation available and the functional requirements of the median. Often, consultation with the relevant planning authority is required prior to agreement of the width and function of the median. Preferred standard median widths are given in Table 5.3.

Intermediate Wide

2.0 6.0 12.0

Table 5.3 Preferred Standard Median Widths (m):

Narrow Narrow medians are those in the range 1.2m to less than 4.0m and are used in restricted conditions. Medians 1.2m wide do not provide a refuge area for pedestrians but do provide the minimum space permitted for clearance of opposing traffic provided the lane edge is kerbed. Narrow medians are used where there is a need to provide a divided road, but where the available reservation does not permit a greater median width. Narrow medians are not wide enough to provide effective left turn lanes. The minimum allowable median width to provide a safe pedestrian refuge is 3.5m. Pedestrians ability to cross at narrower medians shall be controlled or actively discouraged by the provision of barrierslhigh kerbs, continuous planting and other features.

- --



It is not recommended that narrow medians are used on rural roads.

A narrow median should not be considered if it is possible to provide an intermediate or wide median at that particular location. Acceptable standards for the remaining cross section elements should be maintained.

Intermediate Intermediate width medians are those in the range 4.0m to less than 8.0m and are generally wide enough to provide for a left turn lane. A width of 6.0m is the desirable minimum to provide a left turn lane and a residual median, and a width of 8.0m is the desirable minimum to shelter a crossing vehicle undertaking a U-turn manoeuvre.

Wide Medians 8.0m or greater in width provide space for effective landscaping and may be used for signing, services and drainage. Wide medians may also be used to absorb level differences across the road reserve. Rural medians should be a minimum of 8.0m wide with a central safety barrier.

A disadvantage of wide medians occurs at signalised junctions, where the increased time for vehicles to cross 'the median may lead to ineffective signal operation.

Wide medians should not be implemented at the expense of reduced verge widths. Verge widths are required for pedestrian walkways, installation of services, traffic signs, drainage channels, parking etc. Any significant reduction in verge width may result in hazards in the verge which negate the advantages of a wider median.

It is recommended that urban medians should be kerbed and that rural medians should be provided with an edge strip and not kerbed. A kerbed median is desirable where there is a need to control left turn movements and is also used when the median is to be landscaped. In the rural situation, a depressed median is preferred as this improves drainage of the road.

Special attention should be given to drainage of medians. If the median is kerbed, the median surface should be designed to have slopes of 2 percent, and should fall towards the centre of the median if unpaved, or slope out if paved. Depending on whether the median is paved or open, or planted or not, the drainage should not interfere with the operation of the highway. Paved medians may require positive drainage systems incorporating manholes, pipes etc. Nonpaved medians may be self-draining, but

January 1997 . -

SECTION 5 -- again, consideration should be given to the provision of additional storage capacity or

f - outlets to allow for storm conditions. All L' drainage inlets in the median should be designed with the top flush with the ground, and culvert ends provided with safety grates so they - will not be hazardous to out of control vehicles that run off the road.

It is common practice to landscape medians. This is seen to provide a better environment and reduce driver stress. Careful consideration should be given to the choice of planting to ensure that visibility and stopping distances are not impaired. Furthermore, the upkeep of the landscape and growth of the plants should be designed for minimal maintenance and hence less disturbance to the road user.

L-

Watering shall not require tankers to obstruct the trafficked lanes at any time.

Where two abutting sections of highway have i

different carriageway widths it is desirable that a smooth transition should accommodate this difference. The transition should be as long as b

possible for aesthetic reasons and preferably occur within a horizontal curve.

5.7 VERGES L

The verge is a width of the reservation which facilitates additional functions essential for the - operation of the road. As a minimum verges must be able to accommodate highway signs, structures, utility services such as water,

L-

electricity, Q.TEL, drainage, and additionally such items as traffic signals and street lighting. Where a verge is adjacent to a development a set back may be required. Verge widths may - vary from a desirable minimum of 3.0m up to the limits of the reservation, which could be in excess of 15.0m. Paved verges should be .-

designed with a 2% fall towards the carriageway for diainage purposes. However, in larger paved areas, falls shall be designed to specific drainage collection points in the verge. L

It is important to ensure that whatever is installed in the verge (such as structures, signs . -

or landscaping) does not affect the sight distances required for the particular design speed of.the road. Additional care should also

. .L- be taken at traffic signals and junctions where more signage is implemented.

Verges may be paved, landscaped or graded L- - depending on the intended use of the verge.

It may be necessary to increase the verge width if soakaways are to be installed within the verge.

L

Page 511 4 .-

Due investigation of utilities to be accommodated shall be made at the design stage.

5.8 PARKING BAYS AND LANES

The need for parking is determined by the existing and future development of the immediate surrounding area. Consultation will be required with the Traffic Section and the Planning Department to determine the future development plans and the amount of on-street and off-street parking required.

Where possible, parking shall be provided away from the carriageway and in convenien,tly located, specific lots or along service roads. Parking should not be provided near junctions or opposite access points as this is likely to increase the probability of accidents and also hinder sight distance.

Provision for parking is achievable by the following methods.

Parking Lanes (parallel parklng) Parking lanes may be provided adjacent to the inside lane of the carriageway (ie. the slow lane). The standard width required for a parking lane is 2.5m, each bay being nominally 6.5m in length. Care should be taken when providing a parking lane to ensure that the design speed is appropriate to allow a safe stopping distance, if for example, a passenger were to accidentally step into the carriageway whilst embarking or disembarking a vehicle. It is recommended that parking lanes should only be provided on single carriageway roads, with posted speeds of 50 kph or less. The lane provision, design speed, stopping sight distance and traffic volumes should also be appropriate to allow minimal interruptions to traffic flow when vehicles are entering or leaving the parking lane.

Parklng Bays (angled parking) If the width of available reservation allows, consideration should be given to the provision of parking bays. Parking bays should not be permitted on the main through carriageway of dual carriageways. The perpendicular parking bay should be made up of stalls 3.0m wide and 6.0m in length. The dimension requirements for angled parking are shown in Table 5.4 and Figure 5.9. Parking in bays requires greater adjacent lane width to accommodate the turning movement depending on the choice of parking angle. .

Stall widlh. parallel I A ( 4.25 I 3.50 I 3.25 1 3.00 1 I to aisle

Dimension

I Stall depth to wall ( C 1 '6.40 1 6.70 ( 6.60 1 6.00 1

On Figure

5.9

stall lmes

Stall depfh interlo$

Angle

Mod Je, wall lo interlock

Table 5.4 Parking Bay Dimensions (m)

45-

E

Bumper overhang H (typiml)

H

Intarbck to Karb Modulo Modulo Modulo

X = Ct.1 not avmlbblo h 8art.b Imyouts

F

Figure 5.9 Parking Bay Dimensions for , 3.0m x 6.0m Stalls

80.

5.30

'Dimensions for 3m by 6m stalls

0.60

Service Road Parking If there is sufficient reservation width, consideration should be given to the provision of a service road to access either a parking lane, parking bays or designated car park. This results in a safer highway 'and fewer interruptions to through traffic, and enables flow to be maintained more easily. Refer to Clause 5.1 1 for service roads.

18.20

Parking on Access Roads where residential development is dense and the requirement for additional on-street parking is likely, then the standard parking lane width of 2.5m shall be used. The minimum parking .lane width is 2.2m. The designer should bear in mind that the very low 'number of vehicles using, access. .roads means occasional on-street parking by visitors or delivery vehicles will not cause congestion. In fact, their presence will help to keep the speed of other vehicles low.

75'

5.95

0.70

January 1997

?om

17.65

Page 511 5

620

0.75

6.00

19.90

0.75

20.00


Parking Exclusions Parking shall be excluded from the following locations:

a Junctions; to provide space for pedestrians to cross and to maintain adequate visibility. See Figure 5.1 0

• Bends; to maintain adequate forward visibility for drivers

a Pedestrian crossing points; to minimise . crossing width and enable crossing pedestrians to be seen clearly by drivers

, Any other location where parking would cause .. .. unsafe conditions.

-

'In all cases parking must not encroach on visibility splays.

~ i~ure '5 .10 Typical Parking Lane Treatment at T-Junctions.

5.9 SIDE SLOPES

Side slopes fall into two categories, embankment and cutting. They serve two maln functions; firstly they provide structural stability to the road, secondly they provide a surface on which out of control vehicles may travel and recover, minimising the chance of overturning.

Where possible the side slopes should fall away from the verge at a slope of 1 in 5. It is usual to consider the provision of a safety fence when slopes are steeper than 1 in 5 and/or the height of the slope is greater than 6m. Safety fencing is discussed in Clause 5.15. Generally, it is better to use flatter slopes, providing there is adequate fall for drainage. Slopes in cutting should not be steeper than 1 in 2 and preferably should be 1 in 3 to allow mechanical maintenance equipment to be used on the slope. If there is insufficient width which would require slopes steeper than 1 in 2, then partial or full retaining walls should be used or some method of slope stabilisation. Retaining walls should- be set back from the carriageway to avoid a constricting feeling and reduce stress for the driver.

SECTION 5

Cut and fill dopes should be flattened as appropriate with the topography and be consistent with the overall type of highway. The intersection of slope planes in the highway cross section should be well rounded to simulate natural earth forms. The rounding and flattening of slopes minimises drifting and wash out of loose material such as sand and hence reduces maintenance costs.

It is recommended to carry out an adequate geotechnical investigation prior to specifying slopes. The investigation will determine the maximum slopes for cut and fill and the criteria for benching or erosion protection if required.

Benches should ideally be 4.0m in width and laid to falls of approximately 1 in 20 to avoid ponding of water and consequential slip failure.

In rock cuttings it is recommended to include ditches and a debris verge to provide a safe landing and catchment area for possible rock fall, and removal of surface water run off. This additional width also provides a useful area for rock face maintenance. It is becoming common practice in the UK for rock outcroppings to be left in place for reasons of economy or aesthetics. This may be considered for application in Qatar. However in such situations this may prove lethal if a vehicle were to collide with the outcrop. It is recommended that at all such locations a safety fence be provided. Refer to Clause 5.1 5 for safety fences.

For details of sand slopes, wind blown sand and dune control refer to the Kingdom of Saudi Arabia, Ministry of Communications, Highway Design Manual, Book 2, Section 1.1 6, Sand Dune Control.


QATAR HIGHWAY DESIGN MANUAL . .

, n .

SECTION 5

5.1 0 AUXILIARY LANES

Auxiliary lanes serve as speed change lanes, storage lanes or a combination of the two. They may also be either right turn or left turn facilities at junctions (refer to Section 6 Junctions). A speed change lane is primarily for the acceleration or deceleration of vehicles entering or leaving the through traffic lanes. A speed change lane should be sufficient in length and width to enable a driver to make the necessary change between the speed of operation on the through highway and the lower speed necessary to turn, with minimal disruption to the speed of following vehicles. Speed-change lanes can have different layouts depending on the alignment of the highway, frequency of intersections and the distance required to effect the necessary change of speed.

. .

Refer to Section 6 Junctions for further details on the following topics.

Deceleration Lanes A deceleration lane consists of a taper and a full lane width. The length of deceleration lanes should be. determined according to the design speed of the highway and the design speed necessary to make the turn. The greater the difference between these speeds the longer the deceleration lane should be. Deceleration lanes on approach to at-grade intersections can also function as storage lanes for turning traffic.

Acceleration Lanes Design considerations for acceleration lanes are similar to those for deceleration lanes. Acceleration lanes are provided to permit an increase in speed before entering the through- traffic lanes and also to serve as manoeuvring space, so that a driver can take advantage of an opening in the adjacent stream of through-traffic and join it.

Left and Right Turn Lanes The provision of separate left and right turning lanes should be determined by a capacity analysis for the junction under consideration. Acceleration and deceleration tapers should be used with these turning lanes. The length of turning lanes shall depend upon the length required for speed change and the number of vehicles to be stored. Typically the storage length is based on the number of vehicles that are likely to accumulate in two minutes, as determined by the capacity analysis, and is calculated by the following formula:

Where S = Storage length (m) N = Design volume of turning

vehicle (vehicles per hour) L = Length in metres occupied by

each vehicle (7m for passenger vehicles, 12m for trucks)

For further details of junction design and lane capacity refer to Section 6 Junctions and the Qatar Traffic Marrual.

5.11 SERVICE ROADS

Service roads are roads which run roughly parallel with, and are connected to the main through highway; They are generally of low design speed and preferably restricted to one- way traffic. Figure 5.7 shows a typical reservation with a service road.

Service roads provide a number of functions depending on the development of the surrounding area. The provision of service roads reduces the number of access points onto the main highway and segregates the higher speed through traffic from the lower speed local traffic. This reduces interruption of traffic flow, makes the best use of road capacity and results in a safer road.

service roads may also provide an alternative route if maintenance is required on the through road or in case of an emergency.

The width of the service road is dependant on the classification of traffic expected to use the service road such as light vehicles, delivery lorries or heavy goods vehicles. Further consideration should be given to the turning requirements of such vehicles, the type and number of access points and type of street parking, if required.

Service road connections to the main road should be. designed as at-grade junctions in accordance with the guidelines given in Section 6 Junctions.

Where one-way service roads'are to be installed within the reservation the absolute minimum width of outer median permitted is 1.2m provided no signing is required. This distance allows for the provision of a central pedestrian barrier only. If traffic signs are required or other street furniture the desirable minimum width is 2.lm. A wider outer median is preferred, but this will depend on the width available within the reservation. Wider outer medians give greater scope for landscaping which enhances the appearance of both highway and the


QATAR HIGHWAY DESIGN MANUAL - . . - . .. . .. SECTION 5

appearance of both highway and the development adjacent to the highway.

Refer to HMSO publication, Designing for Deliveries for detailed explanation and guidelines of requirements for service roadslareas, and turning movements for different vehicle types.

5.1 2 . PEDEST RlAN FACILITIES

Pedestrian facilities are generally found within the verge and at road crossing points. The provision of paved pedestrian areas is related to the function of the roadside development. It is often difficult to obtain reliable estimates of pedestrian volumes and movements. For this reason, studies should be carried out at the concept and preliminary design stage. All urban roads and junctions shall allow space for footpaths unless the area strictly forbids walking. A width of 2.0m should be provided depending on pedestrian needs. The width of paved pedestrian areas should be increased to a minimum of 3.0m near schools, large sports venues; commercial areas or other areas with high pedestrian volumes. Footpaths may be constructed of paving blocks or concrete and laid to crossfalls of 2% towards the roadway to permit drainage.

Where possible a separation area should be included within the verge which acts as a buffer between vehicular and pedestrian traffic. The separation width should be designed to discourage pedestrians from standing at the kerbside. This is achievable by providing a number of obstacles such as low planting, raised blockwork or pedestrian barriers. A minimum separation width of 1.2m is desirable. A separation width is not required in commercial areas with on street parking where wider footpaths are usually provided.

Pedestrians should be actively discouraged from 'crossing roads along the length of dual carriageways. Special pedestrian refuge sections should be provided at selected points, or ideally at junction locations. It is recommended that these refuge areas be a minimum of 3.5m wide and should be staggered so that pedestrians are not able to approach and cross both carriageways in one line.

On roads with a posted speed of 60kph or less, it is recommended to provide a pelican crossing (signalized pedestrian crossing) or a zebra crossing (pedestrian crossing defined by road markings) as a crossing point for pedestrians. These crossings should be located, signed and marked in accordance with this manual and with the Qatar Traffic Manual.

In areas with high volumes of pedestrian traffic, footpaths should be provided on both sides of the road. Some urban areas and most frontage roads can be served with a footpath on one side only. In these areas, footpaths must be continuous for the full pedestrian route.

On rural roads, footpaths are not usually required, except along sections of road where there is substantial residential or commercial development. In such situations, footpaths are usually located between the bottom of the embankment and the property line.

Pedestrlan Ramps In order to provide adequate and reasonable access for the safe and convenient movement of pedestrian and handicapped persons, including those in wheelchairs, kerb ramps should be included at all pedestrian crossing points. Kerb ramps should be at least 0.9m in width, sloped at the rate of 1 in 12 or flatter, and located on the pedestrian side of the kerb face.

The edge of the ramp facing the carriageway shall be flat and set 25mm above the level of the road pavement. Drainage equipment such as gratings should not be placed in ramp areas where they may caused a hindrance to wheelchairs.

Structures for Pedestrlan Movements The need for a pedestrian grade separated structure such as a footbridge or underpass must be investigated in some depth for each particular situation. The investigation should cover studies of pedestrian generating sources, travelling patterns, crossing volumes, classification of road to be crossed, land use, location of adjacent crossing facilities, and social and cultural factors. The structure to be provided must accommodate handicapped pedestrians and those with wheelchairs. Ramps should be provided to a preferred grade of 1 in 12. However, a maximum grade of 1 in 10 may be used in difficult locations. Level landing areas of 1.5m length should be installed such that no individual ramp section is longer than 9.0m. Handrails should be provided on all steps and ramps. The width of the walkway should be a minimum of 2.5m between walls or railings. It may be necessary to install pedestrian barriers in the vicinity of the structure to deter pedestrians from crossing the road at-grade.

A pedestrian overstructure is preferred to an underpass. An over structure should be designed'to be in keeping with the surrounding area in terms of geometry and architecture. The required headroom clearance for overst ructu res is detailed in Clause 4.6. Lighting and fencing should be considered on a site by site basis.



pedestrian underpasses shall be well lit with clear unobstructed visibility. A pavement or ramp approaching an underpass should provide a clear view through the underpass. The desirable headroom clearance through the underpass is 3.0m.

Specific consideration needs to be given to the drainage of underpasses both for the removal of rainwater and effects of high groundwater levels.

5.1 3 UTILITIES

Road corridors are given in Figures 5.1-5.9. These are intended to provide adequate space for road cross section requirements and at the same time allow the public utilities. sufficient space for existing and proposed plant. Where space for utilities is limited, "wayleavesn outside the road resenration may be obtained by contacting the planning department.

The public and private utilities to be accommodated include the following:

- Telephone (Q.TEL)

9 Cable television

Electricity - distribution

- Electricity - lighting

o Electricity - transmission

- . . . Sewerage

o Return effluent

9 Surface water and land drainage

- Water

9 Oil and gas.

Each utility has their own working procedures and works specifications. These shall be referred to when designing the road construction and drainage facilities.

Particular consideration may be required to position soakaways if the reservation width is restricted. Refer to the typical cross sections shown in Figures 5.1 to 5.7. Where space is limited, soakaways may be lowered, by the addition of rising sections, to allow shallow utilities such as Q.TEL to pass above the soakaway chamber. However, in new roads, priority is to be given to road related utilities, eg. drainage, lighting etc.

? , . , . , . . . . . -. . . . . SECTION 5 '

5.14 USE OF KERBS

There are a number of types and combinations of kerbs available, each with particular applications. Some of the details in regular use are listed below.

- Raised kerb

- Raised kerb with channel block ..

. .

o ~ d ~ e 'kerb

- Channel block

- lush kerb

- Dropper kerb

- Dropped kerb

Vehicle barrier unit (VBU). ,

The standard kerb unit is available in a range of sizes and shapes. The shape is varied to enable kerbs to be installed on a range of radii. It is recommended to check the availability and dimensions of kerbs with the manufacturer as a full range may not be available in Qatar.

Kerbs provide a number of functions which are: to define and provide structural support to the edge of carriageway; to control highway drainage; to segregate vehicles and pedestrians.

Kerbs are tobe used on all urban roads and" only at special locations on rural roads, such as. junctions where there is a need to give a clear delineation of the road edge.

Where there is a need to install a safety fence alongside a kerbed section of road, the fence design, kerb design and drainage design should be carried out together. The kerb may affect the choice of safety fence type, and it is important to ens.ure that the combined drainagekerb facility does not reduce the safe operation of the safety fence.

. .



5.15 SAFETY FENCES

General A safety fence is a longitudinal barrier used to shield motorists from natural or man-made hazards located along a road. It may also be used to protect bystanders, pedestrians and cyclists from out of control vehicular traffic. Safety fences may be located in the verge or median depending on the particular requirements and location. Refer to Figure 5.1 1 for the definition of terminology used in safety barrier design.

The safety fence is designed to prevent an errant vehicle from leaving the carriageway and striking a fixed object or feature that is considered more hazardous than the barrier itself. This is accomplished by containing and redirecting the-errant vehicle.

On a divided road, a safety fence is located in the median to separate opposing traffic.

Safety fences should only be installed if they reduce the severity of accidents. This may appear subjective, but generally a barrier should be provided if the consequences of a vehicle striking a fixed object, or running off the road are determined to be more serious than hitting the safety fence itself. Other considerations are trafric speed and traffic volumes and a cost ,analysis.

'The cost analysis is based on:

• Removing or reducing the hazard so that it no longer requires protecting

Installing an appropriate safety fence

• Leaving the hazard unprotected.

Median safety fences are generally provided where the median width is relatively narrow and the traffic volumes and speeds are high. They may also be provided where the separated carriageways are at different levels, as the likelihood of an accident increases the steeper the slope between carriageways. It is important to provide gaps in the median fencing for emergency use and maintenance.

Embankments Embankment height and side slope are factors ih determining safety fence need. The provision of safety fencing should be considered when slopes are steeper than 1 in 5 and/or the height of the slope is greater than 6m, refer to Figure 5.12. Rounding slopes reduces the chances of an errant vehicle becoming airborne. The optimum rounding may be defined as being the

January 1997

SECTION 5 . .

minimum radius a standard. size car can negotiate without losing tyre contact. This is dependant on approach angle and speed as well as the characteristics of individual vehicles.

Roadside Obstacles A safety fence should only be installed if it is clear that the result of a vehicle striking the barrier will be less severe than the accident resulting from hitting the unprotected object.

Generally, if an object is greater than 1 Om from the travelled way, it does not requlre protection.

Table 5.5 summarises of the various needs for safety fencing.

Pedestrians The most desirable solution to protect the innocent bystander is to separate pedestrians and vehicular traffic. If this is not achievable then consideration of safety fencing should be given at schools, busy commercial and retail centres, sports venues and other locations where high pedestrian movements are anticipated or observed.

. . , . . . . . . Page 5120

. .

Figure 5.1 1 Definition of Safety Fence Elements

. . . . . . . . . . . . . . . . . ,

, . - . .' , .* .


TRAVELLED WAY , SHOULDER / FILL SECTON EMBANKMENT

R = ROUNDING

.

. .

Figure 5.1 2 Requirement for Safety Fences on Embankments

QATAR HIGHWAY DESIGN MANUAL , .

. SECTIONS

. . . . ,

0 n

.Om 3m 6m 9m 1 2m 15m 18m Height (m)

Barrier not Required for Embankment. However, Check Barrier need for other Roadside Hazards.

- January 1997 . Page 5/22

-

QATAR HIGHWAY DESIGN MANUAL . . . . . - SECTION 5

Types of Standard Sections of Safety Fence Safety fencing is usually classified as flexible, .semi-rigid or rigid.

Flexible systems are generally more forgiving than other categories, as much of the impact energy is dissipated by the deflection of the barrier and lower impact forces are imparted on the vehicle. There are two basic types of flexible system:

The first is a cabled fence, normally comprising 4 strands of tensioned cable. Cable fences redirect impacting vehicles after sufficient tension is developed in the cable, with the posts . in the impact area providing only slight resistance. The cable fence returns to ,its original position and damaged posts are easily replaced. . . .

The second type utilizes a standard steel beam section mounted on relatively weak posts. This system acts in a similar manner to the cable safety ferrce; It retains some degree of effectiveness after minor collisions due to the rigidity of the beam rail element. However, after major collisions it requires full repair to remain effective. As with the cable system, lateral deflection can be reduced to some extent by closer post spacing. This system, as with all barriers having a relatively narrow restraining width, is vulnerable to vaulting or vehicle under- ride caused by incorrect mounting height or irregularities in the approach terrain.

Semi Rigid Systems work on the principle that resistance is achieved through the combined flexure and stiffness of the rail. Posts near the point of impact are designed to break or tear away, distributing the impact force to adjacent posts. Deflection of this type of beam is up to approximately 1.5m (test data; 26 degrees, 95kph, 1.8 Tonnes)

Strong post fences usually remain functional after moderate collisions, thereby eliminating the need for immediate repair. There are a number of different types of semi rigid fence on the market, each system having its own performance requirements and capabilities. A few examples are listed below:

Box Beam

Open Box Beam

W-Beam (corrugated type of fence)

Blocked Out W-Beam

Self-Restoring Safety Fence

The self-restoring safety fence is a high performance fence designed to be maintenance free for most impacts and capable of containing and redirecting large vehicles. The combination of high initial cost and high performance makes this barrier more suited for use at high accident frequency locations.

When traffic speeds are expected to be greater than 50kph the semi rigid system should be tensioned. Tensioned systems usually require a minimum length to be effective and are unable to be installed on sharp radii (typically 50m length and 150m minimum radii). Individual barrier manufacturers specifications should be adhered to.

Table 5.5 . Consideration for the Provision of Safety Fencing !

Object

Bridge piers, abutments and railing ends

Culverts, pipes, headwalls

Cut slopes (smooth)

Cut slopes (rough)

Ditches (transverse)

Embankments

Retaining wall

Signs and iuminaire supports

Traffic signals

. Trees and utility poles

Permanent bodies of water

Rigid Systems offer no deflection when hit by a vehicle. The impact energy is absorbed by the vehicle. For high angle and high speed impacts, passenger size vehicles may become partially airborne and in some cases may reach the top of the barrier. For shallow angle

Comment

Protection generally required

Judgement required based on size, shape and location of hazard

Generally protection not required

Judgement required based on likelihood of impact

Generally protection required, ditch profile to be considered

Judgement required based on height and slope

Judgement required based on relative smoothness and anticipated maximum angle of impact

Generally protection required for non-breakaway supPo@

lsokited traffic slgnals on high speed rural roads may require protection

Protection may be required depending on site by site conditions

Judgement required based on depth of water and likelihood of encroachment

January I997

QATAR HIGHWAY DESIGN MANUAL . SECTION 5

impacts, the roll angle toward the barrier imparted to high centre of gravity vehicles may be enough to permit contact of the top portion of the vehicle with objects on top of or immediately behind the fence, eg. bridge piers. Commonly used rigid systems are the New Jersey Barrier in the USA, and the British Concrete Barrier in the WK.

Typically the system is relatively low cost, has generally effective performance for passenger- sized vehicles and has maintenance-free 'characteristics.

End Treatments The untreated end of a safety fence is extremely hazardous if hit, as the beam element can penetrate the passenger compartment and will generally stop the vehicle. A crashworthy end treatment is therefore considered essential if the safety fence terminates within 10m of the travelled way andlor is in an area where it is likely to be hit head-on by an errant vehicle. The termination of the safety fence should not spear, vault or roll a vehicle for head-on or angled impacts. For impacts within the length of need, the end treatment should have the same redirectional characteristics as the standard safety fence, which means that the end must be also properly anchored.

There are a number of different types of end treatments which work on a range of principles, some of which are listed below:

- Breakaway Terminals

- ' Turned Down Terminals

- . Energy Absorption Systems

- Special Anchorage for Cable Fence

- Anchorage into Embankment

Further reference is essential to select the most appropriate system for each particular situation.

Transitions Transition sections of safety fence are necessary to provide continuity of protection when two different barriers join, when a barrier joins another barrier system (such as. a bridge rail) or when a roadside barrier is attached to a rigid object (such as a bridge pier).

The transition section should be the same strength or stronger than the two systems.

The transition should be long enough so that significant changes in deflection do not occur within a short distance. Generally the transition

January 1997

length should be 10 to 12 times the difference I in the lateral deflection of the two systems in question eg. for a beam dellection of 1.5m the transition should be around 15m.

G- I -

I Drainage features such as ditches should be - avoided at transition positions as they may initiate vehicle instability.

The stiffness of the transition should increase \\,. -

smoothly and continuously from the less rigid to \.

the more rigid system. This can be achieved by decreasing the post spacing, increasing post - size or strengthening the rail element.

Selectlon of Safety Fence L

The selection process is not easily defined but the most desirable system is one that offers the required degree of protection at the lowest total

L

cost. Table 5.6 summarises the factors to be considered.

* - 4

I

L

Page 5/24 -


I Design Speed ( Setback from Edge of Pavement (m)

. . SECTION 5 .

Placement Lateral offset: As a rule, safety fences should be placed as far from the travelled way as conditions permit. This gives the errant driver the best chance of regaining control of the vehicle without having an accident. It also provides better sight distance. Table 5.7 gives suggested lateral offsets related to the design speed. Other factors may override these suggested figures.

1

2

3

4

5

6 a)

b)

c)

d)

7

8

I 50 I ' 1.0 I Note: Rigid system is not recommended for design speeds greater than 100kph

Table 5.7 Desirable Lateral Clearance for Safety Barriers from Edge. of'Travelled Way.

Criteria

Performance Capability

Deflection

Site Conditions

Compatibility

Cost

...

Maintenance: Routine

Collision

Materials Storage

Simplicity

Aesthetics

Field Experience

The desirable minimum distance between back of fence and rigid hazards should not be less than the dynamic deflection of the safety fence for impact by a vehicle at impact conditions of approximately 25 degrees and 100kph.

Comments

Fence mus! be structurally able to contain and redirect design vehicle

Expected deflection of fence should not exceed available room to deflect

Slope approaching the fence and distance from travelld way may preclude use of some fence types

Fence must be compatible with planned end anchor and capable of transition to other safety fence systems

Standard fence systems are relatively consistent in cost, but hlgh performance railings can cost significantly more

Few systems require a significant amount of routine maintenance

Generally, flexible or semi- rigid systems require signi f icant ly more maintenance after a collision than rigid or high performance fences

The fewer the different systems used the fewer inventory items and storage space required

Simpler designs cost less and are more iikely to be reconstructed properly on site

OccasionaHy safety fence aesthetics are an important consideration in its selection

The performance and maintenance requirements of existing systems should be monitored to identify problems that could be lessened or eliminated by using a different fence type

Specific manufacturers requirements must be followed. However, as a guideline, the '

clearances set out in Table 5.8 are typical.

I Ba*erType Clearance from Back of I Fence to Hazard (m) I

Tensioned wire rope

Tensioned beam

Box beam

Rigid I O* I 'Minimum clearance of oMects behind the barrier to travelled way must be maintdined.

Table 5.8 Typical Manufacturers Table 5.6 Selection Criteria for Safety Clearance Requirements

Fences On embankments care should be taken to ensure that at full deflection of the fence the wheels of the vehicle do not overhang the edge of the slope.

The combined use of kerbs and flexible safety fences together shotlld be avoided. The use of kerbs and semi-rigid or rigid safety fences shouid generally be avoided. However, if the face of the safety fence is within 22Smm of the kerb face, a vehlcle is not likely to vault the fence.


QATAR HIGHWAY DESIGN MANUAL ' I - . .

A safety fence is considered flared when it is not parallel with the carriageway. Flare is normally used to locate the barrier terminal section further from the carriageway, to minimise a driver's reaction to a hazard near the road by gradually introducing a parallel safety fence installation, to connect a roadside barrier to a hazard nearer the carriageway such as a bridge parapet or railing, or to reduce the total length of rail needed. Reference Figure 5.1 1.

Flare rates are a function of design speed and safety fence type. Bearing this in mind, Table 5.9 shows typical flare rates.

Design flare Rate Flare Rate for Fence Speed for Fence (kph) within

1-:x System

1:17 1:15 1:13

1:14 1:11 1:17 1:13 1:8 1:7

'Refer to manufacturers technical literature for special conditions.

Table 5.9 Typical Flare Rates , .

The length of safety fence required should be such that it protects the vehicle for the full extent of the hazard. This includes the length of the approach flare, the length of the hazard and the runout length beyond the hazard. The runout length is particularly important on single camageways where protection is required for vehicles travelling in the opposite lane.

Underground Obstructions Where there is a risk of driven posts or standard concrete footings interfering with cables, ducts and pipes and the alignment of the safety fence cannot be adjusted to avoid the obstruction, or the depth of pavement construction is such that the standard driven post or concrete footing would not penetrate into the subgrade, special posts or footings shall be provided with the approval of the Director of Civil Engineering Department.

Existing Systems With the development of technology and understanding of this subject, it is a fact that older installations are sub-standard and do not always meet current recommended performance levels. These deficiencies usually fall within two categories, those that have structural inadequacies and those that are improperly designed or located.

SECTION 5 . . .

These installations will require upgrading to current standards and each installation should be considered on a site by site basis.

For further reference on the different types of safety fencing refer to the British Department of Transport document TD 19/85, Safety Fences and Barriers, and the American Association of State Highway and Transportation Officials publication, Roadside Design Guide. For details of specific safety fences the manufacturers' technical literature should be referred to.

5.16 CRASH CUSHIONS

Crash cushions or impact attenuators are protective devices designed to prevent errant vehicles from impacting fixed object hazards. This is achieved by gradually slowing down a vehicle to a safe stop (from possible head-on impacts) or by redirecting a vehicle away from the hazard (for side impacts). Crash cushions are ideally suited for use at locations where fixed objects cannot be removed, relocated or made to breakaway, and cannot be adequately protected by a normal safety fence.

Crash cushions primarily serve to lessen the severity of accidents rather than to prevent them from happening.

Crash cushions work on one of two principles, either absorption of kinetic energy or transfer of momentum. In the first instance the kinetic energy of a moving vehicle is absorbed by crushable materials. This can be achieved by the use of water filled containers. Crash cushions of this type require a rigid back stop to resist the impact force of the vehicle. -

The second concept involves the transfer of momentum of a moving vehicle to an expendable mass of material or weights. This may be sand filled containers. Devices of ,this type require no rigid back stop.

The design procedure is relatively straight fornard and basically relates to the number of crash cushion units being able to slow down a design vehicle, at a design speed under an acceptable deceleration force. Most manufacturers have design charts to select an appropriate layout.

The most common application of crash cushions is at an exit ramp at an elevated or depressed structure, ' where a bridge pier requires protection. However, they may also be used at temporary r o d works or used to slow a vehicle down on a slope when the brakes have failed. For optimum use, the crash cushion should ideally be placed on a relatively flat surface. Kerbs should also be



avoided as this may cause the vehicle to become airborne.

The effective use of crash cushions is restricted to cars travelling up to speeds of IOOkph, and not applicable for large trucks and buses.

There are many different manufacturers of crash cushion systems, each with there own particular merits and applications. However, the engineer in the selection process must consider the site characteristics, cost, maintenance and the structural and safety characteristics of the different systems.

For further reference on the different types of crash cushions refer to the American Association of State Highway and Transportation . Officials . publication, Roadside Design Guide. For details of specific crash cushions, manufacturers technical literature should be referred to.

5.17 FENCING

There are many different types of fences used within the road reservation, each type having particular applications. The main types of fencing are listed below:

Right of Way Fencing to delineate and separate private property from the road reservation

Safety Fencing erected where considered necessary. Refer Clause 5.15

Animal Fencing prevents animals from entering the highway reservation. The size and type of fencing is dependant on the type of animal the fencing is intended to control, eg. camel or goat

Acoustic Fencing may be required in sensitive locations such as residential areas to lower the traffic noise level. The' fence forms a barrier and the sound is reflected away from the sensitive area

Headlight Barriers may be implemented at locations where it is desirable to minimise the glare of the headlights of oncoming vehicles, such as at unlit bends on rural roads

Pedestrian Access Fenclng . may be required where there are significant numbers of pedestrians such as on commercial streets, outside schools or

. .- . . .. . . .. . . .- .-. 1 .

January 1997

. .- .

SECTION 5

: large sports complexes where crowds may gather. The fencing controls the movement of pedestrian traffic and lowers the risk of a pedestrian accidentally moving onto a live carriageway.

5.18 ROAD CLOSURE AND PARTIAL CLOSURE

The main aims of full or partial road closure are to:

Deter non-access traffic from using residential roads as through routes

Limit the nurr~ber of minor accesses onto major routes

-' Remove the crossroad type junction which is generally considered unsafe. Refer Section 6 Junctions

Although these aims are common to the design of new roads, the approach here is different as established route patterns, many having been in use for years, have to be broken and

'

reformed elsewhere. Provision of clear,' concise warning andlor diversion signs are advised during the first two to three months of operation. This will help re-educate the driver who wasfamiliar with the old road layout. "

The most basic way to prevent traffic using a particular route is to close the road, either at a particular point or along a certain length. It is usual to close a road at an existing junction,' ie. at the end of a block of properties, unless the block is very long, eg. 250-300m, in which case "No Through Roadn signs must be displayed at the open end(s) of the road.

End of block closures could be made simply by the use of "No Entry" signs, but these may prove to be ineffective, particularly if drivers approaching a closure can see traffic moving beyond it. Hence it is preferable to provide a physical barrier to prevent drivers violating the restriction. This may be in the form of a traffic island with a sign showing the direction that vehicles must now follow. It is important to ensure that the arrangement is in keeping with the area and consideration shoi~ld be given to the provision of landscaping.

Where a closure is made at a mid-block position,' provision must be made for large vehicles, such as refuse vehicles to turn around.

Typical turning heads are shown in Figure 5.1 3. The choice of layout is dependant on the width

. .

Page 5/27

QATAR HIGHWAY DESIGN MANUAL : : . .

SECTION 5 .. . . . I .-. . -: . , ! ! 4 ; 7 . . * . , * .... :.. A L . * . . . & . . 4 . . 9 G . * < - - - .... -.,..-. - , . . . , . , - -.

of carriageway available and the positions of existing property accesses that have to be accommodated by the closure. , - .

Any barriers or turning heads shall be designed in such a way as to ensure that emergency vehicles are able to gain access. This is achievable by the use of lockable barrier gates or demountable bollards. Whichever is chosen, it must be capable of preventing private vehicles from passing through the restriction. For 'this '

reason, solutions such as a route through a landscaped area are not recommended as they are open to abuse, particularly by drivers of four wheel drive vehicles.

Whatever the designed restriction, adequate access and parking shall be provided for residents. . , .

Partial closure allows access into areas. However, by the use of width restriction or raised road humps it is made unattractive for general road users.

Partial closure is often incorporated at undesirable locations along the major road to discourage use such as at accesses near to major junctions. Where the minor road has to remain open due to emergency vehicle access requirements or limited access routes into the development then partial closure is an easy way to control general use.

. .- January 1997 . . . . .. Page 5/28

. . . . I

Figure 5.1 3 Typical Turning Head Details


l+l Fd3 e

P-I I

All dimension in metres

Note : A central island radius of 10 metres will just allow the vehicle to turn about. In view of the restricted area available, the island may be reduced or omitted altogether.

Minimum Dimensions for Turnina Heads in situations where larger vehicles have to be accommodated, these dimensions should be increased to take account of the larger turning radius and swept path area.


i # .

I

QATAR HIGHWAY DESIGN MANUAL . : - . . . . . . . .- -..- .-.. . . - SECTION 5

5.1 9 LANDSCAPING

Apart from the amenity benefits, the landscape treatment of medians, junctions and verges can have practical advantages. By ground modelling, perhaps in conjunction with planting, the layout of the road can be made more obvious to traffic.

Landscaping can play an important part in aiding drivers waiting to exit the minor road by providing reference points or features by which to judge the speed of drivers approaching on the major road. 'This is particularly useful where a major/minor junction is located in an open landscape, where there is a lack of natural reference points. Planting can also provide a positive background to the road signs around the junction, whilst visually uniting the various component parts. It is important that a wider view does not distract from the developing traffic situation as the driver sees it.

pecialised planting, which might be more ppropriate in an urban area, generally requires reater maintenance effort if it is to be uccessful. The preferred maintenance method an automatic irrigation system connected to a turn effluent main. Approval for any such

cheme must be sought from the Director of the Civil Engineering Department and the Drainage Division. If a return effluent main is unavailable, care should be taken so that watering does not require tankers to obstruct trafficked lanes at any time.

A well defined maintenance programme should be developed if extensive planting is used to ensure that such planting does not obscure either opposing traffic or traff ic signs at any time.

In rural areas, planting should be restricted to indigenous species and be related to the surrounding landscape. In the desert, for example, any planting of other than local species would appear incongruous and landscape treatment would normally be restricted to ground modelling.

At roundabouts, the areas required for visibility envelopes can be planted with species having a low mature height, with higher and denser species of bushes and trees towards the centre of the island. Due allowance for the situation that will develop with matured growth must be made.

Apart from the amenity benefits, the landscape treatment' of roundabouts can have practical advantages. By earth modelling, perhaps in conjunction with planting, the presence of the roundabout can be made more obvious to approaching traffic. The screening of traffic on

January 1997

the opposite side of the roundabout to the point of entry can, without restricting necessary visibility, avoid distraction and confusion caused by traffic movements of no concern to a driver. Planting can provide a positive background to chevron signs and direction signs on the central island while visually uniting the various vertical features and reducing any appearance of clutter.

Generally the planting of roundabout central islands less than 10m in diameter is inappropriate as the need to provide driver visibility leaves only a small central area available. Such a restricted area of planting is out of scale with the roundabout as a whole, and becomes an incongruous "blob".

Recent experiments with a ring of black and white paving laid in a chevron pattern inside the central island perimeter at a gentle slope have proved successful in improving the conspicuity of central islands and they can be effective from a safety point of view (Figure 5.1 4).

It is common to construct features such as coffee pots etc. in roundabouts. They become a focus for the traveller, and if designed and positioned correctly will prove an asset to the surroundings.

Lighting of central islands or any landscape feature is important, though care should be taken to avoid distraction or dazzle to drivers.

Sectlon X-X

Figure 5.1 4 Contrasting Chevron Markings for Roundabouts

. - ..... . . , , . . . Page 5/30


SECTION 6 JUNCTIONS

6.1 GENERAL

The scope of this section of 'the QHDM is to identify the main types of majorlminor junction which can be used in the design of new and the improvement of existing roads.

Advice is given on the main factors which affect the choice 'between different types of majorlminor junction, on the siting of such junctions and suitable types of layout.

To ensure a consistent approach to the design of the majorlminor junctions, a series of recommendations covering the geometric design of the key elements of the junction, and how these can. be best combined to produce a good overall design, have also been included.

Junctions are widely recognised as one of the primary locations of accidents on #all roads. Safety is therefore of paramount importance during the development of any junction design. A number of safety issues such as: visibility; driver perception; signing and road markings; traffic control and pedestrian access, need to be ' considered as part of the design development process. More detailed guidance on these and other relevant factors is given elsewhere in this section.

6.1.1 Junction Spacing

The frequency at which junctions are located on a main. road .is usually a function of the surrounding area and its current or future development, i.e. rural or urban environment. In general terms, urban environments are characterised by a mixture of residential properties, and commercial and industrial developrnentsloutlets. There is usually a high demand for through traffic and local traffic movements. Consequently there is a high demand for access across, onto and off of the main road from the local road network.

In contrast, rural environments generally have few residential properties that are interspersed intermittently with industrial and commercial developmentsioutlets. The demand on the main road is for through traffic with local traffic movements catered for chiefly by the local road network. As demand for links with the main road are lower than urban environments, junctions occur much less frequently.

The spacing of junctions, particularly in urban situations is critical to ensure that disruption to traffic on the main road is rninirnised. It is

- . . . . SECTION 6

however, important to ensure that the minor road traffic movements are still adequately provided for. Spacing between consecutive junctions is best considered in terms of the minimum distance that allows traffic travelling on the main road and traffic leaving it or joining it, to do so in an easy, efficient and safe manner.

In determining this distance, due consideration must be given to:

Design speeds

Weaving lengths for mergingldiverging traffic flows .

Horizontal and vertical geometry of the main road for visibility

Provisions for turning traffic wishing to cross, join or leave at the junction.

The minimum spacing between consecutive simple T-junctions on access roads and service roads is 80m, and across a staggered T- junction 40ml Refer to Clause 6.7.15 for additional information on stagger distance and refer to Clause 7.4.9 for additional 'information on weaving sections.

Consideration should also be given to the spacing of the deceleration lanes and the acceleration lanestof junctions along the main carriageway. Refer to Clause 6.14 for information on divergelmerge distances. The spacing of these junctions should relate to the weaving characteristics of:

Traffic on the mainline

Traffic entering the mainline from the first junction

Traffic leaving the main road to the second junction.

When improving existing roads it may be necessary to reduce the number of junctions on the route. This may be achieved by:

. . Provision of service roads to collect minor roads .

~losure'of minor roads and provision of turning heads, refer to Section 5.


QATAR HlGHWAY DESIGN MANUAL

6.1.2 Traffic Flows

An important factor that governs the choice of junction type at a given location is the volume of traffic that is currently using the main road and side roads, and the predicted future traffic demand. Before any detailed evaluation can be made it is important to obtain the best estimate of all the relevant traffic flows and turning movements for the junction.

In situations where this data is not readily available it will be necessary to undertake traffic surveys, or use traffic modelling to predict the likely traffic flow levels.

The composition and turning movements of traffic will. influence the geometric layout adopted. A high proportion of heavy goods vehicles for example will dictate the minimum lane width and corner radii to be adopted at the junction. A high proportion of tuming traffic may require the provision . of a segregated or dedicated turning lane at the junction; to ensure that adequate through traffic capacity is maintained.

Predicted future traffic flows are important because they:

Enable the design to be tailored to provide sufficient capacity to meet the future traffic flow demands

Enable a decision to be made to constrain the traffic flows at the given location for a particular reason

Identify the need to allow for current or future junctions.

Guidance on acceptable traffic flows for junction types and layouts are given throughout this Section.

6.1.3 Design Vehicles -

An obvious but often overlooked aspect of the design of junctions is the type of vehicle that will be using the junction. Different sizes and classes of vehicle have varying swept paths and turning circles. All junctions need to be designed to allow the vehicle with the greatest swept path, that will regularly use the junction to turn in a safe and easy manner. For example a 36 tonne articulated lorry is unlikely to be a regular user of a residential road. In this example the most likely largest vehicle would be a refuse vehicle or a school bus. Generally, the design vehicle is likely to be a heavy goods, .public senrice, or refuse vehicle and it is the

SECTION 6 . ,

corner radii and lane widths that are likely to be affected. Swept paths should be checked using standard templates or a computer software package.

The vehicle classification to be used in Qatar is shown in Table 6.1.

Failure to make adequate provision is likely to result in:

A reduction in the junction capacity as the larger vehicles are forced to straddle two traffic lanes to facilitate the turning movement at the junction

Overrunning of kerbs

Reduced visibility for other traffic approaching or negotiating the junction.

These design principles should be extended to the positioning of street furniture such as signs, splitter islands, traffic signals and lighting columns.

Allowance shall be made for the swept turning paths of long vehicles where they can reasonably be expected to use a junction. Consideration shall also be given to the manoeuvring characteristics of these vehicles in the design of staggered junctions.

All of the geometric parameters given in this section for use in the design of a majorlminor junction have been developed to cater for a 16.5m long articulated vehicle, whose turning width is greater than for most other vehicles that regularly use these junctions.

The turning requirement of a 20.0m long drawbar trailer combination are less onerous regarding road width.

In cases where hardstrips are present, the design vehicle is assumed to use 'these on some turns, and at some simple junctions, it may encroach into opposing traffic lanes.

Where buses or other long rigid vehicles form a significant portion of total or peak time traffic, and their tuming movements within these dimensions would be awkward or present a hazard or significant delay. Then corner radii and lane widths should, be increased based on the use of appropriate swept path templates.

January 1997 . .. Page 612

- : QATAR HIGHWAY DESIGN MANUAL : . . . . . $ : . . . . , ;. . . . ., ;. I : sEcnoN 6 .

\

L-

6.1.4 Siting of Junctions

,-, / - The siting of junctions for new build and

\ improvement schemes is very important. - Failure to choose a suitable location can reduce the effective operation and safety of the junction. It is essential to include engineering

L considerations in the early planning stages to help minimise poor land use.

Sites that should be avoided include: L

Where the major road is on a sharp v curve and visibility may be impaired by !

C waiting vehicles

1 Where the minor road approaches are

i skewed less than 70" or greater than 11 0" to the main road.

? , , At the top or bottom of gradients greater L- than 4% on the main road

Where the minor road approaches the ? - main road on an up or down gradient

greater than 2%

Where junction frequency is excessive.

The problems listed above while not exhaustive cover the more commonly occurring situations, and they can usually be overcome by modifications to the horizontal and vertical alignments.

In situations where, because of site constraints, it is not practical to fully apply these principles, then a comppmise will need to be established that minimises the potential risks to drivers approaching the junction. Measures such as reduced speed. limits, alignment constraints, additional signing and road markings can help to minimise the potential hazards to the driver.

January 1997 : Page 613

QATAR HIGHWAY DESIGN MANUAL 1 . . . . I - . . . . . , . . :"" SECTION 6

Table 6.1. Vehicle Classification (Sheet 1 of 2)


a

*;3 - * E

z5: 3 : z

!%

a ; p ;!

= z pa E L - r":tz

.- + .-

I

N

V)

2 - P I

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* V ) V )

0 0 0 0 0 0

t::

.- f 0 ? Y - P

i ? E t

L 1

c 0 - 1 (I)

310

e 0 - g : 5 m -

U

- E - I 9 P

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> = Y C m 2 - O X - 5 3

.- + .-

n

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Y .-

h .-

* t

0 0

w F

N

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1113

1 - 0

m 0. a I- m - U I :

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Y ? F! 0

N

V) .-

9 Iy

N

b

0 0 a V)

m

U

2 I

r r u a r o ~ / s o m g

N + C

9 '? tn .-

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0

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n C

0 0 V) 10 C

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9 7 Y O

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z

Y w

0 0

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r l o ~ r r r s p o o l rut-qprrj

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n t

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n a

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0 0 0 0 .-

m

rr 4 i! - m - 4 N

raioluan

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Table 6.1 Vehicle Classification (Sheet 2 of 2)

January 1997 . Page 615


6.2 TYPES OF JUNCTION

There are seven basic types of junctions that should be considered for use.

There are advantages and disadvantages to each of the seven types and the engineer should carefully consider the suitability of each type for the intended location and purpose.

The seven basic junction types are as follows:

The T-Junction, of which there are five main variants, is an at-grade junction of two roads where the minor road terminates at the major road at right angles. It is the most common type of approach road junction and is a suitable solution for coping with most traffic flow requirements. Traffic control is generally provided by "Give Way" or "Stopn signsfroad markings on the minor approach but could include traffic control on all approaches.

In certain urban situations where traffic, pedestrian or safety requirements dictate, signalization may be requir'ed. The type of traffic control is determined through a "warrant analysisn (refer to the Qatar Traffic Manual).

'6.2.2: Simple Crossroads

The crossroad is an at-grade junction of two roads that cross approximately at right angles. Simple crossroads are not safe junctions because of the high number of traffic movement conflicts that can occur at the same location. For this reason, the use of crossroads is not recommended. A safer solution, location permitting, is to provide a roundabout or signal control.

6.2.3 Staggered Junction

A staggered junction is an at-grade junction of three roads, where the major road is continuous through the junction. The minor roads intersect the major road forming two separated T- junctions on opposing sides of the main road.

This type of junction is the preferred alternative to a simple crossroad. However, should future traffic volumes be expected to increase, then a roundabout or signalisation may be preferable from the outset at certain locations.

SECTION 6

6.2.4 Skew or Y-Junction

This type of junction is an at-grade junction of two roads, where the minor road approaches the major road at an oblique angle. In a similar manner to the T- junction, traffic control is provided by "Give Way" or 'Stopn line road markings in conjunction with "Stopn or "Give Way" signing on the minor road.

As skew angle to the main road.decreases, the junction becomes less safe.

6.2.5 Roundabouts

A roundabout is a special form of at-grade junction characterised by a one-way circulatory carriageway around a central island located at the intersection of a maximum of six roads. Traffic flows around the central island on the circulatory carriageway in an anti-clockwise direction until it reaches the required exit point. Entry onto the roundabout from the approach roads is controlled by the appearance of gaps in the circulating traffic flow. Traffic wishing to enter the roundabout must give way to-traffic already on the circulatory carriageway.

6.2.6 Grade Separated Interchange

This type of junction removes the principle vehicle conflict by the provision of grade separation between some of the turning movements. These junctions are complex and include extensive connecting roads and loops. Grade Separated Interchanges are discussed in Section 7 of this manual.

6.2.7 Traffic Signdls

Whilst not strictly a junction type, traffic signals may be implemented on a number of junction types to control the movement of traffic. Junctions may be specifically designed for signal control or signal control may be added a later stage.

The design of physical features of this type of junction, excluding the signal . design, are covered within this manual. An introduction to signalized junctions is given in Clause 6.1 6.

January 1997 :

Page 616


6.3 JUNCTION SELECTION

The selection of a junction type at a given location is important for operational, economic and safety reasons.

The engineer should carefully select the junction type for the location in accordance with the considerations listed below.

However, in some circumstances, local conditions such as driver behaviour may also influence the engineers choice of junction type for a particular location. Where it is felt that drivers may ignore 'Stop" or "Give Way" signs. a different or higher type of jllnction could be selected.

Apart from the basic selection considerations given below, the engineer should also consider the possibility of planning benefits to be gained by the selection of junction types at locations that promote the use of the roads in the hierarchy defined in this manual.

The following points should be considered:

6.3.1 Status of lntersectlng Roads

~estrictions are placed on the categories of road that may meet. As a result, for any given permitted combination of road types, only certain junction types will be appropriate for use. Table 6.2 below outlines acceptable carriageway and junction combinations.

SECTION 6

6.3.2 Continuity of Standard .

In the interests of safety, the sequences of junctions on a section of road or neighbouring roads of sirr~ilar standard should not involve many different layout types. A length of major road comprising roundabouts, single lane dualling, ghost islands and simple priority junctions would inevitably create confusion and uncertainty for drivers, and may result in accidents. The safest schemes are usually straightforward, containing no surprises for the driver.

6.3.3 Junction Capacity

The form that a junction takes is greatly influenced by the volume of traffic predicted to pass through it. All junction layouts will need to be analysed to ensure they have sufficient capacity. This analysis should be carried out using a standard software package (eg. ARCADY for roundabouts and PICADY for majorhinor junctions).

Junction selection by capacity is given in Figure 6.1. It is based on capacity and on UK; congestion acceptance levels. Engineers may consider that higher standard facilities should be provided than that indicated by the nomograph for operational or safety reasons.

The detailed geometry of junction types relating to capacity is given in Clause 6.7.

Key: T T-Junction R Roundabout I Interchange Ts T-Junction with Signals Tu T-Junction with U-Turn V Service Road

Table 6.2 Possible Junction Types for Different Major Road Carriageway Configurations

Major


Minor

7.3m Carriage-

Way

1 1.3m Carriage-

Way

Dual-2

Dual9

Rural *

Urban

Rural

Urban

Rural

Urban

Rural

Urban

7.3m Carriageway

Rural

T;R

T,Tu,R,I

T,Tu,R,I

11.3m Carnageway Dual-2

Urban

T,Ts,R

T,Ts,R

T,Ts,V,R,I

T,Ts,V,R,I

Rural Rural

RBI

RBI

Dual9

Urban

T,Ts,R

T,Ts,R

T,Ts,V,R,I

T,Ts,V,R,I

Urban

Ts, R,I

Ts, RBI

Rural

RBI

Urban

Ts, RBI .


V ) c 0 .- .r In

( A ~ M - o w ) l a w ( o L x MOlA aVOU UONIW

----

Figure 6.1 Junction Selection By Capacity



6.4 MAJORIMINOR JUNCTIONS - GENERAL

'This section gives advice and standards for the geometric design of majorlminor junctions with regard to traffic operation and safety.

6.5 SAFETY AT MAJOFUMINOR JUNCTIONS

Vehicular and pedestrian accidents mainly occur at majorlminor junctions. More accidents occur in the urban environment than the rural.

These accidents are mainly associated with poorly judged left turn movements onto and from the major road and with incautious overtaking manoeuvres.

.. . Various methods to enhance safety can be introduced at majorlminor junctions. The engineer should review each junction on an individual basis. ,

Ghost islands and single lane dualling (physical islands) to shelter left turning traffic and discourage overtaking are discussed in Sections 6.6 and 6.7: Other safety measures that could be adopted are as follows:

The use of road markings, double white lines, raised rib markings, narrow central hatching, block paving, ceramic studs, refuge islands with keep right bollards or different coloured surfacing to discourage overtaking manoeuvres on the major

. road

Skid resistant road surfaces

At urban locations where pedestrian movements occur, pedestrian barriers, central refuge islands and at some

. locations, pedestrian crossings and controlled pedestrian crossings

At some locations where safety is an issue, the majorlminor junction may require traffic signals.

In addition, in rural areas, problems occur with driver perception of the termination of the minor road. Drivers at night, on unlit rural roads are mostly involved with this type of misjudgment. The engineer shall ensure that there are no physical obstructions to the path of such a vehicle.

SECTION 6

6.6 MAJORIMINOR JUNCTION TYPES

6.6.1 The Simple T-Junction

Figure 6.2 Simple T-Junction

A simple T-Junction is without any ghost or physical islands in the major road, and without channelizing islands in the minor road approach. Refer to Figure 6.2.

Simple T-Junctions are appropriate for most . minor junctions on single carriageway roads, but not dual carriageways. For new rural junctions, they shall only be used when the design flow on the minor road does not exceed 300 vehicles AADT (two-way) and on the major 'road does not exceed 13000 vehicles AADT (two-way).

At existing rural and urban junctions upgrading to a left turning facility, ghost island or single lane dualling should be considered when safety considerations dictate or where the minor road flow exceeds 500 vehicles AADT (two-way).

6.6.2 T-Junction with Ghost Island

A T-Junction with widening on the major road to accommodate a ghost island and an extra central lane for turning traffic. The minor road approach should also have a channelizing island to direct vehicles to the correct position for turning movements. Refer to Figure .6.3.

Ghost islands will enhance safety of the junction by giving shelter to left turning traffic from opposing vehicles and vehicles approaching from behind. Measures to discourage overtaking at ghost island widening could be the use of physical traffic' islands, double white lines, different coloured surfacing and ceramic studs.



Figure 6.3 T-Junction with Ghost Island

Figure 6.4"' T-Junction with Single Lane DuallingfPhysical Island

Ghost islands, however, should not be positioned where overtaking opportunity is restricted either side of the junction because drivers may use the wide ghost island hatching and central lane as .a place to overtake. If a ghost island has to be positioned at these locations then an alternative such as single lane dualling should be considered.

Ghost island junctions should not be used where traffic tuming left out of the minor road needs to make the manoeuvre in two stages. This can bccur when the major road flow exceeds 18000 AADT (two-way).

6.6.3 T-Junction with Single Lane Dualling

Single lane dualling (physical islands) can be used on rural single carriageway roads to shelter left turning traffic on the major road and prevent overtaking. It can also be used where

'the traffic turning left out of the minor road needs to make the manoeuvre in two stages. Refer to Figure 6.4. *

Single lane dualling does, however, bring in other safety issues. With the improved highway layout there may be a tendency for drivers on

the major route to speed up through the junction where slow vehicles may be crossing. Care needs to be taken when siting the junction.

The single lane dualling carriageway width is 6m, where 4m is the running carriageway and there are I m hard strips on both sides. Some drivers may try to overtake in this width and hatching of the I m strips will discourage such manoeuvres.

There may be certain conditions when single lane dualling could be misinterpreted by drivers:

Where a length of road contains alternating single and dual carriageway sections

Where single lane dualling is proposed within 3 kilometres from the end of a long length of dual carriageway.

In these cases, other forms of junctions should be considered.

January 1997 -. . - . Page 6/10

Figure 6.5 T-Junction with Dual Carriageway with Median Opening (Signalized Only)

Figure 6.6 T-Junction on a Dual Carriageway with Carriageway Separation

6.6.4 T-Junctlon on a Dual Carriageway with Median Opening (Signalized)

'These T-Junctions may be used on two or three lane dual carriageways. This layout shall only be implemented with traffic signals. Refer to Figure 6.5.

Short lengths of dual carriageway just to incorporate a junction should not be provided.

On continuous dual carriageways the median width is usually between 2 and 8m. If required, this width can be widened to provide space for a left turn lane and waiting space for vehicles turning left into the minor road.

6.6.5 Taunction on a Dual carriageway with Cartiageway Sepatatlon

On dual carriageways, the left turn manoeuvre from the minor road is prevented by the median, unless the minor road warrants signalization to

be incorporated. The turning facilities should be provided nearby at another junction. The nearby junction may be grade separated, a roundabout, signalization or a U-Turn where traffic speed and traffic flow conditions are different. Refer to Figure 6.6. .Acceleration and deceleration lanes from and to the minor road should be designed in accordance with through traffic volumes and speeds.

6.6.6 Crossroads

As discussed earlier in Clause 6.2.2, simple crossroads are not recommended. Staggered junctions are always considered a much safer alternative, especially if a significant proportion of the flow on the 'minor roads is cross movement. In residential areas, consideration should be given to closing off one of the arms of the crossroads to create a preferred simple T- Junction.

- -


n o b u y y u l uu JUI I W ~ I W I I WVI I I ~ I 1 3 ~ 3 a I I I ~ J V I I WSIU ~IUIIIU WI I r l I= I I I ~ J W I luau. rttzctzi ru r ~ y u i ~ u.r . passing through the junction with opposed T- Junctions on either side. figures 6.7 to 6.1 1 RighULeft Stagger show variations of staggered junction layouts. A simple righffleft staggered junction should not

be considered. However, the rightneft LettlRIght Stagger staggered junction with ghost island or single The lefttright stagger is preferred because the lane dualling would be an alternative. Refer to two left turning traffic streams on the major road Figures 6.1 0 and 6.1 1.

Figure 6.7 Simple LefVRight Staggered Junction

Figure 6.8 LefVRight Staggered Junction with Ghost Island

Figure 6.9 LefVRight Staggered Junction with Single Lane Dualling

. - -- . . . . . . . . . . . . . . . January 1997 . - . . . . _ . . Page 6/12 .. -

i

. . QATAR HIGHWAY DESIGN MANUAL

* . . ' ' SECTION 6

Figure 6.10 RightLeft Staggered Junction with Ghost Island ...

- -

. . . . Figure 6.1 1 RightLeft Staggered Junction with Single Lane Dualling

6.6.8 Right and Left Hand Skew Junction This form of junction can be a solution when an existing minor road joins the major road at a

Figure 6.12 shows a left hand skew junction skew angle. It is sometimes called a Y- .with a ghost island; The junction could also be Junction. right handed.

The existing junction is improved on safety grounds by channelizing the minor road with islands and road markings, and connecting it to the major road at right angles for optimum* visibility.

Typically skew angles of 70' or greater do not require straightening to approach the main road at 90'. As skew angles become smaller a large area is required in order to achieve an effective 90' junction.. . . ,

Other combinations o i skew junctions could combine staggered junctions, single lane dualling and dual carriageways.

Figure 6.12 Left Hand Skew Junction

- . . . . , - ! * - January 1997 Page 6/13


6.7 -MAJORIMINOR JUNCTION ELEMENTS

6.7.1 General

This section outlines the geometric design elements to be considered in the design of major/minor junctions. Many of the elements are dealt with separately, and the engineer should work systematically through the design procedure prior to assembling the component parts. This is an iterative process, and it may be necessary to alter part of the junction design covered previously in order to achieve a satisfactory design.

6.7.2 Design Speed

When considering geometric standards for the design speed of the major/minor road junctions, it is the design speed of the .major road that governs.

6.7.3 Visibility I

Clear visibility on the approach to, at and travelling through a junction is essential for the safe and efficient use of that junction.

L.

In determining the correct visibility requirements for a junction, the engineer must consider both the layout of the junction and the vehicles that will use it. The visibility and intetvisibility requirements provided within this clause are related to the design speed of the major road and little benefit is to be gained by increasing them. However, each junction must be considered on a site-specific basis with an assessment made of additional visibility to be provided due to factors such as:

Width of major road to be crossed

a . Traffic control on the minor approach road

Turning movements to be made at the junction

Gradient of the approaches and departures

Type of vehicle that will be using the junction, eg. large, slow speed vehicles require additional visibility.

As well as having adverse safety implications, poor visibility reduces the capacity of turning movements.

SECTION 6 . , . ., . . .

Drivers approaching a major/minor junction from both the major road and the minor road shall have unobstructed visibility in accordance with the following clauses. The envelope of visibility for driver's eye height is as described in Section 2.

Major Road Drivers approaching a majortminor junction along the major road approaches shall be able to see the minor road entry from a distance corresponding to 1.5 times the stopping sight distance (SSD) for the design speed of the major road as described in Section 2. This intetvisibility allows drivers on the major road to be aware of traffic entering from the minor road in time for them to be able to slow down and stop safely if necessary.

The concept of adequate visibility to make safe turning movements also applies to vehicles turning left into the minor road from the major road.

Minor Road Minor road traffic has to approach the junction and join or cross the major road when there are gaps in the major road traffic streams. It is therefore essential that minor road drivers have adequate visibility in each direction to see the junction layout and oncoming major road traffic in sufficient time to permit them to make their manoeuvres safely.

The principle of providing the required visibility for drivers approaching the junction from the minor road has three distinct features (refer to Figure 6.1 3):

W: Approaching drivers should have unobstructed visibility of the junction from a distance corresponding to the stopping sight distance (SSD) for the design speed of the minor road. This allows drivers time to slow down safely at the junction, or stop, if this is necessary. Where a "Give Way" or "Stop" sign is proposed, the visibility envelope shall be widened to include the sign.

Z: A driver approaching the junction should be able to see clearly the junction form and those peripheral elements of the junction layout. This provides the driver with an idea of the junction form, possible movements and conflicts, and possible. required action before reaching the major road. This point is called the '2' point which is 15m back along the centreline of the minor road measured from the continuation of the line of the nearside edge of the running carriageway of the major road (not from the continuation of the back of the major road hardstrip, if this is present).

January 1997 . . . . . Page 6/14

X, Y: The distance back along the minor road

Q from which the full visibility is measured is known as the 'x' distance. It is measured back along the centreline of the minor road from the continuation of the line of the nearside edge of the running carriageway of the major road. The 'x' distance shall be desirably 1 Om.

From this point an approaching driver shall be able to see clearly points to the left and right on the nearer edge of the major road running carriageway at a distance given in Table 6.3, measured from its intersection. with the centreline of the minor road. This is called the 'y' distance. Relaxations are not available for this distance.

If the line of vision lies partially within the major I

road carriageway, it shall be made tangential to the nearer edge of the major road running carriageway, as shown in Figure 6.14.

In difficult circumstances, the 'x8'distance may be taken as a relaxation from 10m to 7.5m for lightly trafficked simple junctions, and in exceptionally difficult circumstances, to 5.0m back from the nearer edge of the major road running carriageway. In some urban locations where only light vehicles are involved, the 'x'

i distance can be further reduced to 23m.

The 'x' distance, from which full 'y' distance visibility is provided, should preferably be not more than 1 Om as this induces high minor road approach speeds into the junction, and leads to excessive landtake.

Similarly, although the 'y' distance should always be provided, there is little advantage in increasing it, as this too can induce high approach speeds and take the attention of the .

minor road driver away from the immediate junction conditions. Increased visibility should not be provided to increase the capacities of various turning movements.

These visibility standards apply to new junctions, and to improvements to existing junctions.

If the major road is one way, a single visibility splay in the direction of approaching traffic will suffice. If the minor road serves as a one-way exit from the major road, no visibility splays will be required, provided that forward visibility for turning vehicles is adequate.

Vehicles parked within splay lines will obstruct visibility. Parking and access should be designed to prevent this. Care should also be taken in the placing of signs, landscaping and street furniture within the visibility splay areas to. ensure that their obstructive effect is minimised

Design Speed of Major Road

(kph)

140 120 100 80 70 60 50

< 50 tote : h ail cases the preferred 'x' distance is 1

'y' Distance (m)

Minimum 'x' Distance

(m)

with Clause 6.7.3.

- ~ a b i e 6.3 Minimum 'x' and 'y' Visibility Distances from the Minor Road

350 295 21 5 160 120 90 70 50

I B ~ # aamf I QQ7 Darrs C I I K

10 10 10 10 7.5 7.5 5.0 . 2.5

lm. The minimum 'x' distances given shall only be used in difficult circumstances, in accordanc

QATAR HIGHWAY DESIGN MANUAL . *- ' . . . , .

SECTION 6

i Figure 6.1 3 Visibility Standards . i t

Lines over which unobstructed visibility should be provided

- - - ------- -+------ --.-.L

. a.

I . . . . . . . .

------- Tangent edge of carriageway

x 'x' Distance y 'y' Distance

1

-.

W

Figure 6.14 Visibility Standards with a Curved Major Road


- A X

z = 1 5 m

v .. .

v

. -

i

: I ' . . :I: . . #

;l: i I

* I

x 'x' Distance y 'y' Distance w Minimum Stopping Distance (SSD)

for Approach Road Design Speed

8 .

QATAR HIGHWAY DESIGN MANUAL . . . SECTION 6

6.7.4 Corner Radii

For simple junctions, where no provision is to be made for large goods vehicles or buses, it is recommended that the minimum circular corner radius shouid'be 6m in urban areas and 10m in rural areas. Where provision is to be made for large goods vehicles or . buses, the recommended circular corner radius is shown in Table6.4 and Figure 6.1 5. , . ,

These radii' only apply where there are no diverge tapers or lanes, or merge tapers. Refer to Sections 6.7.13 and 6.7.14. / -

Alternatively, where large goods vehicles comprise a significant proportion of the turning v. movements, use of the compound curve shown inFigure 6.1 6 is recommended. .. - .

i- Table 6.4 Circular Corner Radii

b

Junction TW

Urban Simple

Junction

Rural Simple

Junction

Ghost Island

Junction

! R = Corner Radlus ! L - Length of Taper ! T = Taper Rate

L 0

! 1

. - I Figure 6.1 5 Circular Corner Radii i

Staggered Junction

All Other 20

Taper Rate

T

1:s

1:lO

Figure 6.1 6 Design of a Compound Curve

6.7.5 Carrlageway Widths

All of the geometric parameters defined in this clause can be seen for the three main types of majorlminor junctions in Figures 6.1 7 - 6.1 9.

Length of Taper (rn)

L

30

25 Through ~ a n e s At ghost and physical island junctions, the through lane in each direction shall be 3.65m wide, exclusive of edgestrips.

Corner Radius (m)

4 R

10

15

At dual carriageway junctions, the through lane widths remote from the junction shall be continued through the junction.

Minor Road Approaches On a minor road approach of nominal width 7.3m, where a channelizing island is provided, both lanes shall be 4.0m wide at the point where the hatched markings surrounding the channelizing island begin. (Refer to Figure 6.1 7).

At the point where the channelizing island commences, the widths on either side shall be as follows:

a) On the approach to the major road, 4.0m = wide for a ghost island or 4.5m wide for

single lane dualling or a dual carriageway, exclusive of hardstrips. If the approach on the minor road consists of two lanes, this dimension shall be 5.5m.

b) On the exit from the major road, 4.5m wide for a ghost island, or 5.0m wide for single . lane dualling or a dual carriageway, exclusive of hardstrips.

- . \ I January 1997 Page 6/17

QATAR HIGHWAY DESIGN MANUAL . . .I SECTION 6

a. ?.am Womkrl Wldlh b. 4.0m k AH Carer 1 4.5m For Shgla Lana Oualllnfi

of Ova1 Canlagoway

If there are no channelizing islands igthe minor Where the minor road approach is a dual road, the nominal approach $dth should carriagewa$3t, should be either reduced to a continue up until the tangent point-af the curve single cahageway before the junction (see to join the edge of the major road running Figure 6.1 8), or signalized. carriageway.

Where 16.5m long articulated vehicles (eg. Class 8) are anticipated, but are likely to form only a very small percentage of the total number of vehicles and where conflicts will not occur on bends, the carriageway widths should be designed to cater for the lower class vehicle that will regularly use the junction with an additional 1 m allowance for variation in vehicle position. Alternatively, figures from Table 6.5 could be used.

An articulated car transporter will turn in the widths shown, but where provision is to be made for this type of vehicle, street furniture above 2.5m high should be set back at least 1 m from the edge of the minor road carriageway at the bellmouth (this does not apply for channelizing islands) to allow for the projection of the trailer over the tractor cab.

Figure 6.17 Minor Road , . -. ..

Approach Reduced to Single Cartiageway Approach Incorporating U-turn Facility

# rm'

Figure 6.1 8 Minor Road Dual Garriageway Approaches ?- F


QATAR H~GHWAY DESIGN MANUAL SECTION 6

Carriageway Widths Around Comers Where carriageways are taken around corners and short radius curves, added width shall be provided to cater for the swept path-of largei goods vehicles and the "cut in" of trailer units. Table 6.5 shows the recommended minimum widths for various nearside curve radii based on the Class 12 design vehicle. For radii above 1 OOm, the standards set out in Table 3.5 shall be used.

On single lane sections greater than 50m in length, the allowance given in Table 6.5 shall be made'for broken down vehicles. However, the engineer must be careful not to use' this additional width in locations that may encourage .

2 lane flow to develop, eg. at intersection right turn lanes.

Table 6.5 Minimum Corner and Curve Radii and Carriageway Widths

a b - e

C , -

- ----------- - x----------- --------

a. Turning Length (+Queuing Length c. Through Lane Width (6.7.5) if required, see clause 6.7.8)

d. Turning Lane Width (6.7.6) b. Deceleration Length (6.7.10)

e. Direct Taper Length (6.7.9)

Inside ComerICurve

Radius (m)

10

15 =.. 20

25

30

40

50

75

100

I Figure 6.1 9 Major/Minor Junction with a Ghost Island

Two Lane Width for One Way or Two Way Traffic (Excluding Edgestrip Provision)

(m)

i


Single Lane Width (Excluding Edgestrip

Pmklon) (m)

8.4

7.1

6.2

5.7

5.3

4.7

4.4

4.0

3.8

lnslde Lane

8.4

7.1

6.2

5.7

5.3

4.7

4.4

4.0

3.8

Slngle Lane Width with Space to Pass Stationary

Vehlcles (Including Edgestrip Provision)

(m)

10.9

9.6

8.7

8.2

7.8

7.2

6.9

6.5

6.3

Outslde Lane

6.5

6.0

5.6

5.2

5.0

4.6

4.3

4.0

3.8

Total

14.9

13.1

11.8

10.9

' 10.3

9.3

8.7

8.0

7.6


6.7.6 Central Islands - Major Road

Ghost Islands For new junctions, the desirable width of a ghost island turning lane shall be 4.0m, but a relaxation to 3.0m is permissible. At urban and suburban junctions, it can sometimes be advantageous to use a greater width not exceeding 5.0m to allow a degree of shelter in the centre of the road for large goods vehicles turning left from the minor road to execute the turn in two separate manoeuvres.

For improvements to existing junctions, where space is very limited, a reduced width may be unavoidable. The width of ghost islands shall not be less than 2.5m.

At rightneft staggered junctions, the deceleration lengths would overlap but the width of the ghost island shall not be increased to make them lie side by side. The starting points of the left turning section shall be joined by a straight line, which will mean at higher design speeds, the full width of the turning lane will not be developed until the end of the diverging section (as shown in Figure 6.10). The width of the turning lane shall be the full width of the ghost island.

Physlcal Islands At single lane dualling and dual carriageway junctions, the width of the central island at the crossing point shall be 10.0m, including median hardstrips. This width will shelter most large goods vehicles turning left from the minor road, except for very long vehicles. In exceptional circumstances where use by very long vehicles is expected and a roundabout is not feasible, a width of 14.0m including hardstrips will be needed to shelter the largest articulated vehicles (1 6.5m) and a width of 16.5m including hardstrips will be required to shelter drawbar trailer combinations (20.0m). The minimum width of a physical island, usually located at the end of the direct taper shall be 3.5m.

Crossing left turn movements within the central island can usefully be separated by physical or painted guide islands set out with road markings so that the number of traffic conflicts at any point is reduced. Painted guide islands'can be enhanced by the use of coloured surfacing or textures within them, block paving, road marking or traffic studs. However, designs which have numerous small traffic islands should be avoided as they are confusing and tend to be ignored.

one time. This can lead to greater safety. For the separation to be effective, the junction must be large enough for drivers to identify in adequate time those vehicles which will conflict with their, intended path and those that will not. If this is not so, gaps in the flow cannot be used effectively by traffic entering the junction.

6.7.7 Central Island Tapers

Central islands, whether for ghost islands (Figure 6.20) or single lane dualling (Figure 6.21) should normally be developed symmetrically about the centreline of the major road to their maximum width at the tapers shown in Table 6.6. The maximum island width should continue through the junction to the tangent point of the minor road radius and the edge of the major road carriageway.

Table 6.6 Tapers for Central Islands (1 :T)

Design Speed (kph)

50 60 70

. 80 100 1 20

,140

T R -1

.. -0

0 ~\\~ixX!!=-- - -

R

T. Ghost Island Taper (1:T)

R. Rounding (50mR Typical) I

Figure 6.20 Ghost Island Development

Taper for Ghost Island and Single

Lane Dualling

1 :20 1 :20 1 :20 1 :25 1 :30 -- --

For single lane dualling, the central island should be introduced by means of hatched markings until there is sufficient width to accommodate the appropriate sign on the nose of the physical island with the required running clearances to it.

Taper for Dual

Cartiageways

1 :40 1 :40 1 :40 1 :45 1 :50 1 :55 1 :60

Preventing or minimising conflicts by separation means that drivers are only faced with simple decisions on their choices of movement at any

. January 1997 Page 6/20


T E -1

T. Physical lrland Taper (1:T)

Figure 6.21 Physical Island Development

6.7.8 Turning Length In Median (a)

The turning length is provided to allow long vehicles to position themselves correctly for the left turn. The. turning length should be a minimum of 10m long irrespective of the type of junction, design speed or gradient, measured from the centreline of the minor road. It is shown on Figure 6.1 9.

Where capacity calculations indicate that for significant periods of time there will be vehicles queuing to turn left from the major road, the turning length shall be increased to allow for a reservoir queuing length to accommodate such vehicles. For simplified calculation of storage length refer to Section 5.10.

Where reservoir provision appears desirable at a junction with ghost islands, consideration shall be given to providing physical islands instead to

- afford greater protection to turning traffic. Where site conditions prevent this, the reservoir space may still be provided.

6.7.9 Direct Taper Length (e)

The direct taper length is the length over which the width of a left turning lane is developed. For ghost islands and physidal islands in single lane dualling and dual carriageway junctions, left turning lanes shall be introduced by means of a direct taper whose length is part of the deceleration length and depends on the design speed. This taper length is given in Table 6.7 and illustrated in Figure 6.1 9.

6.7.1 0 Left Turning Lanes

Design Speed (kph)

50 60 70 80 100 120

. 140

Left turning tapers and lanes in the centre of ghost islands, and single lane dualling are especially useful as they provide a convenient space for vehicles to slow down and wait before turning off the major road. These junction layouts can also assist the left turn out of the minor road.

Direct Taper Length - e (m)

5 5 15 15 25 30 35

The overall length of a left turning lane provided at ghost island, single lane dualling and dual carriageway junctions will depend on the major road design speed and the gradient.

It consists of a turning length, as described in Clause 6.7.8, and a deceleration length. This component shall be provided in accordance with Tables 6.8 and 6.9, in which the gradient is the average for the 500m length before the minor road.

Table 6.8 Deceleration Length - b (m) for Ghost Island and Single Lane Dualling

Design Speed (kph)

50 60 70 80 100 120

Table 6.9 Deceleration Length - b (m) for Dual Carriageways

Design Speed (kph)

50 60 70 80

The deceleration length can be seen on Figure 6.1 9. The deceleration length is based on the assumption that vehicles will slow by one design speed step on the trunk road before entering the length. The deceleration rate on the level is assumed to be 0.3759. 'There is no . reaction time as this is a planned manoeuvre.

Up Gradient

Table 6.7 Direct Taper Length - e .

'

0.4%

25 25 40 55 80 110

Down Gradient

Above 4%

25 25 25 40 55 80

04% .

25 25 40 55 80 110

Up Gradient

Above 4%

25 25 40 55 80 110 -

04%

Down Gradient

Above 4%

0-4%

25 25 40 55 80 110 150

55 80 110

1 00 1 20 140

Above 4%

25 40 55 80 110 150 200

80 110 150

25 25 40 55

25 25 25 40


6.7.1 1 Median Openings illustrated in Figure 6.29. Opposite the refuge openings, dropped kerbs shall be installed to aid pedestrians. The opening in the median for single lane

dualling at the crossing point shall be 15.0m wide. . . 6.7.1 3 Nearside Diverging Tapers and

Auxiliary Lanes Problems have been experienced with driver confusion over priority within the median, particularly where the width of the physical island has been increased to cater for large goods vehicles.

Nearside Dlverging Taper Major road traffic, when slowing down on the approach to a junction in order to turn into a minor road, may impede following vehicles that are not turning. It is helpful, therefore,to permit the divergence of the two streams at a small angle by the provision of. a nearside diverging taper.

Measures to regularise the priority arrangement within the median opening include channelizing the central area.

Consideration may also be given in these circumstances to introducing differential coloured surfacing or studs to enhance 'the road markings or indicate the area of allowable overrun for large goods vehicles. However, such coloured surfacing should also be visible at night and in poor weather conditions.

Nearside diverging tapers are of less benefit in terms of operation and safety than left turning lanes because the right turn from the major road does not cross an opposing traffic stream and is rarely. impeded. However, nearside diverging tapers should always be considered for higher speed roads or on gradients.

Nearside diverging tapers shall not be provided at simple junctions (Clause 6.6.1). They shall be provided at junctions between major and minor roads where the design speed for the major road is 80 kph or above. They shall be provided at other junctions in the following circumstances for traffic in the design year:

6.7.1 2 Traffic Islands , .

Traffic islands can be ghosted or kerbed (physical) and should be provided in the mouth of the minor road at major/minor junctions (except at simple junctions) to:

Give guidance to long vehicles carrying out turning movements Where the volume of right turning traffic

is greater than 600 vehicles AADT (one- way)* Channelize intersecting or merging traffic

streams Where the percentage of large goods vehicles is greater than 20%, and the volume of right turning traffic is greater than 450 vehicles AADT (one-way).

Warn drivers on the minor road that a junction is ahead

Provide shelter for vehicles waiting to carry out manoeuvres, such as waiting to turn left

Where the junction is on an up or down gradient of greater than 4% at any design speed and the volume of right turning traffic is greater than 450 vehicles AADT (one-way). .

Assist pedestrians.

Physical traffic islands should be positioned in urban situations only, shall have an area of at least 4.5 square metres, and shall be treated to be conspicuous in poor lighting conditions. Smaller areas. should be defined by road markings. The risk of overriding the islands can be reduced by offsetting the approach nose from the edge of the vehicle paths., ,

Where the major road traffic flow is greater than 7000-8000 AADT (one-way), then the figures given above for turning traffic should be halved.

Nearside diverging tapers shall not be provided when the minor road is on the inside of a curve where traffic in the diverging lane could adversely affect visibility for drivers emerging from the minor road. They shall generally not be provided where the design speed for the major road is less than 80 kph nor where the cost of provision is excessive. In this case, adequate warning of the junction ahead must be provided.

Where a traffic island serves as a refuge for pedestrians, it shall be at least 1.5m wide and have openings in the centre at carriageway level to make the crossing easier for pedestrians (see Clause 5.12). The recommended layout and details of the design of channelizing islands are

January 1997

QATAR H~GHWAY DESIGN MANUAL , SECTION 6

Nearside diverging tapers shall be formed by a direct increase to a width of 4.0m contiguous to the corner Into the minor road. A radius of at least 20m should be used where the main road design speed is 8Okph and at least 40m above this speed. The width around this corner will depend on the radius selected. The length of this lane is defined as being from the beginning of the taper up to the start of the radius, as shown in Figure 6.22.

The desireble length of a nearside diverging taper shall be that of the relevant deceleration length given in Tables 6.8 and 6.9.

Auxiliary Lane At major road flows of over 7000-8000 AADT (one-way), vehicles decelerating on the main carriageway and moving into the diverging taper

'

to a point where there is a full lane width available in the diverging taper may have a significant effect on the capacity of the through carriageway by impeding following drivers.

In this instance, consideration should be given to the provision of a nearside auxiliary lane instead of a taper for diverging traffic. The provision of an auxiliary lane, as shown in Figure 6.23, would allow turning traffic to move off the mainline prior to any deceleration.

The auxiliary lane should be of sufficient length to allow for the speed change from the major road to the turn into the minor road and would not normally be less than 80m. Its length may also depend on any need for reservoir space for turning traffic. The auxiliary lane should commence with a direct taper (Figure 6.23), the length of which shall be determined from Table 6.7. The taper should be that used for a left turning lane for a single lane dualling or dual carriageway junction, with the. relevant deceleration length given in Tables 6.8 and 6.9.

E =? *

a. Deceleration Length

Figure 6.22 MajorIMinor Junction with Nearside Diverging Taper

E 9 .4

b - a

3

a. Deceleration Length b. Direct Taper Length

Figure 6.23 Major/Minor Junction with Nearside Auxiliary Lane


6.7.14 Merging Tapers

Merging tapers permit minor road traffic to accelerate fully before joining the faster traffic streams on the mainline where the joining traffic may otherwise impede flow or be a hazard (Figure 6.24).

Merging tapers shall only be used at dual carriageway junctions. They shall be provided generally where the major.road design speed is 80 kph or above, or when and the volume of right turning traffic in the design year exceeds 600 vehicles AADT (one-way).

However, where the merging taper is for an upgradient of greater than 4% or where the percentage of large goods vehicles exceeds 20%, the threshold value may be reduced to 450 vehicles AADT (one-way).

Merging tapers shall never be used at single lane dualling junctions.

At some junctions on dual carriageways, there may be safety benefits in providing merging tapers at lower flows.

SECTION 6

A separate turning lane, with a radius of at least 25m where the main road design speed is 80 kphj and at least 30m above this speed, shall be used to introduce the merging taper from the minor road. The .initial width of the lane, which will depend on the radius of the turning lane (determined from Table 6.5), should. be decreased at a constant taper depending on the design speed.

The lengths of the tapers to be used are give'n in Table 6,10. The minimum initial width of a merging taper shall be 4.0m.

On dual carriageways, with a design speed of 120 kph or greater, the merging taper may be preceded by a short nose of 40m length formed between it and the end of the 30m approach curve. The back of the nose should' have a minimum width of 2m (Figure 6.25).

Table 6.10 Merging Length - a

Design Speed (kph)

80 100 120 140

, -1

a - -

-------------------------------------- - - - - - - - - - - - - - - - - - - -

E 9 *

a. Merging Length

Merging Length-a .

(ml

90 ' 110 130 150

Figure 6.24 MajorIMinor Junction with Nearside Merging Taper



Nose 2m minimum

9 * a. Merging Length b. Nose Taper Nose

.. . .- I Shoulder

Figure 6.25 MajorNinor Junction with Nearside Merging Taper (Alternative for Dual Carriageway with a Design Speed of 120 kph) -...

6.7.1 5 Stagger Distances I

The stagger distance of a 'junction is the distance along the major road between the centrelines of the two minor roads.

ieft/F3ight stagger For simple major/minor junctions with a IefVright stagger, the minimum stagger distance shall be 40m.

For a ghost island junction the stagger distance shall be 50m and for a junction with single lane dualling it shall be 40m. These are based on the distance required for manoeuvring the 20.0m drawbar trailer combination design vehicle between the two minor roads, and shall be provided on all new staggered junctions, including the upgrading of rural crossroads, where large vehicles are expected.

RightLeft Stagger The minimum values for staggered righffleft majorlminor junction are given in Table 6.1 1. For higher design speeds, the distance is based . on the sum of the two deceleration lengths lying side by side plus the turning lengths (and queuing lengths, if appropriate) at each end, as indicated in the table. Otherwise'it is based on the manoeuvring requirements of the design vehicle.

Table 6.1 1 Minimum .Stagger Distance for RightLeft Staggered Junction

6.7.1 6 Skew Junctions

The design parameters where the minor road. approaches at an angle other than 90°, for both left hand and right hand skew junctions, are shown in Figure 6.26. The geometric parameters are set out in Clauses 6.7.5 to 6.7.1 2.

. . Figure 6.26 MajorIMinor Junction with Skew Minor Road'. .

. - . . . .

r

January 1997' . Pane 6/26

.

QATAR HIGHWAY DESIGN MANUAL SECTION 6 I

. .

----- -----

. . .

A b - a: - , t . . ..

b

I

-.. -

----- - - -.- ------ . . -_ 4

b .

. .

c. Through Lane Width (6.7;s). a. Turning Length (+ Queuing Length . . if required, see clause 6.7.8) d. Turning Lane Width (6.7.6)

b. Deceleration Length (6.7.1 0). e. ~ i n o r Road Entry Width (6.7.5)

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1 , * ! * . *.. :,.;.: ;: . , -. .., < I . . . - l * . " e , .; QATAR HIGHWAY DESIGN MANUAL SECTION 6

6.7.17 T-Junctlon with Carriageway Separation

On dual carriageways, left turn crossing manoeuvres at the junction are prevented and facilities shall be provided nearby for turning traffic. as highlighted in Clause 6.6.5. One method of achieving this is to provide an interchange, the principle of which is shown in Section 7. The design of such crossings is outlined in the following paragraphs and the right-idright-out connections to the mainline are illustrated in Figures 6.27 and 6.28.

Preventing left turns removes the need to signalize the carriageways on the major road to cater for these movements. The major road camageway can pass through the junction at a constant width. Two right-Wright-out connections are used with an overbridge or underpass. These junctions should be designed in composite form, as described in this section, catering for the right turn movement only.

For the right turn merge to the main road, the minor road channelizing island shown in Figures 6.27 and 6.28 shall be designed so as to provide a constant width of turn into the major road. The width shall be determined from Table 6.5. The detail of the island as approached along the minor road is as set out in Clause 6.7.18. If there is a merging taper as shown in Figure 6.28, the widths and tapers shall be as set out in Clause 6.7.14. The hatched markings shall be extended from the minor road centreline to link with those for the merge taper, the channelizing island being provided within them, as in Figures 6.27 and 6.28.

For the right turn diverge from the major road, the channelizing island described in Clause 6.7.12 and shown in Figures 6.27 and 6.28 shall be designed so as to provide a constant width around the turn to the minor road. The width shall be determined from Table 6.5. Where a nearside diverging taper or nearside auxiliary lane is present (see Figures 6.22 and 6.23), the hatched markings should be extended along their current path until the intersection with the centreline of the minor road, and the channelizing island shall be provided within them. This is shown in Figures 6.27 an 6.28.

The right-infright-out connections can also be used with the compact interchange detailed in Section 7. ' It offers a cheaper but more restricted form of grade separation where the economic case for a full interchange cannot be fully justified. The connector roads between the

right-idright-out connections shall be designed in accordance with Section 7.

Traffic shall be introduced to the right turn lane by a nearside diverge or auxiliary lane in accordance with Clause 6.7.13. ..

. .

Traffic leaving the right turn lane should 'Give Way" or merge with the major road traffic in accordance with Clause 6.7.14,. or join an added lane, depending on the major road design speed, traffic flows and layout.


- - - - - - - _ I - - - - - - -

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ChanneAshg kland flared to gke conrtant carriageway width around the turn

Figure 6.27 T-Junction with Carriageway Separation

c b - - a

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---------------------------------------

a. Diverge Taper c. Merge Taper

b. Nose Taper d. Cunre Widened Lane

:

Figure 6.28 T-Junction (Alternative for Dual Carriageway with a design speed of 12Okph).

January 1997 . . . . - . .- Page 6/28

6.7.1 8 Channelizing Islands b)

T-Junctlon The recommended channelizing island layout for T-Junctions or staggered junctions, where the minor road centreline is inclined to the major road at an angle of between 70' or 1 10°, is shown in Figure 6.29: This should be read in conjunction with Tables 6.1 2 and 6.1 3. C)

The following points should also be noted: d)

a) "Edge of major road carriageway" means edge of major road travelled way.

b) The circular arc R, is tangential to the offset, d, from the minor road centreline and the offside edge ofthe through traffic

'

lane on the major road into which left e) turning traffic from the minor road will turn.

c) By striking a circular arc of radius (R, + 2) metres from the same centre point as arc R, to intersect the edge of the major road carriageway, point A is established where a straight line drawn from the centre point of arc R, to this intersection crosses R,.

d) 'The circular arc R, is tangential to the offside edge of the major road offside diverging lane and also passes through point A.

e); -Radius R, is normally the same value as R, but should be designed to ensure that the island nose is positioned between 2 - 4 metres from the edge of the main carriageway and that the width of the island lies between 2- 5 metres.

f) The design ensures that left tuming traffic from the major road will not clash with traffic waiting to turn left from the minor road.

Skew Junctions The design of a channelizing island for skew junctions is similar to that outlined above, but the following points should be noted:

a) The centreline of the minor road is turned with a radius of at least 50 metres td meet the edge of the major road at right angles.

For right hand skew junctions, the island should be about 15 metres long. The left hand side of its tail (viewed from the minor road approach) should touch the curved minor road centreline and be rounded off at a radius of 0.75m to 1.00m.

The offset, dl for right hand skew junctions is 4.5 metres.

For left hand skew junctions, the circular arc R, touches the curved minor road centreline and is tangential to the offset edge of the through traffic lane on the major road into which left turning traffic from the minor road will turn.

The island should be about 15 metres long. The tail is offset about 1m to the left of the curved minor road centreline (viewed from the minor road approach) and rounded off with a radius of 0.75m to 1.00m.

Januarv 1997 Pane 6/29


Figure 6.29 Design of Channelizing Island

Table 6.12 Channelizing Island Offset

Minor Road Inclination - 8"

70 80 90 100 110

Offset - d (m)

1.5 2.0 2.5 2.0 1.5

Table 6.1 3 Design of Radius R,

Width of Major Road Carriageway at

Junction - w (m)

7.3

1 1.3 (Ghost Island)

17.3 (Single Lane Dualling)

January 1997 . Page 6/30

Radius - R, (m)

12

14

22

. - . .. QATAR HIGHWAY DESIGN MANUAL

6.7.1 9 SplitterIRight Turn Islands

Splitter/right turn islands may be used to chanelize traffic flows and separate conflict points within a junction.

They have additional benefits of reserving space for signing and aiding pedestrian movement by providing a refuge at busy junctions. Refer to Figure 6.30.

Comer radii and carriageway widths given in Table 6.5 shall be used to construct the right turn lane. The raised island shall be constructed to give shy distances to travelled ways as illustrated in Figure 6.30.

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' -, . SECTION 6

Traffic leaving the right turn lane should "Give Way" or merge with the major road traffic in accordance with Clauses 6.7.4 or 6.7.14, or join an added lane, depending on the major road design speed, traffic flows and layout.

6.7.20 Drainage and Crossfail

From considerations of surface water drainage and driver comfort, the road camber on the major road shall be retained through the junction and the minor road graded into the channel line of the major road. Checks shall be made for flat areas at all changes of gradient, superelevation of crossfall. Refer to Section 8.

Splitter islands are particularly useful at signalized junctions where minor road right turn traffic can be controlled by "Give Way" signs and markings rather than signals.

Traffic shall be introduced to the right turn lane by a nearside diverge or auxiliary lane in accordance with Clause 6.7.1 3.

- . L

. ..

Edge of Travelled Wa

R = Inside Corner Radius

RW E Outside Comer Radius

W = Lane Width

r = 0.- Radius

Hatched Marking

Figure 6.30 Typical Layout of Splitter Island

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Januaiy 1997 Page 6/31

QATAR HIGHWAY DESIGN MANUAL SECTION 6 . . . . . .

6.7.21 Traffic Signs and Road Markings

The need for, and layout of, traffic signs and road markings is an integral part of the design process and no junction design is complete without these features having been included. Advance direction and warning signs shall be provided, and care must be taken with the positioning and size of signs at the junctior?. itself. so that they do not interfere with drivers' visibility requirements.. These matters need to be considered from the earliest stage as they can fundamentally affect layout and hence land acquisition requirements. Advance signing on minor roads may need particularly careful consideration. Refer to the Qatar Traffic Manual for details of signing and marking.

6.7.22 Road Lighting

Road lighting is normally provided at majorlminor junctions in rural areas only when an intersecting road has lighting. ,When an existing junction is being modified, the lighting provision should be checked for suitability with the new arrangement. Any alteration should be carried out prior to, or at the same time as the roadworks. Refer to Section 10.

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6.8 ROUNDABOUTS - GENERAL

This section defines the main types of roundabouts that can be used for an at-grade junction of any class of road.

The requirements are defined in relation to the size of roundabouts, effect of approach speed, visibility, entry width, entry deflection and ttie circulatory carriageway.

The recommendations for siting of roundabouts are given in Clause 6.1.4.

6.8.1 General Principles

The principal objective of roundabout design is to secure the safe interaction of traffic between crossing traffic. streams with minimum delay. This is achieved by a combination of geometric layout features that, ideally, are matched to the volumes of traffic in the traffic streams, their speed, and to any locational constraints that apply*

There are two broad regimes of roundabout operation. The first occurs in urban areas with high peak flows, often with marked tidal variations and physical restrictions on the space available. The second regime occurs in rural areas and is characterised by high approach speeds, low tidal variation and few physical constraints.

Entry width is an irnportant feature that determines entry capacity and often needs to be larger in urban situations than in rural cases. On the other hand, the most important determinant of safety is vehicle deflection imposed at entry because this governs the speed of vehicles through the junction. It is particularly important whenever approach speeds are high. Entry deflection is related to the entry path curvature and limiting this radius of curvature in the vicinity of the entry to 100m maximum ensures that sufficient deflection will be undergone by entering vehicles to limit through speeds.

The characteristics of roundabout accidents and their frequencies in relation to geometric layout design and traffic flows have been studied in the UK by Transportation Research Laboratory (TRL). 'The relationships derived from these studies have provided insights into how various aspects of design interact to influence the types and frequencies of accidents at roundabouts. These relationships therefore, constitute the fundamentals of design for safety. As relationships between aspects of design are not always mutually compatible, minimising the likely incidence of a particular type of accident

may increase the potential for another. Design, therefore is a trade-off between operational efficiency, minimising delays at the junction, and various safety aspects within whichever location constraints apply. The latter are often the dominating factor when designing improvements to an existing junction, particularly in urban areas.

Consideration of the need for, and layout of traffic signs and road markings should be an integral part of the design process. Reference should be made to the Qatar Traffic Manual.

The provision of road lighting at roundabouts should normally be regarded as an essential safety requirement. Sometimes lighting requirements may conflict with environmental considerations. However, it should be recognised that roundabouts are generally safer than other forms of at-grade junctions and the decision to use a roi~ndabout should not be abandoned solely because of lighting problems. In sensitive locations it may be possible to adopt alternative lighting methods and other measures to make the roundabout more visible. When an existing roundabout junction is being modified, the lighting layout should be checked for suitability with the new road arrangement and any alteration carried out prior to, or at the same time as the roadworks. It is irnportant that approaching drivers see and perceive that they . are approaching a roundabout and are not misled by the projection of the lighting layout, particularly at times of poor visibility.

6.8.2 Types of Roundabout

Definitions The preferred main type of roundabout to be used in Qatar is the Normal Roundabout. There are other forms such as Mini and Double Roundabouts, and other variants of these basic types, ie. Ring Junctions, Interchange Roundabouts and Signalized Roundabouts.

Normal Roundabout A roundabout having a one-way circulatory carriageway around a kerbed central island 4m or more in diameter and usually with flared approaches to allow multiple vehicle entry. (Figure 6.31).

The number of entries recommended is either 3 or 4. Roundabouts perform particularly well with 3 arms, being more efficient than signals, provided the traffic demand is well balanced between the arms.


a Traffic deflection

. , island

. . QATAR HIGHWAY DESIGN MANUAL . - ' . SECTION 6 I

Figure 6.31 Normal Roundabout . . .

I

If the number o f entries is above 4, driver comprehension is affected and the roundabout becomes larger with the probability that higher circulatory speeds will be generated.

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Interchange Roundabouts The foliowing examples of interchanges are discussed in Section 7 Interchanges.

Two bridge roundabout . . One bridge and two roundabouts'

. (dumbbell)

Ring junction interchange.

Signalized Roundabout As with Majorminor Junctions, traffic signals can be installed at roundabouts to improve safety or traffic capacity. Traffic signals can be used at one or more of the approach arms or even on the circulatory carnageway on some '

large roundabouts.

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6.9 SAFETY AT ROUNDABOUTS

6.9.1 General

It is generally known from studies that fewer accidents occur at roundabouts than at signalized junctions of similar traffic flows. The severity of accidents is also much less than at other junctions.

Care must be taken in layout design to secure the essential safety aspects. The most common problem affecting safety is excessive speed, both at entry or within the roundabout. The most significant factors contributing to high entry and circulating speeds are:

Inadequate entry deflection . . .

. A very acute entry angle which encourages fast merging manoeuvres with circulating traffic

Poor visibility to the "Give Way" line

Poorly designed or positioned warning and advance direction signing

"Reduce Speed Now" signs, where provided, being incorrectly sited

More than four entries leading to a large configuration.

Additional safety aspects to be considered in designing a layout include:

Angle between arms: The accident potential of an entry depends on both the angle (anticlockwise) between its approach arm and the previous approach arm, and the traffic flows. A high flow entry should have a large angle to the next entry, and a low ftow entry a smaller angle in order to rninirnise accidents

Gradient: Whilst it is normal to flatten approach gradients to about 2% or less at entry, research at a limited number of sites has shown that this has only a small beneficial effect on accident potential

Visibility to the left at entry: This has comparatively little influence upon accident risk. There is nothing to be gained by increasing visibility above the recommended level

Crest Curves: Junctions should not be sited on crest curves where the approach sight to the roundabout is impaired

' SECTION 6

~ e s i ~ n to encourage slow entry to the junction and quick exit to leave the junction clear for the next users.

Measures to reduce accidents at existing roundabouts with poor safety records include:

Repositioning or reinforcement of warning signs

Provision of map type advance direction signs

Making the "Give Way" line more conspicuous.

Moving the central island chevron sign further to the right to emphasise the angle of turn, placing another chevron sign above the normal position, and placing chevron signs in the median in line with the offside lane approach on dual carriageways. Chevron boards can impinge on circulatory visibility but the effects can be minimised by positioning the boards (and associated turn right sign) 2m back from the central island kerbline

When approach speeds are low (usually in urban areas), a ring of contrasting paving can be laid in a chevron-pattern inside the central island perimeter at a gentle slope, refer to Clause 5.1 9.

In rural areas it is not recommended to install raised kerbed chevrons. on roundabouts. Experience has shown that physical obstructions such as' chevron kerbing will be hit inadvertently at night time by vehicles whose drivers are not aware of the junction ahead. Chevron signs should be placed in these locations only

Landscaping where approach speeds are high in urban areas can provide a useful supplement

Provision of "Yellow Bar Markings" on fast dual camageway approaches has shown that a 57% reduction in accidents can be achieved. This is from studies carried out in the UK by the TRL

Provision of appropriate levels of skidding resistance on the approaches to roundabouts and on the circulatory carriageways

Januarv 1997

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Paae 6/35


It should be noted that at the speed of traffic on a circulatory carriageway, skidding resistance is derived from the surface texture of the aggregates which form the surface of the road (the micro-. texture). Particular consideration should be given to ensure that the aggregates used have skid resisting properties appropriate to the circumstances .

The deep surface texture (the macro- texture) necessary for good skid resistance on high speed routes is not required for circulatory carriageways. Deep surface texture is required however, on the approaches to roundabouts if the design speed of traffic is greater than 120 kph

. .

Avoidance of abrupt and excessive superelevation in the entry region

Reduction of excessive entry width by hatching or physical means

Provision of "Reduce Speed Now' signs andlor 'Count-down" markers

Reduction of the circular width by insertion of a central island collar.

Care should be taken with the choice of kerb type for roundabout design. A safety problem can arise where certain specialist, high profile kerbs are used around a central island as they can be a danger to vehicles over-running the entry. - Observations have shown that these kerbs can result in loss of control or overturning of vehicles unless the approach angle is small and actual vehicle speeds are low. Where high profile kerbs are to be used on approaches, the kerbs can be hazardous for vehicles and pedestrians, and consideration should be given to the provision of pedestrian guardrails. Care should be taken to ensure that visibility sightlines are maintained.

High circulatory speeds cause associated entry problems and normally occur at large roundabouts with excessively long and/or wide circulatory cartiageways. Excessive circulatory speeds can also be caused at smaller roundabouts by inadequate deflection at previous entries. The solution to high circulatory speeds usually has to be fairly drastic, involving the signalization of problem entry arms at peak hours. In extreme cases the roundabout may have to be converted to a ring junction in which the circulatory carriageway is made 2-way and the entrieslexits are controlled by individual normal roundabouts or traffic signals.

If entry problems are caused by poor visibility to the left, good results can be achieved by moving the 'Give Way" line forward in conjunction with curtailing the adjacent circulatory carriageway by hatching or extension of the traffic deflection island.

6.9.2 Two Wheeled Vehicles

Though roundabouts have an impressive overall safety record for most vehicle types, this does not apply equally to two wheeled vehicles. Research has shown that at four-arm roundabouts in the UK, injury accidents involving two-wheeled vehicles constitute about half of those reported. The proportion of accidents involving cyclists is about 15%, although they typically constitute less than 2% of the traffic flow.

The.accident involvement rates for two-wheeled vehicles, expressed in terms of accidents per road user movement, are 10-1 5 times those of cars, with pedal cyclists generally having slightly higher accident rates than two-wheeled motor vehicles riders.

The study at four-arm roundabouts by the TRL in the UK has shown for example that, in 50 and 60 kph posted speed areas, there are differences in pedal cycle accident involvement rates for different categories of roundabouts. Engineers should be aware of the following:

Normal roundabouts with small central islands and flared entries have accident rates which are about twice those of normal roundabouts with large central islands and unflared entries. This relationship appears to apply consistently for all types of vehicular road users. As previously stated, analysis of accident data suggests that when all types of accident are considered, entry deflection is the,most important factor

70% of pedal cycle accidents at smaller normal roundabouts are of the 'entry/circulating' type, for example, motor vehicle entering roundabout collides with pedal cycle crossing entry

At dual carriageway roundabouts, the accident involvement rate for cyclists is about two to three times greater than that at dual camageway traffic signals, but for cars, the opposite is true.


. . . . QATAR HIGHWAY DESIGN MANUAL . I , _ . _ < , . .

. . - . . SECTION 6'

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6.9.3 Large Goods Vehicles : i . .

The problem of large goods vehicles overturning or shedding their loads at roundabouts has an obvious solution in relation to layout geometry. Whilst in the UK there are only about 60 personal injury accidents a year in this category, there are considerably more damage-only accidents. Load shedding often involves great congestion and delay, and is expensive to clear, especially if occurring at major junctions. Experience suggests that roundabouts where these problems persist usually exhibit one or more of the following features:

Inadequate entry deflection leading to high entry speeds

. . Long straight sections of circulatory carriageway leading into deceptively tight bends .

Sharp turns into exits

Excessive crossfall changes on the circulatory carriageway

Excessive adverse crossfall on a nearside lane of the circulatory carriageway.

An incipient problem for some vehicles may be present even if high speeds are not occurring. Research has shown that an articulated, large goods vehicle with a gravity height of 2.5m above the ground can overturn on a 20m radius bend at speeds as low as 24 kph. Particular attention should be paid to ensure that pavement surface tolerances are complied with and that abrupt changes in crossfall are avoided. It is good practice to make the exit radii greater than the entry radii.

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January 1997 Paae 6/37


6.10 ROUNDABOUT ELEMENTS

6.1 0.1 Definitions

Entry Wldth: el is measured from the point A along the normal to the nearside kerb, see Figure 6.32.

A. Polnt of Maxlmum Entry v. Appmach Half Dbfleclon at Left Wldth Hand End of Olvm Way r. Enty Radlu~ Unm D. In~uibed

a. Entry Width Clrclm Diambtbr

I' Average Effective Flare Length

Figure 6.33 Average Effective Flare Length

inscribed Circle Diameter: Dl is the diameter of the largest circle that can be inscribed within the junction olitline, see Figure 6.32. In cases where the outline is asymmetric, the local value in the region of the entry is taken.

The Entry Angle: a, serves as a geometric proxy for the conflict angle between entering and circulating streams. For roundabouts having a curved circulatory carriageway,

Figure 6.32 Geometric Design Features

Approach to Half Width: v, is measured at a point in the approach upstream from any entry flare, from the median line (or offside edge of carriageway on dual carriageways) to the nearside kerb, along a normal, see Figure 6.32

Average Effective Flare Length: It, is found as shown in Figure 6.33. The line GF'D is the projection of the nearside kerb from the approach towards the "Give Way" line, parallel to the median HA and at a distance of v from it. BA is the line along which e is measured (and

is therefore normal to GBJ), and thus D is at a distance of [e-v] from B. The line CF is parallel to BG (the nearside kerb) and at a distance of [e-vy2 from it. Usually the line CF' is therefore curved and its length is measured along the curve to obtain 1'.

Sharpness of Flare: S, is defined by the relationship:

4 Entry Angle

Figure 6.34 Entry Angle

The line BC is a tangent to the line EF, which is midway between the nearside kerbline and the median line or the edge of any median island on the offside, where this line intersects the "Give Wayw line. is measured as the acute angle between BC and the tangent to AID' at the point of intersection between BC. and AtD' shown in Figure 6.34.

and Is a measure of the rate at which extra width is developed in the entry flare. Large For all other, roundabouts, the construction is values of .S correspond to short severe flares shown in Figure 6.35. The line BC is the same and small values to long gradual flares. as in Figure 6.34. The line GH is the tangent to

the line JK, which is in the following exit, 'midway between the nearside kerb and the

January 1997 - . . . . Page 6/38

, . - . QATAR HIGHWAY DESIGN MANUAL

median lineor the edge of any median island on the offside, where this line intersects the outer edge of the circulatory carriageway. BC and GH intersect at L. 9 is then defined by:

when the right hand side of the equation is positive.

Figure 6.35 Entry Angle

When the right hand side of the equation is zero or negative, @=O. Angle BLG is measured on the "outside" of the roundabout, that is, on the side facing away from the central island..

Entry Radius: r is measured as the minimum radius of curvature of the nearside kerbline at entry, see Figure 6.32. For some designs the arc of .minimum radius may extend into the following exit, but this is not important provided that a half or more of the arc length is within the entry region.

Minimum Stopping Sight Distance: as defined in Section 2.

Entry Path Curvature: This is a measure of the amount of entry deflection to the right imposed on vehicles at the entry to a roundabout, see Clause 6.1 0.8.

Traffic ~ef16ction Island: a raised area (usually kerbed) on the carriageway, which is located and shaped so as to direct and also separate traffic movements onto and from a roundabout.

Ghost Islands used for Subsidiary Traffic Deflection: a shaped area, flush with the road surface, delineated by road markings, and within the entry width of the approach to a roundabout, so located to deflect and direct traffic movements into the circulatory carriageway.

SECTlON 6

6.1 0.2 Entries

The design of roundabout entries is a complex procedure, there are several variables which need to be addressed to ensure a design which is safe and has adequate capacity.

The designer has flexibility in the application of the parameters to best meet the particular site requirements and constraints. The variables are:

- Entry Width

- Flare Length

- Entry Angle

- Entry Radius

- Approach Carriageway Half Width.

6.10.3 Entry Width

It is good practice to add at least one extra lane width to the lanes on the entry approach, but as a general rule, not more than two lanes should be added and no entry should be more than four lanes wide. The relationship between entry width and capacity is quite significant. Entry width is the largest single factor, apart from approach camageway half width, affecting capacity.

There may be some cases, usually associated with low predicted flows, where increased entry width is not operationally necessary, but in these circumstances it is still recommended that two entry lanes be provided. This will give added flexibility at abnormal flow periods in the future, a passing facility in the event of breakdown, and will ease the problem of space provision for long vehicles turning.

Lane widths at the 'Give Way" line shall be not less than 3m. Lane widths should be tapered back in the entry flare to a minimum width of 2m. It is generally better to use wide lane widths because they are more suitable for large goods vehicles. For example, at a 10m wide entry, 3 x 3.33m lanes are better than 4 x 2.Sm lanes.

The development of entry lanes should take account of the anticipated turning proportions and possible lane bias since drivers often have a tendency to use the nearside lane. The use of lane bifurcatfon where a lane widens into two should maximise use of the entry width. The use of short offside lanes is not recommended.

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Paae 6/39


The alignment of entry lanes is also critical. On rural roundabouts, where design speeds are relatively high, the kerbline of the deflection island (or central reserve in the case of a dual carriageway) should be on an arc which, when projected forward, meets the central island tangentially. In urban areas, where design speeds are lower, this is less important, but nevertheless should be achieved where possible. Care should be taken to ensure that the resultant entry angle is not too low and that entry path cutvature is not too great.

For capacity assessment, the entry width should be taken as the width which drivers are likely to use. Where the offside kerbline forms a vehicle path which is tangential to the central islands, the entry width and effective entry width are the same.: '

It is usual to consider design flows 15 years after opening for highway schemes. This can result in roundabout entries with too many lanes for earlier year flows and lead to operational problems. A design year layout will determine overall geometry and land requirements for the roundabout, but for the early years, it may be necessary for the designer to consider an interim stage. This approach can result in reduced entry widths and entry lanes. Consideration can also be given to an interim reduction of the circulatory carriageway width, either by an increase in diameter of the central island, or by extending islands fonrvard into the circulatory carriageway.

6.10.4 Flare Design at Entry

Flares on the approach to roundabouts shall be such that:

a) The maximum entry width shall not exceed 10.5m for single and 15.0m for dual carriageway approach roads

b) The average effective flare length shall not exceed loom, but it should be noted that beyond 30 or 40m any expected extra capacity is derived from extrapolation beyond the bounds of experimental data and should therefore by treated with caution.

The capacity of an entry can be improved by increasing the average effective Flare length, though this is of limited effect. A minimum length of about 5m is desirable in urban areas, . whilst a length of 25m is considered adequate in rural areas. Flare lengths greater than 25m may assist in geometric layout but have little effect in increasing capacity. Flare lengths should not be areater than 100m. as bevond

SECTION 6

this the design becomes one of link widening. Where the design speed is high, entry widening should be developed gradually, avoiding any sharp angles. In urban areas the use of long flare lengths is often not possible due to land constraints and capacity may have to be achieved using wider entries and shorter flares.

As a rough guide, the total length of the entry widening (BG) should b e about twice the average effective flare length I' (Figure 6.33).

6.1 0.5 Entry Angle

The effect of entry angle on entry capacity is negative; as the angle increases capacity decreases slightly. However, care should be taken in the choice of entry angle since high and low angles may result in increased accident potential.

The angle should, if possible, lie between 20 and 60 degrees. Low entry angles force drivers into merging positions where they must either look over their shoulders to their left or attempt a true merge using their mirrors (with the attendant problems of disregarding the 'Give Way" line and generation of high entry speeds).

High entry angles produce excessive entry deflection and can lead to sharp braking at entries accompanied by "nose to tail" accidents, especially in rural areas. The best entry angle value is about 30 degrees. Figures 6.36 and 6.37 show two extreme cases.

Entry Anale i8 Defined a8 (90'-812)

. .

Figure 6.36 Example of Too Low an Entry Angle and also Substandard Entry Deflection.


. . * . . .. :: . . , .'- . . . , , ,'. ' , " . " ." C ' . . . . ..


4 Entry angle

Figure 6.37 Example of Too High an Entry Angle and also Excessive Entry Deflection

6.1 0.6 Entry Radius . . I

For small roundabouts entry capacity increases with entry radius up to about 20m, higher radii result in very little increase in capacity. The minimum entry radius should be 6m, a good practical design is about 20m. Where a roundabout is designed to cater for large goods vehicles in particular, the entry radius should not be less thanJ0m. For large roundabouts (40-60m diameter), large entry radii will almost certainly result in inadequate entry deflection, for example it will not be possible to achieve the deflection standard if the entry radius is lOOm or more.

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1

L . 6.10.7 Entry Kerbing

As entries are almost always kerbed, hardstrips should be terminated when entry widening begins. The simplest procedure is to place the. kerbs at the back of the hardstrip and then terminate the hardstrip edge line by profiling it back towards the kerbs in a short smooth curve or taper. (See Figures 6.38 and 6.39). This is not appropriate where there is regular use by cyclists who may wish to continue to the edge of the circulatory carriageway by using the hardstrip. .

, r - ! . , " . . L .. . January 1997 Page 6/41

QATAR HIG'HWAY DESIGN MANUAL SECTION 6

----------

d Edge of Carriageway

Figure 6.38 Method of Terminating Edge Strip on Single Carriageway Approach to a Roundabout

d

d

d

.................... - .

I ' m b d

a Kerbs ,

b Edge Lines c Edge Line Profiled Back towards the Kerb d Edge of Carriageway

Figure 6.39 Method of Terminating Edge Strips on Dual Carriageway Approach to a Roundabout

January 1997 . . . . Page 6/42

QATAR HIGHWAY DESIGN MANUAL . -

, . . . SECTION 6

6.1 0.8 Entry Deflection

Entry Path Curvature is one of the most important determinants of safety at roundabouts. It is a measure of the amount of entry deflection to the right imposed on vehicles at entry to the roundabout.

For design purposes only, at both new and improved 'normal' type roundabouts, the vehicle entry path shall be such that when inscribed in accordance with the following construction, the tightest radius of the entry path curvature shall not exceed 100 metres.

Constructlon of the Vehicle Path The method of.construction and measuring the entry path curvature is described below, and shown in Figures 6.40 to 6.43. Figure 6.41 shows an approach with negative curvature, Figure 6.42 shows an approach with positive approach curvature, and Figure 6.43 a roundabout at a 'Y' junction.

Assume:

a) The entering vehicle is 2m wide and will be taking the 'straight ahead' movement at a 4 arm roundabout and across the head of the Tee at a 3 arm roundabout

b) That there is no other traffic on the approach and on the circulatory carriageway

c) That the driver will negotiate the site . . constraints with minimum deflections and

that lane markings by the 'Give Way" line will be ignored

d) The initial approach position for entry path curvature measured from a point not less than 50m from the 'Give Way" line is within the range:

- 1 m from the nearside kerb

- 1 m from the centreline of a single carriageway or 1 m from the offside kerb of a dual carriageway

This will ensure that all approach alignments are examined and that no vehicle path can exceed the recommended maximum radius of curvature

e) 'That the vehicle proceeds towards the "Give Way" line, then:

- It proceeds towards the central island of the roundabout passing through a point not less than 1m from the nearside channel or kerb, the position of which relative to the starting point depends on the amount of approach flare to the right (Figure 6.40 and 6.41)

- The vehicle is then assumed to continue on a smooth path with its centreline never passing closer than 1 m from the central island (it may be more in some configurations).

Draw, to a scale not less than 1/500 using a flexible curve of equivalent, the centre line of the most realistic path that a vehicle would take in its complete passage through the junction on a smooth alignment without sharp transitions. More than one independent assessment of the vehicle paths shall be carried out.

This tightest radius shall be measured by means of suitable templates. See "To Measure the Entry Path Curvaturen.

The exact path drawn will be a matter of personal judgement and the results should be examined for compliance and consistency with the appropriate clauses in this section.

One convenient method of construction of the required path is to imagine the advance of all the channel or kerb lines and centreline in the case of single carriageways (together with central islands and deflection islands) into the carriageway by 1 m.

The vehicle path will be the line of least resistance, whose centreline will normally, but not always, be tangential to these construction lines; in the entry, at the central island and in the exit. Any reverse of curvature in the vehicle path around the central island must be drawn so that there is no sharp deviation between that curve and the entry curve. Particular care in checking entry path curvature is required when considering small central island designs.

To Measure the Entry Path Curvature The entry path curvature is measured on the curved length of path in the vicinity of the 'Give Way" line (but not more than 50m in advance of it) between points X and Y (see Figures 6.40 to 6.43) about 20m to 25m but not less than 20m in length, over which the tightest radius occurs.


QATAR HIGHWAY DESIGN MANUAL SECTION 6 . . . . ..

Figure 6.40 Determination of Entry Path Curvature L

Figure 6.41 Determination of Entry Path (On a Curved Approach Arm with Negative Approach Curvature)

January 1997 . . .- . . . . . .

Page 6/44 . . . .. - .

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. . . . . . . . . . . . . . . . . . . . ( , ' , . . ' j 3 ; . . . - - . . . . . .


Figure 6.42 Determination of Entry Path C u d u r e (On a Curved Approach Arm with Positive Approach Curvature) . .

path in the vicinity of the Give Way line but not more than $Om In advance of It.

b. Commencement polnt 1 rn from the ofkide kerb for D2 or 1m from centre line for S2L, not ierr than SOm from the Give Way line.

c. VehWe entry path cumture.

i

Figure 6.43 Determination of. Entry Path Curvature for a Roundabout at a Y Junction


6.1 0.9 Achieving Entry Deflection

A good method for creating entry deflection on new schemes where there are no other constraints is to stagger the arms, as shown in Figure 6.44. This will help with the overall design, reduce the size of roundabouts, minimise land acquisition and assist with the construction of 'easyn exits.

It is not good practice to generate entry deflection by sharply deviating the approach roads to the left close to the roundabout and then to the right at entry. Approach curves should be fairly gentle, but there are cases when horizontal radii below the minimum for the general design speed of the approach link may be used, provided always that they are proceeded by the 'Roundabout Ahead' waming sign as defined in the Qatar Traffic Manual. However, tight radii will require large amounts of verge widening to provide adequate forward visibility and add to the verge maintenance requirements.

There is evidence to suggest that a gentle left hand bend leading to a right hand deflection at entry is more safe than a gentle right hand bend.

In urban areas, the restrictions on space available coupled with the turning width requirements of large goods vehicles may necessitate small normal roundabouts which cannot provide sufficient entry deflection to the right by means of the central island alone. In these cases, deflection should be generated by means of enlarged traffic islands in the entry, (Figure 6.45).

a. Crntmlnr Omrt 1620m (Not. Paaf Exb)

Figure 6.44 Entry Deflection by Staggering Approach Roads

Figure 6.45 Example Showing How Island Design can Increase Entry Deflection at an Existing Roundabout

January . . 1997 Page 6/46 - .

QATAR HIGHWAY DESlGN MANUAL SECTION 6

6.1 0.1 0 Visibility

The forward visibility at the approach to a roundabout shall be as indicated in Section 2 for the appropriate design speed. This Minimum Stopping Sight Distance (SSD) is measured to the "Give Way' line as shown in Figure 6.46.

The following guidelines represent good practice concerning the provision of visibility and, when subject to relaxation, there is a need for additional signing to alert drivers of all vehicles to potential hazards.

Eye and Object Heights Visibilities, with the exception of visibility to the left at entry and across the central island, shall be assessed in accordance to Section 2. Visibility to the left and across the central island shall be obtainable from a driiets eye height of 1.05m to an object height of 1.05m, and the envelope of visibility shall extend to 2,Om above the road surface.

Where traffic and direction signs are to be erected on a central reservation, verge, or deflection island within the envelope of visibility, including to the left, the mounting height shall not be less than 2.0m above the carriageway surface and the envelope checked on sites with changes of gradient.

Visibility to the Left Drivers of all vehicles approaching 'the "Give Way' line shall be able to see the full width of the circulatory carriageway to their left from .the "Give Way" line for a distance appropriate to the sight stopping distance for the circulatory traffic (measured along the centreline of the circulatory carriageway) as indicated in Table 6.14, and shown in Figure 6.47. This also applies to roundabouts that have parapet walls on either side of the circulatory carriageway.

This visibility shall be checked from the centre of the offside lane at a distance of 15m back from the "Give Way' line, as shown in Figure 6.48. Checks shall be made that crossfall design or consiruction and sign location do not restrict visibility.

It should be noted that excessive visibility at entry or visibility between adjacent entries can result in approach and entry speeds greater than desirable for the junction geometry. Consideration shall be given to limiting in particular the visibility of adjacent entries to that from 15m back on the approach, and the visibility along the approach to no more than the stopping sight distances for the design speed of the approach, by the selective use of landscaping.

-.. . . . . . ' ' . ...:: . ....

January 1997

Forward Visibility at Entry Drivers of all vehicles approaching the 'Give Way" line shall be able to see the full width of the circulatory carriageway ahead of them for a distance (measured along the centre line of the circulatory carriageway) appropriate to the size of the roundabout (as indicated in Table 6.1 4). The visibility shall be checked from the centre of the nearside lane at a distance of 15m back from the "Give Way" line as shown in Figure 6.49.

Circulatory Visibility Drivers of all vehicles circulating on a roundabout shall be able to see the full width of the circulatory carriageway ahead of them for a distance appropriate to the size of roundabout (as indicated in Table 6.1 4). This visibility shall be checked from a point 2m in from the central island as shown in Figure 6.50. It is often useful to improve the conspicuousness of central islands by the use of landscaping, but this could obstruct circulatory visibility. The circulatory visibility envelope will encroach onto the height of vegetation or surface treatment. In these situations, limited penetration into the visibility envelope by vegetative growth of a dispersed nature would not be unacceptable.

Table 6.14 Visibility Distance

Inscribed Circle Diameter . (m)

< 40

40-60

60-1 00

> 100

Pedestrian Crossing Visibility Driven of all vehicles approaching a pedestrian crossing across an entry shall have a minimum distance of visibility to it of the Stopping Sight Distance for the design speed of the link (see Section 2). At the 'Give Way" line, drivers of all vehicles shall be able to see the full width of a pedestrian' crossing across the next exit if the crossing is within 50m of the roundabout (see Figure 6.51). In urban areas, adjacent roadside development may however prevent 'this visibility splay being fully established.

Visibility Distance (m)

('a' in Figure 6.49)

Whole Junction

40

50

70

- QATAR HIGHWAY DESIGN MANUAL SECTION 6

. . Visual Intrusions Signs, street furniture and planting shall not be placed within the visibility envelopes so as to obstruct visibility, but infringements by isolated

C, slim projections such as lamp columns, sign - supports or bridge columns can be ignored provided they are less than 550mm wide. The only exception to this will be positioning of - bollards on deflection islands and staggered chevron boards on central islands. Where possible, care shall be taken to minimise the effects of pedestrians on visibility requirements. -

- Page 6/48 - January 1997 . . , .

. . . .. . . .


7.3m Dual Carriageway

11.3m Single Carriageway

7.3m

7.3m Single Carriageway

Vehicle Position Centre of Nearside Lane s Desinble Minimum Stopping Sight Distance (SSD) for Approach Road Design Speed

Figure 6.46 Measurement of Stopping Sight Distance

.. . January 1997 Page 6/49

QATAR HIGHWAY DESIGN MANUAL 3cb I IUN o

. .

. .

a. Sight Stopping Dlrtancr for Circulating Tratfic b. Half Lane Width

Figure 6.47 Visibility to the Left Required at Entry (From "Give Way" Line)

- -

Aroa oIcirculetoy crniapeway over whkh vislbilily shall be obtained korn viewpoint 4

8. Sbht Slopping Distance for CkculeUnp TraMc b. Half Lene WUU, 8. Limit of VlrlblIMy Splay

w

Figure 6.48 ~ i i i b i l i t ~ to the Left Required at Entry (15m back from Oive Way" Line)

January 1997 . - . .- . . . Page6150

QATAR H ~ G ~ W A Y DESIGN MANUAL ' SECTION 6'

a. Sight Stopping Distance for Circulating Traffic b. Half Lone Width c. Limit of Visibility Splay

1 Figure 6.49 Forward Visibility Required at Entry L .-

t

Area 01 clrcuiatory carriageway over which visibility shall be obtained . from viewpoint 4

. 8 Dlstance Related to Circulatory Speed b Llmlt olVlsibility Splay

Figure 6.50 Circulatory Visibility

. . . . . ._ . . .lanuarv 1997 Page 6/51

--

. 8 Minimum 8ma over whleh unobstructed vlrlblltty h required from vlewpolnt 4 when crotrlng h wlthin 6Om of exit

b H a l lane width c LlmR of vlslbllfty rpky


Figure 6.51 Visibility Required at Entry to Pedestrian Crossing at Next Exit

CJ I

January 1997 Page 6/52 . .


6.1 0.1 1 Circulatory Carriageway

The circulatory carriageway should, if possible, be circular in plan, avoiding deceptively tight bends.

The width of the circulatory carriageway shall not exceed 15m. However, block paving 'collars' around the central island can be used to provide additional width if long vehicle turning movements need to be catered for on smaller roundabouts.

The width of the circulatory carriageway shall be constant and lie between 1.0 and 1.2 times the maximum entry width. However, see Clause 6.1 0.1 2 i f small lnscribed Circle Diameters (ICDs) are being contemplated.

It is normal practice to avoid short lengths of reverse curve between entry and adjacent exits by linking these curves or joining them with straights between the entry radius and the exit radius. One method is to increase the exit radius. However, where there is a considerable distance between the entry and the next exit, as at three entry roundabouts, reverse curvature may result (see Figure 6.50).

'There may be situations where the turning proportions are such that one section of circulatory carriageway will have a relatively low flow. In this case, there may be an over provision in circulating carriageway width and an area of carriageway, usually adjacent to an entry deflection island, becomes unused. It would be possible to reduce the circulatory carriageway width by extending the deflection island and advancing the 'Give Way" line. This method of reducing circulatory width may also be adopted as an interim measure in the early . years of a scheme.

For larger roundabouts, this reduction in circulatory width can be achieved by the use of hatch markings and is often associated with taking out of use the offside entry lane. If such measures are to be considered as an interim geometric design feature for early years traffic flows, consideration should be given to the use of contrasting hard surfacing for these areas.

For smaller roundabouts it is more appropriate to consider interim circulatory carriageway reduction by increasing the size of the central island. If this is to be introduced from the outset, a preferable measure would be the use of contrasting hard surfacing but hatch markings could also be considered. .

6.10.12 Inscribed Circle Diameter (ICD)

The following advice is based on the swept turning paths generated by a 16.5m long articulated vehicle with a single axle at the rear of the trailer. This is referred to below as the "Design Vehicle".

The turning width required by this type of vehicle is greater than that for all other vehicles within the normal maximum dimensions permitted in the classifications given in Table 6.1, or likely to be permitted in the near future. The requirements for other vehicles (including a 12m long rigid vehicle, 12m long coach, 20m drawbar trailer combination, and a 16.5m articulated vehicle) are less onerous.

The smallest ICD for a normal roundabout that will accommodate the "Design Vehicle' is 28m. It should be noted that it may be difficult, if not impossible, to meet the entry deflection requirement with normal roundabouts which have lCDs up to 40m. In this case consideration could be given to the installation of a low profile central island which would provide adequate deflection for standard vehicles but allow overrun by the rear wheels of articulated vehicles and trailers. Such islands should have the same profile as the circulatory camageway with a maximum upstand of 50mm.

The turning space requirements for the "Design Vehiclen at normal roundabouts from 28m to 36m ICD are shown in Figure 6.52. For lCDs above these values, and/or where low profile central islands are to be installed, the circulatory carriageway width should be checked against Table 6.5. But usually the rule in Clause 6.10.1 1 will provide more than adequate width.

6.1 0.1 3 Exits

The spacing of an exit and the preceding entry shall not be less than that which results from the combination of the minimum entry radius (6m) and the minimum exit radius (20m), though desirable radii of 20m, and 40m respectively should be used where possible. If an existing roundabout is to be modified to include an additional entry, care must be taken to ensure that this does not affect safety at the proceeding entry and following exit. It may be necessary to redesign the whole junction if adequate spacing between adjacent entry/exit cannot be achieved.

'The principle of "easy exitsn shall be applied. A nearside kerb radius of about 40m at the mouth of the exit is desirable but for larger rural roundabouts this may be increased to suit the



overall junction geometry. In any case, this radius shall not be below 20m or greater than 200m.

At the beginning of an exit, its width, measured normally to the exit radius, should, where possible, allow for an extra traffic lane over and above that of the link downstream.

For example, if the downstream link is a single 2 lane or wide single 2 lane carriageway, the width at the exit should be 7.0m or 7.5m, and if the link is a 2 lane dual carriageway, the width should be 10m to 11 m. This extra width should be reduced on the nearside in such a way as to avoid exiting vehicles encroaching onto the entering carriageway at the end of the traffic deflection island. Normally, this would be at a taper of 1:15 to 1:20, though where the exit is on an up gradient, the local widening may be extended to reduce intermittent congestion from slow moving larger vehicles and to provide an overtaking opportunity for faster, vehicles. Similarly, if the exit road is on a right hand cunre, it may be necessary to extend the taper length and the length of the traffic deflection island. Within single carriageway exits, a minimum width of 6m, measured normally to the nearside kerb, should be provided adjacent to traffic deflection islands to allow traffic to pass a broken down vehicle. Figure 6.53 shows a typical single carriageway exit embodying some of the above principles. On exits, the edge line should continue along the line of the kerbing once this is terminated (see Figures 6.38 and 6.39).

SECTION 6

January 1997 . . . , - . Page 6/54, ..

. t - . , . . QATAR HIGHWAY DESIGN MANUAL SECTION 6

..-

. . a Main central Island b Low profile subsidiary central island where provlded c Remslning circulatory carriageway width 1.0-1.2 x maximum entry width d Design vehicle e l m clearance minimum f Inawlbed circle diameter (ED)

- 3 Figure 6.52 Turning Widths for Smaller Normal Roundabouts

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January 1997 Pane 6/55


6.1 0.1 4 . Crossfall and Longitudinal bradlent

Steep gradients should be avoided at roundabout approaches or flattened to a maximum of 2% before entry. Crossfall and longitudinal gradient combine to provide the necessary slope that will drain surface water from the carriageway. Thus, although the following clauses are for simplicity written in terms of crossfall, the value and direction of the

. greatest slope must always be taken into account when considering drainage. Superelevation is arranged to assist vehicles when travelling round a curve. Its values, when used, are equal to or greater than those necessary for surface water drainage.

Superelevation is not required on the circulatory carriageways of roundabouts whereas crossfall is required to drain surface water. However, on the approaches and exits superelevafion can assist drivers to negotiate the associated cuwes.

SECTION 6

Normal crossfall for drainage on roundabouts should not exceed 2% (1 in 50). Crossfall should not exceed 2.5% (1 in 40). To avoid ponding, longitudinal edge profiles should be graded at not less than 0.67% (1 in 150), with 0.5% (1 in 200) considered the minimum.

The design gradients do not in themselves ensure satisfactory drainage, and therefore the correct siting and spacing of gullies is critical to efficient drainage.

For Entries Here, curves may be tightened, (see paragraph 6.10.9) and the degree of superelevation should be appropriate to the speed of vehicles as they approach the roundabout but superelevation should not exceed 5% (1 in 20). In cases where superelevation is used, it should be reduced to the crossfa,ll required merely for drainage in the vicinity of the 'Give Way' line, since with adequate advance signing and entry deflection, speeds on approaches should be reducing.

To provide comfort and enable drivers to remain in control, the maximum algebraic sum of opposing crossfall gradients should not be greater than 5%.

Figure 6.53 Typical Single Carriageway Exit

January 1997 . . . . _ . . . . _ . . . . . Page 6/56 - .

. - . . . QATAR HIGHWAY DESIGN MANUAL SECTION 6

. . b

1 Sectkn X-X

a Crown Line b Smooth Crown

Figure 6.54 Typical Example of Crossfall Design Using One Crown Line Which Joins the Traffic Deflection Islands by Straight Lines

For Circulatory Carriageway Values of crossfall should be no greater than those required for drainage, although it is good practice at normal roundabouts, to arrange for crossfall to assist vehicles. To do this, a Cross line is formed where the entry and exit carriageways meet the conflicting crossfall of the circulatory carriageway. This line can either join the end of the traffic deflection islands from entry to exit (Figure 6.54), or divide the circulatory carriageway In the proportion 2:1 internal to external. The conflicting crossfalls at the crown lines have a direct effect on driver comfort and may also be a contributory factor in load shedding and large goods vehicle roll-over accidents. The maximum recommended algebraic difference in crossfall is 5% although lesser values are desirable, particularly for roundabouts with smaller ICD. Care needs to be taken. during 'dotailed design and at the constmctlon stage lo ensure a satisfactory carriageway profile, without sharp changes in crossfall. is achievtrtl. A smoothed crown is essential:

In some cases with tvnall ICDs it may be more appropriate apl'l~ crossfall across the full circulatory cania9e~v11~ width either towards the .-. . . . . . . . . jnnuary 1997

central island or away from it. This should onli apply where vehicle speeds are relatively low.,

For Exits Superelevation, related to the horizontal alignment, should be provided where necessary to assist vehicles to accelerate safely away from the roundabout. However, as with entries, crossfalls adjacent to the roundabout should be those required for surface water drainage. If the exit leads into a left hand curve, superelevation should not be introduced too quickly and to such a value that vehicles tend to encroach into an adjacent (dual or opposing single carriageway) lane.

Adverse Crossfall Adverse Crossfall is crossfall that acts against the desired movement of a vehicle when turning. It can lead to driver discomfort and even safety hazards and should, if possible, be eliminated from the paths of the main traffic movements at norrnal roundabouts. Smaller normal roundabouts In urban areas are often superimposed upon existing pavement profiles and in these cases, the cross section of the existing roads will influence crossfalb at the roundabout. T-Junctio'ns require particular

. . . . . Page 6/57


atiention. Some adverse crossfall can be accepted in order to fit the existing levels provided approach speeds are low. Limited adverse crossfall at these roundabouts can assist in making the form of junction more conspicuous to drivers.

6.1 0.15 Segregated Right Turning Lanes

Segregated right turn lanes are a useful method for giving an improved service to vehicles intending to leave a roundabout at the first exit after entry. Their use should be considered when more than 50 percent of the entry flow, or more that 300 vehicles per hour in the peak hours, tum right at the first exit. However, when considering the use of these lanes, vehicle composition should be examined. If the right turn vehicles are predominantly light and there is a high proportion of cyclists and/or large goods vehicles leaving the roundabout, there could be problems with differential speeds at the merge, particularly if this is on an uphill gradient. If segregated lanes are to be used in these situations they should finish with a "Give Way' line at the exit to the lane.

The use of these lanes in urban areas where pedestrians are expected to cross should be carefully considered. In no circumstances should pedestrians be expected to cross right turn lanes segregated by road markings.

If pedestrians are anticipated they should be channelled with the use of guard rail to a safer crossing point. If this is not possible the segregation should by a physical island of sufficient width to accommodate the anticipated peak number of pedestrians.

There are two basic types of segregated right turn lanes, namely segregation by road markings and physical segregation. In both types, vehicles are channelled into the rlght hand lane by lane arrows and road markings supplemented by advance direction signs, and vehicles proceed to the first exit without having to 'Give Way" to others using the roundabout. Segregation by road markings is more common, but is less effective because it is subject to abuse. It is essential that the operation of the segregated lane is not impaired by traffic queuing to use the roundabout itself. The designer should ensure that the approach arrangements are sufficiently clear so that they are relatively self-enforcing.

Segregated right turn lanes should not induce' high speeds. The design speed should not exceed that of either the entry or exit link, and any desirable speed reduction should be achieved at the entry to the lane rather than

SECTION 6

within it. Forward visibility throughout the segregated lane should be the appropriate stopping site distance for the design speed. Where the large goods vehicle proportion is low, the lane width may be reduced to 3.5m but should not be less than 3.3m. Where road markings are used to create the lane segregation, the overall width of the marking should normally be a minimum of 1 .Om. Where the large goods vehicle content is higher, the lane width must be checked to ensure that it can accommodate the swept paths of larger vehicles, especially where physical segregation occurs. Further information on the widening of lanes on curves is given in Table 6.5 and Section 3.

It is not necessary to make allowance for broken down vehicles. With segregation by road markings, such vehicles can be overtaken with caution. Where physical segregation is introduced, this should not prevent a right turn at the roundabout in the normal way from the non-segregated part of the approach.

These lanes have been observed to handle 1300 vehicles per hour with ease and for design purposes a maximum capacity of 1800 light vehicles per hour may be assumed where the exit is free running. Segregated lanes need not be considered as part of the entry when calculating capacities for other traffic movements.

The merging between vehicles from a segregated right turn and other vehicles exiting the roundabout should take place within 50m of the roundabout, where speeds are still comparatively low. Ideally, there should not be a forced merge. However, running the two streams alongside each other is only possible where the exit link can provide two lanes in the same'direction.

In other cases the segregated right turning traffic has to merge with the other stream, giving way where necessary. This merging length should be at least 10m long. Segregation by road markings is not recommended if vehicles have to give way at the merge point. Where street furniture is placed on the island in the vicinity of the merge, it should not obstruct visibility.

In the improvement of an existing urban T- Junctions, the signing on the segregated right turning lane must clearly indicate to drivers that they have to 'Give Way" to vehicles leaving the roundabout.

January 1997 . , . . . . . . . . Page 6/58

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SECTION 6. '

6.10.16 Road Markings . ,

Road markings are used to channelize traffic and, where required, to indicate a dedicated lane. Lane indication arrows to reinforce the advance map type direction signs at entries can be beneficial where heavy flows occur in a particular direction.

Lane dedication by arrows and markings on the circulatory carriageway is not normally recommended. Where a roundabout is particularly extensive and partially signalled and it is tending to a gyratory system, then some degree of channelization by road markings may prove beneficial operationally.

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Januar) 1997 Page 6/59


6.i I U-TURNS - GENERAL

The provision of U-tum facilities are appropriate to a limited number of situations in rural locations on dual carriageways and when combined with other forms of junction in urban situations. We shall consider rural U-turn facilities only in this section.

Generally rural U-turns should be provided-in advance of or beyond junctions as follows:

Beyond a junction to enable drivers to retum to an important junction if they miss .their turning

Beyond a junction to accommodate left turn traffic movements not otherwise catered for at the junction

In advance of a junction where through and other turning movements would be hampered by the U-turn movement

. To facilitate maintenance operations, use by emergency services etc.

One of the key requirements for a satisfactory U-turn design is that the width of the carriageway, including the shoulder or tuming bay, be sufficient to permit the turn to be made without encroachment beyond the outer edges of the road pavement. The minimum median width for a U-turn is 11.6m. This allows space for physical islands each side of traffic waiting to turn. U-Turns should be positioned at least 400m in advance of or beyond any junction. Figure 6.55 illustrates the standard U-Tum layout.

Wherever a U-turn facility is to be provided, consideration should be given to providing a reciprocal U-turn. This enhances safety by reducing the likelihood of any illegal turning movements that may have resulted from the provision of a single U-turn facility and presents a consistent layoutto drivers.

6.1 2 SAFETY AT U-TURNS

Safety is a major concern at all junctions, particularly on high volume, high speed roads. Where U-turn facilities are to be provided on these roads, the hazard created by the turning vehicles and their interference' with through traffic must be minimised. Designs that enable vehicles to be in a protected position while waiting to turn are safest. As are those that make the turning vehicle cross and leave the opposing carriageway before returning to the near side lane with a standard merge movement.

January 1997

SECTION 6 ..., .

The area of median in the vicinity of the U-turn should be kept uncluttered and free from obstructions that are over 1 .Om high and wider than 500mm, with the exception of signs. The visibility requirements are given in Table 6.1 5.

This measure will help to ensure that drivers exiting from the U-turn are able to see vehicles approaching from their right, and for them to be seen by drivers on the major road.

U-turns, in a similar fashion to left turns, contribute to congestion by drawing slow moving tuming traffic into the offside lane. They also add to the accident hazard particularly where U-turning movements are heavy or of slow moving vehicles. However, U-turns often afford the best economically available solution to a given problem.

6.13 U-TURN ELEMENTS

6.1 3.1 General

The main elements in the production of an acceptable U-turn facility are:

Median width

The length of the median opening

Use of acceIeration/deceleration lanes or tapers

The nature of the turning traffic

The design speed of the main road.

Figure 6.55 and Table 6.15 detail standard U- Turn layout arrangements for rural locations.

6.13.2 Direct Taper Length (d)

The direct taper length is the length over which the width of a left turning lane is developed. Left tuming lanes shall be introduced by means of a direct taper whose length is part of the deceleration length and depends on the design speed. This taper length is given in Table 6.1 6.

6.13.3 Width. of Physlcai Islands in the Median

The width of median at the turning point shall be a minimum of 11.6m including hardstrips. This width is sufficient to shelter most large goods vehicles using the U-turn facility. The minimum width of a physical island, usually located at the end of the direct taper shall be 3.5m. The minimum width of physical island separating the storage lane from the through lanes shall be 1.2m or that necessary to incorporate signing.

Page 6/60


6.1 3.4 Left Turn Lane

The length of the left turning lane will depend on the major road design speed and the gradient. It consists of a median opening length, a storagelqueuing length and a deceleration length. The deceleration length shall be provided in accordance with Table 6.17, in which the gradient is the average for the 500m length before the U-turn opening.

6.1 3.5 Medlan Openings (a)

The opening in the median at the crossing point shall typically be 11 .Om wide, as shown on Fig 6.55. However this should be adjusted to suit long 'vehicles or those with abnormal loads when required.

. . .

6.1 3.6 StoragelQueulng Length (b)

The storagelqueuing length shall be determined in accordance with the requirements of the Qatar Traffic Maunal. The queuing length shall be separated from through traffic by a physical island on each side and the queuing lane width shall be 5.0m.

6.13.7 Merging Length (e)

The merging length shall be constructed in accordance with Clause 6.7.14. The merge length commences a minimum distance of 45m from the inside radius of the median opening, or if the major road design speed is 120kph or greater, the merge nose taper commences at this point. The distance of 45m is that required for the design vehicle to be parallel to the major road carriageway following the U-turn movement. '

The width of shoulder on the exit of the U-turn shall enable the design vehicle to make the U- turn without using excessive steering lock whilst maintaining a 1 m hardstrip from the outside wheel to the edge of surfacing. To aid vehicle direction, the shoulder should be marked or studded to guide vehicles to the merging length.

6.13.8 Pavement Construction

The pavement construction for the entire U-turn facility shall be a minimum of that used for the major through road construction. Where consistent heavy loading is expected, the engineer should consider more durable pavements. Refer to Section 9.

. . . . SECTION 6

6.13.9 Road Lighting

It is particularly important that U-turns are clearly visible to through traffic. In all cases, street lighting shall be provided. Refer Section 10.

6.1 3.1 0 Traffic Slgns and Road Marklngs

U-turns shall be clearly signed in accordance with the Qatar Traffic Manual. Consideration should be given to providing additional signing for the traffic on the through route to indicate that vehicles may be crossing the road ahead.

6.13.1 1 Drainage and Crossfall

To allow for surface water drainage and driver comfort, the road crossfall on the major road shall be continued through the U-Turn. Checks shall be made for flat areas at all changes in gradient, superelevation or crossfall. Surface run-off shall not be allowed to collect in streams and flow from the U-Turn across the major through road, or to collect on or cross the U- Turn lane so as to present a hazard to vehicles manoeuvring and braking. In addition, the rural situation requires the engineer to carefully consider the maintenance requirements of any drainage system he adopts. Refer to Section 8.

. . . .. . . , . i . . . . . . . . . . . I_. . ..,.. _ . . . . .(. . . , . . .. . . ... . : . . . . . ' . : 9 . .

Januarv 1997 Paae 6/61


Figure 6.55 Typical U-Turn Layout

8 b C - r

d

.-----------------------------------------------------------------------------. ------------------

20

Table 6.1 5 Visibility Distances

Design Speed on Major Road

(kph)

0-45 45-60 60-80 Over 80

d. Diracl taper Imngth a. Marping k n g h (norm kngh when ngulrmd) b l and S2. Vislblmy dimnun

7

8. M mdhn opmnhg b. Queuing kngth c. D8cmbr8Uon imngth + dhetspmr lmngth 8

S l (m)

50 75 125 175

Table 6.1 6 Direct Taper Length - d

Design Speed (kph)

50 60 70 80 100 120 140

S2 I (m)

5.0 7.5 10.0 10.0

Table 6.17 Deceleration Length - c (m) for Dual Carriageways

- Design Speed

- (kph)

' 50 60 70 80 100 120 140 -


45 m k

Note: Rounding shall be applied to the kerblines, typically SOmR.

Direct Taper Length (m)

5 5 15 15 25 30 35

l ,.

Up Gradient

04%

25 25 40 55 80 110 150

Down Gradient

Above 4%

25 25 25 40 55 80 110

04%

25 25 40 55 80 110 150

Above 4%

25 40 55 80 110 150 200


6.14 URBAN ROAD - SERVICE ROAD

Service roads should be provided in the urban situation where through traffic on a district distributor or higher classification road will be significantly affected by traffic manoeuvres from developments lying adjacent to the through road. The function of the service road is therefore twofold:

Collects connecting minor roads and concentrates the entrances and exits to a limited number of locations along the major road, thereby allowing major road traffic to flow more freely

Provides road users with a safer environment adjacent to developments by separation from higher speed through traffic.

Service roads typically run parallel to the major road: However, their vertical alignment is often governed by a lower design speed and can therefore be used to match threshold levels in existing development situations.

Service roads should preferably be connected to major roads using the majorlminor junctions criteria listed earlier in this Section. However, limited reservation space usually requires the junction to connect at a skew to the major road. This creates the following undesirable situations which the engineer should recognise in preparing service road designs:

Angled diverge off the main carriageway encourages high speed entry into the service road and consequent danger to other service road users

Angled merge onto the main carriageway requires the driver to make use of his mirrors to effect a safe merge with major road through traffic.

The above points can be mitigated to some extent by:

Eliminating parking and providing uncluttered visibility in the area of merges and diverges

Introducing a chicane type manoeuvre at the entrance to a service road therefore slowing traffic entering the service road

Increasing the conflict angle where vehicles entering and vehicles using the service road meet

. . . . . - - . " - - . . . . . .

January 1997

Avoiding long straight service roads

Providing satisfactory divergelmerge lengths

Siting diverges and merges away from. other junctions or traffic generation points (both on the major and service roads).

Figure 6.56 shows a diverge and merge for a service road off an urban road of design speed 100 kph or greater. The spacing of diverge nose to merge nose is also fixed by the design constraints of the facility. Major road hardshoulders continue across the junction as a painted hatched marking.

~ igure 6.57 shows a similar diverge and merge for a service road off an urban road of design speed 80 kph or greater but less than 100 kph. The spacing of diverge nose to merge nose for this design speed is fixed by the design constraints of the facility. Major road. hardshoulders are shown with 45' tapers at distances, set backs and shy distances shown.

The minimum weaving length between merges and diverges is given in Table 6.18.

Table 6.1 8 Minimum MergelDiverge Weaving Length

Design Speed (kph) .

120 . . 100

80 70 60 50

The minimum weaving length in metres between successive MergelMerge or DivergeIDiverge measured between the tips of the noses shall be:

Weaving Length (min) = 3.75V

Minimum MergeIDiverge

distance (m) -

500 417 333 292 250 208

Where V = design speed of main road (kph)

Note: Junction spacings may only be reduced below these minima on the express approval of CED Roads.

The distance given by the above formula may be increased if the minimum requirements for effective signing are p.rovided.

Note: Service roads would generally be one way in the same direction as the major road, the major road always being a dual carriageway or minimum 11.3m wide single carriageway.

However, where space permits, a service road may be two way with normal T-junction entrylexits onto the major road.

. . . -. -.. . < . -

Page 6/63

Figure 6.56 Service Road DivergeIMerge for Speeds 2 100kph


Deceleration Y = 4.0m

__C

__C Major Road

Deceleration

__C - Service Road

Y=4.0m P a r k i n g or Shoulder Merging Length

Paint Marking

- Service Road One Way

Y=4.0m

3m Shoulder

Service Road - One Way

Figure 6.57 Service Road DivergeIMerge for Speeds 2 80 kph < 100 kph

4-


Merging Length

__C

Major Road

- __C

3m Shoulder

One Way

'0 Y = 4.0m



6.1 5.1 Residential Areas

In urban areas and, in particular, within residential areas, where there is the likelihood of pedestrians crossing the road and where pa,rldng may be on-street, careful consideration is required at road junctions. .

The most commonly used junction to access developments and the most appropriate is the T-Junction. There are two basic forms of access layout.

In the first form, the major traffic flow is on the through route (eg. a local road with access roads joining), as shown in Figure 6.58.

In the second form, shown in Figure 6.59, all traffic is distributed to the residential access roads. This is the preferred method,of treating access roads, as the short lengths of straight, combined with the turning movements required at the junction, serve to restrict vehicle speeds and the number of accesses onto and off the major route.

Figure 6.58 Multiple Access Roads Joining a Major Road

Figure 6.59 Access Roads Concentrated Prior to Main Road Juncfion

As already discussed in Clause 6.2.2, the use of 'simple' crossroads is not encouraged as there is a tende-ncy, particularly in areas of low

traffic flow, for drivers on the minor road to fail to obey the priority signing and drive through the junction, thereby creating a hazard to traffic on the major road.

The preferred form of vehicular crossing movement is the staggered crossroads. Wherever possible the offset should be to the left so that vehicles making the cross- movement first turn left then right. This is discussed in Clause 6.2.3.

Roundabouts may be used at the jilnctions of local roads with local roads and of local roads with access roads.

However, roundabouts are generally only required where the volume of traffic on the minor road approaches is of the same order as that on the major road, and where the overall level of traffic is such that vehicles on the minor road experience severe delay. If the residential road network is properly planned, this should not occur.

6.1 5.2 Older Residential Areas

Many existing older residential areas in Qatar have particular requirements. When considering recoristruction of these roads, the following shall be noted:

Narrow road resewations giving rise to poor visibility, especially at junctions

High parking requirement

Street system of ill defined through-ways, . crossroads and rat-n~ns

Poor utility records and poor utility condition

Highly variable threshold levels, often adjacent.

No existing surface water drainage

Existing development in low lying flood areas

Poor illumination.

Faced with this nurr~ber of considerations, it is essential that the engineer carefiilly plan the revised road system to meet the requirements of the area. Traffic should be restricted from areas where it is undesirable, rat-runs should be closed, parking regulated and surface water effectively collected. The following are typical actions:

January 1997

. . . . . . . . . .

Page 6/65


Close or partially close one or more legs of a crossroads

lntroduce parking at every opportunity

Close some minor access roads leading .' into the development from local or district distributors

Identify areas such as schools, shops, mosques, etc that may require specific consideration for parking or access

lntroduce one-way systems where appropriate

lntroduce traffic calming if required

Introduce-block paving as a road surface to identify areas where pedestrian traffic is a dominant factor

lntroduce effective surface water removal

Consider utility requirements for future developments and reconstruction

lntroduce street lighting.

Many of the factors identified for older residential areas can be satisfactorily applied to any older area. Engineers should first identify the area uses and needs and apply suitable solutions to arrive at a well thought out, safe and useful environment.

6.15.3 Other Road ~ g e r s '

' '

The principle road users in Qatar are vehicles. However, it is important that the'engineer also considers the requirements of other users of the road system, particularly cyclists and pedestrians, where major/minor junctions including roundabouts present a particular hazard.

Measures to improve cyclist and pedestrian safety are described below:

Provision of cycle lanes adjacent to the running carriageway will go some way toward protecting the cyclist. This lane should be identified with the cycle lane marking. At junctions the minor road 'Stop" and "Give Way" lines should be set back out of the way of cyclists

Provision of a displaced cycle track/footpath for shared use by pedestrians and cyclists with uncontrolled or controlled crossings at junctions

' SECTION 6

Provision of a sign posted alternative cycle route away from a junction

Grade separation (eg. in urban areas) by means of a footbridge or subway. This could be combined for use by both pedestrians and cyclists.

If provision of any of these is not possible, then greater emphasis should be placed on safety with carefully selected crossing places. At roundabouts, where cyclists are always at risk, motorists should be made aware of their presence by road markings and signing, especially where segregated right turning lanes are used.

Pedestrian requirements at major/minor junctions including roundabouts should be carefully considered.

Although it is preferable to provide separate pedestrian routes away from junctions, where road crossing widths are less and traffic movements more predictable, this is rarely practical.

Suggested facilities for improved pedestrian safety at junctions are given below:

Provision of a minor road central refuge at an unmarked crossing place with dropped kerbs and tactile paving, if in a busy pedestrian area

Provision of a pedestrian crossing, with or without a central refuge. These should not be of excessive length or angled to the road

Provision of displaced controlled pedestrian crossings

Provision of a subway or footbridge.

At-grade pedestrian crossing points should not be placed in the mouth of the junction. Instead they should be located away from the mouth where the carriageway is relatively narrow. In urban areas, with low pedestrian flows, it is possible to provide a central refuge in the hatched area of a ghost island junction, though it is important to check for the design vehicle movements.

If a crossing giving pedestrians priority is provided close to the entryiexit points of a roundabout the safety of pedestrians will be compromised and traffic operation problems may become evident with the roundabout. Where a crossing must be provided within the layout of a roundabout, a non-signalized

January 1997. . . Page 6/66

. . . . . . . . . a . ! . ...,... . . " A . . . . (i . . . . .

QATAR HIGHWAY DESIGN MANUAL . . -. . . . . . . . . _ . , _ & , . . , _ ^ 'SECTI'ON'~ ' " i . .' - .

pedestrian crossing is preferred. A signalized pedestrian crossing may be confusing to drivers approaching the "Give Way' line of a roundabout. If a signalized pedestrian crossing is provided, it should preferably be of the divided crossing type to minimise delays at the exits.

In urban' areas, where large numbers of pedestrians are present, pedestrian barriers would prevent pedestrians from crossing indiscriminately across the junction. They should direct the pedestrians to a controlled, safer place to cross. Pedestrian barriers should be of the standard CED design and positioned so that the drivers view of the pedestrians is maintained and vice-versa.

L

The type of-. safety facility selected for I pedestrians and cyclists at majorlminor i junctions (including roundabouts) will depend

upon the expected volume and movements of pedestrians, cyclists and vehicular traffic.

. 4:. . . . . . . . . . . . . . . . . -... . ,.._ ._.. . . . . . . . . . . . . . . . . January 1997 Page 6/67


6.16 SIGNALIZED JLlNCTlONS

6.1 6.1 Introduction

Design of signalized junctions brings together the highway engineer and the traffic engineer. In Qatar this requires the close involvement of the Civil Engineering Department - Roads Division. The highway engineer is responsible for the geometric parameters of the road design on the approaches to and through the junction. 'The traffic engineer is responsible for the specific layout of the junction in terms of capacity, tliming movements, signing, marking, pedestrian considerations, specification and position of signals.

Reference shall be made to the Qatar Traffic Manual and a concept layout should be agreed with the Director of Civil Engineering Department prior to proceeding with the preliminary and detail design stages.

6.1 6.2 Basic Requirements '

When designing traffic signal installations, care should be taken to ensure the following:

Drivers have sufficient advance warning to know exactly which direction to take at the junction

Drivers are guided into the intended lane or lanes by road markings

Drivers have a clear view of the signals at the junction itself

The junction layout allows easy visual recognition of correct exit lanes and required vehicle trajectory.

. Movement from 'Stop" line to exit lane is a natural flowing.movement and does not interfere with other movements allowed at the same time.

6.1 6.3 Typical Layout Features

It is impractical to deal with all possible variations of junctions. The various features mentioned is this clause may be considered for most situations.

The size of traffic islands and pedestrian refuges is important. Adequate clearance between the kerb and any street furniture is needed to prevent damage by vehicles having a lateral overhang.

The lane width on the approach to the junction shall be in accordance with Clause 5.2.

The number of lanes at the stop line shall be maintained across the junction to the exit lanes.

The possibility of introducing slip roads at the corners of a junction should always be considered. These allow right turning traffic to 'Give Way" or "Stopn rather than wait for the signals. They also provide larger turning radii than would otherwise be the case and can be beneficial to pedestrians when provided with clearly defined crossing points.

If U-Turns are to be provided at the junction, lane widths and turning movements of different vehicles should be considered and the position of pedestrian refuge points checked against possible cpnflict.

It is preferable that left turn lanes and through lanes are segregated by physical islands for the entire queuing length. It is also preferable that entry and exit traffic on opposing carriageways is segregated by a median or physical island.

Minimum visibility requirements to the primary signals are detailed in the Qatar Traffic Manual.


' ,

7.1 INTRODUCTION

For new roads with high predicted traffic flows, consideration can be given to grade separation. Grade separation removes conflicts between the major vehicle flows thereby improving safety and capacity of an intersection. For existing at- grade junctions, grade separation can also be considered to improve safety and capacity if these particular problems have been identified.

It is important for the engineer to use the correct terminology. The principle definitions relating to grade separation are given below:

Intersection: The meeting point of two or more roads. . .

Junctlon: The treatment of the road alignment at the intersection to enable traffic to negotiate the intersection in the defined manner.

Grade Separatlon: Removes conflicts arising from an intersection by the provision of a bridge.

Interchange: When grade separation is used but a connection is maintained between the roads, this combination of grade separation and junctions is called an interchange.

lnterchanges may be complex and include extensive connecting roads and loops. They will only be required for the highest range of traffic flows.

This section sets out the requirements for the design, layout and size of Interchanges. It is essential that the engineer produces safe designs that provide adequate capacity.

lnterchanges are generally required between primary routes and between primary and secondary routes although they may be positioned at the intersection of any urban or rural road. The major selection criteria are always safety and capacity.

lnterchanges may be considered to improve an existing junction for a number of different reasons. For example:

a) To remove a hazardous main at-grade junction in order to improve safety

b) To' eliminate traffic delays at a bottleneck caused by the volume of crossing and turning traffic

SECTION 7

c) To improve the alignment of a road

d) To standardise junction types when upgrading a corridor to motorway status.

7.2 TYPES OF INTERCHANGE

7.2.1 General

The decision to provide an interchange and the type and detailed design of an lnterchange will be specific to a particular site. The selection of the most suitable facility for a particular site and the'associated design parameters depend upon a number of controlling factors which include:

- Safety - Road classification for the connecting

routes - , Design speed - Traffic volume and mix - Required junction capacity - Number of junction legs - Topography - Land available, the type of land and its

present use - Economics - Lighting - Environmental impact - Access to local communities - Pedestrians, farming and cyclists.

Safety is always the most important factor followed by capacity.

Layouts will vary for different locations. It is unlikely that the layout for one site could be directly applicable for another. The traffic and topography are unlikely to be the same. However, it is desirable to standardise layouts along a particular route wherever possible to attempt to reduce confusion to drivers and thereby improve safety.

The two forms of lnterchange considered provide a wide variety of types available to the engineer. These have been classified into the following generic types for selection of the most suitable form:

Full lnterchanges

Full interchanges combine grade separation of major conflicts with slip or loop roads that begin and end with diverges and merges.

- Full cloverleaf interchange - . Directional interchange with variants - 3-leg junction types including trumpets - Partial cloverleaf.

. . , ' , : . Januarv 1997

QATAR HIGHWAY DESIGN MANUAL SECTION 7 . .

Compact lnterchanges

Compact lnterchanges combine grade separation of major conflicts with connector roads that either begin or end with a form of junction other than a diverge or merge.

- Diamond junctions and variants - Roundabouts and variants - Half cloverleaf and variants - Compact 3 and 4 leg grade separation

unctions and Weaving Sections The main aim of grade separation is to remove the conflicts between turning vehicles thereby improving safety and capacity. Therefore particular attention must be paid to the design of those areas of an interchange where this conflict cannot be removed.

Junctions are the areas of carriageway where traffic joins or leaves the main road and are the locations where accidents are most likely to occur.

On a single carriageway road the length between successive junctions is called the stagger distance. Refer to Section 6.

On a dual carriageway the distance between any combination of successive junctions is called a weaving section. This is the length of carriageway in which drivers change lanes in advance of tuming off the main road. Due to lane changing, weaving sections must be carefully designed in order to give drivers sufficient time to make their manoeuvres safely. Refer to Clause 7.4.9.

7.2.2 Full lnterchange

This form of interchange provides uninterrupted movement for all tuming traffic by the use of interchange links.

Full Cloverleaf lnterchange

Figure 7.1 ~ u l l Cloverleaf lnterchange

A typical full cloverleaf interchange is shown in I

L,

Figure 7.1. It is a 4 leg interchange which provides free flow movements for all traffic. It

c- completely eliminates all left turn conflicts. i

4

Inner loop/slips are provided for the 4 left turn movements and outer loop/slips are provided for the 4 right turn movements. - Advantages:

a) . All left turn movements are provided for L

with one grade separated single structure

b) All traffic movements are free flowing L

c) The interchange may be built in stages I

d) Trafficsignalsarenotrequired.

Disadvantages: . . . L.

a) ~equires large land take

b) Weaving lengths on both routes are greatly reduced. A collector distributor road would help weaving by reducing the traffic speed, but would increase the structural costs

c) Multiple merges and diverges complicate traffic signing

d) Short deceleration lane lengths for inner loops

e) The design speed of the inner loops is generally low

f) Provision for U-turn movement is restricted until fully constructed

g) Significant environmental impact due to the size of the junction.

Trumpet Junctions Trumpet junctions can be of varying forms. Typical layouts are shown in Figures 7.2 and 7.3.

Advantages:

a) 'The layouts generally provide a relatively - L

high speed semi-direct connection for large traffic flows

-- - b) Only one structure is normally required

L

c) Successive merges and diverges are avoided therefore no weaving lengths are ' '. L

required C J j ,

t - - January . . 1997 Page 712

QATAR HIGHWAY DESIGN MANUAL .- .

SECTION 7

d) ' Traffic signals are not required

e) Can orientate the largest radii loop to the larger traffic flow

f) All movements are free flow

g) Relatively easy to sign although some gantry type signs may be required

h) Lower environmental impact than cloverleaf and variants is achievable with landscaping and sympathetic topography.

Disadvantages:

a) Should be used with care when the terminating leg is on a high speed route

b) The abrupt end of a high speed route requires ~ubstantial speed reduction with a potential for reducing safety

c) Can only sensibly be used for 3 leg interchanges

,

d) U-turn movement is not provided for unless the interchange includes additional loops and slips that can be very long.

Figure 7.2 Right Hand Trumpet lnterchange

Figure 7.3 Left Hand Flattened Trumpet, Restricted Movement.

7.2.3 Compact lnterchange

This form of interchange involves the grade separation of the major turning movements combined with low-grade junctions at the commencement or termination of slip roads or loop roads to accommodate the turning movements on the minor road. The low-grade junction element, whether a major/minor junction, roundabout or signalization, can produce three' main types of compact interchange.

Dlamond lnterchange and Variants A diamond interchange has slip roads parallel to the major road. The major route is grade separated over or under the minor road. Turning movements are catered for at T- junctions between the slip roads and the minor road. '

The diamond is the simplest form of interchange and is usually the most common.

It is essential that the forward visibility and stopping sight distance of the slip road approaching the at-grade intersection should be designed to the correct standards. Most accidents occur at the at-grade junctions where vehicles cannot stop in time.

A typical diamond interchange layout is shown in Figure 7.4.

Figure 7.4 Diamond Interchange . I

Advantages:

a) High standard merges and diverges can be provided in advance and beyond the structure -

- . . . . .



b) Economic landtake and low construction costs

c) Single exit slip road simplifies signing

d) No weaving lengths are required on the major road

e) No acceleration or deceleration tapers required on or under structures

f) Simple structures can be achieved

g) Can improve capacity of the at-grade intersection by providing extra lane width at entry, segregated turning lanes and traffic signals.

a) Lower capacity on the minor road due to left turning movements

b) Many points of conflict on the'minor road increasing the accident potential. Traffic signals will help reduce conflict

c) With the many tuming movements at two locations on the minor road, visibility and intervisibility is difficult

d) Possibility of traffic tuming the wrong way down slip roads

e) Turning traffic from the primary route has to stop at the secondary route with the possible requirement of wider lanes for storage capacity

f) Little possibility of future expansion of the junction.

lnterchange with Roundabouts and Variants Interchanges with roundabouts can provide a more flexible junction arrangement than a diamond interchange. The roundabout element can cater for varying turning volumes, thereby reducing the overall delay to vehicles in comparison with simple T-junction elements. They are particularly useful when there is a large percentage of left turning traffic.

The two most common forms of roundabout interchange are the two bridge and the dumbbell type. The dumbbell type is the most economic because of the single structure and reduced landtake, however the two bridge type is safer for larger volumes of traffic. These are shown in Figures 7.5 and 7.6.

. SECTION 7

Figure 7.5 Dumbbe l l Roundabout lnterchange

Figure 7.6 Two Bridge Roundabout lnterchange

Advantages:

a) The dumbbell roundabout is very economic with a signal structure and very small landtake

b) The two bridge roundabout, although not so economic is safer with a less confined "Give Way" area

c) High standard merge and diverge can be provided in advance and beyond the structure

d) Single exit slip roads simplify signing

e) No weaving lengths are required on the major route .

1) No acceleration or deceleration tapers on . or under structures

g) Simple 'structures can be achieved

January 1997 -

. . Page 714


h) Can improve capacity of two bridge roundabout by providing extra lane width at entry, segregated turning lanes and traffic signals.

Disadvantages:

a) The efficiency of the roundabout relies on drivers being aware of how roundabouts operate. Drivers must give way to traffic on the roundabout to their left and must not queue across the exits which would cause the roundabout to lock

b) Difficult to enter large, two bridge roundabouts if circulatory speeds are high.

Compact 3 and 4 Leg Partial Cloverleaf Interchanges Compact partial cloverleaf intersections can be used in rural or urban locations. They are simple, low speed versions of partial cloverleafs with the same advantages and disadvantages except that they have smaller land take and lower cost. Typical compact partial cloverleaf interchanges are shown in Figures 7.7 and 7.8.

4th Quadrant

Connector Road 2nd Quadrant 3rd Quadrant

i

Figure 7.7 Compact Partial Cloverleaf lnterchange

4lh Quadnnt

2nd Quadnnt 3rd Quadmnt

Figure 7.8 variant of ' compact Partial Cloverleaf lnterchange

The objectives of compact partial cloverleafs are as follows:

a) Provide a safe means of crossing a high speed route

b) ~educ6 the ekironmental impactof full . interchanges :by providing, a compact . junction layout. . , .

c) Regulate and maintain vehicle speed for minor route traffic through the junction at a level appropriate to the layout standards

d) Remove the left turn manoeuvres from the major route

e) Provide a junction with minimal land take

9 Provide an operational, efficient junction layout.

g) Provide an economic solution for modifying an existing junction to grade,

. 'separation standards. '

The only disadvantage is that high speed traffid .on the major route will exit on a tight loop. radius. ~ d e q ~ a t e advanced signing, good visibility and chevron signing at the exit point will reduce the safety hazard. If all such junctions along a primary route are the same then drivers would be very aware of the tightness of all such loops and would adapt accordingly. .

it is only when the% i s inconsistency in the design standards and types of junctions that drivers are confused and safety is compromised.

January 1997

. .. . . . - . . . . . . . . - . . . . , .. - . .

Page 715


7.3 SELECTION OF INTERCHANGE TYPE than the minimum weaving length as defined in Clause 7.4.9.

7.3.1 General 7.3.4 Initial Information Requlrements and

Declsions This section outlines the design procedures for selecting a form of interchange most suitable for a particular location. The geometric design of the elements are covered in Clause 7.4. A series of preliminary designs shall be prepared for comparison before final selection and production of a detailed design.

The following information must be collated to form the basis for the selection of the most appropriate type of-interchange for a particular location.

Required Information:

a) Define the classification of the roads approaching the intersection

b) Define the carriageway cross-section of the roads on each side

c) Define the design speed of the roads

d) * Define the proposed opening year for the new facility'

7.3.2 Traffic Flows and Design Year

The major factor influencing junction design is safety. However, for the road network to operate efficiently, new junctions must have sufficient capacity. It is not possible to ensure at the time of design that' a new junction has sufficient capacity indefinitely. Instead, new and improved junctions shall be designed on traffic levels predicted to occur in the Design Year, typically 20 years after the opening of the schemes, to ensure that they are free of conge'stion for a reasonable period. e) Obtain the existing traffic volumes must

- be obtained for the peak hour and apply ' growth factors. . Predicted traffic flows shall be based on the

existing, observed traffic flows growthed up to model the Design Year flows. All Junctions and Interchanges shall be designed using the peak hour flows. The use of peak hour flows will model the worst case for traffic congestion. Of particular 'vportance to junction design is the volume of traffic undertaking each turning manoeuvre. All predicted traffic volumes and turning volumes for the Design Year shall be agreed with CED Roads.

f) Define the location of any constraints to the scheme. These include land ownership, existing and proposed utilities, planning constraints, topography, dry wadi courses, flood plains and ground conditions.

g) Define the environmental constraints. These include proximity to dwellings, severance of communities, plants of particular importance, animal habitats and regularly used animal tracks and migration routes.

7.3.3 Junction Spacing Within the Network

In deciding on the form of the interchange the engineer must consider the location within the overall road network. 'The aim must be to produce a consjstent junction strategy across the network that maximises safety. Guidance on the junction strategy for a particular location shall be sought from CED Roads.

Having collated the above information, the following decisions must be made before finalizing the form to be used.

Initial Decisions:

a) Agree the overall strategy with CED Roads

The minimum spacing of c~nsec~utive junctions on a multi-lane road is defined in Clauses 6.1.4 and 7.4.9, and is based on safety requirements for weaving movements. This minimum spacing will also allow the design of effective traffic signing and lighting schemes for each junction. These clearances shall be achieved between the maximum extent of the consecutive merges and diverges for each junction. In no circumstances.shall spacing between junctions of consecutive interchanges be reduced lower

b) Agree predicted traffic volumes and turning volumes with CED Roads

c) Decide which turning movements will be accommodated

. .

Page 716 January 1997

d) Decide which movements will be given priority with grade separation and high- grade links, and which minor movements will be accommodated by low-grade links and junctions

e) Confirm horizontal and vertical clearances for structures.

7.3.5 Type of Interchange for Preliminary Design

The type of facility must be selected before preliminary designs are prepared. The various ,

types of junction and their relative advantages and disadvantages have also been discussed in Section 6. For a given location two or more types of facility may be worked up into preliminary designs for evaluation.

No fixed rules can be given for the selection due to the multitude of criteria that must be considered. Each location will have different governing criteria and it is for the engineer to use his experience to select the most appropriate type for evaluation.

Safety will always be the highest priority. However, adequate capacity is also important to reduce congestion and thereby improve safety. Refer to Section 6.3.

7.3.6 Preliminary Designs

. Preliminary designs are prepared for alternative arrangements to assess suitability and relative costs. The main elements of the facility must be defined in sufficient detail and at a suitable scale to determine the landtake required.

The items to be defined in the preliminary design include:

a) Safety implications for road users and non-road users

b) Number of lanes required for each movement

c) Radii of links and loops

d) Vertical and horizontal clearances for structures and maximum carriageway gradients

e) Lengths of loop roads, slip roads and merges and diverges

f) Lengths of weaving sections

.. . . . . - . . . - - ... . .'

January 1997

g) Provision for "on-motorway 'traffic and non-motor vehicle road users

h) . Estimate of construction costs

The engineer'must also consider:

i) Method of construction

j) Method of maintenance

k) Environmental effects including landscaping

I) Lighting and signing principles

m) Provision of safety fences and barriers. 8 .

Preliminary designs will be discussed with CED Roads and approval granted before the engineer progresses to detailed design. Certain elements of the preliminary designs may need to be worked up into more detail at the request of CED Roads to fully assess the relative merits of the prehminary designs.

. . , - . . . . .... ..,- _. . ... . . .

Page 7/7


Loop Road: A particular form oi connector road where the carriageway turns through an angle of approximately 270" - in order to accommodate the traffic movement.

7.4 DESIGN ELEMENTS

7.4.1 Definitions

Interchanges are made up of distinct elements, each sewing different purposes. Any one facility may have any number of these elements. The detailed design of each of these separate elements is covered in this section. For clarification, they are defined below:

Weaving Section: The length of carriageway between successive merges and diverges where traffic changes lanes in order to reach its chosen exit.

Main Road: The carriageway ' or carriageways that are given priority, generally by nature of carrying the highest volume of traffic. Physical Nose: The point where the

carriageway surfaces of the main line and the merge or diverge separate.

Minor Road: . . . The carriageway or carriageways that are not given priority, generally by nature of carrying low volumes of traffic.

Painted Nose: The length of chevron \ marking from the physical

nose to the intersection of the merge or diverge with the main road travelled way.

Merge: . The area of, tapered carriageway where traffic joins the,main road.

Diverge: The area of tapered carriageway where traffic leaves the main road.

7.4.2 Design Speed

Design speeds for slip roads and link roads are related to the design speeds for the main road as shown in Table 7.1.

Auxiliary Lane: An additional lane added parallel to the main road and used in conjunction with a merge or diverge carrying higher traffic volumes to provide extra capacity.

Connector Road: he length of road that joins merges, diverges, 'Give

' Way" or "Stopn junctions within an interchange. Slip, Link and Loop roads are types of connector road.

Main Road Design S p a

Type of Connector

Road

Design

Speed

Table 7.1 Design Speed for Link and Slip Roads

Link Road: A particular form of connector road that joins diverges and merges within a full interchange to provide uninterrupted movement for turning traffic.

Where two alternative design speeds are shown, the engineer may use the lower if it is considered that safety will not be compromised. Where transition curves are used between design elements within the Interchange, the transition curve relating to the higher design speed must be used. The appropriate Stopping Sight Distance must always be used. Design speeds on slip roads-must not be reduced below the stated values as they terminate with "Stopn or "Give Way" junctions and would compromise safety.

Urban a) l20kph b) lOOkph

Slip Road: The length of carriageway between the end of the merge or diverge and the "Give Way" or "Stopn line on the junction within the overall interchange.

Link Road

a)120or100

b)100or80

Rural (a) l4Okph (b) l20kph

January 1997- . -

Slip Road

a)70

b)70

Unk Road

a)140or120

b)120or100

. . . Page 718 - .

Sllp Road

a)80

b)80

. . , ... .


7.4.3 Lane Provision and Capacity

Lane provision for the main road, slip roads, link roads and loops shall be based on the agreed traffic flows as defined in Clause 7.3.2. For interchanges, the minimum number of lanes provided on any particular element of the junction shall be based on 1600 vehicles per lane per hour. The number of lanes shall be rounded up to the nearest whole number.

The engineer may wish to increase the lane provision above the minimum defined above for operational reasons. The 1600 figure is based on UK acceptable congestion standards for all purpose roads and may not be suitable for all locations in Qatar.

Lane provision' for the main road or roads through the junction shall not be less than the provision either side of the junction except with the approval of CED Roads.

Where the minimum lane provision is one lane, the engineer may wish to add an extra lane to reduce the potential for problems with broken down vehicles blocking the carriageway or restricted space for maintenance. Any proposed changes from the minimum lane provision shall be agreed with CED Roads.

For the majority of interchanges, the maximum number of lanes provided for connector roads, is likely to be two. If the lane provision for any particular connector road, is more than two, the engineer may have incorrectly defined which is the main road and shall refer to CED Roads for guidance.

7.4.4 Hard Shoulders and Edge Strips

Where hard shoulders or edge strips (Reference to Section 5.4 and 5.5) are provided on the main road either side of the interchange, they shall be continued through the interchange. For connector roads, the provision of hard shoulders or hard strips shall be in accordance with Table 7.2.

On the main road, the hard shoulder or edge strip shall continue immediately after the chevrons for the painted nose.

Table 7.2 Provision of Hard Shoulders and Edge Strips on Connector Roads

Sllp . Roads

Link Roads

7.4.5 Merges and Diverges at Interchanges

Within interchange areas, merges and diverges are the locations where accidents are most likely.@ occur. It is essential for the engineer to pay particular attention to their layout. Traffic should be able to leave or join the main road as smoothly as possible. To this end, the speeds of traffic joining or leaving the main road must be similar to that on the main road. Acceleration or deceleration to the appropriate speed should take place 'on the slip road or link road before the merge or after the diverge. The geometry of the carriageway or other conditions in the vicinity of the merge or diverge must not impede this smooth flow. Queuing in the area of the merge or diverge must be avoided.

I I physical nose. 1 1 Reduce at 1:30 to 1 .Om edge strip.

As link roads As lmk roads

Provision on Main Road

Two alternative types of merge and diverge shall be used depending on the volumes of traffic as defined in Clause 7.3.2. They are the standard taper and the auxiliary lane layout.. The auxiliary lane layout has an additional lane ' parallel to the main road to increase capacity of the merge or diverge taper.

Hard Shoulder

Terminate hard shoulder opposite physical nose. Redwe at 1:30 to 1.0m edge strip. Terminate edge strip 10m before 'Give W a r or 'Stop' line

If both main roads have hard shoulden, continue them along the fink road. If not, terminate hard shoulder opposite

To select a merge layout, hourly flows for the merge and the upstream mainline are inserted into the nomograph Figure 7.9. The intersection point of the merge and upstream main line flows will fall within a segment of the nomograph from which the number of lanes required on the connector road, and need for an auxiliary lane are determined.

Edge Strip

Continua edge strip to 10m before 'Give way' or 'Stop' line .

If both main roads have edge sMps continue them along the link road.


To select a diverge layout, the procedure is repeated using the hourly flows for the diverge and the downstream mainline, and the nomograph Figure 7.10. The mainline lane capacity is based on a flow of 1600 vehihour.

Generally the auxiliary lane layout is used in locations with higher volumes of traffic. The auxiliary lane shall be the same width as the nearside lane of the main road but may be reduced to a minimum width of 3.5m in urban areas on approval of CED Roads.

Where the existing mainline lane capacity is already at a maximum or where exceptionally large merge or diverge flows are expected, provision of a lane gain a lane drop may be required. In these instances, the engineer should refer to'the 'Design Manual for Roads and Bridges, Volume 6 Road Geometry, Section 2 Junctions, Part I TD22192 Layout of Grade Separated Junctionsw and the merge and diverge layouts should be agreed with CED Roads.

The standard taper and the parallel taper merge and diverge are shown in Figures 7.1 1, 7.12, 7.1 3 and 7.1 4. The geometric parameters for setting out are shown in Table 7.3 for merges and Table 7.4 for diverges.

Stopping Sight Distance in accordance with the higher design speed from the adjacent elements shall be provided over the whole length of the merge or diverge

SECTION 7 . - -

L - January 1997 . . . . . . . . . . - Page 7/10

' . C . _ . . . . . . . . . .

. - . . . , a

. . , . . . :: \ .: . . . . .

Q ~ A R t i i G H V i l ~ ; ~ DESIGN MANUAL . . ,

h L

3 2000 ,, ............ 0

5 Q)

2 1500 ,, 2 - LL

............ al F 1000,- Q)

r

Lane 1 Lane 2 Lane 3 Lane 4 Upstream Mainline

I 1 I I 1

I I

I I I I I I

0 1000 2000 3000 4000 5000 6000

Upstream Mainline Flow (Vehhour)

A = standard Taper B = Auxiliary Lane

Figure 7.9 Merge Design

. . . . ;. . . . . . . . . . . . . . . . . . . . . . . . . . . _ . . . . . . > . .a ........ . ,.' . I .


Div

erge

Flo

w (V

ehlh

our)

.

. - QATAR HIGHWAY DESIGN MANUAL - SECTION 7

Table 7.3 Geometric Parameters for Merges

Road Class

Rural 140 kph 120 kph 100 kph

Urban 120 kph 100 kph 80 kph

Figure 7.11 Standard Taper Merge

Painbd Physical Auxhry Lone

(4) 6 (5)

IC - - -- - - - - - - - - - - ---- - ----C- -------------------------

Figure 7.1 2 Auxiliary Lane Merge

Length of entry taper (m)

(1)

205 150 130

130 95 75

Painted nose length (m)

(3)

115 85 75

75 50 40

Taper for min angle at

physical nose (2)

1 :40 1 :30 1 :25

1 :25 1:15 1:12

Table 7.4 Geometric Parameters for Diverges

Physical N;=~-N;-~ Painted p e r I (1) 6 (2) - ----- * ------ ........................

Min auxiliary lane length (m)

(4)

230 190 160

160 125 loo.

, - Figure 7.13 Standard Taper Diverge Figure 7.1 4 Auxiliary Lane Diverge

Length of aux lane per lane provided (m)

(5)

75 55 55

55 40 40

Min auxiliaty lane length (m)

(5)

200 170 150

150 125 100

Painted nose length (m)

(4)

80 70 70

70 50 40

Road Class

Rural 140 kph 120 kph 100 kph

Urban 120 kph 100 kph 80 kph

. . . . . . r t . .


Length of aux lane per lane provided (m)

(6)

75 55 55

55 40 40

Taper for min angle at physical

nose (3)

1:15 1:15 1:15

1:15 1:15 1:12

Length of exit taper (m)

1 lane (1)

170 150 130

130 95 75

2 lane (2)

1 85 150 130

130 110 90


In addition to the above setting out information the engineer should also ensure that the crossfalls in the vicinity of the merge and diverge areas are suitable for the safety of high speed vehicles crossing at angles. Ideally the main road crossfall should be continued across the merge or diverge up to the physical nose. If this cannot be achieved the maximum permitted difference in crossfall between the mainline and the merge or diverge is 4% for design speeds less than l2Okph and 2.5% for designs speeds of 120kph or greater.

The radius of the edge of carriageway or kerbline on merges and diverges may be reduced to that applicable to the next lowest design speed over a maximum distance of 50m in the immediate vicinity of the join to the slip road or link road. Transitions in this vicinity may be used at the engineers discretion.

7.4.6 Slip Roads

Slip roads shall be designed in accdrdance with the design speeds from Table 7.1. If a slip road is longer than 750m it shall be designed in accordance with the method for Link Roads.

Horizontal and vertical design of slip roads shall be in accordance with the parameters for main roads but with the exception that the maximum vertical gradient may be increased to 6%. Guidance shall be sought from CED Roads if steeper gradients are proposed on slip roads.

The appropriate Stopping Sight Distance relating to the design speed must be provided over the whole of the slip road between the physical nose and the "Give Way" or "Stop* line. The minimum length of slip road is therefore the Stopping Sight Distance relating to the appropriate design speed.

The engineer shalt check that the slip road is of sufficient length in order to develop the level difference required for grade separation from the main road. The cross-section between the slip road and main road should also be checked to ensure that it does not exceed the maximum safe angle of the slope which will depend on the nature of the soil.

7.4.7 Link Roads

Link roads must ensure the smooth flow of traffic between the main roads. They shall be designed in accordance with the appropriate design speeds from Table 7.1 using the parameters for main roads.

SECTION 7

7.4.8 Loop Roads

The minimum radii for loops are given in Table 7.5, measured in accordance with Figure 7.1 6. They are not related to design speed. For the purposes of Stopping Sight Distance on loops the driver must have uninterrupted sight to a point a minimum of 100" around the loop from his current position, measured in accordance with the standard method for Stopping Sight Distance. Refer to Clause 2.2. Consideration shall be given to the use of 7% superelevation on loops in accordance with the requirements of Clause 3.4.

The maximum gradient applicable to loops shall be 6%. The gradient along the loop shall be constant from the physical nose to the end of the loop and shall be determined by the required vertical clearance between the two roads.

Table 7.5 Minimum Radii for Loops

A typical arrangement for a loop is shown in Figure 7.1 5.

Primary Roads

''/Off Main Road

75m

Figure 7.1 5 Typical Loop Arrangement

E - E - - - - - -

. . R = Radhta (Rater T8ble7.5) L = Lane WMO, (Rmfer Table 0.6) E - Edge SMp (Refer Table 8.5)

Secondary Roads

Figure 7.16 Measurement of Loop Radii

On to Main Road

30m

January 1997

Off Main Road

50m

. .-,. . . . Page 7114

I . . .


7.4.9 Weaving Sections > . I

7 N =- (QnW +Qw, +aW2(2% + I )) The 'minimum spacing between junctions is D La,

defined in Clause 6.1.1. This distance is

Weaving sections are those areas of carriageway, within interchanges, between successive merges and diverges that allow traffic to safely change lanes in order to accomplish their next turning movement. They are measured between the points shown on Figure7.17. ' '

required to ensure that the traffic changing lanes as a result of the first junction does not interfere The parameters in the above equation are

- ; . -4

Figure 7.17 Measurement of Weaving Length

with the traffic changing lanes for the second. In effect, the minimum spacing between junctions eliminates the need for specially designed weaving sections. It also allows for the provision of suitable signing.

defined in Figure 7.1 8.

I L

.- The minimum weaving distance shall be provided between all merges and diverges

I within an interchange. This distance is defined 1 - as that which is covered in fifteen seconds by a

vehicle travelling at the design speed. Weaving I --, lengths are summarised in Table 7.6.

: . . . .?, ------5----

C

Table 7.6 - Minimum Weaving Length

Design Speed (kph)

140 1 20

. 100 80

In all cases the minimum weaving length shall be checked against the following equation for the number of lanes. The length of the weaving section shall be increased such that no more than one auxiliary lane is required to accomplish weaving.

Weaving Length

( L , to L,,)

Min Weaving Length (m)

583 500 41 7 333

Flow 3 Flow 1 Flow 2

, Flow 4

Where:

N = Number of traffic lanes

Q,,, = Total non-weaving flow (vph)

Qwl = Major weaving flow (vph)

Q, = Minor weaving flow (vph)

D = Max mainline flow (vpMane)

L,, = Desirable min weaving length for the road class.

La, = Actual weaving length available (m). (where L,, >LmJ

Q,, = Flow 1 + Flow 2

Qwr = Greater of Flow 2 or Flow 3

Q, = Lesser of Flow 2 or Flow 3

-C

Figure 7.1 8 Weaving Parameters



7.5 OTHER DESIGN CONSIDERATIONS

7.5.1 Clearance and Headroom

Clearances and headroom shall be designed in accordance with Clause 3.7 and 4.6. The engineer shall seek guidance from CEO Roads to define if any additional clearance or headroom is required for specific structures to accommodate abnormal load routes.

7.5.2 Superelevation

Superelevation and camber shall be designed in accordance with Clause 3.4. Special consideration shall be given to the superelevation on adjacent design elements. The engineer must ensure that the entire carriageway will drain efficiently and that there is minimal risk of long vehicles grounding at changes of superelevation.

7.5.3 Safety Fenclng

Safety fencing sha.11 be provided at locations defined in Clause 5.1 5. Special consideration must be given to measures at the physical nose of diverges. High speed vehicles crossing the painted nose are at particular risk. The ends of safety barriers at these locations must be given special treatment to reduce the dangers of head-on impact. Consideration shall be given to the provision of energy absorbing terminations for these locations.

Direction and warning signs for interchanges may be large and possibly gantry mounted. Consideration must be given to the protection of isolated signs and gantry legs.

In addition to safety fencing designed to mitigate accidents, consideration should be given to provision of safety fencing to prevent illegal. movements within the interchange. Illegal movements across the verges between slip or link roads are highly dangerous to all traffic and must be strongly discouraged.

7.5.4 Signing

Effective and clear signing is essential for the safe operation of any junction. This is particularly relevant to interchanges where vehicle speed and traffic volumes are high. Signs at such junctions will be large and possibly gantry mounted. Adequate clearance must be provided for the large foundations required.

Detailed guidance on signing is provided in the Qatar Traffic Manual. As a general point, the engineer must consider signing requirements at the preliminary design stage. At this stage the engineer can build in suitable locations and visibility splays for the signs.

7.5.5 Llghtlng

Suitable roadway lighting greatly reduces the potential for accidents throughout the road network. Lighting design is detailed in Section 10. As with signing, the engineer must consider lighting requirements at the preliminary design stage. Lighting columns can have very large bases which may need special consideration.

Information must be obtained from the Utility Authorities at an early stage of the design. Diversion or modification to existing or proposed equipment can have a major impact on the design and the cost of an interchange. Utility Authorities may require service reservations to be provided through the interchange to accommodate future equipment not yet detailed.

7.5.7 Emergency Vehicles

At the preliminary design stage the engineer must consider how emergency vehicles could reach the scene of an incident, particularly if the carriageway is blocked by other vehicles held up by that incident. Provision of additional lateral clearances at structures could be considered along with emergency median crossovers wtth demountable safety fences.

7.5.8 Malntenance Provisions

Maintenance of the carriageway is an important long term objective for the network. The engineer must consider the implications of maintenance strategies and traffic management on the layout of the proposed interchange. He must ensure that the facility will be safe to maintain and that turning movements can be reasonably accommodated whilst maintenance is taking place.

7.5.9 Environmental Issues

Environmental issues shall be considered at the preliminary stage. All reasonable efforts shall be made to design'out unacceptable environmental impacts. The remaining impacts shall be mitigated as far as reasonably practical.

January 1997 . .

Page 7/16

- QATAR HIGHWAY DESIGN MANUAL . ' 'SECTION 7

One main impact of interchanges is visual intrusion due to their size. Careful landscaping can reduce the impact of large structures above ground level. A combination of hard and soft landscaping can usually achieve the best results. Materials in keeping with the surroundings should be used, with careful consideration of colours, textures and styles. In proposing soft landscaping, the engineer must consider how it could be safely maintained thmughout the year, including regular watering.

The design of hard and soft landscaping must not interfere with the operational requirements of the facility. No landscaping features shall obstruct stopping sight distances, visibility of signs or the effectiveness of roadway lighting.



SECTION 8 DRAINAGE Reduces the damaging affect of pore water build up in the pavement, formation or subgrade

8.1 INTRODUCTION

8.1 .1 Functions of Highway Dralnage Prevents pavement weakening due to ingress of salt lenses from the lower subgrade layers.

In the case of exceptional rainfall events the road surface itself can be used as a storm carrier:

The requirement for satisfactory road drainage has a direct bearing on the ability to use the road during and after a rainfall event, long-term serviceability of the road structure, provision of an acceptable urban environment and minimising health risk caused by long term surface ponding. - Prevents damage to property in

flood prone areas Construction of a highway shall not be allowed to increase the risk of flooding to properties. - Concentrates flood water to

discharge basins for easy removal.

The highway drainage system must therefore be considered as providing four primary functions, which due to land use constraints are usually dealt with differently in urban and rural situations, namely:

The engineer shall undertake the following minimum studies for each highway using the criteria set out in the clauses in this section:

Collect precipitation falling on the highway reservation, adjacent side roads and catchment and convey to a suitable outfall:

Determine the total amount and rate (Q) of storm water run-off reaching the poinf under consideration

Select appropriate criteria on limits and frequency of acceptable flooding

Reduces the danger of standing water to traffic

- Maintains the use of all trafficked lanes

- Reduces sediment build up at the road side

- Reduces percolation into the road structure.

Guide surface water run-off safely across or under roadways:

- Minimises disruption to traffic

Minimises damage to the pavement or embankment structure

Determine points of concentration, discharge and hydraulic controls, together with method of entry into and exit from the drainage system

Determine the requirement for the provision of sub-surface drainage. ' '

8.1.2 Minor and Major Systems

Drainage of highways is the joint responsibility of the Civil Engineering Department's Roads Division and Drainage Division. Each Division has defined responsibilities and procedures which shall be adhered to when designing highway drainage. These are explained in the following clauses. - Guides surface water run-off to

suitable discharge points Minor System The Roads Division is responsible for the design of the Minor System, namely the road drainage, comprising gullies, soakaways, connecting pipework and storage areas required prior to discharge into the Drainage Division Network. The highway drainage system shall be designed using parameters defined in this section. The point of discharge and discharge parameters listed below, will be provided by the

Minimises road impact on the . natural surface hydrology in

rural areas.

Remove water percolating through the pavement, lower ground water and prevent capillary rise:


QATAR HIGHWAY DESIGN MANUAL SECTION 8 I . -

CED Drainage Division:

Diameter of trunk sewer - Allowable discharge volume Invert level of trunk sewer - Location of tn~nk sewer

o Acceptable method of discharge into the trunk sewer.

Major System CED Drainage Division is responsible for the Major System which comprises all the drainage components beyond the agreed interface point with the minor system:

Trunk, surface water sewer network Surface water pumping stations

o Ground water control networks o Surface water storage retention

areas/tanks.

'The preferred drainage method is by a positive system. However should this not be practical due to distance from a suitable discharge point or economics, agreement to discharge water to the ground or adjacent areas may be sought from the Director of the Civil Engineering Department.

8.2.1 ~ydro lo~ ica l Data

Rainfall Characterization Long term rainfall records for Qatar commenced in 1962 and are recorded daily, together with other weather information, from a number of locations by the Civil Aviation and Meteorology Department of the Ministry of Communications and Transport.

Summaries of recorded data are issued regularly.

Qatar lies in an arid region and annual rainfall may vary.from 20mm to over 300mm per annum. Individual storms occasionally as intense as 124mm in a 24 hour period Bnd 54mm in a 3 hour period, have been recorded. Rainfall is therefore characterised by:

High variability

Severe thunderstorms of limited geographical extent.

January 1997 -

For the purpose of highway drainage design the country shall be considered as having the same rainfall characteristics for all regions.

The Total Rainfall and Maximum Rainfall in 24 hours data (Table 8.1 a & b) provided from Doha International Airport Meteorological Station provides the longest available rainfall record and'shall be referred to for design purposes. However, a more onerous review may be required in specific cases where flood damage to strategic highwayso or property would be severe.

Intensity-Duration-Frequency Data regarding individual storm events in Qatar is scarce and generally inadequate. However, statistical analysis and comparisons by a number of researchers has established an intensity - duration - frequency relationship which is generally found to stand comparison with Bahrain data and to some extent, the Bilham Formula. See Figure 8.la & b.

I = 25.4 ~1.a x ~ m ) * - * ~ - a 11 T

Where I = rainfall intensity (mmlh) T= duration of storm (hours) N = Probable number of

occurrences in 1 0 years

Run-off Coefficients (C) Typically, for densely built up areas, there is a high run-off for all rainfall intensities. However, as development becomes more sparse or ground conditions more pervious the total run- off will reduce. Run-off is also affected by storm intensity.

Calculation of surface water run-off shall be made using Figure 8.2 which gives values for run-off coefficients which reflect the above situations.

Page 0i2 .. . -

Total Monthly Rainfall (mm)

Table 8.1 a Total Rainfall - Doha International Airport 1962 - 1995 (Data to be reviewed at regular intervals)

. - . . January 1997 Page 813

QATAR HIGHWAY DESIGN MANUAL SECTION 8 I -

Table 8.1 b Maximum Rainfall - Doha International Airport 1962 - 1995. (Data to be reviewed at regular intervals)

State of Qatar Minisly d Communlcalions 6 Transport

Departmen1 of Civil Aviation 6 Meteorology

Muimum Rainfall in 24 Hours (mm)

Staticn: Doha International Airport Lac 25 15N Long: 5134E

January 1997 Page 814- -

YW&bntIl

1962

1 983

1%4

1965

I966

1907

1908

1969

1970

1971

JAN

' 0.0

0.0

47.0

3.0

0.0

. 0.0

0.0 .. . 58.0

8.7

0.6

1979

1880

1981

1082

1983

1984

1985

1968

1987

1988

1969

loo0

1991

1992

lo03

1994

1995

H W * t

FEE

0.0

0.0

15.0

0.8

17.6

1 .S

25.0

0.2

0.0

5.8

MAR

0.2

0.0

13.0

0.0

0.0

1.5

0.0

0.0

1.5

0.0

JUN

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

4.5 1 0.1

7.2 20.2

6.4 2.4

1.8 9.9

6.0 4.1

Tmce Trace

1.7 0.0

3.7 6.2

0.5 0.1

4.1 41.3

Trace

6.8

1.3

1

APR

0.2

0.9

2.5

30.0

2.4

6.1

14.0

6.4

0.0

7.4

46.6

3.0

12.7

40.1

17.5

15.2

0.5

3.4

28.0

2.3

5.0

0.6

14.7

1.6

1.9

8.8

58.2

58.2

0.1 I 0.0

JUL

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

MAY

0.0

64.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.5

12.0

Trace

Trace

Trace

2.1

5.0

Trace

Trace

17.1

Tram

8.7

2.6

2.3

0.6

1.2

2.0

2.0

3.1

34.4 58.0 44.6

AUG

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

Trace

0.7

1.6

0.0

0.9

0.2

Trace

Trace

Trace

0.0

0.0

Tram

0.0

0.1

1.6

8.6

TI-

64.0

Year

, 0.2

64.0

80.1

30.0

17.8

6.1

25.0

58.0

6.7

7.4

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

SEP OCT NOV

0.0

5.6

0.0

13.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

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Figure 8.la Bilham Formula, Intensity - Duration - Frequency Chart (0-4h)



Figure 8.1 b Bilham Formula, Intensity - Duration - Frequency Chart (1-24h)

- January 1997 Page 816

1.0

0.B

0.8

0.7

C5

2 LL

0.6 0 Z 3 oL U.

0.5 U) I- Z w_ 0 - LL

0.4 W 0 C)

0.3

0.2

0.1

0.0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

RAINFALL INTENSITY ( I ) mmlh

- ,

, L

\ 0

'2 Figure 8.2 Run-off Coefficients for Urban Catchments L

+ . . QATAR HIGHWAY DESIGN MANUAL SECTION 8

. ., Januarv 1997 Paae 817


Catchment Area (A)

Rural The area to be considered shall incorporate two parts:

The area of the road corridor subject to direct precipitation

The broader natural catchment area within which the road runs. Specifically, the effect the road may have on the natural surface and subsurface drainage of the area.

Reference to topographic mapping should be made to assess the catchment area.

. - . Urban The area to be considered shall incorporate two parts:

. --.

--The area of the road corridor subject to direct precipitation

. The additional adjacent area assessed by reference to the Development Plans ,

and topographic mapping for the area.

The additional area will be dependent on factors such as intensity of development, provision of ' flood storage areas, and contribution from adjacent roads and developments.

Surface Run-off (Q) . .

Highway drainage areas to be considered in Qatar are typically less than 50 Hectares. For these. areas surface run-off (Q litres/second) shall be calculated using the formula:

Q = 2.78CIA Where

C = Run-off coefficient B I = Rainfall intensity (mrnlh)

A = Area (hectares)

For areas larger than 50 Hectares, mostly rural conditions, consideration should be given to assessment of run-off by a combination of historic observation and generation of storm hydrographs. The method used shall be agreed with CED.

Time of Concentration (Tc) The engineer wishing to size a drainage system must ascertain the peak rainfall run-off from the catchment under consideration for the designated design storm return period.

January 1997

At a chosen point the peak flow generally occurs at the instant all parts of the catchment are contributing to the flow.

The Time of Concentration (Tc) is defined as the interval in time from the beginning of the rainfall to the time when water from the most remote part of the catchment reaches the point under consideration by the engineer.

The Time of concentration is a function of the average slope, length 'and roughness of the catchment.

A number of equations have been developed for computation of the Time of Concentration for various methods of flood analysis. However, it is recommended that where the Rational Method is employed, Manning's equation is used for the calculation of flow velocity in gutters, drainage channels or pipes.

Manning's Equation:

v= FIS)sH n

Where V = Mean velocity of flow (rn/s) n = Manning's coefficient of

roughness R = Hydraulic radius (metres) S = Slope (percent)

Time of Concentration:

Where Tc = Time of Concentration

(seconds) V = Meanvelocityof Row (m/s) L = Length of flow path from the

point of consideration to the furthest catchment extremity (metres)

For easy reference, when preparing drainage computations to the Rational Method, the engineer may use the nomograph given in Figure 8.3.

When considering short duration storms the rainfall intensity changes rapidly with only a small change in storm duration, (Figure 8.la). Therefore it is important that for small drainage areas an accurate assessment of Time of Concentration is made. However, due to the necessity for the surface to receive rainfall and reach a flowing condition the Time of Concentration shall not be reduced to less than 3 minutes.


Permeability (P) Permeability of the ground shall be determined by in situ geotechnical testing as described in Appendix B of this manual.

Ideally the permeability will be assessed at a number of locations along a project site and soakaway sizes optimised on the basis of the test results.

Should geotechnical data not be available then reference to Table 8.2 and to records held by CED Roads and Drainage Divisions should assist the engineer. However, where existing records are used, this should be verified by site permeability testing during construction in order to confirm the design values used.

I Soil ~ i e Permeability I

Desiccated and fissured clays

Clean sands and sand- gravel mixtures

Very fine sands, silts and clay-silt laminate

' Special measures required in this range.

Table 8.2 Typical Permeability

8.2.2 Design Return Period

The design of an economic surface water drainage system is related to the acceptable risk against flooding. Though Qatar is an arid country, when storms occur, the disruption and damage caused can be considerable. However, to provide a complete, risk free, surface water drainage system would be prohibitively expensive.. The following Design Return Period tables list the minimum storm return periods to be used in the design of surface water systems.

The engineer may chose a reduced level of risk if a specific project requires this.

SECTION 8

The run-off that a positive highway drainage system shall be designed for is determined by the Time of Concentration and reference to the acceptable frequency limits provided for the different highway classes in Table 8.3.

Table 8.3 Design Return Period - Positive System

Highway Classification

Primary

Secondary

Tertiary

Where a positive drainage system is not available and drainage is to soakaways, then the 24hrs total rainfall figures given in Table 8.4 shall be used.

Situation

Rural Urban

Rural Urban

Rural Urban

Table 8.4 Design Total Rainfall - Soakaway . System

Storm Return - Period

(years)

1 in 10 1 in10

1 in5 1 in5

1 i n2 1 In2 '

Area Description

Residential Areas & Minor Roads

Major Roads & Commercial Areas

Where the highway is required to cross a water. course, the acceptable frequency limits against flooding and damage from natural water courses given in Table 8.5 shall be maintained:

Rainfall

12mm in 24 hours

18mm in 24 hours .

Table 8.5 Design Retum Period - Natural Surface Run-off

. -

Highway Classification

Primary

Secondary

Tertiary

* -. . . .. . . . .. . .- . . .

Jariuary 1997 - Page 819 ,

Situation

Rural U@an Rural Urban Rural

. 'Urban

Storm Return Period (years)

l ' i n 50 1 in 50 1 In 20 1 in 20 1 In 10 1 in10


If there is a .requirement to utilise the road as a storm run-off carrier in the case of a major rainfall event then advice regarding the acceptable frequency limijs for individual situations should be obtained from the CED Drainage Division. This will typically reflect those shown on Table 8.6.

SECTION 8

• Catchments where the contributing area does not increase uniformly with time can produce erroneous results.

Hydrograph Methods Suitable for larger urban catchments where storage in pipes and above ground bec~mes significant, and for calculation of overland flow in larger rural catchments for the sizing of culverts and retention ponds.

Storm hydrographs should be built up from existing known storm data. However, this information is currently not widely available in Qatar and hydrographs such as a UK summer storm are considered generally equivalent to Qatar storms and therefore suitable for use in hydrograph models.

t

Classification of Area Subject to flood

Hospital I Airport Industrial Prestigious Commercial Government Offices and Private Offices Residential & Llght Commercial

Table 8.6 Design Return Period - Areas .. .

If an area forms a boundary with no natural outlet for surface run-off then higher acceptable frequency limits may apply. CED Drainage Division should be consulted further for advice.

Storm Return Period

1 in 100 1 In 50 1 in 20 1 in 20 1 in 10

Design of larger diameter piped systems should take account of pipe storage and proprietary computer software models should be used at the direction of CED Roads to optimize system design.

8.2.3 Design Method Retention ponds, storage tanks and hydraulic restrictors shall be modelled using methods as agreed with CED Roads.

Surface water drainage design should be submitted to the CED Roads Design Section for approval as part of the project detail design report. Detail design should utilise the information provided within this Design Manual. Basic design methods to be used are as follows:

Soakaway Design Soakaways should be considered for surface . water drainage in areas where a positive system is not available or economics preclude the use of a positive system. Lloyd Davis Rational Method

Suitable for the majority of surface water drainage systems envisaged in Qatar, eg. Minor branch connections to a major trunk sewer designed by others. The relevant storm and catchment parameters given in this section are used to calci~late surface water discharge flows and the piped system is sized to suit these flows.

However, areas of high groundwater table shall not be considered suitable for soakaways. In these areas positive systems shall be provided with outfalls to EFA's, storagehetention tanks or purnping stations.

Where permeability has been accurately assessed with confidence and where its long term availability through maintenance is without doubt, then ground permeability can be considered within the design of the soakaway. In all other situations the soakaway shall be considered a storage chamber and shall be capable of storing the total rainfall requirement of Table 8.4, below carriageway formation level.

A standard calculation sheet to be completed and submitted with designs is given in Figure 8.4.

A number of important points need to be considered by the engineer utilising this method.

Simple to use

Larger catchments can provide conservative results, typically when chosen pipe diameters exceed 600mm

Each gully shall be connected to an individual soakaway, except at junctions where areas to be drained are reduced due to gullylchannel requirements.

Care should be taken in selecting run-off coefficients and rainfall intensities for use in the equations.

Soakaways shall be positioned in accordance with the reservation cross-sections given in Section 5.

-

Page 8/10

. , QATAR HIGHWAY DESIGN MANUAL 'SECTION 8

- ! n Soakaways can introduce localised subsidence j/~ due to wash out of fines. As such, they shall not

, . be positioned under the camageway, shoulder or i parking area or within 5m of a structure (subject

to geotechnical advice).

L Soakaways shall be sized and located so as not to introduce water to the pavement construction.

Soakaways should be constructed with a rising L piece to enable shallow utilities to pass above

the main chamber.

I L

In particular situations the engineer can consider linking soakaways by pipe connections at invert

I or intermediate levels. However, he should ensure that this is not going to merely

L concentrate the surface water at the road valley .. . point.

I

Where the existence of a perched water table has been established by geotechnical investigation, CED Drainage shall be'consulted regarding the use of combinations of boreholes and soakaways to discharge to lower aquifers. It should be noted that in some areas lower aquifers may be under a piezometric head or utilised for potable water purposes.

When a road is reconstructed or a piped drainage system is installed in an existing road, the original soakaways are unlikely to be either efficient or undamaged by corrosion and will need to be removed or renovated.


QATAR HIGHWAY DESIGN MANUAL . -

SECTION 8 I

Figure 8.3 Time of Concentration - For Use with Rational Method

L- ( $

r

7em

arks

Ve

loci

ty

Full

Velo

city

%pa

city

(rn3/

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(mls

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8.3 URBAN DRAINAGE

8.3.1 lntroductlon

Drainage of highways in urban areas of Qatar is achieved using the major and 'minor systems described in Clause 8.1.2 and constructed and maintained by the CEO Roads and Drainage Divisions.

Highway drainage shall be provided for all urban roads.

Rainfall falling within the catchment area shall be collected and disposed of within the highway limits or to a designated outfall point. Surface run-off shall not be allowed to shed outside the highway reservation unless to a specified discharge point.' Surface- water shall not be allowed to stand within the highway reservation for an extended period of time so as to cause public nuisance or a health hazard.

It is important that the highway' drainage requirements are established early in the design process to ensure that adequate reservation space is provided and service utilities routed to avoid possible clashes, particularly with soakaways. Refer to typical cross-sections in Section 5.

Drainage problems can often be alleviated by the engineer considering the layout of the road system and planning of a new development in harmony with the natural drainage of a catchment.

The roadway can be used to provide the following functions:

Guide overland flow

Isolate drainage catchments into manageable sizes

Increase the drainage path and hence time of concentration

Provide additional flood storage area

Provide a drainage reservation to the area discharge point.

Open areas such as parks, school yards, car parks etc. can provide storage areas should the drainage system be unable to cope with area surface water run-off. Their location should therefore be carefully chosen at the planning stage to make the best use o.f topography and drainage constraints.

8.3.2 Urban Catchment

Urban development causes changes to the run- off process by both altering the route and surface characteristics over which the run-off flows.

Highways form a part of the urban catchment and the highway engineer must carefully consider adjacent development and its discharge points and characteristics in order to accurately assess the total catchment that may be contributing to the highway drainage system under design.

The urban catchment provides the engineer with further points for consideration; that of availability of discharge points for the collected water, and the environmental damage due to increasing build up of pollutants washed into the highway drainage system.

8.3.3 Positive Drainage

Positive drainage is preferred in all urban situations. Water collected is piped or channelled to a discharge point from whence it can be collected and discharged away from roads and developed areas.

Highway drainage by positive means involves discharging run-off to a point advised by the CEO Drainage Division for onward transmission by the Trunk Sewer System.

8.3.4 Drainage of the Carriageway

Rain falling on the road surface builds up and presents a hazard to vehicles both during and after storms. It is therefore necessary to provide drainage to the carriageway by a combination of transverse and longitudinal gradients, shedding to water collection points and a distribution system.

Typical topography in urban areas of Qatar, where roads are kerbed, requires slack gradients to minimise the appearance of a roller coaster road and reduce fill requirements. The minimum gradient criteria to be used are given below:

Transverse gradients of 2% are provided as normal for drainage off the travelled way to the channel



Minimum longitudinal gradients of 0.3% should be provided to drain the edge of a travelled way to a discharge point

However, a desirable minimum longitudinal gradient of 0.5% is to be provided, where practical

Care shall be taken at junctions and areas of superelevation to ensure that the combination of transverse and longitudinal fall does not create a flat zone in the carriageway

In particular cases, a rolling crown may be used as an alternative to superelevating channel lines to avoid flat zones, Figure 8.5. The length of the rolling'crown is determined using the same formula as that for applying superelevation (refer to Clause 3.4).

k

. . X-x

--em---

Crniagowry Crossbfl

C~rrirgewry Crossfrl Longth of

Rolllng Crown

Figure 8.5 Typical Detail of a Rolling Crown Across a Single Carriageway

Drainage collection points in urban areas should typically be provided by gullies located along the channel or gutter. On gradients of 0.5% or less the flow of water to the gullies can be aided by the use of channel blocks. Gully spacing is a function of grating size, road gradient and crossfall and acceptable flow width at the channel. Standard gully spacings and criteria are given in Figure 8.6.

Where standard criteria do not apply, the engineer should consider reducing the gully spacing or referring the specific case to more detailed calculation procedures.

On roads with longitudinal falls, valley points shall be provided with double gullies to aid water collection.

To maintain gully performance under the influence of wind borne debris and dust and to improve collection under the effect of high rainfall intensity, it is preferred that gullies are constructed as pairs.

Valley points of large catchments should be located in areas where flooding would present minimal hazard or disruption, or where additional . water storage or dispersion is available. ie Emergency Flood Areas (EFA), parks and gardens, trunk storm sewers etc.

Gullies shall be linked to the disposal system, by piped connections.

The preferred minimum gradient for gully connections is 1 %. However, gradients of 0.5% are acceptable should situations dictate.

Maximum gully. connection length is 36m. Should longer lengths be required then intermediate manholes or catchpits shall be included in the scheme to facilitate cleaning.

Utilities shall be located so as not to provide a hindrance to the drainage system installation and maintenance or increase the chance of damage during utility maintenance works.

Storm sewer design shall be in accordance with CED Roads and Drainage Divisions' design guides and specifications. Storm sewers shall cater for the flows computed from the design criteria in this Section and any additional flows advised by CED Roads or Drainage Divisions at, the project commencement.

. _... ? q .


QATAR HIGHWAY DESIGN MANUAL . , SECTION 8 . I

200 .

190

180

170

160

,150

..- 140

130

h 120

E Y

13 110 z 0

2 100 cn

- 3 4 90 3 13

80

70

60

50

40

30

2 0

10

0 2 3 - 4 5 m6 7 8 9 10 11 12

'9 m 2 '?

t-

IMPERMEABLE WIDTH (m)

-

Longitudinal gradient at channel given as % - Flood width of 1.Om

Figure 8.6 Gully Spacing

- January.1997 .. . . .- , . . . ., . , Page 8/16. --


8.3.5 Dralnage of Medians, Footways and Verges

Medians Medians in urban areas are normally paved or landscaped with planting. Paved medians shall be sloped to shed run-off onto the adjacent carriageway for collection by the carriageway drainage system. Landscaped areas in medians shall be edged so as to prevent run-off from these areas taking soil and plant debris onto the carriageway.

Footways Footways shall normally be sloped at 2% towards the carriageway to shed run-off onto the carriageway.

Where new highways are to be constructed in areas of existing development, care must be taken to ensure road levels are set to allow the footway to slope from the property threshold to the carriageway. Areas of wide paving may require sloping to additional collection points away from the carriageway. These collection points must be suitable for pedestrian traffic to cross without risk of injury and must be situated so as not to be a hindrance to maintenance access.

It is the duty of adjacent property owners to prevent significant run-off across the footway by the introduction of collection channels. This is particularly relevant in the case of polluting run- off such as from petrol station forecourts.

Collected water may be added to the highway drainage system once cleaned of grit, oil and other. pollutants.

Verges Verges with hard landscaping shall be sloped to shed water towards the carriageway. Where soft landscaping is provided then it shall be edged and sloped to prevent run-off from depositing soil and plant debris onto the adjacent pedestrian or trafficked surfaces, or into property thresholds. Areas of raised planting which incorporate drain holes shall incorporate a filter membrane to prevent washout of soil onto adjacent areas.

8.3.6 Emergency Flood Area (EFA)

Emergency Flood Areas are portions of land set aside, within or adjacent to the highway reserve, that are used for additional storage of exceptional run-off generated by storms greater than those normally designed for.

. SECTION 8

They are to be used in situations where run-off from. sizeable catchments would become trapped at a valley point and consequential flooding would cause damage to adjacent properties or render a road impassable with no equal adjacent route available for detours.

Water should not be allowed to pond for extended periods so as to cause a health hazard.

Emergency Flood ~ i e a s shall therefore be provided with:

A location where water can be easily pumped by tanker or temporary pumping station.

, Borehole soakaways to aid discharge to the ground water table, where investigation has shown this is achievable.

Permanent surface water pumping station and rising main connected to the trunk sewer system.

In order to make the best use of land in developed areas it is normal practice to design EFA's as sports fields, parks, playing fields, car parks etc.

EFA's that are not landscaped or utilised for other purposes have a tendency to collect rubbish and become an eyesore.

L

EFA's should be considered a potential . drowning and disease hazard. Where possible they should be kept shallow and spread over a large area. This helps evaporation and dissipation and presents a less deep water hazard. Side slopes should be gentle to allow easy exit and marker posts should be located around the rim to identify the deeper area in times of heavy flooding.

Prior to designing EFA's the prevailing groundwater table should be ascertained to ensure the excavation does not allow standing water to remain. Soakaways or boreholes can be constructed in the base of the EFA to encourage water dissipation.

. . . . . . . _ .. *


SECTION 8

8.3.7 Maintenance Strategy .. .

All highway drainage systems shall be designed with future maintenance procedures being considered.

8.4.3 Dralnage of the Carrlageway

In order to reduce surface build up of rainfall and the consequent hazard to vehicles both during and after storms, it is necessary to provide drainage to the carrlageway by a combination of transverse and longitudinal gradients shedding onto the verge and adjacent land:. .. .

Routine maintenance will be required due to build-up of wind blown debris and settled sediments in gutters, gullies and pits. .

Exceptional maintenance should be limited by good design and construction practices.

Transverse gradients of '2% are provided as normal for drainage of the travelled way.

CED Highway Maintenance Section are the responsible authority for the maintenance of the highway drainage system, including EFA's and storage areas not in the Trunk Storm Sewer System. .. .

Longitudinal gradients are not considered for drainage purposes on unkerbed roads. However, care must be .taken during the deslgn of superelevated sections to avoid flat zones in the camageway. CED Drainage Division are the responsible

authority for maintenance of the Trunk Storm Sewer System. In areas where carriageway edge run-off collld

damage verges or steep embankments then edge kerbing or edge channels shall be provided to collect water to discharge points.

8.4 RURAL DRAINAGE

8.4.1 Introduction Discharge points would include gullies and precast channels. Drainage of highways in rural areas of Qatar

can be considered as two cases: 8.4.4 Drainage of Medians and Verges

Drainage of rainfall falling onto the road and highway reservation Medians

Medians in rural areas would normally be unkerbed and unpaved. Drainage of natural overland flows.

The median should be sloped away from the carriageway to prevent run-off washing soil debris onto the road.

Drainage of run-off from the road and highway reservation shall normally be achieved by shedding onto adjacent land.

Where run-off is collected from long sections of gradient, median outlets should be provided at wadi and valley points to prevent water ponding and flooding onto the carriageway. Alternatively the median may be broken into individual catchment segments and surface water allowed to percolate into the embankment or evaporate. Median ditches, if required, should have a maximum side slope of 1 in 6 and shall be designed such that water in the ditch cannot percolate into the road construction, see Figure 8.7.

8.4.2 Rural Catchment

The engineer is not usually faced with the problem of catching and dissipating rainfall as in urban situations, but is allowing run-off to flow generally unimpeded on its natural course.

Rural catchments are often extensive and can build considerable volumes of water in their lower reaches during even moderate storm events.

Considerable care should be taken in assessing the size, slope and surface characteristics of the catchment (refer to Figure 8.2) and applying the appropriate design storm (refer to Table 8.5).

Where ditches are required to facilitate subsurface drainage, it is important to ensure that adequate outlets or storage volume is provided.



Fig 8.7 Typical .Median Ditch

Verges and Ditches Verges in rural areas shall be sloped to shed water away from the carriageway.

At the back of the verge a shallow ditch may be provided to both collect and transport carriageway run-off and catch minor area run- off for transport to wadiis along the route.

The designer shall ensure that ditches are not located so they can introduce surface water to the pavement construction. Normal practice is to ensure the ditch invert is a minimum of 0.3m below the carriageway formation level at the outer edge of the carriageway.

Ditch dimensions and shape shall be designed following consideration of its location and impact on highway safety together with the following hydraulic considerations:

- contributing catchment - appropriate storm duration - gradient - roughness coefficient of lininglsurface

In most cases it is expected that rural ditches will be unlined. Permissible depths of flow for unlined channels are given in Figure 8.8.

Shallow side ditches are not normally graded to provide a fall but follow the road profile.

Ditch slopes should not present a significant hazard to traffic leaving the road during an accident. Side slopes of 1 in 6 or shallower should suffice for this.

In areas of steep cutting, ditches should be located so they are not filled with loose debris from the cutting. In areas where natural surface run-off is high it may be necessary to install a ditch setback from the top of cuttings to prevent rainfall damaging the cutting face.

8.4.5 Natural Surface Drainage

Where a highway crosses a wadi, the wadi catchment characteristics, design storm and class of road will determine the type of road crossing required. It is normal practice to allow run-off even from small catchments, to cross under the road so as to minimise disruption to the natural surface flow.

Culverts A culvert is a covered channel or pipeline used to convey a watercourse under the road. It consists of an inlet, one or more barrels and an outlet.

Typically, culvert barrels will be constructed from concrete or steel pipes or boxes. Inlets and outlets may be constructed with gabions, mattresses, stone pitching or concrete.

The hydraulic characteristics of a culvert are complex due to the number of flow conditions that can occur. The highway engineer shall consult specialist literature in his design of culverts and shall choose the most appropriate culvert for the specific purpose considering the following general constraints:

Preferred minimum pipe culvert diameter 800mm

Minimum pipe culvert diameter 450mm

Flooding against embankments is acceptable short term. Freeboard to edge of carriageway to be a minimum of 0.5m for the design storm. , .

Embankment slopes of 1 in 6 or greater do not normally require protection against washout due to short term ponding. Long term ponding may require embankment slopes of 1 in 10.

The engineer shall balance embankment height with culvert height to .provide a satisfactory technical and economic solution.

January 1997 . Page 811 9


Fords i /--

Where wadi flows are exceptionally high or the ..-/ road requires a low storm design return period r

l

and is lightly trafficked, culverts may prove I S

5-

impractical. The engineer may therefore consider incorporating a dry ford or vented dry ford. In designing a dry ford, care must be exercised to ensure driver awareness of the - potential hazard. Guide posts should. be positioned adjacent to the caniageway to assist . ,

traffic positioning and advance signing should b

be used to indicate the dry ford to approaching drivers.

Specific attention must be paid to minimising scour and the prevention of carriageway surfacing and edge loss. Verges, medians and embankment slopes should be protected by impervious l*ers or rock. Washout of embankment fines should be prevented by the use of filter layers or impermeable membranes.

January 1997 ., . Page 8/20

s 0 F

s LD

- .I

0 C/) .c, C m s 3-,

CV V) ..- cn - 0

E cr s. E -- .- V) 6 0 S Q L w

o v , I - a, I c

sg c 0 0 0 a

Q)

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v .- u

0 0 L

W

s v

v- v 0

0 z 0 0

(w) 'yldaa MOI-J lunw!xeyy

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L

,? . - Q \

Figure 8.8 Permissible Depths of Flow for Unlined Channels


Januajr 'l997 Page 8/21


8.5 JUNCTION DRAINAGE

8.5.1 Introduction

Effective drainage of the carriageway at junctions is particularly necessary for two reasons:

The need to retain surface grip to enable the safe stopping, starting and turning manoeuvres routinely undertaken by vehicles at these locations.

The need to maintain the traffic system capacity, particularly at major junctions makes it essential that flooding of lanes and reduction in junction capacity is avoided.

The following criteria must be considered to satisfy the above requirements:

Satisfactory transverse gradients must be maintained, particularly on the approach to "Stop" or "Give Way* lines

Longitudinal gradients must be carefully chosen to keep slack sections of channel to a minimum

Where slack gradients are unavoidable the transverse gradient should be a minimum of 2%

Collection points must be carefully sited to avoid ponding or run-off across carriageways from one channel to another

Collection points must link to an easily maintainable disposal system with adequate capacity.

Junctions should preferably be situated away from valley points for large catchments to prevent flood concentration at these points. Locating junctions adjacent to trunk sewers or EFA's to provide additional drainage facilities should also be considered.

Urban junctions should always be kerbed and are therefore drained by gullies to the disposal system.

Rural junctions would normally be kerbed however an economic collection and disposal method may be achieved by flush kerbs located at collection points with shallow lined channels removing the water to the adjacent ground..

January 1997

Lightweight Glass Reinforced Concrete (GRC) embankment channels are easily installed to prevent washout of embankment slopes at areas of run-off concentration such as at kerb ends.

8.5.2 Drainage at Junctions

Carriageway crossfalls and longitudinal gradients at junctions are used to channel water to collection points. The following are examples of satisfactory crossfall layouts with typical collection points:

T-Junctions (Figure 8.9)

Constant camber maintained on major road

, Longitudinal gradient on major road maintained across minor road throat

Longitudinal gradient maintained on minor road to major road channel line

Constant transverse gradient on minor road maintained to radius tangent points

Gully positions chosen to prevent flow crossing the minor road entrytexit.

It is preferred to maintain the major carriageway transverse gradients through cross roads or small signalized junctions.

Figure 8.9 Typical Drainage at T-Junctions

MAJOR ROAD

----------

Page 8/22

I I

I I

I

U W O R R O A D

QATAR HIGHWAY DESIGN MANUAL SECTION 8 i _ . .

Large Signalized Junctions (Figure 8.1 0)

a Transverse gradients to be maintained at approach to "Stop" lines & pedestrian crossings

a Longitudinal gradients to be satisfactory to prevent a large flat area being created at the intersection point

a Transverse gradients on right turn slips to provide superelevation

- - b a Valleys created in slips to have I

I

4% adequate collection and disposal points

- . a Additional gullies placed at collection 1

1 . points serving a large surface area +

a Gully positions chosen to prevent flow

i ̂

crossing carriageways. . . L

Roundabouts (Figure 8.1 1)

a Transverse gradients maintained at approached to "Give Way" lines

a Longitudinal gradients to continue to be maintained on approaches and departures

a Channel of central island to fall to one collection point

a Transverse gradients provide superelevation for right turners or those

- circulating

! L a Gullies positioned to prevent cross

.-. ,. ., carriageway run-off.

-$-

January 1997 - I

- . . . Page 8/23 . . . . . i , .

9

QATAR HIGHWAY DESIGN MANUAL ' " ' SECTION 8

Figure 8.1 0 Typical Drainage at Large Signalised Junction

Figure 8.1 1 Typical Drainage at Roundabouts



8.6 SUBSURFACE DRAINAGE

8.6.1 lntroductlon

Water can be introduced to the pavement by:

Rainfall permeating through the wearing and base courses (old pavements)

Rainfall permeating through the verges and medians

High groundwater table at the formation due to natural water table or seasonal ponding

Capillary rise from groundwater near the formation.

Where these situations are present, subsurface drainage is required to prevent build up of pore water within the pavement, formation and subgrade. lncrease in pore water q n weaken the pavement by:

Transferring loads to lower (weaker) sections of the pavement through increase in pore water pressure

Washout of fines by movement of pore water

Increase in salt content in pavement layers and subsequent swelling due to capillary rise when appreciable quantities of salt are present in the subgrade

Swelling in susceptible material, followed by shrinkage or drying out.

Subsurface drainage is not normally detailed in Qatar as it is rarely a problem. Low lying areas are normally filled prior to development to raise them sufficiently above the groundwater table. Roads are generally constructed on embankments in areas of high groundwater as they are usually subject to flood inundation during storms.

Subsurface drainage is therefore only considered necessary in the following situations:

Areas of existing development and rising groundwater levels

SECTION 8

Tidal coastal areas where the water table varies close to the surface.

It is preferred that a soils investigation is undertaken to assist in deciding the need for subsurface drainage.

8.6.2 Subsurface Drainage Methods

High Groundwater In areas of existing development where high or rising groundwater is likely to bring moisture to the formation level, a collection and disposal system shall be installed to lower the water table.

It is normal practice that this is performed by the installation of a perforated land drain below the carriageway, together with a positive surface water drainage system. 'This would normally be undertaken by the CED Drainage Division as part to the Trunk Sewer Network. In these locations, soakaways shall not be used for drainage.

Alternatively, in rural areas, the provision of side ditches can serve the dual function of intercepting overland flow and aid in the lowering of groundwater local to the road structure.

Coastal Areas In tidal coastal areas, sabkha is likely to be . present as an indication of a high groundwater table.

In these situations capillary rise of up to 1 .Om can draw saline water up to the road formation level, depositing salt lenses and increasing pore pressure.

'This is generally prevented by:

Construction of high embankments

Introduction of a granular capillary break layer below the formation.

General Deslgn Conslderation The highway engineer should consider the introduction of water to the formation as likely to happen due to annual rainfall and irrigation of plants in the median and verges.

In most cases the dry granular nature of the typicai Qatar subgrads layers means the pore pressure rise due to percolation is slight.


. . . , . * : ; i : . ..-.. .. -..: i '- - . ..... . _ . . < . . . _ . . . .. .. . . , . . ... .*- -. .:, . -. .

QATAR HIGHWAY DESIGN MANUAL . . . . .. : , , . . . ..': ,.& ,: ..; SECTION 8

However, to provide an additional safety factor against this occurrence, the following measures should be considered:

Slope the formation todrain away from the carriageway to the verge or median

Avoid steps in the formation that could lead to water concentration points -

Keep planting areas separated from the pavement construction to prevent moisture transfer

Ensure planting area watering is effectively controlled to prevent over watering

~tilise=*surface water drainage details that will reduce the chance of accidental damage and maintenance problems

Ensure soakaways do not introduce water to the pavement construction.



SECTION 9 PAVEMENT DESIGN

9.1 INTRODUCTION

9.1 .1 General

The pavement designs described in this manual replace those given in the 1989 Highway Design Manual issued by the Civil Engineering Department of the Ministry of Public Works. Unlike the previous designs, based on the road hierarchy and a standard subgrade, the new designs described in this section are based on the cumulative traffic over a definite design life (normally 20 years) and three subgrade strengths. 'The designs are set out in catalogue format and the technical basis for these is described in an Annex at the end of this section.

All materials, --methods of construction and tolerance used for road pavements must be in accordance with the Qatar Construction Specification (QCS). The Civil Engineering Department (CED) laboratory should be consulted during both the design and construction stages of any project to ensure that the latest material specifications are being used.

The various types of pavement constructions described herein may be used for kerbed or unkerbed roads, in locations with or without positive drainage. Any requirement for kerbing and drainage will depend upon the exact nature and location of the road - refer to Section 8.

.9.1.2 Typical Pavement Structures

A typical flexible pavement structure is shown in Figure 9.1. It comprises a wearing course laid upon roadbase and sub-base layers, and the subgrade.

'The wearing course must provide a skid resistant running surface and should be both crack and rut resistant. However, due to its exposure to the extremes of temperature and high wheel load shear stresses, the wearing course will probably deteriorate and require replacement before the rest of the pavement. Resurfacing is likely to be required at intervals of approximately 6-8 years during the life of the road.

The roadbase is the main structural layer of the pavement and may consist of either asphaltic concrete or granular material (gravel or crushed stone) for medium traffic levels, but only asphaltic concrete for high traffic levels. Its thickness is determined by the amount of traffic which is expected during the design life.

SECTION 9

The sub-base is a granular layer to support the roadbase and its thickness is determined by the strength of the underlying subgrade. In addition to providing adequate support to the roadbase, the sub-base must be able to carry construction traffic without developing excessive ruts.

'The subgrade is the top layer of the earthworks and depending on the road geometry, will be either cut or fill.

In rigid pavements, the asphalt wearing course and roadbase are replaced by a high quality concrete slab, with or without reinforcement. The sub-base is normally cement bound rather than just granular, to ensure a robust surface on which to erect side forms and joint assemblies and to minimise any pumping of fine material through slab joints.

Flexible-composite pavements consist of a cement bound roadbase with asphalt surfacing. As the cement bound material normally cracks transversely due to shrinkage and temperature warping, the surfacing must be thickened to provide insulation, to reduce the temperature gradient in the roadbase, and to prolong the period for crack development through the surfacing.

In pre-cast block paving, the asphalt surfacing is replaced by a layer of concrete blocks bedded on a course of sand. This pavement type is only used in areas of low speed traffic, typically in parking areas, or when a contrasting appearance is required for areas such as median strips.

9.1.3 Road Deterloration

Generally, pavements gradually deteriorate with time under the influences of environment and traffic. The environmental deterioration can take the form of hardening of the bitumen in the surfacing which can lead to excessive brittleness and cracking, or to salt damage of thin surfaced roads built on or with salt-rich materials. Poorly designed or maintained drainage-can lead to weakening of pavement layers or the foundation. which then deforms under traffic loading. Traff ic-related deterioration can take many forms including the development of ruts, general unevenness of the road surface, with a consequent loss of riding quality, and cracking which can lead to pop-outs and potholes.



Wearing Course (40mm layer of asphalt concrete, Gabbro aggregate)

Roadbase (Generally, asphalt concrete, Limestone aggregate 110 - 230mm thick dependant on traffic. For traffic less than 5 million standard axles, granular material can be used for part of this layer)

Sub-base (Granular layer, varying between 100 and 200mm depending on subgrade strength)

Formation 1 . 1

.,, v,. ,.- 1 ? 1

Figure 9.1 Typical Pavement Layers

Determining when a pavement has "failed' or is no longer providing the intended level of service is not simple. Generally the deterioration is very slow and variable. Criteria for "failure' can be set such as rut depth, roughness, deflection or even the level of maintenance expenditure or total quantity of patching.

Occasionally, major deterioration can occur over a relatively short period of time when, say, a low quality, moisture susceptible sub-base becomes wet due to surface cracking or a rise in groundwater level. However, distress at the surface of the pavement does not necessarily indicate the structural failure of the road. Surface cracking and rutting within the wearing course material may be treated without the need for major structural maintenance, as the main structural layer of the road, the roadbase, could be completely undamaged.

9.1.4 Variability in Materials and Road Performance

Road pavement performance is a very variable process due to a number of factors. Variations in the thickness and quality of the pavement layers and variations in the strength of the foundation all contribute to this, even though materials may comply with the relevant specifications. Also, uncontrolled factors such as the long term ageing of the bitumen cause variations in performance.

The random nature of variations in each layer should ensure that most deficiencies in thickness or strength do not coincide, or very

rarely so. The importance of good practice in quarrying, material handling and stockpiling to ensure this randomness and also to minimise variations themselves cannot be over emphasised.

Sometimes a road fails to carry traffic satisfactorily to the end of its design life because the traffic is considerably greater than predicted. Proper axle load assessment and reliable traffic forecasting are essential to prevent this. However in some circumstances this is very difficult and either a generous contingency will have to be provided or the traffic and/or pavement regularly monitored so that strengthening can be carried out before the pavement is seriously weakened.

9.2 TRAFFIC ASSESSMENT

9.2.1 Introduct~on

Pavement deterioration under trafficking is due to both the magnitude of the wheel loads and the number of times the load is applied. For pavement design purposes, it is essential to consider not only the number of vehicles that will use the road over the design life but also the axle loads of these vehicles. This is done by converting each axle load to an equivalent number of 'standard axlesQf 80 kN using an empirical relationship and totalling these over the life of the pavement. The conversion to standard axles is described in more detail in Clause 9.2.5. Light vehicles cause negligible damage - an axle load of 10 kN (1 tonne) has a damaging effect of only 0.00024 standard axles


QATAR HIGHWAY DESIGN MANUAL . . .

compared to the normal maximum axle load of 130 kN (13 tonnes) which has the effect of 6.45 standard 80 kN axles.

The pavement designs in this manual are selected on the basis of the cumulative traffic to be carried over the design life expressed in standard axles. The determination of this number is done in three stages:

1 The traffic for each class of vehicle which is expected to use the proposed road, both at opening and subsequently over the design life, must be forecast

2 The axle loading of each class of vehicle over the life of the road must be estimated

3 The cumulative number of standard axles to be carried over the design life must be calculated from stages 1 and 2.

These stages are described below.

9.2.2 . Design Life

The design life for the majority of pavements will normally be 20 years. In this period it should not be necessary to either strengthen or reconstruct the pavement provided that the traffic volume and axle loads have been as forecast. At the end of the design period the pavement should still have sufficient integrity to allow overlaying, rather than full reconstruction, to extend the life for further service. However, some surface deterioration, generally rutting or cracking, will occur in this period. The ruts could be caused by slow or stationary vehicles (at junctions), high temperatures and over-rich mixes (where the mix parameters have drifted to high bitumen or low voids within the specified limits) and could develop early in the pavement life before the bitumen has aged and stiffened. Cracking will normally arise (after 10 years) as a result of ageing of the bitumen in the high temperature environment. Depending on the status of the road and the extent and degree of surface deterioration, resurfacing by a thin overlay (40mm) or inlay (planing off and replacing the surfacing) may be necessary within the design life.

There may be situations where the future traffic loading may be very uncertain depending, say, on the siting or timing of some major development. In this case it may be prudent to consider a shorter design period and make provision for possible strengthening overlays when plans are more definite. The CED should be consulted in cases where a design period different to 20 years appears appropriate.

At the end of the 20 year design period, the great majority of pavements will continue to be used, but will probably require strengthening. The precise works will be determined by evaluation as described in Clause 9.6, but will probably take the form of an overlay of 50 to 150rnm, with or without planing the existing surfacing. Outside urban areas, with minimal kerbing and ironware and generous shoulders or verges, a raised road surface will not present any significant problems. However, in urban areas or adjacent to and under over-bridges, raised surface levels could be difficult or expensive to accommodate. In these areas, an increased initial pavement thickness would allow inlays to be used and thus avoid the need for overlays and changes in level.

This is an uncertain process, particularly in a country with a developing economy such as Qatar. To forecast traffic growth, the following three traffic categories must be considered. Any one of these could be dominant or insignificant, depending on the site.

1 Normal traffic, which would pass along the route even if no new pavement was provided

2 Diverted traffic, which is attracted to-the route because of the improved pavement

3 Development traffic, which arises from either planned or unplanned development along the road corridor. (The latter type is sometimes termed generated traffic).

Normal traffic can be assumed to continue to grow according to current trends, either as a fixed number of vehicles per year or as a fixed percentage of the current total. Diverted traffic can be considered from an economic perspective. It can be assumed that all vehicles which would save either time or money by switching from an existing route to the new pavement would choose to do so. Diverted traffic is normally forecast to grow at the same rate as the traffic on the road from which it has been diverted. The quantity .of planned development traffic can be estimated from the details of policy plans. The quantity of unplanned development traffic, sometimes called generated traffic, will be far more difficult to predict but will be influenced by the availability of land for such development and by experience from previous road projects. Allowance must also be made for the construction traffic which will be associated with both types of development.



Traffic forecasting must differentiate between light, medium and heavy goods vehicles as their growth rates may be differed and their pavement damaging effects are iery different. Whilst most routes will have approximately similar traffic in both directions over a period of time, checks should be made for any directional effects.

9.2.4 Traffic Counts'

The requirement for counts of present traffic will depend on the type of road project being considered and the relative magnitude of the three types of. traffic expected to use it. Forecasting normal and diverted traffic will require knowledge of the flows and vehicle composition on existing roads running parallel to, or in the vicinity of, the proposed road. Obviously, development traffic cannot be counted, but traffic resulting from planned development should be quantifiable if the general details of the planned residential, commercial and industrial projects are known. For most roads it is likely that there will be some relevant traffic data available but this will probably have to be augmented updated by further counts. t . Conventional traffic counts, to justify or to geometrically design a road project, are usually based on manual or automatic methods where all vehicles are combined to produce a single Average Daily Traffic (ADT) figure. The ADT is defined as the total annual traffic summed for both directions and divided by 363. However, for pavement design purposes, i t is essential that classified counts are carried out so that the heavy goods vehicles which cause most of the pavement damage can be clearly quantified. The counting process must yield separate ADT values for each vehicle class. Also, for pavement design it is the traffic in one direction or individual lane, rather than the two-way flow, which is of interest.

Manual classified counts should be carried out using the Qatar standard 16 classes indicated in Table 9.1. In order to ensure that the ADT and composition percentages are representative of the yearly traffic, the following method is suggested:

Traffic counts are performed on six consecutive working days (excluding Fridays), for both travel directions

Times of abnormal traffic activity should be avoided such as public holidays, etc.

During the six days at least two counts should be for a full 24 hours. Ttie count totals for the other days should

January 1997

SECTION 9

be factored up to obtain 24 hour totals : 4

Tqe'.average of the six 24 hour counts (total or vehicle class) in each direction should be considered to be the one- way Average Daily Traffic (total or vehicle class)

On important road schemes, the six- day counts should be repeated several times throughout the year to ensure accuracy in the ADT values.

9.2.5 Standard Axles

For pavement design purposes the damaging effect of vehicle axles is expressed in terms of a "standard axlen. This was originally defined as one carrying 18,000 Ib (8,160 kg), in the AASHTO road trial in the USA in 1956-8 (Croney and Croney, 1991). Subsequently this load has been rationalised in SI units to 80 kN (equivalent to 8,157 kg). In order to determine the cumulative axle loads over the design life of the pavement, it is necessary to convert the

each ,class of heavy vehicles that -r'o"<d;,jo an equivalent number of %-axles. Axle loads are related to

the standard axle using the following relationship :

standard Axles = ( Axle Load(kg) 8157

)

For example, axle loads of 5, 8, 10 and 13 tonnes ar. equivalent to 0.14, 1.00, 2.26 and 6.45 stan&d . axles, . respectively.

11

12

Table 9.1 - Qatar Standard Vehicle Classes For each vehicle class, a representative number

14 1 Trailer

15 Tredlef

Page 914

kctic. Lorry &

&tic. ~orry

Note: Relu elso la T.bh 6.1

3

4

4

5

+2+22

+22+22

1 +22+2

1 +22+22

2.0 - 7.0

2.0 - 10.0

1.5 - 7.0

2.0 - 7.0


need to be weighed and the average number of standard axles for that class determined. This is then 'applied to all the vehicles of that class for the design period. The values can vary considerably depending on the proportions of the various vehicle classes and the degree of loading. On some routes, the loading is very directional, eg the approach to a quarry may have slmilar vehicle flows in both directions, but empty lorries in one and fully laden in the other, hence axle load surveys are essential.

At present, Qatar has no legal limits on either axle or gross vehicle weights. A considerable amount of overloading, relative to the designed vehicle weights occurs. Local surveys have found extreme cases of vehicles being loaded to nearly twice their designed gross vehicle weights. Overloading causes a big increase in wear to the pavement. In the case of a 5-axle articulated truck, this can increase from about 4 equivalent standard axles, for the designed weight limit, to 160 for the overloaded case. Obviously, not all vehicles will be overloaded to this degree, but the average number of equivalent standard axles per vehicle for each traffic class will generally be higher than in places where legal limits, related to the vehicle design, are imposed and enforced. An indication of the likely range of average values for Qatar in each of the classes is shown in Table 9.1. (Classes 1 and 2, consisting of cars, 4-wheel drive vehicles, light pick-ups and taxis cause negligible pavement damage and have been omitted.) The wide ranges are due to the varying proportions of loaded, part loaded and empty vehicles and the extent of overloading. The mix will vary with vehicle class and route.

Axle load surveys, using portable weighbridges, should be carried out to determine the axle load distribution of a sample of the heavy vehicles in the vicinity of the road. Data collected from these surveys can then be used to calculate the mean number of standard axles for a typical vehicle in each class. These values can then be used In conjunction with traffic forecast to determine the predicted cumulative standard axles that the road will carry during its design life. Alternatively, there may be data available from the CED, who should be consulted on the need for specific load surveys.

Axle loads can also be measured and counted by weigh in motion (WIM) systems. These involve the embedment of load sensitive strips or pads, flush with the road surface, across the wheel path. These systems are very attractive because axle loads are measured while vehicles travel at normal speeds. However, WIM systems require careful, regular calibration and the measurements are affected by the

speed of the vehicles, the transverse position of the vehicle wheel and the smoothness of the road surface. In UK, trials of WIM systems have shown substantial unexplained variations in average vehicle loads between sites with similar traffic. Moderate errors in weight measurement will be converted to much larger errors in the equivalent standard axle values. If WIM systems are used, it is strongly recommended that check weighing of a sample of the heavy vehicles be carried out using conventional weighbridges, either permanent or portable types. This is in addition to the calibration already mentioned.

9.2.6 Determlnation of Cumulative Standard Axles

In order to determine the cumulative "standard axles' over the design life of the road, the following procedure should be followed;

L

1. ~ e t e h i n e the daily traffic flow for each '

dass of vehicle weighed using the results of the traffic survey

2. Determine the average daily one- directional traffic flow for each class of vehicle

3. Make a forecast of the one-directional traffic flow for each class of vehicle to determine the total traffic in each class that will travel over each lane during the design life

4. Determine the mean equivalence factor for each class of vehicle and for each direction from the results of the axle load survey

5. The products of the cumulative one- directional traffic flows for each class of vehicle over the design life of the road and the mean equivalence factor for that class should then be calculated and added together to give the cumulative "standard axle" loading for each direction. The higher of the two directional values should then be used for design.

For dual carriageways it should be assumed that the slow lane will carry all the heavy vehicles unless local experience indicates othewise or the one-way ADT traffic flow exceeds 13000 vehicles per day. In the latter

. case 90% of the heavy traffic should be assumed to travel on the slow lane. AH lanes of the carriageway should be designed for the slow lane traffic. Each carriageway can be designed for a different number of standard axles.

QATAR HIGHWAY DESIGN MANUAL , SECTION 9

,

However, the differences would have to be at least 50% before pavement thicknesses were altered significantly. In pract!ce, the largest number of standard axles in eitheruslow lane would determine the design for all lanes.

9.2.7 Design Traff lc Classes

Accurate calculations of cumulative traffic are difficult to make due to inaccuracies in the traffic forecasts and average numbers of standard axles for each vehicle type. Consequently the pavement designs are provided as a set of discrete thicknesses for defined ranges of traffic rather than as a graph of thickness versus cumulative standard axles. Each range of cumulative axles is termed a class and these are summarised in Table 9.2, expressed in millions of standard axles (msa). For comparison, the pavement classes used in the previous design manual are also shown. When the forecast number of axles is considered fairly reliable, and is within 10% of one of tCle class boundaries, it is acceptable to use a design based on the average of the adjacent classes.

Table 9.2 - Design Traffic Classe! .zL,, - - e . 9.3 PAVEMENT MATERI~L~S~ I . .

9.3.1 Qatar Construction Speciflcatlon (QCS)

The full details of the materials to be used in pavement construction and the subgrade are given in the QCS together with the applicable test . methods, based mainly on British Standards. Brief descriptions of these materials are given below.

9.3.2 Subgrade

Qatar generally has high strength natural soils consisting of weathered limestone or sands. Historically, it has been possiblg to construct earthworks, or at least the upper. la~r$~ using material with a minimum soaked ~alifomia Bearing Ratio (CBR) of 25% and the previous pavement designs were based solely on this strength. However it is becoming impractical or expensive to always provide this standard. In some locations, such as cuttings, a significantly higher strength of in situ subgrade is possible.

Accordingly, the present pavement designs include three classes of subgrade defined by CBR: ; +;7

S1: 2 15% and < 25% S2: 2 25% and < 50% S3: 2 50%

The CBR values are measured using the BS 1377 method; on soaked subgrade samples statically compacted to 95% of the maximum dry density (MDD), determined using the BS 1377 4.5 kg rammer method. There are also grading and Atterberg Limit requirements, detailed in the QCS. The in situ subgrade must also be compacted to the same relative compaction, namely 95% of MDD (4.5 kg rammer).

The specified subgrade strengths must be sustained for a depth of at least 300mm and the material below this must have a CBR, at the in situ.density, of at least 10%. This can be easily confirmed using a simple hand operated Dynamic Cone Penetrometer (Kleyn and Savage, 1982), rather than the much more l a b o U J i ~ ~ ~ i ~ d : o f recompacting laboratory sainpre f saame'density.

Where the above conditions are not fulfilled, either some of the subgrade material must be replaced with higher quality material, or the amount of cover (fill height) increased. The necessary replacement or cover thickness can be determined on the basis of providing the same $iffness at formation level (top of the &... . earthwprk)' as. !or the standard CBR 25% subgrid=. b a i l s for this procedure aie given in the Annex to this section. The proposals for these non-standard subgrade situations must be discussed with the CED.

9.3.3 Granular Material for Sub-base and Roadbase

The same material is used for both layers and may consist of either crushed stone or gravel, or natural gravel, or a mixture of these. There are requirements for aggregate hardness, durability, cleanliness, grading, shape and strength, given in the QCS. The principal requirement is for the material to achieve a CBR value of not less than 60% when compacted to 100% of the maximum dry density (MDD) determined using the BS 1377 4.5kg rammer method. This material is used as subLbase for all pavements, except the concrete slab designs, in thicknesses ranging from 100 to 200mm. depending on subgrade. strength. The in situ sub-base must be compacted to the same density as the CBR test. namely 100% of MDD (4.5 kg rammer).

January 1997 - --

Page 916

SECTION 9

9.3.4 Roadbase - Asphalt Concrete

The standard form of pavement construction uses a type MD1 asphalt concrete roadbase between 100 and 230mm thick depending on traffic loading. This material must comply with a given grading envelope (maximum particle size 37.5mm) and will be proportioned using the Marshall Design method to meet the following criteria:

Minimum Stability: 8 kN Maximum Flow: 4 mm Air Voids: 3 t o 6 % Voids Filled with Bitumen: 60 to 75 %.

'The required grade of bitumen is 60170 Pen with a binder content typically between 3.2 and 5.0%. ' 'The QCS specifies additional requirements for particle shape, soundness, particle strength, water absorption and abrasion resistance.

The criteria for compaction on th'e road will result in average voids from 5 to 6 % in the laid material before trafficking.

9.3.5 Cement Bound Material

This is used as sub-base in the concrete slab pavements and as roadbase in flexible composite pavements. A fairly wide grading envelope is specified for the material which may consist of, any or all of, sand, gravel or crushed rock. This is mixed with cement either in-place or in an off-road mixer. A modest cube strength of 7.5 N/mm2 at 7 days; is specified.

This material has not previously been much used in Qatar, but is now included for use as sub-base for concrete slab pavements and it may also provide a cheaper roadbase. Limits on grading, cleanliness and durability are given in QCS. For both sub-base or roadbase use, this material must be compacted to 95% of MDD (4.5-kg rammer).

9.3.6 Wearing Course

A standard surfacing of MD4 asphalt concrete, laid as a 40mm course, is used on all flexible and flexible-composite designs. The nominal maximum aggregate size is 14mm and the mix proportions are determined in a similar manner to the asphalt concrete road base, but with the following difference. Imported gabbro aggregate must be used for the coarse fraction, to provide adequate skid resistance and resistance to polishing. This last requirement raises the cost of the material considerably, and justifies the thickness of only 4Omm.

The required grade of bitumen is SOU0 Pen with a binder content typically between 4.0 and 5.0 %. Compaction requirements are the same as for asphalt concrete roadbase and the laid material should have voids of about 5 to 6 % before trafficking.

9.3.7 Concrete for Rigld Pavements

Rigid construction is included for use in local areas with a high risk of'rutting. It may be adopted more widely in the future. Concrete slab pavements require high quality concrete, sometimes termed pavement quality concrete (PQC), with a 28 day cube strength of 40N/mm2. High quality mix constituents, good quality control and thorough curing are necessary to ensure that the required standard is achieved.

In order to reduce the risk of cracking due to imperfect curing or joint construction, reinforced jointed slab construction has been adopted. Longitudinal reinforcement to BS 4483 is required at the rate of 600mm2/m width. The reinforcement also reduces the slab thickness compared to an un-reinforced slab and reduces the number of transverse joints. 'The reinforcement is placed with 50 to 60mm of cover below the slab surface and maintaining a minimum cover of 30mm below any longitudinal joint sealing groove. Longitudinal joints must be provided to limit slab widths to less than 5.0m for limestone aggregate. Most other aggregates with higher coefficients of expansion must be limited to 4.0m. Transverse expansion and contraction joints must be installed alternately at 15m intervals and proper transitions provided between sections of concrete and asphalt construction. Details of these features, defied from the UK Highway Construction Details (DOT, 1991), are provided in the QCS.

9.3.8 Precast Pavlng Blocks

These are manufactured from Portland Cement concrete i n two thicknesses, 60 and 80mm. The thickness to be used depends on the level of traffic. The average compressive strength must be not less than 40 N/mm2 and individual blocks not less than 35 N/mm2. Other requirements, including preferred shapes and dimensional tolerances are given in QCS.

The paving blocks are laid on a compacted course of sand, normally in simple herring-bone bond. The laying course sand may be either natural sand or crushed rock fines, complying with the grading envelope in Table 9.3. The sand is laid so that after compaction it forms a layer 30mm thick. After placement, the blocks are compacted using a vibrating plate compactor and finally, sand is vibrated into the joints.

Page 9l7


Table 9.3 - Sand Gradings for Block Paving

Full details of the laying procedure are given in the QCS, based on BS 6717, Part 3.

9.4 DESIGN CHARTS

9.4.1 General I

The designs for the various types of construction are presented as a series of charts, Figures 9.2 to 9.6. Knowing the subgrade class (refer Clause 9.3.2) and the traffic class (refer Clauses 9.2.6 and 9.2.7) the thicknesses of the layers can be easily read for each pavement type. Not all types of pavement are considered appropriate for every traffic class.

Pavement construction should be constant across all running lanes as the savings to be made by reducing the roadbase thickness are not great. In rural situations, where the hard shoulderledge strip is not expected to have heavy usage, its pavement thickness may be reduced. In urban areas, where parking is expected, a reduction of the pavement construction for the hard shoulder is not recommended.

The design requirements for staged construction is dealt with in Clause 9.5.

9.4.2 Asphalt Concrete Roadbase (Figure 9.2)

This type of construction will suit all classes of traffic and is similar to past pavement practice in Qatar. The basis of these designs are discussed in the Annex to this section.

9.4.3- Asphalt and Granular Roadbase (Figure 9.3)

This type of construction is restricted to roads expected to carry no more than 5 million standard axles and with only a small propottion of heavily loaded vehicles.

January 1997

SECTION 9

These pavements do not satisfy conventional analytical strain criteria but have performed satisfactorily in other areas of high temperatures. The designs in Figure 9.3 are based on those in Figure 9.2, but with some of the asphalt concrete thickness replaced by twice this thickness of granular roadbase. This is in accordance with the structural number concept of the AASHTO design method (1 993) in which the reduction in thickness of one layer is compensated by increasing another, in proportion to the material coefficients. In this instance the granular layer (CBR 60%) has a coefficient of 0.1 3 whilst 'the asphalt (stiffness 1.0 GPa) has one of 0.26.

9.4.4 Flexi ble-Composite Roadbase (Figure 9.4)

This type of pavement has not previously been used to any great extent in Qatar although it is very common and successful in some countries. The cement bound layer will crack transversely soon after construction through a combination of drying shrinkage and thermal gradient warping. The successful performance of this type of pavement depends on the shrinkage of the cement bound roadbase being small and the asphalt roadbase being tolerant of the cracked roadbase. The low strength of 7.5 Nlmm2 and the use of limestone, with a low coefficient of thermal expansion, should result in narrow roadbase cracks. 'The high temperatures are likely to assist the asphalt surfacing in resisting the development of reflection cracks.

Thick asphalt surfacing will reduce the development of cracking by insulating the cement bound layer and reducing the temperature gradient and warping stresses.

The material thicknesses shown in Figure 9.4 are based on UK practice, However, it is probable that the asphalt surfacing thickness couM be reduced in future designs, after some experience of satisfactory performance is obtained.

9.4.5 Reinforced Jointed Concrete Slabs (Figure 9.5)

Rutting of conventional asphalt pavements at the approaches to junctions or at roundabouts is a significant problem in Qatar. It results from the high ambient temperatures, inherent properties of the asphalt concrete and high axle loads. Although it may be possible to reduce deformation by mix re-design, or by the use of bitumen modifiers, there will be uncertainty over performance and the increased stiffness may could cause other problems in later life.

Page 918


Concrete slab pavements at these problem locations will provide guaranteed, rut-free performance. Concrete pavements require considerable attention to mix quality, placement, joints and curing to be successful. Jointed reinforced concrete slab construction has been selected in preference to un-reinforced slabs as the reinforcement will provide more tolerance to any workmanship deficiencies and will also reduce the slab thickness and number of joints.

The designs shown are based on UK practice (DOT, 1994) which is based on the work of . Mayhew and Harding (1 987). The concrete slab (40 N/mm2, 28 day cube strength) rests on a cement bound sub-base (7.5 N/mm2, 7 day minimum cube strength). This is to ensure that there is a robust surface on which to erect side forms and joint-assemblies, and that pumping of sub-base or subgrade fines through joints is minimised; Joint details and reinforcement around openings shall be as shown in the UK Highway Construction Details, Series C (DOT, 1993) or as specified by the CED. '

The UK un-reinforced slab designs agree closely with USA practice (Portland Cement Association, 1984). It has not been possible to directly verify the reinforced slab designs as the Portland Cement Association manual does not cover this type.

If properly constructed, concrete pavements should last longer than asphalt pavements and 5s cheaper to maintain because they should not require resurfacing or re-texturing for at least 30 years. However the joints will probably require periodic resealing at 15 year intervals.

9.4.6 Precast Block Paving (Figure 9.6)

Block paving may be used for the construction of car parks or parking bays, median strips and verges, laybys and access roads. Selection of the appropriate design will be on the basis of both total traffic and the incidence of heavy vehicles. Granular roadbase has been selected as this will be a more practical material than asphalt for working in small areas, which will often be the case with this type of roadwork. In addition, any fuel or oil spillage will not affect the structural layers.

The designs are based on German practice (Roads and Traffic Research Association, 1986) and are only suitable for the stated levels and types of traffic. Where block paving is required for locations with substantial numbers of heavy vehicles, such as ports or industrial areas, other designs such as those of the British Ports Association (1994) should be used. The proposed designs for such situations should be discussed with CED.


QATAR HIGHWAY DESIGN MANUAL - .' J' " SECTION 9

Figure 9.2 Asphalt Concrete Roadbase Designs


. . . . w-$#-J$$ QATAR HIGHWAY DESIGN MANUALs-- "". " SECTION 9

Traffic Classes TI T2 T3 T4 T5 T6

Standard Axles (millions) . <1 1.2 2-5 5-10 10-20 20-50

greater than 15%

Layer definitions Notes

1. Standard Axles are 80 kN. Wearing Course (Asphalt Concrete MD4)

2. All thicknesses in millimetres. Upper 'Oadbase

3. These designs are only to be used when the

Lower Roadbase (Granular Base Material) proportion of goods vehicles, with equivalent standard axles of 12 or more, does not exceed 5% of all vehicles.

Sub-base (Granular Material)

Subgrade (CBR at 95% of MDD (BS 1377, 4.5Kg rammer, soaked))

Figure 9.3 Asphalt and Granular Roadbase Designs

January 1997 ' Page 911 1

1

QATAR HIGHWAY DESIGN MANUAL SECTION 9 i _

Notes

1. Standard Axks a n 80 kN. Wearing Course (Asphalt Concrete MD4)

2. Ail thicknesses in mlilimetres. Upper Roadbase (Asphalt Concrete MDl)

3. The asphalt concrete inhibits the Lower Roadba8e (Cement-bound Material development of reflection cracking. cube strength of 7.5 Nlmm' at 7 days)

4. A low strength Cement-Bound Sub-base (Granular Material) Material has been selected to

minimire reflection cracking. Subgmde (CBR at B5K of MDD

(BS 1377,4.5Kg rammer,soaked)) 5. The coat of thir form of construction Is simliiar to Asphalt Concrete Roadbase, but could vary depending on local circumstances.

~igure-9.4 Flexible-Composite Roadbase Designs

January 1997 Page 9/12 ..

. . p , . ,+-+>t, , * . 1 - t * <

QATAR'HIGHWAY DESIGN MANUAL . I;*- - SECTION 9


1. Standard Axles are 80 kN. Concrata Slab (40 Nlmm' cube strength at28 days wDth 600 mmzlm of longitudinal reinforcement 2. All thicknesses in milllmetres, to 8s 4483)

3. Transverse joint spacing shall be not greater Cement-bound Sub-base than 15m. (7.5 ~ l m m ' cube strength at 7 days)

4. These pavement designs are intended for use Subgrade (CBR at 95% of MDD at junctions or other areas with a high risk

(BS 1377.4.5Kg rammer, soaked)) mtting.

5. The deslgn given for Class T31Sl provides the minimum construction thicknesses to be used.

Figure 9.5 Reinforced, Jointed Concrete Slab I3esigns

Subgrade Class 5 3

CBR, greater than 50%

,


Same as S1

Same as S l

Same as S l

Same . as S1



1. Standard Axles are 80 kN. Precast blocks (60 or 80mrn)

2. All Ullcknesres in mlllirnetres. 30mm sand laying course

Roadbase (Granular Material) 3. TO Traitlo Cksm lnciuda8 resid8nUal roads and

parking areas with minimrl heavy vehicka.

Sub-base (Granular Material) 4. T I Traffic Chs8 Includes laybys, dual carriageway median strips and areas with appmchbb heavy vehicles.

Subgrade (CBR at 95% of MOD (BS 1377,4.5Kg rammer, soaked))

Figure 9.6 Precast Block Paving Designs



9.5 ' SPECIAL PAVEMENT SECTIONS

9.5.1 Staged Construction (Single Layer Construction)

Sometimes it is appropriate not to construct the full pavement thickness at one time for one of the following reasons:

A road may initially be required to carry only limited traffic. After the completion of related development (other roads or industrial or residential projects) traffic volumes will increase

A new road may carry construction traffic in the first few years of its life and thereafter normal traffic. The application of the wearing course could be delayed until after the construction traffic has ceased to avoid rutting of the final surfacing

An anticipated change to kaffic flow patterns may require extensive changes to road markings.

Assessments should be made in each case of the traffic over the whole design life and in the initial period. The sub-base would be designed for the whole life but roadbase and surfacing would be matched to the initial level of traffic. The balance of the asphalt would be added in due course.

However, other factors must also be considered:.

Any ironware in the carriageway will have to be lifted when the overlay is applied

Kerbing; if present, must either -be installed high or also raised when the overlay is applied

Depending on the status of the road, the initial top course of asphalt may have to be a conventional wearing course containing gabbro aggregate. This will involve extra cost to provide two, rather than one, asphalt courses with superior aggregate

Sufficient overbridge headroom must be provided to allow for the oueriay thickness

The approaches to underbridges should be constructed to full thickness to avoid either overlaying the structure or full depth reconstruction of the approaches. .

Pavements for temporary roads can often be constructed to lower standards than conventional pavements because performance expectations will be lower (deeper ruts or more cracking will be tolerable). However, the following should also be considered:

Design period may be very unpredictable

Design traffic may also be very ' unpredictable

Savings may not be very substantial.

9.6 PAVEMENT EVALUATION

9.6.1 lntroduction

As the road network reaches maturity, there will be fewer new roads to design but more existing pavements to rehabilitate. Increasingly pavement engineers will be required to evaluate existing pavements and devise appropriate resurfacing or strengthening measures. Pavements deteriorate in different ways and at different rates depending on traffic, pavement thickness, material quality, drainage etc. The visible deterioration does not always give a reliable indication of the underlying cause(s) and some investigation is needed. A four- stage, highway pavement evaluation procedure is outlined below:

1. Routine Monitoring

2. Detailed Survey

3. Detailed Investigation (planning, Execution and Interpretation)

4. Interpretation and Remedial Works Design.

9.6.2 Routine Monitoring

The objective of routine monitoring is to id en ti^ those parts of the road network which are showing signs of surface or structural deterioration and require further investigation and possible maintenance. Routine monitoring of most of the network should be carried out at intervals of 2 to 4 years, depending on the age, condition and importance of the road, and the traffic usage. The monitoring will be by visual surveys with written records of the condition supplemented by photographs or video tapes. Some indicative rut measurements should be made. In rural areas the survey will normally be carried out during a slow (20 kph) drive-through, with occasional examinations on foot at junctions, structures or any locations with serious defects. , In urban areas the surveys will



be carried out mainly on foot, from the verges or footways. The results of all Routine Monitoring should be stored in the Pavement Management System (PMS) being implemented by the CED in 1996.

9.6.3 Detailed Survey

Where any significant pavement deterioration is discovered, a Detailed Survey should -be carried out over the affected length and adjacent area. The objectives of this survey are to obtain a good description of the deterioration (type, degree and extent) and an indication of the likely causes. The survey will consist of a more detailed visual survey carried out on foot, including rut measurements. Non- destructive testing of these pavement lengths, using either Benkelman Beams or a Falling Weight Deflectometer (FWD), may also be useful at this stage. Deflections can be used to check if there is any change in pavement stiffness between a sound and deteriorated section and should assist in deciding whether the deterioration is confined to surface layers or affects the whole pavement structure. The FWD will give more detailed structural information as it measures the deflections bowl of the pavement in response to a dynamic load. Using appropriate software, it is possible to backcalculate the stiffnesses of the pavement layers, provided that the thicknesses of these are known. In order to produce consistent measurements and layer stiffnesses, the recommendations given in the FEHRL (1996) publication should be followed. The stiffness of asphalt layers are strongly influenced by temperature and the results of all deflection measurement must be corrected to a standard temperature. To do this, temperatures in the asphalt layers must be measured at the time of test.

Where the deterioration is considered serious or is worsening, strengthening or resurfacing work will be necessary. However, a Detailed Investigation will be required to provide further

* information to decide precisely what work is necessary. If the pavement condition is not too serious, it may be appropriate to merely repeat the Detailed Survey after, say, one year.

9.6.4 Detailed lnvestlgation

The objective of the Detailed Investigation is to explain the pavement deterioration, including the identification of the layer(s) responsible for the deterioration and thus provide information to enable any strengthening to be economically designed. It will normally involve coring and test-pitting of selected areas of the pavement

, together with in situ and laboratory testing of the pavement layers. If deflection testing has

January 1997

SECTION 9

not already been carried out at the Detailed Survey stage, it should now be carried out.

'The investigation must be properly planned and effort concentrated at locations to produce data which will be relevant to explaining the deterioration. Before planning the investigation, as much background information as possible, applicable to the length of interest, should be assembled:

Original construction details, including specifications

Local subgrade and drainage conditions

Maintenance history

The results of any previous pavement surveys or investigations

Past and current traffic flows and composition.

Some or all of this information should be available from the CED Pavement Management System. If there are major omissions in this information, then the Detailed Investigation may need to be expanded to include traffic counts and additional cores or test pits. Where thicknesses are unknown, ground penetrating radar may be of assistance but this technique needs careful calibration against known thicknesses for each type of pavement being surveyed.

The standard investigation strategy is to compare deteriorated and sound sections of pavement (20 to lOOm in length) carrying similar traffic and of similar construction (the selection of such sections, itself, can sometimes indicate a possible cause of deterioration).. Appropriately sited cores andlor test pits should reveal any differences in material qualities or thicknesses which may explain the different performance. Depending on the variation of traffic and construction within the length of interest, a number of pairs of comparison sections may be necessary. Where available, deflection and FWD data may be used to select pairs of sections with high and low deflections. However, adequate explanations for the different stiffnesses are not always found. The majority of cores or .pits should be in -the deteriorated sections, sited right on the deterioration (cracks, ruts etc) to determine exactly which layers are affected. In the case of cracking, it is important to know the depth of crack propagation and for rutting, whether or not this is present in both the asphalt and underlying granular layers. To determine which layers are contributing to a rut, or other

Page 9/16


deformation, will require a set of three or more cores, straddling the rut.

'The following points should be considered when planning and executing the investigation:

The cores and test pits are fundamental to the whole investigation and should all be carefully examined and logged by a competent materials or pavementengineer. The core log sheets should include a photograph with a scale, full details of asphalt thickness and condition, including texture, segregation, voids, layer bonding, width and depth of cracks, stripping, soft or otherwise deleterious aggregate, bleeding and any other peculiarities

Granular layers (sub-base and subgrade) can be rapidly and cheaply assessed by in situ testing using either a hand operated Dynamic Cone Penetrometer (DCP) (Kleyn and Savage, 1982)(Jones and Rolt, 1991) or a portable dynamic plate bearing tester (PDPBT) (Roads and Traffic Research Association, 1992). The DCP test can be carried out through a core hole but the PDPBT will require a test pit to expose an area of 0.5 by 0.5m.

Static plate bearing tests or in situ California Bearing Ratio tests could

- also be carried out in place of the DCP or PDPBT but are slower, more costly and technically no better than these hand methods

Decisions on the number and type of laboratory tests should be made after the assessment of the field data. Samples of suspect foundation material should be obtained during the excavation of the test pits, but not necessarily tested. Decisions on what laboratory tests should be carried out would be made after the field data has been reviewed

Density testing of sub-base or subgrade layers will be helpful where the strength of these layers is unexpectedly low and low compaction is suspected to be the cause. A maximum dry density value (determined in the laboratory) will also be necessary to determine relative compaction.

9.6.5 lnterpretatlon and Design of Rernedlal Works

The interpretation of the data from the investigation must address the following issues:

What is the nature, extent and degree of the deterioration?

Is only the surfacing or the whole pavement affected?

What has caused it?

What remedial treatment is needed?

Provided that the Detailed Survey has been thorough and the Detailed Investigation has been properly planned, the first two issues should be answered by a proper presentation o f . the survey/investigation data.

Answers on the causes could be very obvious such as an under-designed pavement, poor quality asphalt containing segregated aggregate and voids, or soft and friable sub-base. In other cases the causes may be more subtle requiring detailed laboratory testing to identify. In practice, interpretation should commence with the completion of the Detailed Survey and continue during the planning and execution of the Detailed Investigation to ensure that relevant and sufficient data is obtained to answer the main questions. Successful interpretation leading to robust conclusions depends strongly on having carried out the right field work, sampling and testing in the first place.

In addition to evaluating the existing pavement, the future design traffic must be estimated before deciding what thickness of overlay will be required. The methods described in Clause 9.2 for new roads are appropriate. Normally, pavement strengthening should be designed for a 20 year life, subject to the comments made in Clause 9.2.2. .

A possible method of determining overlay thicknesses is to compare the existing pavement thickness with that required to carry the total of past and future traffic (AASHTO, 1993). The overlay will provide the difference in thickness between the existing pavement. Allowance should be made for any difference in quality of existing material and the current QCS, due either to deterioration or a lower original specification. Defective or deteriorated wearing course should be replaced before overlaying. Roadbases with moderate deficiencies could be retained but with a reduced allowance of thickness.

January 1997 Paae 9/17


In cases where reconstruction is proposed, this should be designed in accordance with the requirements of the rest of this section. Granular sub-base is not subject to fatigue and, provided that it complies with the current specification, is unlikely to require replacement.

9.7 REFERENCES

AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS (1 993). AASHTO Guide for design of pavement structures. Washington, DC.

BRITISH PORTS ASSOCIATION (1 994). The structural design of heavy duty pavements for ports and other industries, 2nd edition. London.

.. . CRONEY D Qnd P CRONEY (1991). The design and performance of road pavements, 2nd edition. McGraw Hill International, Maidenhead, UK.

DEPARTMENT OF TRANSPORT (1 991). Manual of contract documents for highway works, Volume 3, Highway construction details, HMSO, London.

DEPARTMENT OF TRANSPORT (1 994). Design manual for roads and bridges, Volume 7, Pavement design and maintenance, HMSO, London.

FEHRL - FORUM OF EUROPEAN NATIONAL HIGHWAY RESEARCH LABORATORIES (1 996). Harmonisation of the use of the falling weight deflectometer on pavements, Part 1. FEHRL Report No. 1996/1. Crowthome: Transport and Road Research Laboratory.

JONES CR and J ROLT (1991). Operating instrirctions for the TRL dynamic cone penetrometer (2nd edition). TRL Overseas Centre Information Note. Crowthome: Transport Research Laboratory.

KLEYN, EG and PF SAVAGE (1982). The application of the pavement DCP to determine the bearing properties and performance of road pavements. Proceedings of the International Symposium on Bearing Capacity of Roads and Alfields. Trondheim.

MAYHEW, HC and HM HARDING (1987). Thickness design of concrete roads. Research reporf 87. Crowthome: Transport and Road Research Laboratory.

MINISTRY OF COMMUNICATIONS of the KINGDOM of SAClDlA ARABIA (1990). Highway Design Manual. Riyadh.

PORTLAND CEMENT ASSOCIATION (1 984). Thickness design for concrete highway and street pavements. Skokie, Illinois, USA.

ROAD RESEARCH LABORATORY (1 970). A guide to the structural design of pavements for new roads. Road Note 29. HMSO, London.

ROADS AND TRAFFIC RESEARCH ASSOCIATION (1 992). Technical test specification for soil and rock in road building, Part B 8.3, Dynamic plate-load test using the light falling-weight device. (In German.) Cologne.

TRANSPORT and ROAD RESEARCH LABORATORY (1990). A users manual for a program to analyse dynamic cone penetrometer data. TRRL Overseas Road Note 8. Crowthorne: Transport and Road Research Laboratory.

January 1997 Paae 911 8


)" ANNEX9A BASIS OF THE DESIGN

METHOD FOR ASPHALT ROADBASE

9A.1 DESIGN METHODS

Analytical and empirical methods can both be used to determine the thicknesses of pavement layers to carry a specified amount of traffic. In the first, the materials to be used in the pavement are characterised by their stiffnesses- and fatigue laws, ie. the relationship between strain and the number of load cycles to produce failure. The pavement is then proportioned so that strains at critical depths, due to standard wheel loads, do not exceed permissible values for the required number of load repetitions (the horizontal strain at the base of the roadbase and the vertical strain at the top of the subgrade are normally considered to be the critical criteria). The design documents produced from analytical methods may consist of either a detailed calculation procedure or an easily read "catalogue" of diagrams or graphs relating layer thicknesses to traffic and layer properties.

Although the analytical method is technically attractive, there are considerable practical difficulties:

Determining stiffness values ' is complicated. Asphalt stiffness varies with temperature, rate of loading and age of the bitumen. For unbound materials, the stiffness varies with moisture, stress history and confining stress

There is no standardisation of fatigue measurement and a wide variety of tests are in use, hardly any of which are compatible (Tangella et al, 1990). Consequently, each analytical design method has its own load cycles/$triin relationship based on a specific fatigue test method

The field evidence of fatigue failure, in the manner assumed in the analytical method, is not conclusive.

In the empirical method, the performance of trial pavements is monitored to determine the amount of traffic which can be carried before the condition is considered unacceptable. Sometimes the traffic is accelerated by continually trafficking by heavy vehicles, as in the AASHTO Road Trial, or occurs normally, as in the trials carried out in the UK on public roads. The latter method is the more reliable, however, the trial results are only strictly applicable to the trial conditions. Analytical

SECTION 9

methods are sometimes used to extend the empirical results to wider ranges of traffic or layer thicknesses, of to slightly different pavement types.

The design documents produced from empirical performance studies are usually in "catalogue" format with the exception of the AASHTO method in which traffic, pavement thickness and material quality. are related by an empirical equation.

In practice, design by either method is often checked to some degree by the other.

9A.2 DESIGN STRATEGY

Conditions in Qatar differ from the temperate environments, where both the analytical and empirical methods have been most practised, and need to be reflected in any design for local use:

Qatar has a much hotter climate which will greatly affect the stiffness of any asphalt and will affect bitumen ageing

Subgrade strengths are generally high due to the prevalent limestone and sand, and many roads are constructed on low embankments of good fill material

A significant proportion of heavy vehicles are overloaded causing significantly more damage than the same types of vehicle elsewhere.

The material standards in Qatar are simijar to mainstream practice elsewhere.

9A.3 APPLICABLE METHODS

The first stage in determining asphalt roabase . pavement designs for Qatar was to review

established methods or "catalogues" which could apply to the hot conditions, either because they were empirically derived. from the performance of pavements in a hot climate, or allow the input of low stiffness values. The methods reviewed are listed in Table 9A.1.

The methods all quantify cumulative traffic on the basis of equivalent 80kN (or 8 tonne) standard axles using a 4th power law. The first three methods do not require specific temperature or asphalt stiffness input but the last three do.



Table 9A.1 - Design methods

The Shell method, No. 5, gives temperature data for Bahrain which is applicable to Qatar. The weighted mean monthly air temperature is given as 28°C which results in an effective pavement temperature of 40°C for a 200mm thickness of asphalt. Two methods of determining asphalt stiffnesses at this high effective pavement temperature have been considered:

The standard stiffness nomographs (Van der Poel, 1954 and Bonnaure et all 1977) indicate stiff nesses between 1 and 3 GPa for MD1 asphalt roadbase

Back calculation of falling weight deflectometer data from Qatar indicates an average roadbase stiffness of over 3 GPa for mature asphalt. Similar analysis of data from 18 month old Malaysian pavements indicates asphalt roadbase stiffness of 1.5 to 3.5 GPa at 40°C.

These are appreciably higher than the values of only 0.3 to 0.6 GPa indicated in the Shell method, which are considered to be too low. Partly as a consequence of these low stiffnesses, the Shell method indicates much greater pavement thicknesses compared to all the other methods except for Saudia Arabia. The satisfactory performance of roads in hot environments with much thinner asphalt roadbases than the Shell designs suggests that the method is conservative for these conditions.

An asphalt roadbase stiffness of 1.0 GPa has been used in the AASHTO and Austroads methods. 'The South African, AASHTO and TRL Overseas methods all indicated very similar thicknesses of 11 0 to 270mm of asphalt for 1 to 50 million standard axles. All the others, in varying degrees, we& thicker:

9A.4 SPECIFIC METHOD FOR QATAR

The second stage in determining asphalt roadbase thicknesses was to set these slightly greater than the 110 to 270mm values and then adjust to ensure that the roadbase and subgrade strains did not exceed permissible values. The fatigue laws from the Austroads manual were used for this because:

The manual reflects more recent experience

Many Australian pavements are built in a fairly hot climate

The Austroads manual produces designs for hot climates which accord

.' reasonably well with performance. 1 -

The fatigue laws are:

Transverse strain at the bottom of the roadbase

PC,, = (6532)/(p) for asphalt with a stiffness. of 1.0 GPa, and

.. .

Vertical strain at the top of the subgrade

( p~ = microstrain and N = number of load repetitions. )

The layer stiffnesses and Poisson's ratios used to determine the strains are shown in Table 9A.2.

Table 9A.2

(5% voids, 4% of 60170 Pen bitumen at a temperature of 40°C)

(1 GPa = 1 Gigapascal= 1x109 N/d and 1 MPa = 1 Megapascal = 1x1 d N/H)

The standard approximate relationship for subgrade stiffness, E (MPa) = 10 x CBR (%), only applies to low strength material. The above



values are based on those determined either from plate bearing tests or back analysis of falling weight deflectometer data.

In all cases, asphalt fatigue was found to'be the critical criterion.

9A.5 WEAK SUBGRADES

In Clause 9.3.2, the minimum subgrade strength included in the design charts was set at a CBR of 15% at in situ density which is generally achievable. However, in the small number of cases where the in situ subgrade strength falls below this, it will be necessary to provide a layer of stronger material called "capping" between the subgrade and the sub-base. The capping will normally be either the 15% or 25% CBR subgrade material used in the standard designs and will have the same stiff nesses as above. In cuts or where the road surface is close to ground level, some of the subgrade will have to be removed and replaced with capping. In fill situations, the upper earthworks layers must be constructed with the capping material. A method of determining the necessary capping thickness for either case may be based on the surface stiffness at formation level, ie immediately below the sub-base.

The minimum strength standard subgrade (Class S1) consists of at least 0.3m thickness of CBR 15% material (or stronger) resting on material with a CBR of at least 10%. A 40kN single wheel load at formation level will produce a surface deflection of 1.49mm. The thickness of the capping layer required for a weaker subgrade will be that which produces the same deflection for the same load. The thickness will be determined by trial and error using an elastic layer programme to model the stiff nesses of the subgrade layers. For the cases of subgrade CBR values of 7%. 5O/0, and 3%, the required thicknesses of CBR 15% capping will be 0.5, 0.9 and 1.9m. For the weaker subgrades of CBR 5% and 3% it will be more effective to use the stronger CBR 25% capping in thicknesses of 0.35 and 0.7m respectively. Other capping thicknesses are possible depending on specific strengths or stiffnesses, but for prabtical reasons the thickness should not be less than 0.2m.

The stiffnesses and Poisson's ratio used in this analysis are shown in Table 9A.3.

Table 9A.3

Capping material of greater strength may be used. However, In determining the thickness, higher stiffness v'alues'should be used with great caution as the in situ stiffness is dependent not just on the quality of the capping but also on the stiffness of the underlying material.

9A.6 REFERENCES

AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS (1 993). AASHTO Guide for design of pavement structures. Washington, DC.

AUSTROADS (1992). Pavement design - a guide to the structural design of road pavements. Sydney, Australia.

BONNAURE F, G GEST, G GRAVOIS and P UGE (1977). A new method of predicting the stiffness of asphalt' paving mixtures. Proceedings of the Association of Asphalt Paving Technologists, Vol. 46.

COUNCIL FOR SCIENTIFIC AND INDUSTRIAL RESEARCH (CSIR) (1 985). Structural design of interurban and rural road pavements. Technical recommendations for highways (TRH 4). Pretoria, South Africa.

SHELL INTERNATIONAL PETROLEUM CO. (1978). Shell Pavement Design Manual, London.

TANGELLA SCSR, J CRAUS, JA DEACON and CL MONlSMlTH (1990). Summary report on fatigue response of asphalt mixtures. Strategic Highway Research Program, Report SHRP- AIIR-90-011. National Research Council, Washington, DC, USA.

TRANSPORT and ROAD RESEARCH LABORATORY (1 993). A guide to the s t ~ t u r a l

. design of bitumen-surfaced roads in tropical and sub-tropical countries. Overseas Road Note 31, fourth edition. Crowthome: Transport and Road Research Laboratory.

VAN DER POEL C (1954). A general system describing the visco-elastic properties of bitumen and its relation to routine test data. Journal of Applied Chemistry,' Vo1.4.



SECTION 10 ROADWAY LIGHTING

10.1.1 Reasons for Lighting

Highway lighting is provided to aid the safe and orderly movement after dark of all road users, both vehicular and pedestrian.

For the driver, properly designed lighting will increase his range of vision, reveal hazards more effectively, reduce fatigue and particularly in the case of high-speed roads, increase the traffic carrying capacity. Pedestrians will be able to orientate themselves and to detect vehicular and other hazards. From the police point of view, crime directed against the person and property . will be discouraged, whilst sunreillance and recognition will be greatly enhanced, particularly if good colour rendering is provided.

10.1.2 Justification

In considering whether a road should be lit, from an engineering point of view, the following factors should be considered.

The nature of the road (eg. motorway or mixed traffic road) as determined by its geometry and also by its night traffic accident rate.

The traffic intensity and composition (eg. fast traffic only or mixed traffic).

The danger points and other special situation, such as junctions, crossings for cyclists and pedestrians and other interruptions in driving continuity which may present drivers with unexpected situations.

It is particularly important to avoid sudden changes in the visual field of the drivers as far as determined by the lighting and to allow drivers to prepare themselves well in advance for manoeuvres which suit the situation to be met over the next stretch of road.

Factors such as traffic volume, speed, road use during the night, night accident rate, road geometry, and general night visibility conditions are important when considering highway lighting.

Justification for lighting is also based on the economics of lighting as compared to the cost of not lighting. Economic returns for lighting are

measured in terms of reduction in personal injuries, fatalities, property damage, and other costs to society. More effective usage of the road and the possible increase in its capacity are also considered.

10.1.3 Scope

This section of the Manual sets out the performance requirements and standards which shall be adopted for the design of lighting on all types of highway in Qatar, except for those footpaths which are separated from vehicular routes.

10.1.4 Complementary Standards

This section of the Manual requires the use of BS 5489 : Road Lighting : Parts 1-1 0 : 1992.

10.2 PERFORMANCE REQUIREMENTS

10.2.1 Summary of Road Classifications in Qatar

Individual roads in the State of Qatar each fulfill certain functions within the overall network. A hierarchy exists which defines their various roles and the position of a road within this hierarchy is a measure of its national importance. Route classification is discussed at the front of this manual. Table 10.1 shows the relationship between the classification used in this manual and the classification used by Ministry of Electricity and Water, Street Lighting Section.

Categoly

Ring and Radial Roads I

Class 'A' Motorways or Express Roads (eg Doha-Ruwais Road, Doha Abu Sarnra Road)

I Class 'C' I Camrdal and Shopping 82, TRI. TR2 streets I I

Description

PI, P2

Local Streets. Residential Roads or Access Roads

Refer to Table 1.1 for full descriptions of the Highway Classification.

Highway Class (Refer Table 1)

Table 10.1 Road Classifications in Qatar for Roadway Lighting


QATAR HIGKAlAY DESIGN MANUAL

10.2.2 Lighting Performance Recommendations

Minimum designed performance levels for the various classes of road are given in Table 10.2.

Table 10.2 :. Lighting Requirements for Traffic Routes

For slip roads and shoulders on Class 'A' and Class '6' roads, maintained average luminance values. of 2.0 and 1.25 respectively will be acceptable, but the other parameters should remain unaltered.

10.2.3 . Limitation of Glare and "Light Poiiutlon"

Disability Glare Disability glare, defined and discussed in BS 5489 : Part 1, reduces the contrast between objects and their background, so that their visibility is decreased. An object that is just visible (that is at the threshold of visibility) when there is no disability glare will, in the presence of disability glare, merge into the background. The percentage by which the background luminance has to be increased to render the object just visible again is known as the threshold increment (TI). This provides a notional measure of disability glare from installations.

The value of the TI depends on the light .distribution from the luminaire between 70' and 90' in elevation in the vertical plane at which the luminaire is observed, usually within 10" of azimuth of the transverse axis of the luminaire. It also depends on the road luminance, the layout of the luminaires, the mounting height and the observer position.

Discomfort Glare Control of the TI within the limits recommended in Table 10.2 will generally ensure that discomfort glare, defined in BS 5489 : Part 1, will be adequately controlled.

January 1997

SECTION 10

Light Pollution Another effect of lighting is 'sky glow' which occurs when upward stray light is reflected back to earth. Although some sky glow from major conurbations is unavoidable, special care should be taken when designing road lighting in areas where little exterior lighting exists, to limit the amount of upward or stray light. Such areas should be considered to be environmentally sensitive at night and special light control lanterns specified. As well as hindering astronomers, many people feel that this form of light pollution diminishes the aesthetic properties and value of the dark night scene.

10.3 RECOMMENDED PRACTICE

10.3.1 Decisions Prior to Design

Arrangement and Mounting Height Lantem arrangement and mounting height shall be in accordance with the options set out in BS 5489 : Part 2 within the local geometric, maintenance and environmental constraints that apply. Greater mounting heights shall be considered, particularly for wider carriageways.

Limitation of Glare The performance requirements of Clause 10.2 shall be met by the selection of lanterns as described in BS 5489 : Part 2.

In order to limit the glare factor on roads Class 'A' or 'B' where the surrounds are dark, lanterns must be flat glass type with a distinct cut-off limiting the visual aspect of the light source to an angle of 30" from the horizontal. Lanterns will be mounted at a minimum height of 12 metres from the road surface.

For Class 'C' roads where the surrounds are mostly bright, the mounting height will be a minimum 10 metres and either cut-off lanterns or lanterns having a prismatic controller will be permitted.

Class '0' roads where the decorative and aesthetic aspect dominates (e.g. Pole top lanterns) should use diffusers to eliminate both discomfort and disability glare. The mounting height for such lanterns shall be from 3 to 8m.

Overhang Overhang shall be in accordance with the options set out in BS 5489 : Part 2. Bracket projection generally shall be as small as possible in order to minimise vibration effects on both the lamp and the column itself.

For aesthetic reasons, the bracket arm is usually limited to one quarter of the column height (ie. H/4), as longer arms can give the impression that the column is top heavy.

Page 1012


The Light Source In order to conserve energy and achieve high efficiency, Qatar has standardised on high pressure sodium lamps for Class 'A' and '6' roads and high or low pressure sodium lamps for Class 'C' roads. Class 'D' roads may be lit with sodium, high pressure mercury, metal halide or linear compact fluorescent.

Malntenance Factor Maintenance factors, as defined in BS 5489 : Part 2, shall be taken for designs from Table 4 of that Standard.

The necessity for lantern cleaning at more frequent intervals than lamp changing will be avoided if a minimum degree of ingress protection rating of IP65 is specified.

Road Surface" '

Design tables based on the 'representative British road surface' as given in Table 3 of BS 5489 : Part 2 may be used.

However a more economical lighting design is possible if a concrete road surface is to be provided. If at a later stage the concrete surface may be overlaid with bituminous material then the lighting shall be designed for this initially.

Where design calculations are carried out by computer, a range of characteristic road surface reflection tables may be input from Publication CIE No. 30-2 : Calculation and Measurement of Luminance and Illuminance in Road Lighting. Most proprietary lighting. calculation programs will contain data files for one or more of these standard road surfaces.

10.3.2 Standard Lighting Geometries for . Different Road Profiles

Road authorities are primarily concerned with road lighting for its accident reducing potential. However, these benefits can be seriously diminished if insufficient attention is given to reducing the hazard created by lighting poles near the roadway.

Whilst the development and application of geometric standards for roads and streets has reduced the variation in roadway layout for various classes of roads, the road lighting designer is nevertheless confronted with a large number of road layout features and conditions which will influence the lighting design.

Divided or Dual Carriageway Roads 'This type of roadway layout is most common for high volume urban and rural artetial roads. Such roads may involve cross sections with service roads on one or both sides of the main carriageways, a great range of median and outer separator widths and often with carriageways

(including service roads) constructed at different levels.

The presence and location of existing road furniture and service utilities such as power distribution lines, telecommunications, and various underground services may impose certain constraints on the lighting layout.

The location and form of major intersections, median openings and other traffic facilities such as pedestrian crossings and bus stops must also be considered.

Thus the road lighting designer must be completely familiar with the section of road to be lit and equally important, he should have a good understanding of traffic operations that occur particularly during night time. It is only with this knowledge that he can arrange the lighting layout to best meet the many controls and demands of individual sites and achieve the maximum lighting effectiveness at reasonable cost.'

The general lighting arrangement will of course be dependant on the roadway width, luminaires available and the desired mounting height in accordance with the design rules and procedures as set out in BS 5489 : Parts 2 and 10. However, a choice of several layouts will usually be available to the designer.

On dual camageway roads any of the following arrangements of luminaires. may be appropriate:

a) Single Sided Arrangement.

Single sided arrangement on each carriageway with luminaires mounted on the right hand side. In some cases the mounting height possible even with special brackets will be inadequate for the width of carriageway and an alternative arrangement will be required.

b) Opposite Arrangement.

On dual caniageway roads, an opposite arrangement involving poles mounted along the right (footpath) side of each carriageway may be appropriate where the carriageways are not too wide and the median is narrow.

C) Twin Central.

This arrangement provides the designer with the greatest flexibility in locating luminaires but requires the minimum median width to be at least 1.8m and preferably wider. The choice of mounting height is flexible, as clearances to overhead distribution lines will generally not be a problem.



This layout can be considered the most suitable for dual carriageway arterial roads, particularly those with carriageway widths greater than 10m, because of the following advantages:

The number of poles can be minimised by selecting the highest practical mounting height.

The installation cost is often lower than other layouts because of less underground cabling and only one row

- of poles.

This layout provides excellent route -. guidance.

It is often feasible to install guard fences at hazardous locations where vehicle collisions with poles become a problem.

Undivided Roads Undivided roads form the major length of urban traffic routes. They are usually bordered by relatively narrow verges and footpaths which may contain overhead power distribution lines.

On these roads the designer is often confronted with constraints such as clearance of power distribution lines, location of underground services, location of driveways and commercial entrances and often the presence of trees etc. which will make an optimum layout difficult to achieve.

In general, single sided arrangements will rarely be practical and depending on the width to be lit and mounting height available, a staggered or opposite arrangement must be selected.

On wide undivided roads (and sometimes on dual carriageway roads) there is a tendency by lighting designers to locate the luminaires well out over the carriageway, in an attempt to achieve a single sided arrangement. Such layouts are generally unsatisfactory because of the 'flash' produced as vehicles pass directly under the luminaires and more importantly, the verge and footpath area is often poorly lit as a result of the overhang exceeding H/4, refer to Clause 10.3.1 Overhang.

Curves BS 5489 : Part 2 sets out the requirements for spacing luminaires around curves. This usually calls for the luminaires to be located on the outside edge of the curve which is in conflict with normal road safety requirements to avoid locating obstructions at such locations.

It is suggested that unless the curve is quite sharp (which would be unusual on a traffiC route of reasonable standard) the designer should

forgo the use of the sighting gauge and simply close up the spacing slightly to raise the general ambient light level to compensate.

It should be remembered that true silhouette vision against the road pavement as background will generally not be achieved on curves and drivers will be seeing by either direct vision or by silhouette vision against fences, buildings and trees, etc. along the verges.

Crests The designer will generally follow normal 'even- grade' procedures when crests are encountered on the section of road to be lit. However, i f the crest is relatively sharp, as might exist where the road overpasses another road, consideration should be given to the use of cut- off rather than semi-cut-off luminaires. Often this should involve only one or two luminaires at the top of the crest.

10.3.3 Llghtlng Columns as Hazards

Road accidents involving fixed objects beside the roadway are a considerable concern to everyone involved with roads and traffic.

Table 1 of BS 5489 : Part 1 recommends minimum clearances between columns and edge of carriageway for a range of design speeds. ,'

10.3.4 Typical Lighting Layouts at Junctions

Junctions are particularly important elements of the road system both from the point of view of efficient traffic operation and of road safety. The latter is evidenced by the fact that at least 60% of casualty accidents in urban areas occur at these locations.

It is especially important, therefore, that the lighting standard at junctions be at least as good and preferably somewhat better than that on the intersecting roads. In addition, the importance of minimising the number of poles and/or locating them clear of vulnerable areas cannot be overstressed.

BS 5489 : Part 3 makes specific recommendations in respect to the positioning of key luminaires at simple intersections and the engineer should conform with these requirements as far as practical.

The large variety of channelization layouts, each designed to meet the specific site and traffic conditions at any particular location, makes it difficult to set down standard luminaire arrangements. However, some rules relating to



luminaire arrangement can be formulated to guide the lighting designer in the achievement of good design practice:

At the outset, the engineer must recognise that seeing by silhouette vision is unlikely to occur at junctions and as a reslitlt, the layout should aim at illuminating the conflict area and the objects in and around it, such -as pedestrians, cars, kerbed islands, pavement markings and signs etc. so that they are seen by direct vision

The level of Clluniination and its uniformity should be such that the layout of the islands and the various carriageways and turning roadways are clearly discernable by drivers approaching on the intersecting roads and negotiating the required movements within the junction

Luminaires must be placed to provide the best possible illumination of

. pedestrian crossing areas

The luminaire layout as seen in . perspective should not confuse but

enhance the route of through traffic. .A good layout will provide ro~~te guidance to lead traffic through the junction

Care should be taken to ensure that points where traffic streams merge and diverge are well lit

The number of lighting poles near the conflict area should be minimised. Where traffic signals are installed or being installed, joint sharing of the pedestals should be achieved wherever possible. Where large channelizing islands exist, consideration sho~~ld be given to the use of high mast flood lighting techniques to reduce the number of poles around the junction.

It is very important to avoid locating poles:

Close to the approach ends of narrow residual medians and median islands

In the nose area of islands where traffic streams diverge

In areas where the poles might obstruct the sight lines of drivers waiting to enter or cross another traffic stream

In the vulnerable areas along the outside of curved slip roads

January 1997

#

# . SECXI~NIO

At .roundabouts in the small approach splitter islands, on the central island opposite entry roadways and on the right hand side immediately downstream of an entry point to a roundabout.

Figures 10.1 to 10.6 show typical lighting layouts recommended for standard junction designs in Qatar.

Note: S = design column spacing on the main road.

Figure 10.1 Typical Layout for TJunction .

QATAR HIGHWAY DESIGN MANUAL S E W N 10

A2 - - -----------------------------------------

a - A1 C

Note: S = design column spacing on the main road.

=..- Figure 10.2 Typical Lighting for Staggered Junction

Figure 10.3 Typical Lighting for U-Turn Facility

Figure 10.4 Typical Lighting for Large Roundabout

Figure 10.5 Typical Lighting for Small Roundabout



Pederuian Crorrlng

I - - - - - - - - - - - - - - -

& Equal dlrbncer of

not mom than 1Om .

(a) Staggered

Pederuian Crorrhg

-------- - - - - - - -

Equal dkbncer of not more than 15m

(b) Opposite

Figure 10.6 Typical Luminaire a Positions Adjacent to a Pedestrian Crossing

10.4 SPECIFICATION OF EQUIPMENT

Detailed specification for the supply, installation and testing of equipment will be found in the QCS.

The recommendations for lighting columns are to be based on a windage of up to 100kph and in accordance with the weight of the fitting or fittings installed and the length of the bracket used.

10.5 ELECTRICAL DISTRIBUTION

10.5.1 Supply

Prior to any installation of street lighting, application must be made to the MEW together with a design drawing and specifications. The Planning Section of MEW will, after the approval of the design by the Street Lighting Section, allocate the points of supply.

MEW will allocate a 3 phase and neutral 415V 50Hz supply either directly from an existing 11 kVl415V MV substation panel or more usually from an MV feeder to a street lighting feeder pillar.

10.5.2 Feeder Pillars

Feeder Pillars will be placed at the nearest point to the main supply so.urce.

Frequency of feeder pillars will depend u on the ! maximum length of run of the 25mm cable consistent with permitted voltage drop, which In

turn will be governed by the load rating of individual lighting units in the circuit.

10.5.3 Cables

Cable runs will be of a maximum length which will permit a voltage drop of not more than 30V per phase (i.e. a single phase voltage at any column of 21 OV). A ring main to reduce voltage drop by looping every alternate column in each direction will be permitted.

Cabling between columns will be to a maximum of 25mm2 4 core PVC armoured cable and will be looped into each column by means of an approved street lighting cutout fitted with an HRC fuse of a size appropriate to the load of the light sources plus control gear losses. Consideration must be taken of the fact that the starting load for some light sources can be in excess of 100% of the normal running load.

On 3 phase systems every column of a row of 3 will be connected to a different phase thereby ensuring a balance throughout the installation.

10.5.4 Ducts

Cables crossing roads or structures will be run through PVC ducts of 150mm diameter only.

Unless ducts terminate at cabinets, mounting posts or columns, their ends shall be marked with approved marker blocks or location posts so that their location can be clearly identified without exploratory excavation.

On completion of an installation, 'as-built' drawings showing all cable routes and duct locations will be provided and submitted to MEW for records.

10.5.5 Earthing Systems

The wire armouring of the cables shall be used as an earth conductor (circuit protective conductor). The armour from each incoming cable shall be bonded together by means of an approved earth bonding clip to which a 6mm2 PVC insulated wire shall also be fixed leading from the earthing stud provided in the column. Earthing shall be terminated at the nearest MEW supply point by bonding to the MEW substation earthing point.

January 1997 Page 1OR

QATAR' HIGHWAY ~ZESIGN MANUAL

10.5.6 Safety Standards 1

Engineer's Responsibiiities In order to promote safe working practices for both construction and maintenance, the design engineer shall carry out Risk Assessments for any activities which may endanger personnel or property, including the following where relevant:

- . Working at height

- Use of mobile elevating working platforms

- Storage and use of liquid propane gas

- sto;age and use of highly flammable liquids

- slingini of loads

- Use of lifting equipment

- Use of hand tools

- Use of compressors and pneumatic power tools 8

- Use of portable electrical equipment

- Electrical work up to 41 5 volts

- Installinglreplacing luminaires

- Electrical testing and commissioning

- Disposal of discharge and fjuorescent lamps

- Disposal of waste materials

- Work in the vicinity of underground services

- Work in the vicinity of overhead electric cables

- Work in and with excavations

- Roadworks

- Minor demolition and breaking out of services.

10.6 MAINTENANCE AND OPERATION

10.6.1 Design lmplicatlons

It is often important to consider the implications of the lighting maintenance operation during the planning and design of a road lighting installation. This is particularly so in respect to lighting on motorways and other high traffic volume andfor high speed roadways.

January 1997

Most road lighting maintenance is carried out using elevating platform vehicles (EPV). These are available in various sizes to service up to about 21 m mounting height, but are expensive to purchase or hire.

On most lighting installations, the maintenance vehicle will stand on the carriageway directly underneath the luminaire, thus reducing the trafficable width available during maintenance operations.

Where the mounting height is 12.5m or less and the EPV can be positioned directly beneath the luminaire, outrigging stabilizers may not be required. In other situations, the use of stabilizers will be necessary and will further considerably reduce the trafficked width available.

Depending on the nature of the road in question and the traffic demands, it will be necessary to implement appropriate traffic control measures and possibly even schedule the maintenance work to periods of low traffic flow.

These arrangements can be both inconvenient and costly and the alternatives available should be properly evaluated. The alternatives may involve a different luminaire arrangement at a lower mounting height, the use of hinged poles (which are now available at relatively little additional cost) or the use of a fewer number of greater mounting height columns with luminaire lowering gear (high masts).

10.6.2 Quality of Equipment

When comparing costs of alternative items of equipment, the "whole life' cost ,of the installation should be considered rather than just the initial construction cost. It will often be found that more expensive, high quality equipment requiring less maintenance attention will be cheaper in the long run, as well as causing less inconvenience to the road user in terms of obstruction to the highway during maintenance operations. There may also be safety benefits in using high quality equipment.

Guidance on luminaire maintenance factors is given in Table 4 of BS 5489 : Part 2.

10.6.3 Inventory and Fault, Reports

In order to obtain the most cost effective service from a lighting installation, adequate procedures for the reporting and logging of faults, and the planning of maintenance programmes need to be established.

An essential requirement for these activitles is the provision of an accurate, comprehensive, easily accessible inventory system, nowadays

Page 1018


usually installed on a desk-top computer or PC network. A large amount of manual survey and logging work is involved initially, but this will be repaid within a fairly short time by improvements in efficiency of management.

It is of great benefit when logging fault reports, which may often originate from persons with no technical knowledge, to be able instantly to view details of the installation on a visual display screen.

10.6.4 Cleaning and Lamp Replacement

It is essential that cleaning and lamp replacement routines should be closely followed to maintain the installation. Maintenance programmes should include lamp replacement, luminaire cleaning, renewal of failed parts, checking of gaskets and optical components, lubrication, painting and night inspections.

Apart from the deterioration of luminaire parts, which can be corrected by cleaning, there is also a longer term deterioration which is permanent and cumulative. Restoration of photometric performance may, therefore, require replacement of optical systems or even the whole luminaire. Site tests should be carried out at intervals of not more than five years to check that performance is acceptable.

The procedure according to which lamps are replaced is a matter of local policy, cost and lamp type used. The cost of replacing lamps on demand should be compared with that of group replacement. In making the comparison, the following factors are among those that should be considered:

- The shape of the lamp survival curve for its environment

- The lamp lumen depreciation curve

- Ease of access, e.g. extent of signing and coning required

- Interference with traffic

- The required frequency of patrolling for outages

- The frequency of need for cleaning of luminaires

- The overall proportion of outages that can be tolerated

- The grouping of outages that can be tolerated

January 1997

- The frequency of inspection . for electrical safety.

It will normally be found that lantern cleaning, which is a costly, labour-intensive activity, can be restricted to coincide with the lampchanging operation if a luminaire with enclosure protection to at least IP65 is installed.

10.6.5 Frequency of Inspections

It is recommended that visual, structural and mechanical inspections of street lighting equipment should be undertaken annually with full electrical testing every five years.

10.6.6 Hours of Operation

Road lighting is required during all the hours of darkness, independently of traffic flow' and should normally be in full operation from about 30 min after sunset to about 30 min before sunrise.

Questions of local policy are outside the scope of this guide, which deals only with technical matters. However, it should be noted that lighting serves emergency services, public security and pedestrians as well as drivers and that extinguishing lighting during the hours of darkness is detrimental to these interests.

The practice of extinguishing certain luminaires when the traffic flow is small does not fulfil the lighting needs of vehicular traffic and may increase the likelihood of collision with columns.

Page 1019

QATAR HIGHWAY DESIGN MANUAL APPENDIX A

APPENDIX A SURVEYS

A1 INTRODUCTION

Survey is a specific discipline, the results of which are utilised for a great many purposes from planning to construction.

With regard ,to road design, the purpose of survey is twofold. .. ,.

Firstly, it is required to establish the roadway line within the context of existing land ownership or planning requirements, thus fixing the available corridors for the roadway and associated utilities.

Secondly, it is required to identify elements which exist within and adjacent to a corridor in order that a satisfactory road design can be affected.

To complete this function it is important that the survey contractor provides all the information the engineer needs and that the engineer makes full use of all the survey information available.

Survey work in Qatar is controlled by its own comprehensive specifications and regulations. As such, this appendix is not intended as a survey manual but as an aid to the highway engineer, to enable the production of comprehensive designs whilst having due regard for existing and proposed site features.

Items specifically covered are:

a Government bodies controlling survey within Qatar

a Survey information useful to the highway engineer that is currently available from each organization

• Survey information that should be collected for use on road design projects

Procedures required by the Civil Engineering Department for survey work associated with road design projects.

Survey during road construction is not covered within this appendix. However, the general requirements of as-built surveys are discussed.

A2 . SURVEY IN QATAR

Survey in Qatar is controlled by the Ministry of Municipal Affairs & Agriculture (MMAA) and the Centre for Geographic lnformation Systems (CGIS) who obtain, update and keep the current survey data, and set the criteria by which survey data is recorded and presented.

The Qatar Survey Manual, issued in 1989 by the Ministry of Industry & Public Works, (subsequently replaced by the Ministry of Municipal Affairs and Agriculture) and amending circulars, deal principally with cadastral, control, engineering and hydrographic survey. This includes the specifications, accuracy and working procedures to be used when undertaking these types of survey relating to the Qatar National Grid (horizontal position) and Qatar National Height Datum (level).

In addition to survey controlled by the MMAA, the Centre for GIs produces and maintains the Geographic Information System(GIS) for Qatar.

For convenience, this appendix lists the various survey bodies that offer services and functions useful to the highway engineer. The organizations are illustrated in Figure A1 . Each organization operates its own specific procedures and methods that should be adhered to if interfacing with it.

I - n v u a n r 1007 Paao A l l


Centre Ministry of Municipal

for Affairs & Agriculture

GIs

b

Land Civil Planning

lnformatlon Engineering Department

u Centre Department

Mapping & General Highway Positioning - - - - - - - - - Survey - - Design

Section Section Section

CED Survey

Unit

Figure A1 MMANCGIS - Survey Related Organizations

January 1997 Page A12


A2.1 Centre for GIS - Mapping and Positioning Services

The Centre of GIs was established in 1990 with the target of setting up, operating and maintaining a Geographic Information System for Qatar.

The Geographic information System is an easily accessible digital library of all surface ,and subsurface features in Qatar. It is therefore an important tool for planning and co-ordinating all developments in Qatar.

Because of the link GIs naturally forms with all bodies associated with development, each government discipline that encompasses construction of new features includes a GIs co- ordinator. inL'addition, the Centre for GIs employs survey teams who check and collect new features for inclusion within the digital database.

Functions of the Centre for GIs useful to the highway engineer are listed below.

Topographical Database The digital topographical mapping database is available at nominal scales of 1 :500,000, 1 :200,000, 1 :50,000, 1 :10,000 and 1 :I000 (urban areas only).

The 1:10,000 and 1:1000 high resolution databases are stereo-compiled from aerial photography and form Qatar's GIs Digital Base Map Database (DBMD).

The larger scale digital mapping was created by digitizing existing maps.

-

The DBMD is constantly updated sheet by sheet from aerial and ground observations.

1:1000 mapping has an accuracy of *500mm which is acceptable for most studies and concept road design and is useful as a back-drop for illustration of areas adjacent to the route under conside ration

1 :10,000 mapping has an accuracy of A3m which is suitable for location plans and diagrams.

1 :50,000 mapping has an accuracy of d 5 m and is suitable for presentation- style diagrams.

National Control and Benchmarks 'The 1' - 4" order survey control points and benchmarks situated around Qatar provide co- ordinate and level information for the entire country. A greater density of control is given in the urban areas.

Orthoimagery Orthoimagery comprises digital aerial photography that is assembled to form a visual picture of the landscape. It has an accuracy of i5OOmm with a greater resolution in urban areas. The digital orthoimagery database is not generally made available due to the amount of information contained within the files (typically 6OMBIsheet).

1:1000 orthoimagery is available for urban areas of Doha, Wakrah and Dukhan. This is useful for engineering studies and as a check on field data

1 :10,000 orthoimagery is available for the whole of Qatar. This is useful for engineering studies, particularly relating to the identification of drainage catchments and wadi locations.

Digital Elevation Model The digital elevation model consists of accurately recorded spot heights for the whole of Qatar.

Levels are related to the Qatar National Height Datum and quoted to two decimal places.

Satellite Imagery Available in digital format and posters for the whole of Qatar. Satellite imagery is not generally used in highway design but is useful for specific studies because additional information that is not available on the digital mapping or orthoimagery is presented.

Aerlal Photography The earliest black and white photography taken in 1947 is still available. Complete photographic cover of Qatar dates from 1977 and colour photography is generally available dating from 1980.

Aerial photography for the whole of Qatar is presented at scales of approximately 1 :40,000 and is useful for route and development planning and engineering studies. Wadi conditions, areas of high water table and flooding are clearly identifiable from the aerial photography.

Page A13


Old Mapping Early map series are available on film or paper sheets from archives. The mapping was produced from aerial photography taken during 1971, 1973, 1977, 1980 and 1987. Scales of 1 :200,000, 1 :100,000, 1:50,000, 1:20,000, 1 :10,000, 1:5,000, 1 :2000, 1:1000 and 1500 have been prepared, though not all areas of Qatar are covered by each scale. The engineer should refer to the Qatar Survey Manual for further details of coverage and series.

Old mapping is useful for identifying features such as sink holes, shore lines and low areas since covered by development.

Global Positioning System (GPS) The global positioning system provides position and level of any place in the world from satellite- generated'location information. A minimum of 3 satellites need to be operational over the locality. GPS equipment may be small enough to be hand-held. Varying levels of' horizontal and vertical accuracy are available, depending on the number of satellites read and the occupation time at the station.

The Centre for GIs broadcasts VHF correction information for use with GPS equipment within Qatar to provide real time outputs.

GPS has much use in route finding and strip levei surveys in areas where more accurate control is not available. However, in Qatar where accurate control is widespread across the whole country, its uses are limited by the cost required to achieve the accuracy necessary for highway design.

A2.2 Land Information Centre - General Survey Section (GSS)

The Land Information Centre was created in 1994 and incorporates the General Survey Section.

Functions of the General Survey section useful to the highway engineer are listed below.

Cadastral Database The GSS maintains a database of registered coordinates relating to land ownership boundaries for the whole of Qatar. The information is available in the form of co-ordinated points in text files.

Cadastral information shall be used by the road designer for the production of road corridor and network plans and in the computation of road intersection points and centrelines.

Approval of Survey Companies The GSS is responsible for the approval of private survey companies who can access cadastral information and undertake cadastral survey work for private or government bodies.

Approval of Corridor lntersectlon Points For new corridor alignments the calculation of corridor intersection points and curve parameters shall be made by the highway engineer or surveyor based on adjacent cadastral information.

Where there is no existing adjacent cadastral information, corridor IP's and curve parameters shall be computed from Planning Department policy plans. Existing site features such as walls, pylons, posts etc may be used to define boundaries reflected on the policy plans. The computed corridor IP's and curve parameters shall, in this instance, be reported for the approval of the General Survey Section. Companies that are approved for cadastral survey work by the General Survey Section shall be employed to compute and report these points.

- Highway engineers are reminded that road alignments shall be developed in accordance with the relevant sections of the QHDM. Alignments are therefore not defined by the corridor centreline (Refer to Section 5).

A2.3 Planning Department

The Planning Department is responsible for the co-ordination of all land planning in Qatar including the outline approval of private developments.

Functions of the planning department useful to the highway engineer are listed below.

Policy Plans The Planning Department can provide current policy plans illustrating information regarding land use allocation for the whole of Qatar.

Policy plans are available at scales of 1:1000 for urban areas and 1 :2000 for rural areas.

Paper copies of policy plans are available illustrating the up-to-date land use planning. Digital copies of the policy plans are updated every three months, and are also made available for general use.



A2.4 CED Survey Unit A3.2 ' Services Surveys

The CED Survey Unit operates exclusively for the Roads Division. Its main activities are listed below :

Services survey shall be undertaken utilising electronic radio-detection methods. Line and level of existing services apparatus shall be recorded on services survey plans.

Topographical surveys for in-house design work Services survey drawings shall be prepared at

1:500 scale for urban areas on A1 sheets and in digital format. Scales for use in rural areas should be chosen to reflect the amount of detail required. Layer numbering, line types and symbols shall be in accordance with the Civil Engineering Department standard. The horizontal accuracy of the services surveyed by electronic radio-detection shall be to * 250mm, with vertical accuracy to *100mm. Where services are located by trial pits they shall be surveyed to an accuracy of k5mm horizontally and vertically.

Setting out for grading schemes undertaken by the Direct Labour Organisation.

Functions of the CED Survey Unit useful to the highway engineer are listed below.

Road Intersection Points The CED Survey Unit maintains a database of road intersection points.

IP's computed by the highway engineer from cadastral information shall be submitted to the CED Survey Unit for review.

Location of services lines are to be determined by the co-ordinate of points along the lines.

Co-ordinates may be derived from measuring:

Topographical Surveys Topographical surveys for CED Roads projects are subject to CED Survey Unit review and approval.

a) anglehearing and distance from known control points.

b) offset and chainage from knownJco-ordinated lines (eg. road centreline)

C) distances from 2 or more known points.

As-built Surveys As-built drawings are prepared by contractors with the assistance of supervising consultants and private survey companies. They are recorded in digital and map sheet form and are archived in the CED Prime Document Storage.

Level shall be recorded on the survey plans to national datum at specific points along utility routes. Points shall be levelled and recorded at bends, 'junctions and at 25m intervals along straights.

As-built surveys are reviewed by CED Survey Unit on an ad hoc basis as required.

A3 SURVEY WORK PROCEDURES

In order to maintain consistency between projects, specific procedures are to be followed in surveying, recording and presenting survey information for highway design projects.

All radio-detection survey operators shall be approved by the Civil Engineering Department prior to commencement of the services survey.

Typical survey requirements for highway design projects are listed below. The results of radio-detection surveys shall be

corroborated by manual excavation of trial holes at selected sites in accordance with service authority procedures.

A3.1 Topographical Surveys

The topographical survey shall cover the full extent of the works to be designed and include tie-ins to all existing features.

A3.3 As-built Surveys

On completion of construction, as-built survey drawings shall be produced by the project contractor. ;

Survey data recorded shall be sufficient to enable preparation of survey drawings and shall be prepared in accordance with the specification given in A5. As-built utility information shall be collected

during site works by the contractor and recorded in digital format for line and level by

January 1997

QATAR HIGHWAY DESIGN MANUAL . . APPENDIX A

the supervising consultant. All as-built information shall be collected in a digital format compatible with CED's highway design and draughting software.

As-built survey drawings of principal alignments and visible features shall cover all of the works installed under the contract including utilities, services and all finished alignments and levels, both above ground and subsurface.

As-built surveys shall be undertaken by companies approved by the General Survey Section and shall be in the format approved by the Civil Engineering Department. The construction contract is not normally considered complete until the as-built surveys have been submitted to CED and approved.

A.4 APPROVED SURVEY COMPANIES

The General Survey Section is responsible for the approval of survey companies in Qatar for cadastial survey work.

Companies undertakirrg topographical survey work for road designs shall also be from the GSS approved survey company list.

A5 SPECIFICATION FOR TOPOGRAPHICAL SURVEY

A.5.1 Features to be Observed

The survey contractor shall undertake a detailed topographical survey of the subject areas. The following features shall be observed:

Building extents (including overhangs, walls, fences, gates and entrances) Kerbs, bitmac edges, tracks, footpaths and parking areas Service postslpoles (eg. telephone, electricity, lighting, signals) Road signs (street names, traffic) and billboards Manholes, gullies, hydrants, culverts, service boxes and markers Overhead and buried cablesnines Trees, plant boxes, landscaping limits Water channels, culverts Surface type changes (eg. between natural ground and concrete paving) Slopes, escarpments Spot levels at every 25m and at: a. Gates and entrances b. 'Services covers, gullies, culverts c. Isolated high and low points d. Abrupt grade changes.

For road corridors, cross section levels to the edge of the reservation or agreed extent shall be taken at 25m intervals.

A5.2 Preparatlon of Survey Data

The Contractor shall prepare and submit the data observed as survey plans in the following format:.

Topographical survey drawings shall be produced at 1:500 scale for urban areas. Larger scales of 1 :200 or 1 :I 00 shall be used for areas requiring greater detail such as major junctions. In rural areas, where few features are present, the survey drawings shall be produced at 1:1000 scale or as otherwise agreed

Surveys plans shall be contoured at 0.5m vertical intervals. Additional spot levels shall be indicated at lqw and high points and across flat areas without contours

Levels at 25m intervals shall be indicated on the survey drawings. In larger open areas a grid of levels at 25m centres can be used

In addition, the data collected and survey procedures used shall be submitted in the following format:

AutoCADe .DWG or .DXF plot file of the topographical maps

Printout of Easting, Northing, Level, and Code of all points

Comma-delimited DOS text file of points containing:

- a. . Point Number b. Easting c. Northing d. Level e. Code

Printout of raw data for the topographical survey

meld data, computations and descriptions ' for new control stations~benchmarks

. Job Report describing the work undertaken which includes: a. Location and project limits b. Dates of survey



c. ~e thods and instruments used d. Details about new control

pointslbenchmarks, where established.

For multi-sheet drawings, sheet limits shall be plotted with the current sheet highlighted.

5. Survey Drawing All digital data shall be submitted on 3.5" floppy disks. All surveyed features shall be plotted.

Lines shall be labelled when not specifically identified in the Legend, (Table A5.4). Point features shall be represented by standard symbols and annotated accordingly.

A5.3 Specifications

The Contractor shall comply with the following specifications:

Survey works shall be tied to the Qatar National Grid and Height Datum or the QNDDS co-ordinate system.

North point symbol and grid coordinates shall be plotted such that no part of the drawing is written over.

Establishment of new control stations and benchmarks shall be in accordance with Section 2 of the "Qatar Survey Manual". Levelling closure errors must be better than 15mm JK, where K is the length of the level route in km. Traverses shall have relative accuracies of 1125,000 or better Eastings, northings and levels of detail points shall be within 10mm accuracy

For multi-sheet drawings, match lines and appropriate notes for adjoining sheets shall be provided.

Layers shall be used in the preparation of digital drawings. Each layer shall contain only one feature type and shall be appropriately named in accordance with the typical layering given in Table A5.5

Topographical maps shall be of the format shown in Figure A5.1 and shall contain the following information:

A5.4 Checklng and Verification

All works and resulting survey data shall be subject to the checking and approval of the CED Survey Unit.

Contractor's name, project reference number, surveyor's name, dates of survey, sheet contents, drawing number, plan scale.

2.. Notes

Details relevant to the survey work done (eg. reference system, datum, methods and equipment used).

3. Legend

Listing of line types, symbols and codes used and corresponding descriptions. Tables A5.1, A5.2 and A5.3 list standards for CED survey drawings while Table A5.4 is a typical legend listing.

4. Location/Shee t Index Map

Drawing (typically at scale 1:10,000) showing the area surveyed, name of major roadslstreets, grid markers, coordinates and north point symbol.

Paae A n

5'

(SU

RV

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DR

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1

L QATAR HIGHWAY DESIGN MANUAL APPENDIX A

All lines are 0.2mm, thick unless othewise specified .

Line

----------------- ----------------- ................... - -.-.-.-.- - - - - - - - .-..-..-..-..-

Table A5.1 - Survey Map Line Types

L

L

Description

Solid line

Solid line, 0.25m

Solid line, 0.35m

Two solid lines

Short dashes

Short dashes, 0.35rn

Dots

Solid line and slash

Dash-dot

Long dash-short dash

Long dash-dot-dot

Use

Standing kerb line, step, planter, concrete

paving, SIS limit and other features not othewise listed.

Cadastral plot boundary

Building line

Wal l (line separation equals thickness of wall

Edge of bitmac, flush kerb, change of surface

Building overhang

Toplbottom of bank, change of grade

Picket fence, railing, crash barrier

Road Centreline

Overhead cablelline

Underground cablelline

L ' Table A5.2 - Survey Map Symbols

To be drawn using 0.2mm.pen

Notes: 1. Features exceeding prescribed dimensions shall be surveyed as polygons and plotted with solid lines O.2mm. thick.

2. Signs supported by more than 1 post shall be plotted as solid lines 0.2mm. thick.

Use

Gate (length equals gate width)

UIG cabielduct marker, services and fire hydrant covers not more than O.5m.square

Borehole, gully and circular MH cover not more than 0.5m. diameter

Bollard, marker post not more than 0.5m. diameter

Triangular MH, sides not more than 0.5m.

Services poctlpole (electricity, telephone, street lighting, traffic signal); road sign and sign board supported by single post

Electricity junctionltraffic controller box and telephone booth not more than 1m.square

Survey control or benchmark

Palm tree

Tree, general

Symbol - o

0

a

8

A

o

I

% Q

i .

- January 1997 Pane A19

A

Description

Scaled size

Box, 1.2mm.square

Circle, 9mm'diameter

Solid circle,l.2mm.diam. Solid box,1.2mm.square

Triangle, 1.2mm.sides

Solid circle, 2mm.diam.

Solid box, 2mm.square

Double triangle, 1.2mm.

Scaled size

Scaled size


Annotation Description

B Bollard

BH - Borehole

CB Cadastral boundary

EP Electricity post

FH Fire hydrant

G Gully

GV - Gas valve

IC g Inspection chamber

JB - Electricity junction box

LP * - Lamp post

MH g Manhole; type unknown

MHQ - Q-Tel manhole

MHS , - Sewerage manhole

MP - Marker post

PB - Post box

PC - Pipe culvert

PPB - Pedestrian push button pole

RS - Road sign (street name)

S/S - Electricity sub-station

SB - Sign board

SC Stopcock

SM Underground service marker

SV Sluice valve

TCB Traffic controller box

TEL Telephone booth

TP Telephone post

TSP Traffic lightlsignal post

TS Traffic sign post

WT Water tank

WV Water valve

Notes: 1. Annotations shall be plotted alongside corresponding symbol, line or polygon in the drawing area and listed with appropriate description in the legend section.

2. The following features shall be additionally annotated with heights: -MH covers -Top and bottom steps -Gullies -Pipe culverts -Gates and entrances

Table A5.3 - Survey Map Annotations

January 1997 Page All 0


1 LEGEND:

Kerb line, unless othewise specified. (Level taken at the channel) Cadastral plot boundary Building line Wall

--------------- Edge of bitmac, unless otherwise specified .--------.-.-_-_ Buildlng overhang ................. Toplbottom of bank - Picket fence, railing, crash barrier -.-.-.-.- Road centreline - - - - - - - Overhead cablenine -..-..-..-..- Underground cableAine

w ... Gate A Survey control or benchmark

% Palm tree

0 s - Tree, general B Bollard

BH Borehole

CB Cadastral boundary

EP Electricity post

FH Fire hydrant

G Gully GV Gas valve IC Inspection chamber JB Electricity junction box LP Lamp post

MH Manhole; type unknown

MHQ Q-Tel manhole

MHS Sewerage manhole

MP Marker post PB - Post box PC Pipe culvert

PPB Pedestrian push button pole

RS Road sign (street name)

S/S Electricity sub-station

SB Sign board

SC Stopcock

SM Underground service marker

SV Sluice valve

TCB Traffic controller box TEL Telephone booth

TP Telephone post TSP Traffic lighvsignal post

TS Traffic sign post WT Water tank

WV Water valve

14.55 Spot height

Table A5.4 - Typical Survey Map Legend

Januarv 1997 Page A l l 1


LAYER NAME DESCRIPTION 1 FEATURES COVERED

BUILDING Buildings, houses, shops, bus shelters and corresponding levels

CADASTRL Cadastral points and boundaries

CONTROLS Control stations and bench marks

EX-ROAD Kerb lines, edge of bitmac, islands and corresponding levels

EX-WORKS Excavations, boreholes, temporary construction fences

GATES Gates and threshold lev?ls

IMPROVEM Man-made features and corresponding levels not related to specific layer -

e.g. steps, gardens, drinking fountains, private plant boxes, etc.

NATURAL Trees, waterways, vegetation limits

SERVICES Electricity, water, telephone and other s e ~ c e s , and corresponding levels,

includes : manholes, gullies, hydrants, inspection chambers, valves,

electricity and telephone poles and lines, electricity sub-stations,

junction boxes, postboxes, culverts, ducts, pipelines, services markers

SPOT-HT Spot heights and levels not related to specific layer

ST-FURNI Lamp posts, street name posts, sign boards, marker posts,

public plant boxes; and corresponding levels

TRAFFIC Bollards, traffic signal posts, vehicle detectors, pedestriantroad

markings, crash barriers, railings; and corresponding levels

WALLS Property walls and fences, and corresponding levels

CONTOUR 1 Major contour line

CONTOUR 2 Minor contour line

DESC-TXT Labels and annotations not related to specific layer

GRIDSDAT Map grid lines and coordinates

MATCHDAT Match lines and match line symbols and texts

PLANFORM Drawing margins, legend, title boxes, location map, riotes

ROAD-DES Designed road IPS, center lines and reservations I

. Table A5.5 - Typical Layers for Topographical Survey Drawings

January 1997 Page A112 .

QATAR HIGHWAY DESIGN MANUAL APPENDIX B

APPENDIXB GUIDANCE NOTES TO PREPARE A BRIEF FOR GEOTECHNICAL SITE INVESTIGATIONS

B1 INTRODUCTION

This Appendix primarily is to assist the CED Engineer in the preparation of a brief f.or a geotechnical site investigation.

'The guidelines within this Appendix provide a checklist of items to be considered in the process of preparing a brief. A flow diagram identifying the main points is shown in Figure 61.

Select Route

. . .

Locate Junctions + Locale Structures

I

WalkoverlDrhre Sites

Data Review

Review Structure Locations

Decide on the Information Required to

Enable Design

Select the lnvestigation Procedures Required to ProvMe the Informtbn lo

Enable Design

Review the Scale and Quantify the

investigation

Investigation Brief

Figure 81 Preparing a Geotechnical Brief

Each of the items listed in Figure 61 is discussed in the following sections.

The approach to preparing a brief for concept or detailed design is the same. However, the type of information required and size of investigation varies. The different requirements of both concept and detailed design are identified in the text.

It is essential that all works and specifications comply with the most up to date veisions of the CED approved documentation and procedures.

B2 INI'I'IAL CONSIDERATIONS

Before preparing a geotechnical brief the Engineer must consider the following:

L

Route Selection In Qatar it is often the case that the route corridors are predetermined by the Planning Department. However, the engineer should review the selected route following good practice and guidance given in the Qatar Highway Design Manual. The engineer should at this stage be confident that it is feasible to produce a compliant road design within the route corridor provided.

Locate Junctlons The junction locations are likely to be dictated by one or more of the following:

Existing or proposed routes

Existing or proposed developments such as villas, shops or petrol stations

Service equipment such as electricity pylons, substations, pumping stations, cables or pipelines

Geotechnical conditions.

The geotechnical investigation may reveal ground conditions which result in moving the junction or changing the design. Junctions often involve some form of structure, for example, a full grade separated interchange or an ornamental structure in the middle of a roundabout. So the geotechnical information is quite likely to have a bearing on junction location.

January 1997 Page BI1


Locate Structures The types of typical structures to be found on highway works are:

Bridges

Embankments

Cuttings

Traffic Signals, Signs and Lighting

Box Culverts

Underpasses

Ornamental Structures such as Arches and Feature Structures in Roundabouts.

Before preparing a geotechnicai brief the engineer should have a full understanding of the outline design and be able to identify the type, approximate location and scale of the structures to be built. These are important factor: in defining and quantifying the site Investigation, as most of the investigation will be concentrated at the location of the structures.

WalkoverIDrlve Slte Having determined the route and location of the junctions and structures, the engineer should then visit the site. The site should be walked over or driven through, depending on the scale of the project. The purpose of the site visit is to get a visual impression of the route, locate the junctions and structures and identify any obvious anomalies which may have a bearing on the project. For example a drive through a site may identify lush green vegetation in low areas indicating possible groundwater. This may require additional site investigation to confirm the problem. The site investigation report should identify such topographic features and, as a result of the testing, advise of any problems relating to the design and of any difficulties which may arise during the construction period.

Data Review Following the site visit, the Engineer should review the site notes and, if necessary, amend the design accordingly. Any problem areas should be highlighted and these notes referred to when preparing the site investigation brief. The location of structures should be reviewed against the site visit notes so that if a potential problem exists, either the location is changed, the design of the structure is modified or the site investigation brief increased to cover any additional investigation works.

APPENDIX B

63 PREPARATION OF THE BRIEF

Following the initial considerations (refer Section B2), the engineer must then decide on the information required in the design process. Details of the various methods of investigation and testing listed below are discussed in Section 64.

83.1 Geotechnlcal Investigation Works

The following investigation works may be required:

Desk Study

Geotechnical Walkover

Trial Pits

Boreholes

Samples

Field Tests

Laboratory Tests. '

Each of the above works is described in Section 64 of this Appendix.

Schedules for Geotechnlcal lnvestlgations The following tables quantify typical geotechnical investigations for the following conditions:

Roads - Feasibility Stage, Table 81

Roads - Detailed Stage, Table 62

Structures - Feasibility Stage, Table 83

Structures - Detailed Stage, Table 84.

'The schedules give advice on the frequency of different methods of investigations. These notes are merely guidelines to be used in the preparation of a brief. Each site investigation brief should be considered on its own meriis, taking into account the purpose of the investigation, stage in the design process, scale and design of the project and its location.

January 1997 Page BE


Table B1 . - . Schedule of Geotechnical Investigations for Roads at Feasibility Stage

Road

Dual 2-3 Lane Single 2 Lane

Table 62 Schedule of Geotechnical Investigations for Roads at Detail Stage

Description

Trial Pits Trial pits should be located at 1 OOOm spacing. The trial pits should not all tie located solely along the centreline but should be spread over the width of bath carriageways or the corridor. Trial pits should be concentrated at identifiable problem areas. Trial pits would typically be up to 2.5m deep.

Boreholes These may be considered necessary if the desk study reveals a problematic area. Borehole, quantities and locations should be reviewed by a Geotechnical Engineer, however, the investigation should be concentrated In the problematic area.

Laboratory Testfngr Testing should be undertaken of samples at each trial pit and borehole. This frequency should be reviewed by the Geotechnical Engineer on site and the scope reduced or increased as necessary.

Road

Dual 2-3 Lane Single 2 Lane

Januarv 1997 Paae 813

Notes

It is important to ensure that the proposed road does not interfere with the hydrology of the area. Note should be made of any groundwater in the trial pit and any evidence of collecting water in the area such as evaporation salts or green grass in low areas.

In built up, urban areas, special attention should be given to locating the trial pits to avoid services such as electricity, water or Q-Tel.

-

Description

T& Pits. Trial pits should be located at 500m spacing. The trial pits should not all be located solely along the centreline but should be spread over the width of both carriageways. Trial pits would typically be up to 2.5m deep.

Boreholes Boreholes should be located at 1 km intervals. If the desk study reveals that consistent rock and soil conditions are to be expected, the number of boreholes may be reduced to suit.

Permeability Tests: Falling head, or constant head permeability tests undertaken in boreholes located at 1 krn intervals or in areas of differing ground conditions where surface water from the highway will require collection and discharge.

Dynamic Cone Penetraffon Tests: Where the desk study or walkover survey reveals that soil conditions such as sabkah or alluvium are present, then DCP testing should be considered in these areas, typically at 2Wm centres.

Laboratory Testing Testing should be undertaken of samples at each trial pit and borehole. This frequency should be reviewed by the Geotechnical Engineer on site and the scope reduced or increased as necessary.

Notes

It is important to ensure that the proposed road does not interfere with the hydrology of the area. Note should be made of any groundwater in the trial pits and any evidence of collecting water in the area such as evaporation salts or lush green grass in low areas.

In built up, urban areas, special attention should be given to locating the trial pits to avoid senrices such as electricityyfuater or Q-Tel.


Table B3 Schedule of Geotechnical Investigations for Structures at Feasibility Stage

I


Structure

Interchange

.

Box Culvert

Embankment /Cutting

. .

Special Structures

Description

Boreholes At least one borehole to be located at each proposed abutment position.. The borehole should locate rock head and petrate 5m into rock. Where rock is not present, the borehole should extend a minimum of 1.5 times the width of a shallow foundation. If piled foundations are anticipated, the borehole should extend to rock plus 5m.. A local geotechnical expert can advise on anticipated depths for various locations in Qatar. Groundwater shall be recorded if present. If groundwater is likely to be a problem, it is recommended that the water level Is monitored over a period to allow for seasonal variation.

Trial Pits It is advisable that trial pits be located on selected slip roads and tests undertaken to determine the parameters required to design the earthworks, see Embankment below. Trial Pits would typically be up to 2.5m deep.

Trial Pits: At least one trial pit to be located at the proposed culvert position. Trial Pits would typically be up to 2.5m deep.

Boreholes For embankments/cuttings 2.5m highldeep or greater, at least one borehole to be located at the proposed embankmenVcutting position. If the embankmentjcutting is very long, boreholes should be located every kllometre. Boreholes shwM extend at least 3m beneath the level of the bottom of the proposed ernbankmentkuthg. Borehdes should identify rock head and record groundwater if present. Standard penetration tests are usually recommended to determine hardness.

Trial Pits. For embankmenwcuttings less than 2.5m high, at least one trial pit to be located at the proposed position. Trial Pits would typically be up to 2.5m deep. For cuttings, Investigations should extend a minimum of 2m below cutting base level, or to rock. As such, trial pits may only provide information regarding the material to be excavated.

Site investigations for special structures such as ornamental arches, roundabout centre pieces, gantries or cantilevers for traffic signs will require individual consideration depending on the size of the structure and its location.

Notes

Trial pits alone are not sufficient for major structures.

Structures in the urban location may have the benefit of other geotechnical investigations carried out In the vicinity and so the scope of investigation works may be reduced.

Usually in rural locations, it is important to review topography and hydrology to locate the culvert.

If the cutting Is deep, the engineer should consider the stability of the slopes. Boreholes should therefore be staggered across the cutting and not just follow the road centreline. Laboratory tests should identify parameters for slope stability and settlement to verify that it is possible for an embankmentlcutting to be built


I Structure 1 Description I Notes 1 Interchange

- - p- - - - -

Boreholes At least four boreholes to be located at the site of a typical Interchange In addition to those taken along the main carriageway through the interchange.

Boreholes should locate rock head and typically penetrate 5m Into rock or 5m below formation level, whichever Is the deeper. Groundwater shall be recorded if present. If groundwater is likely to be a problem, it is recommended that the water ievel is monitored over a period to allow for seasonal variation.

Plate Bearing Tests Test to be carried out at foundation level for all fou~~dation locations on the advice of the geotechnical expert.

Permeability Tests: Falling head or constant head permeability tests to be undertaken in boreholes. Necessary h e r e surface water from highways will require collection and discharge. .

Tiial Pits: It is advisable that trial pits be located on each slip road and tests undertaken to determine the parameters required to design the earthworks, see Embankment below. Trial Pits would typically be up to 2.5m deep.

-- -- - -- - - - -

Trial pits alone are not considered sufficient for major structures.

Structures In the urban location may have the benefit of other goetechnlcal investigations carried out In the vicinity and so the scope of lnvestlgation works may be reduced.

Depth of borehole to extend 5m below depth of proposed foundation.

Plate Bearing Tests. Test to be carried out at foundation level for all foundation locations on the advice of the geotechnical expert.

Box Culvert

Boreholes Depending on the findings of the Feasibility Investigation It may be necessary to locate a borehde at the culvert position. Boreholes would typically be extended to 3m below the foundation level.

Trial Pits: At least one trial pit to be located at the pmposed culvert position. Trial Pits would typically be up to 2.5m deep.

Usualiy In rural locations, It Is Important to review topography and hydrology to locate the culvert.

Embankment /Cuttings

p- - -

Borehdes For embankmentsfcuttings 2.5m highldeep or greater, at least one borehole to be located at the proposed embankment/cutting position. if the ernbankment/cutting Is very long, boreholes should be located every kilometre.

Borehdes should typically extend at least 3m beneath the level of the bottom of the proposed embankment/cut'ng. Borehdes should Identify rock head and reoord groundwater if present. Standard penetration tests are usually recommended to determine relative density.

Trial Pits For embankments/cuttings less than 2.5m high, at least one trial pit to be located at the proposed position. Trial Pits would typically be up to 2.5m deep. 1

Laboratory tests should identify parameters for slope stabiri to vedfy that It is possible for an ernbankmentlcutting to be built (Bulk Density determink air/water voids, Shear Strength determines bearing capacity).

Table I34 Schedule of Geotechnical Investigations for Structures at Design Stage

S m a l Structures

Notes

Site Investigations for special structures such as ornamental arches, roundabout centre pieces, gantries or cantilevers for traffic signs will require individual consideration depending on the size of the structure and its location.

1 Whilst detail design information is not usually required at the early stages, it is better to provide as much geotechnical information as possible, as early as possible.

Note : The choice of borehole depth should be at least to the depth of the extent of the pressure bulb set up by the foundation. The final decision on whether to continue the borehole further should be made by the geotechnical engineer on site. '

2 Care should be taken when locating boreholes and trial pits, to ensure that sekices are not damaged during the investigation. This is palticularly important in the urban situatlon.


83.2 Field Tests APPENDIX 8

83.3 Laboratory Tests

Field tests to determine the density, bearing or shear strength of in situ material are very valuable as they can be carried out without disturbing the soil.

Laboratory testing will be required on the samples taken. Table B6 lists the most commonly used laboratory tests and gives guidelines on the frequency of testing. The tests are discussed in Section B4.

Whilst each testing programme must be tailored to suit the particular site investigation, Table 85 gives guidelines on the frequency of testing for the most commonly used tests.

Test

Atterberg Limits

Particle Size Distribution

California Bearing Ratio

Chemical Tests

Dry Density / Moisture Content Relationship

Moisture Content and Density

Triaxial

Standard Penetration 1 Test

Test

throughout depth of

Notes

Plasticity Index, liquid limits

Used in grading and classification of material

Used for pavement design. Shall be carried out in accordance with QCS.

pH, Sulphate & Chloride. Used to check compatibility of materials and aggressiveness of ground and water on concrete structures.

Essential for slope stability in embankments/ cutting

Essential for all testing regimes - relates sample to liquid and plastic limits

Determines shear

Unconfined Compression Test

Frequency

2 tests per trial pit borehole


2 tests per trial pit /borehole




if suitable

Notes

Compression Test

Unconfined Compression - Test

Point Load Test

Gives shear stress of soil . .

Frequency

If cohesive soils. 1m intervals throughout depth of borehde

strength for cohesive soils

Gives shear stress of soil

Determines ground bearing pressure (for rock only)

I California Bearing Ratio

samples recovered

If suitable samples recovered

2 tests at selected boreholes

In situ used as a guide for pavement design. QHDM uses laboratory CBR for design.

'2 tests per trial pit /borehole

Standpipe Plezometer

Monitoring water levels

I test per borehole with regular monitoring

ground bearing structure

Shear Vane Test , .

Measures shear strength of soft soiis

If cohesive soils. Im intervals throughout depth of borehole

Used to determine permeability rates for soakaway design

3 test per borehole

Permeability Test

In Situ Density Test soils soils. 1m

intervals throughout depth of

d also be included at changes of strata.

Table B6 Schedule for Laboratory Tests

It may not be necessary to carry out all the testing listed in Table B6. The engineer responsible for preparing the brief may decide to reduce the scope depending on the information he needs for the design.

; Note: Tests shol

Table 65 Schedule for Field Tests

The testing frequency given in Table 65 is shown as a guideline. It is common practice for the schedule to be revised by the geotechnical engineer responsible for the site investigation as the investigation proceeds.

January 1997 Page B/6


84 ENGINEERING CONSIDERATIONS

84.1 Methods of lnvestlgation

When discussing the procedure for investigation, reference was made to borings as a means of investigation. This is perhaps the most common method of site exploration, but certainly not the only one. BS 5930: 1981 Code of Practice for site investigations provides details of investigation methods to assess ground conditions for construction purposes. Considering new works, from very small to very large contracts, a general guide to exploration would be as follows:

Small works - trial pits up to 3.0m deep

Mediumlo large scale works - borings up to 30m deep, typically 20m

Very large scale works (e.g. grade separation and dams) - a combination of deep borings and pits.

It must be noted that the above is only a guide, the detailed methods of exploration would depend on the type of construction and site involved.

Where rock is expected, borings of various types should be used unless a number of pits would prove more economical. In soils, the normal method of exploration is by boring holes (unless the loads expected are small, then shallow pits will provide adequate samples for testing).

The cost of setting up drilling rigs on site varies from area to area depending on transportation costs.

Before an estimate can be established for site investigation work, the number of boreholes and types of test must be determined. This will be dependant on how much information is already available.

84.1.1 Trial Plts

This is the cheapest form of exploration in shallow depths (e.g. up to 3m). Above 3 metres deep, the cost increases rapidly compared with boring. The main advantage is that soils and rocks can be exposed and examined in situ. This method shows changes in strata much more clearly than by borings. The pits are dug out either by local labour or by a small tractor-mounted excavator. The plan size of a pit depends on method of excavation,

APPENDIX B

but approximately 1.2 x 1.2 m should be dug. Holes should be kept well clear of the position of actual foundations, but should be in the vicinity of important structures such as heavily- loaded walls or columns.

Problems occur in water-bearing soils, particularly sands, and therefore the economies of shoring and pumping pits may outweigh the savings gained against specialist borings. In dry conditions, these pits are particularly valuable since they allow hand-cut samples to be taken, thereby minimising the disturbance of the sample and maximising the conditions for accurate testing.

Deeper trial pits may be used in the investigation of rock fissures or to explore layers of weak rock which cannot be removed intact in normal boring operations. Such deep pits are costly to construct and would be used only in large scale exploration.

Trial pits are often the best method of exploring back filled areas and sites overlain by variable natural deposits.

84.1.2 Boreholes

This type of exploration can be achieved by various methods:

Hand or mechanical auger borings are relatively cheap methods of sub-surface exploration of soils which 'will stand unsupported. Hollow stem augers can be used to support soils in borings. Holes can be sunk to depths up to 30 metres provided there are no obstructions such as boulders. The diameter of the borehole is usually >100mm. This allows soil sampling tubes to be used without difficulty. The mechanical auger is used in gravelly soil, which involves the use of a casing to prevent collapse of the boring.

percussive boring is a method which can be carried out in all types of soils, because the borehole is lined with a thick-walled steel casing. The boring is achieved by using open ended shells in cohesive soils and clack valves in cohesionless soils.

Other tools include chi.sel bits for breaking up boulders. All the tools and sampling tubes are attached to sectioned rods.


If the walls of the borehole require support, the borehole is lined as the hole is bored and the section linings screwed together and driven as the hole deepens.

APPENDIX B.

In boring operations, it is common practice to obtain 'bulk' disturbed samples in order to obtain sufficient sample for compaction and CBR tests, together with full gradings if the soil is granular in nature. This is particularly applicable if the bore is penetrating a proposed cutting.

Percussion boring is the oldest method of boring, in which the formation is broken up by repeated blows from a bit or chisel. Water-is added to the hole as the work proceeds and the resulting debris is removed at intervals by shell, auger or pressure washing. Samples from this type of boring are inevitably disturbed. A new system using a compressed air hammer provides a quick method of boring for permeability tests. This method does not facilitate core sampling.

Undisturbed samples: these are samples removed by methods which preserve, so far as practicable, the natural structure and properties of 'the material. Samples in this category are easily obtained in rock and clay, but difficult in certain other soils. Table B1 .I show the method employed for obtaining samples.

Rotary coring, which is used for the exploration of rocks, can 60 divided into two categories:

SAMPLING METHODS

Core Drilling Core drilling is a process designed to recover continuous cores of rock. Water or compressed air is jetted down the hole through hollow rods and returns up 'the annular space carrying rock cuttings from the coring bit. For hard rock cores, the crown of the drill is usually tipped with industrial diamonds. The continuous cores are laid in wooden core boxes in depth order.

Soil

Rocks

Table B1.l Sampling Methods

84.2 Testing

Disturbed

Undisturbed

Disturbed

Undisturbed

Mud-rotary Drilling In mud-rotary drilling, a mud-laden fluid is pumped in a continuous stream down hollow drilling rods to the rotating bit. The bit is kept in contact with the face of the boring and the fluid carries the debris up the annular space between the rods and the sides of the hole. A steel casing to the hole is not necessary. The cores are obtained by the use of coring tools. This type of drilling is not normally used for site investigation work.

Hand samples Auger samples Shell samples

Hand samples Core samples

Sludge samples from percussion or rotary drills

Hand samples, cores

For any particular location, the engineer must first establish the depth and classification of each strata of subsurface material and compare this with what was envisaged. To do this, a range of tests will be required.

Having completed the tests and reviewed the results, the engineer should consider whether the investigation has confirmed his initial assumptions or whether if has introduced new problems.

Cone Penetration Tests Where a significant thickness of unconsolidated overburden is know to exist, Static 'Dutch' Cone Penetration tests could be conducted to a suitably agreed depth. Methods and equipment in accordance with BS 5930.

842.1 in Sltu Testing

Tests to obtain the density or shear strength of soils in situ are very valuable since they can be carried out without disturbing the soil. Such tests are particularly valuable in sands and silts. The main tests are:

64.1 -3 Samples 'Standard penetration

There are two types of sample.

• Unconfined compression

California Bearing Ratio (CBR)

Monitoring water levels

Disturbed samples: these are samples removed from boreholes with augers or other equipment which interfere with the natural structure of the material. Such samples are useful for visual grading and determining moisture content, and in some cases for laboratory testing. Samples are placed in airtight jars with identifying labels. Plate bearing test


a Shear vane test

a In situ density test

a Permeability test.

Standard Penetration Test As with all penetration tests, this consists of measuring the resistance of the soil ta penetration under dynamic loading. This particular test is made by driving a 35mm (internal diameter) split spoon sampler into the soil at the bottom of a borehole. The sampler, suspended on rods, is first driven 150mm into the soil by a falling standard weight (63.5 kg falling through a distance of 760mm). The sampler is then driven a further 300mm and the number of blows needed to achieve this is recorded as the 'N' value. The test is used to establish the relative density of soil, and for particular soils to design foundations and gauge settlement

Callfornla Bearing Ratio Test This test may be used in the design of flexible pavements and can be carried out on site. The test shows the load-penetration of soils relative to a standard crushed stone sample. The test is normally carried out on soil at least 1 m below ground level (i.e. below the level of any seasonal moisture fluctuation) using a lorry to obtain the necessary reaction load through a screw jack.

The in situ CBR test provides a different result to that obtained in the laboratory under similar conditions of density etc. Road design is normally carried out based on the laboratory CBR only.

standpipe Piezometer Monitoring of water levels is carried out by the use of piezometers. If a borehole is to be constructed to obtain soil information, then unless circumstances dictate otherwise, it should be utilised in order to monitor the fluctuation in ground water level. This may be carried out for several years depending on the time scale of the project. Such information will be invaluable in the future once general trends have been established.

Plate Bearing Test. This type of test was once very popular and is still used on large engineering projects as a means of providing in situ data on the behaviour of soils or rocks at foundation level. The procedure consists of excavating a pit to the level of the proposed foundation and then loading a steel or cast iron plate (usually 600 x 600mm in size) on the bottom of the pit. The

APPENDIX B load can be'applied in either of two ways; the first by loading it with increments of kentledge (concrete blocks or steel billets); the second by means of a hydraulic jack bearing against a heavily loaded beam.

Failure is traditionally assumed when the settlement reaches a depth equal to 10% (some engineers say 15%) of the breadth of the loading plate, this should be verified by plotting a time/load/settlement graph. The safe load (qs) should be taken as one-third of that load which causes failure or the failure load divided by the project factor of safety. For most structures, a generally accepted maximum allowable settlement is 25mm. Terzaghi & Peck have proposed a relationship which enables allowable bearing pressure to be calculated based on a chosen allowable settlement and the loadlsettlement results obtained from a plate bearing test.

The plate bearing test is useful in stony soils where undisturbed sampling is difficult. However, care should be taken to ensure enough tests are taken to be representative where soils may be variable across a site.

The plate bearing test data can also be used to calculate a soils modulus of subgrade reaction.

One disadvantage of this test is the lack of simulation of 'bulb pressuren. The bulb pressure from a test of this nature is usually far smaller than the bulb pressure from the actual foundation. This could lead to error in detecting settlement of a lower weak stratum.

Shear Vane Test 'This test measures the shear strength of soft cohesive soils in situ. The vane is pushed into the soil and rotated by hand at a constant rate. The amount of torque necessary for rotation is measured by a spring balance on top of the rods and the shear strength of the soil is calculated.

In Sltu Density Test Typically sand replacement or nuclear density tests are undertaken in the field. These provide the field density of soils and are useful in assessing compaction and settlement.

Permeability Test . This test enables the permeability of the soil or bedrock to be ascertained. The most common type of permeability test undertaken in Qatar is the falling head test to BS 5930. However, the type of test and the number per borehole should be agreed with the Civil Engineering Department.

Page 619


Dynamic Cone Penetrometer Test This test utilities a hand held drop hammer penetrometer which records cone penetration resistance versus number of blows. Graphical plots of results enable equivalent in situ CBR values of the ground to be determined. (Typically used for depths up to 2.0m). Refer to the TRL specification for DCPT equipment and CBR correlation relationship.

Other Tests Other. in situ tests include the hand penetrometer and hand shear vane.

84.2.2 Laboratory Testing

Laboratory testing is undertaken to establish the following characteristics of soils:

Identification and classification

Measurement of engineering properties

-. Chemical content.

Identification and Classification This analysis involves a number of individual tests, such as:

- . . Visual examination - Moisture content - Liquid and plastic limits - Particle size distribution.

Visual Examinations: made to note the colour, texture and consistency of disturbed and undisturbed samples, these being used later to describe the soil in the engineer's reports.

Molsture Content: important in all soil samples, since it helps to arrange a programme of testing (by relating samples to liquid and plastic limits) so that no doubtful sample will be overlooked. The higher the natural moisture content of the soil, the greater will be its compressibility.

Liquid and Plastic Llmit Tests: made on cohesive soils for classification purposes and for assessing their compressibility. The liquid limit (LL) (BS 1377 Test 3 and 4) determines the amount of moisture content necessary to cause the material to flow or move readily under a given number of vibrations, whereas the plastic limit (PL) is determined by rolling out a 4 mm diameter thread of soil and noting the moisture content which will allow the thread to be rolled out still further until it breaks up due to drying. When both liquid and plastic limits are known, the Plasticity lndex can be established (Plasticity index = LL-PL).

APPENDIX B

Particle Size Distribution: of particular importance when assessing problems of excavation in permeable soils below the water table. It is also useful for assessing the value of non-cohesive soils for use as aggregates and construction materials. The first part of the test is achieved by sifting dried samples through BS 410 sieves. In the case of cohesive soils, a wet analysis is used, employing a hydrometer. The range of particle sizes is compared with a standard chart. PSD is also useful for identification purposes and assessment of material suitable for use as fill.

Measurement of Engineering Properties The foregoing tests give some indication of the engineering properties of a soil or rock, but there are also specific tests which yield more definite information relating to:

- Bulk density of soil - . Shear strength of soil - Consolidation of soil - Laboratory CBR - Laboratory compaction - Point load testing - Unconfined compression testing

(+deformation modulus)

Bulk Density: the weight of material per unit volume, including the weight of air or water in the voids. This information is essential in the design of retaining works, where the weight of a stratum is an important factor (e.g. stability of slopes, formation of earth dams, earth pressure of retaining walls etc). Dry density (weight of solids per unit volume) is used for the determination of optimum compaction in earth dams, embankments and other soil structures, and in the laboratory CBR test.

Typically cone-cutter and 'sand replacement tests are carried out to determine bulk density.

Shear Strength: can be used directly to calculate a soil's bearing capacity and also to calculate the pressure on supports in excavations. There are several tests available for ascertaining shear strength, but the most popular is the triaxial compression test. Triaxial compression tests are suitable for cohesive soils only. Where cohesionless soils have to be tested, the shear box test is used. A sample of soil is subjected to a standard load under which a horizontal force is applied to the lower half of the box until the sample shears.

January 1997 . - . Page BI10


The Triaxial Compression Test can be carried out using any of three different methods:

Undrained

Consolidated undrained

Drained.

In principle, the test consists of subjecting a cylindrical sample of undisturbed soil (75 mm long x 38 mm diameter) to lateral hydraulic pressure in addition to a vertical load. This is achieved by placing the sample in a specially designed plastic cylinder which is subsequently filled with water. Both vertical and lateral loads can be increased as required in order to simulate the in situ stresses. Measurement of the forces needed to shear the sample is used in the calculation of bearing capacity. In the undrained triaxial test (often referred to as the quick test) the sample, encased in a rubber sheath, is capped with non-porous end plates to prevent the pore water escaping and allow axial loading of the ends. Three tests are carried out, one each on three samples (all cut from the same large sample ), each being subjected to a higher hydraulic lateral pressure before axial loading is applied. The results are then plotted in the form of Mohr's circles.

The consolidated undrained triaxial test allows the sample to drain while applying the hydraulic pressure, thereby allowing the sample to consolidate. After consolidation the sample is stressed without further drainage.

In the drained test, the axial load is applied so slowly that the pore water can drain off .without building up any pressure in the sample. The drainage continues throughout the test and the amount of water drained off. is measured. In both cases, where drainage is achieved, the water passes through porous discs at the ends of the sample and then through ducts in the apparatus.

The consolidated undrained test and the drained test have particular application to the behaviour of soil in earth dams and embankments, and also to stability problems in general.

Consolldatlon test: used to calculate the magnitude and rate of consolidation of a particular soil. This is very important in calculating the movement of soil under foundations. The apparatus used is called an 'Oedometer'. The test consists of placing a cylindrical sample (75 mm diameter x 18 mm thick) in a metal ringand capping with porous discs. The sample is placed in a water-filled

APPENDIX B

tray and subjected to load. The load is increased every 24 hours and a time- settlement curve is plotted. Again, this is only suitable for cohesive soils.

Laboratory CBR: shows the load-penetration of soils relative to a standard crushed stone, (see Clause 84.1.3). The test is carried w t in a controlled laboratory situation and is of great importance as it is laboratory CBR values that are referred to in QCS and Section 9 Pavement of QHDM, and which construction materials and subgrade should meet.

Laboratory Compaction: provides the optimum moisture content for a soil sample.

Successive samples of soil are progressively wetted and compacted in a mould. The'dry densitylmoisture content of these successive samples is then plotted to find the optimum moisture content. Typically, the Proctor test is carried out (in accordance with BS 1377) though the modified AASHTO and vibrating hammer techniques are also commonly used.

The value of optimum moisture for the soil is useful for preparing a soil prior to site compaction in order to ensure minimum compactive effort and specification compliance.

Results achieved are also used in other laboratory tests such as the CBR test.

Point Load Testing on Rock: involves the determination of failure strength of rock core samples either by loading axially, diametrically or irregularly. Refer to BS 1377 or ISRM (International Society for Rock Mechanics).

Unconfined Compression Testlng (plus measurement of Deformation Modulus on Rock): involves measurement of failure strength and deformation characteristic of prepared samples. This test can be used either in the site laboratory or in the field, since the apparatus is very portable. This method is therefore particularly useful where a large number of samples are required to be tested. Rock samples 75mm long and 38mm diameter are placed in the apparatus and an axial load applied. The sample is sheared under load and the shear stress is automatically recorded on a chart fixed to the apparatus. Refer to BS 1377.

Sedlmentatlon ~ e s t : used to assess whether material is a silt or a clay. Refer to BS 1377.

Laboratory Permeablllty: used to determine permeability of reconstituted samples, eg. subgrade or roadbase materials.

Page 8/11


Minlature Shear Vane: carried out on borehole samples of cohesive material. Commonly used when samples are not suitable for other testing.

Chemlcal Content: a chemical analysis of soils, rocks and groundwater is carried out to assess the effects, if any, which their composition might have on any material to be used in the proposed works. The tests mainly cover sulphate and chloride content and pH value, although bacteriological analysis .may also be -- required for works in tidal mud flats.

. .. - 84.3 ' Earthworks

Earth moving for roads takes place over a relatively narrow band of terrain and a balance of cut and fill is often difficult to achieve. Constraints to the profile are imposed by the need to provide .required clearances for bridges under or over existing roads or to cross them at their existing level.

Earthworks should be designed to provide an adequate safety factor for shear fdlure and to ensure that any deformation is within acceptable limits. The information required before the cross section of the embankment can be-designed includes:

Ultimate width of top of embankment .' including median, shoulder and verge

Loading on top of embankment

Geotechnical properties of foundation and fill materials

Reservation width

Special considerations, eg. tidal area, sound barriers, services etc.

In the design of approach embankments to bridges and other structures, the superstructures, substructures and associated earthworks should be designed as a whole and not individually. For further reference on earthworks refer to British Standard 6031, Code of Practice for Earthworks. The road design should attempt to minimise earthworks. The aim should be to balance cut and fill requirements, allowing for rejection of unsuitable material, bulking and compaction factors. This will avoid having to dispose of, or

, obtain large quantities of material.

All imported fill material for a CED scheme , must be provided using the services of the

Qatar National Transport Office. The selection of such material and its placing and compaction

' -.'. APPENDIX 8

shall all be in accordance with the Qatar Construction Specification.

The Ground Investigation Report should identify the rock horizon for areas of cut, should suggest methods of excavating the material, and should identify whether the material is likely to be suitable for use as a fill material.

84.4 Retalnlng Structures

~here'sufficient land width is not available to accommodate the full width of the base of the embankment, the provision of earth retaining structures has to be considered. Below is a list of some of the different types of earth retaining structures commonly used.

Gravity walls in mass concrete, brickwork or stone masonry

Reinforced concrete walls, counterforVbuttress

Diaphragm walls

Piling walls

Crib walls

Gabions

Reinforced earth walls.

Gravity Walls Gravity walls are suitable if the soil in the lower part of the cutting can be cut back steeply to a temporary slope to allow the wall to be constructed. Any space between the back of the wall and the temporary slope is then backfilled.

Reinforced Concrete Walls Reinforced concrete walls are suitable if the soil in the lower part of the cutting can be cut back steeply to a temporary slope to allow the wall to be constructed. Any space between the back of the wall and the temporary slope is then backfilled. Alternatively, these walls can be constructed in a timbered trench, the soil in front of the wall being removed after completing the retaining structure.

Diaphram Walls Diaphragm walls, continuous bored pile walls and secant bored piles are suitable for weak, unstable or heavily water-bearing soils where a temporary steep slope cannot be formed or where construction in a trench would cause problems of support or loss of ground.


QATAR HIGHW~Y DESIGN MANUAL

Piling Walls Steel sheet piling may be used as a permanent retaining wall if consideration is given to some measure of protection against corrosion where a very long life is required. Usually, however, sheet piling is used as a temporary support during the construction period.

Crib Walls Precast concrete block walls or crib walls are a form of gravity section and may be economical for sites where suitable broken rock or gravel is available as a fill material for the cribs.

Gablon Walls Gabions are suitable for sites where broken rock, boulders or large gravel are available for filling the wire mesh baskets and where space is available to arrange the baskets in tiers to form a stepped-back retaining wall. A very long life is not possible with gabion walls, but plastic-, covered galvanised wire mesh can provide many years of useful support. The flexibility of a gabion retaining wall is advantageous for sites where appreciable deformation of a slope may occur as a result of stress relief.

Gabions are particularly suitable for construction in conditions where earth slopes are temporarily or permanently flooded and subjected to scour from flowing water.

Reinforced Earth Walls Reinforced earth retaining walls can be formed in the lower part of a cutting slope by excavating at the toe to form a temporary steep slope, then replacing the excavated soil in compacted layers of essentially granular material, each layer being reinfqrced by horizontal metal or plastic ties (refer to Clause 85.3). The steeply inclined face of the retaining wall is protected by metal, reinforced concrete or plastic cladding elements. Reinforced earth retaining walls have the advantage of flexibility and are suitable for soil conditions where appreciable forward movement or heaving of a cutting is anticipated as a consequence of stress relief.

Consideration may also be given to the use of ground anchors or rock b'olts. Information on methods of design and construction of the above types of wall can be found in the British Standard publications, BS 8002 Code of Practice for Earth Retaining Structures and BS 8004 Code of Practice for Foundations.

In all cases and for all types of retaining walls attention should be given to drainage at the back of the wall in order to prevent hydrostatic pressure on the retaining structure and to avoid a general rise in pore pressure in the soil or rock mass behind the wall.

. \ . .

'.' APPENDIX B

84.5 Geosynthetlcs

Geosynthetics are extremely versatile and may be used in the following instances:

Reinforcement for subgrade and sub- base materials in roads

Line drainage facilities, eg. wrapping aggregate around soakaways to prevent loss of fine materials, to separate materials of different grade or placed behind a retaining wall to act as a drainage medium

Reinforcement for soil slopes (cutting or embankment)

Provide a capillary barrier against rising ground moisture.

In all cases the engineer shall refer to the manufacturers technical literature and check the suitability of a geosynthetic for the particular application.

There are a'number of different trade names of geosynthetics. available and the usage and design of such materials is dealt with within their own respective technical literature.

B 5 SAMPLE PRO FORMA . FOR QUANTIFYING . GEOTECHNICAL SITE INVESTIGATIONS

The following five pages show a sample pro forma Bill of Quantities to be used when quantifying a site investigation (with notes). The testing programme shown may be reduced or expanding according to the type of investigation required.

It is important to identify each element of work required in a schedule in as much detail as possible. This serves two purposes. Firstly, it acts as a checklist and enables the engineer to list precisely the requirements of the brief. Secondly, a detailed list with item descriptions enables the brief to be priced by the tenderers on an even basis and reduces the probability of hidden extra costs.

The pro forma has been split into three sections with notes:

Fieldwork

Laboratory work

Reporting.

Januarv 1997

QATAR HIGHWAY DESIGN MANUAL 'APPENDIX B I I -

9 *$

Scope Of Works for Geotechnical Investigation

SECTION 1 - FIELDWORK

Project Code

Project Title

' is A . . . . .

January 1997 -. Page 8/14

Ref

F1

F2

F2.1

F2.2a

F2.2b

F2.3

F2.4

F2.5a

F2.5b

F2.6

Item Descrlption

Wal koverJDes k Study

-#.

Notes

Including all permits, and reporting. Available data from existing geotechnical mapping and any other sources shall be combined with a geological walkover survey of the site. The combined silrvey shall identify such areas as rock outcrop, sabkah, water courses, water collection areas etc. The results are to be marked on topographical plans of 1:2000 scale or 1 :500 scale as directed. One copy of the results are to be submitted to the Engineer as part of the Site lnvestigation Report (refer R1).

I

Qty

1

Unit

Item

Boreholes

Mobilisation

Drilling of Boreholes

Additional Drilling

SPT in Borehole

Standpipe in Borehole

Rotary OpenDrilling

Additional Drilling

Permeability Test in Borehole

1

Rate

Rate'

All items associated with all mobilisation for bore oles including location of boreholes. f, The approximate location of all boreholes, trial pits and surface samples shall be indicated on the contract drawings. The precise positions shall be agreed with the Engineer prior to commencement on site.

Light cable percussion and rotary core drilling to 20m, including hand dig for services as required, liaison with utilities, moves between boreholes, photograpQs, bdrefble logs, reinstatement of boreholes and reporting. The Cor~tractor shall provide full information on the strata and the engineering properties of all soils and rock encountered. See Notes 5,7,8, 9, 14.

Additional drilling depth rate per m below 20m b.g.1.

In situ SPTs shall be made on all cohesionless and non-cemented strata, in accordance with QCS Section 3 Part 4 Soil Sampling.

Installation and monitoring of standpipe in borehole.

Rotary open drilling 100mm diameter, including collection and logging of chipping samples to depths of 30m b.g.l., including reinstatement.

-r

Additional rotary open drilling depth, rate per m below 30m b.g.1.

Falling head test to BS 5930.

Rate (QR)

Total (QR)

-

Item

Nr

m

Nr

Nr

Nr

m

Nr

,

i f ? I,?

- -

I F4 1 Additional Methods I

QATAR HIGHWAY DESIGN MANUAL , APPENDIX B

Pavement Coring

Ref

F3

F3.1

F3.2a

F3.2b

F3.3

F3.4

Cores shall not be less than 150 mm diameter and shall be taken through the full thickness of the asphalt pavement, such that the underlying, unbound material is exposed. All core holes shall be backfilled with fine cold asphalt mixture or similar approved, placed and compacted in layers using a suitable tamper such as a plate attached to vibrating hammer. Backfilling shall take place immediately upon completion of

I testing.

Dynamic Cone Penetrometer Testing on Pavements

Item Description

DCP testing in accordance with TRL Information Note at core locations shall be carried out immediately upon completion of coring and the hole is then sponged dry. A profile of the bearing capacity to a depth of 800mm below the road surface or until resistance to penetrate is such that for 30 blows less than 5mm of penetration is achieved. The DCP plot and profile shall be provided at each location. See Note 10.

Dynamic Cone Penetrometer Testing

Notes

- -

DCP testing in unconsolidated material to a depth of 2m or until resistance to penetrate is such that for 30 blows less than 5mm penetration is achieved. The DCP plot and profile shall be provided at each location.

Trial Pits

Plate Bearing Test

Qty

Mobilisation

Excavation of Trial Pit Hand Excavation

Excavation of Trial Pit Machine Excavation

Reinstatement of Trial Pit

Soakaway and permeability Test

- --

Shear Vane Test

Test to .be carried out at foundation or formation level. Plate to be approximately 600mm sq, loading details to be suitable for project requirements. Contractor to supply reaction load.

Unit

All items associated with mobilisation for trial pits.

Excavation to 1.2m, including liaison with utilities, moves between trial pits, photographs, trial pit logs and reporting. See Notes 6,7,8, 9,14.

Excavation to 3.0m including liaison with utilities, moves between trial pits, photographs, trial pit logs and reporting. See Notes 6,7,8, 9,14.

Trial pits shall be backfilled and compacted in accordance with QCS. All materials shall comply with QCS.

See Note 17.

Test shall be in soft sensitive clays. Vane to consist of four blades 75mm x 150mm. 1 4 . 1 In Situ Density Test

Rate (QR)

1

Total (QR)

Item

Nr

Nr

Nr

Nr

Tests shall be by core cutter, sand replacement or nuclear density metre as appropriate to the soil type.

Tests in accordance with BS 5930. F4.7

January 1997 Page BM5

In Situ CBR

QATAR HIGHWAY DESIGN MANUAL APPENDIX B - . -

SECTION 2 - LABORATORY WORK

SECTlON 3 - REPORTING

Ref

L1

L2

L3

L4

L5

L6

L7

L8 .

L9

L10

L11 .

L12

. * NOTES

Item Description

Atterberg Limits

Particle Size Distribution

CBR

Chemical Tests

Dry Density / Moisture Content Relationship

Moisture Content and Density

TtiaxialTest

Point Load Test in Rock :--

Unconfined Compression Test (with Modulus of Deformation on Rock)

Sedimentation Test

Laboratory Permeability Test

Miniature Shear Vane Test

1 These notes apply to Field Work, Laboratory Work and Reporting.. It is assumed that the rates , ,

for the above items include for the requirements of these notes. . L

R1

2 The purpose of a geotechnical investigation is to provide infomiation to determine parameters sufficient for concept or detailed design, as required. The investigations should enable the A

Consultant to advise the Engineer on the requirements necessary for further investigation work that wiil enable quantification of the project.

Notes

See Note 12.

See Note 12.

See Note 12.

pH, sulphate and Chloride. See Note 12.

See Note 12.

See Note 12.

See Note 12.

See Note 12.

See Note 12.

See Note 12.

See Note 12.

See Note 12.

1

3 L

All works shall be carried out in accordance with QCS Section 3 Ground Investigation.

Item Site Investigation Report

4 The Contractor shall exercise the greatest possible cam to ensure that both field and laboratory work are of the highest quality. - -

Qty

Comprehensive factual and interpretative Geotechnical Report, including photographs, the number of copies to be as specified. See Note 13.

5 The measurement of the depth of the trial pits and boreholes shall be taken from the level at which the pit or bore enters the ground. The positions of all boreholes and trial pits shall be

A

recorded to within an accuracy of 1m together with the ground levels to the nearest 50mm. related to the Qatar National Datum (refer to QCS Section 3). This-information shall be recorded on the plans and submitted to the Engineer as part of the Report.

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January 1997 Page B/16

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6 rial pits shall be excavated to rock level or otherwise to the limit of the mechanical excavator, nominally a depth of 2.5 m. The depth of boreholes may be varied by the Engineer subject to the strata encountered on site. Bed rock in boreholes shall be proved for a minimum depth of 5m. In cuttings remote from structural foundations, the depth of boreholes shall be 3m below proposed formation level.

7 All excavations shall be logged by a fully qualified geotechnical engineer or engineering geologist and such logs shall form part of the Report. Refer to QCS Section 3 Clause 1.6.7.

8 The equipment used for excavation, bodng, sampling and testing shall be subjected to the , approval of the Engineer. Under no circumstances shall water be used to assist boring through

clay.

9 If any object, natural or artificial, obstructs either setting up or progress of excavating and boring the matter shall be reported to the Engineers Representative, who may direct the excavation or borehole to another location to avoid the obstacle.

10 DCP testing shall be in accordance with the UK Transport Research Laboratory (TRL) Information Note, Operating Instructions for the TRL Dynamic Cone Penetrometer, 1991.

. . . Analysis of the DCP reading shall be made using the latest version of the TRL DCP computer programme based on the following relationship between penetration resistance and estimated in situ CBR:

Log,,, (CBR) = 2.48 - 1.057 Log,, (Strength)

It should be noted that this formula may not be applicable to Qatar conditions and results obtained should be treated with caution.

The analysis shall account for the effect of water used in the coring process on the aggregate layers.

11 All rotary core samples shall be retained for a period of six months at the offices of the Contractor for the purpose of inspection. All core samples shall be colour photographed and postcard size prints inserted in each copy of the report. Photographs are to be taken at a distance from core samples to enable a detailed study of the core.

Small disturbed samples shall be taken at changes of strata and at approximately 1 .Om intervals within each type of material.

Bulk disturbed samples of at least 80 kg weight shall be taken in cohesive materials as directed by the Engineer at a change of strata and not greater than 1 .Om intervals within each type of material. One small disturbed sample shall be taken between each two successive bulk disturbed samples. The samples shall be sealed, transported, protected and stored such that no change in moisture content and soil structure occurs.

Surface samples shall be bulk disturbed samples of at least 80 kg weight and these shall be taken in accordance with the recommendations given in BS 5930.

Samples of groundwater of at least one litre shall be taken, and the level at which water is struck and standing water levels shall be observed and recorded

12 All laboratory testing shall be carried out in accordance with the relevant procedures given in BS 1377: 1990, Testing of Soils, save that the method for both compaction tests and recompaction of samples of the CBR test, which shall be in accordance with Central Materials Laboratory method of test CML 12-97 and CML 10-97.

Soil and groundwater samples shall be analysed for the following :

sulphates chlorides pH

. . grading 1 classification (as appropriate)


For each trial pit and borehole, soil samples shall be tested at each change in strata, with a minimum of 2 tests in the overburden above the rock.

Detailed engineering logs shall be submitted, in accordance with QCS Section 3.

13 The Contractor shall submit daily allocation sheets and preliminary logs and test results in accordance with QCS Section 3 Clauses 1.6.1, 1.4.1 and 1.4.3.

As soon as possible after the completion of the Laboratory Testing, the Contractor shall submit 5 copies of his factual and interpretative report, prepared in accordance with QCS Section 3 Clause 1.4.5.

14 The Contractor shall take all reasonable precautions to safeguard all existing on-site services. The Contractor will be held liable for any damage to such services which may be attributable to his negligence. Refer to QCS Section 3 Clause 1.6.6.

15 The Contractor will be expected to carry out the on-site works expeditiously and in one visit.

: i

16 The Contractor shall give a minimum of 48 hours notice, in writing, to the Engineer, before he commences any work on site. r . . .

The Contractor is to carry out the works to the entire satisfaction of the Engineer, and is to work i , . - in such a way that no inconvenience is caused to other contractors, statutory undertakers or the - general public who may be in the locality.

- The responsibility for obtaining Road Opening Permits and the like shall be upon the Consultant, who shall adhere to all the requirements of any authority.

The Consultant shall allow in his fee submission for all requirements of QCS Section 3 Clause < - 1.6.1 including hand excavation to determine the presence of utility lines prior to the commencement of mechanical excavation.

I - 17 In selected trial pits, the Consultant shall undertake tests to determine the suitability of the 1 L

substrata to dissipate water. The results of these tests shall be reported and utilised in the design of stormwater soakaways, positive drainage systems or water ground relief systems. The design of soakaways shall be in accordance the current CEO design practice and BRE Digest

. - 365, modified as appropriate for local conditions.

18 The location of utility lines encountered in the excavation shall be logged and their condition - noted. When trial pits are specified in the Project Brief for utilities location and condition surveys, the Consultant shall ensure that a representative of each utility company is present to confirm the responsibility of the apparatus encountered.

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January 1997 Paae 811 8

qatar highway design manual - 1997.pdf

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