pp2-execution methodology of flexible pavement

Execution Methodology for flexible pavements

of Rural Roads

BySri P. Ganeswara Rao,

M. Tech.,(Structures)

Chief Engineer, PR, Vig & QC

Execution Methodology of Rural Roads

Proper execution of roads leads to construction of good roads without defects and with long life with the same estimate cost.

Hence, understanding and follow of proper execution methodology by field engineers is very essential which enhances the department values.

Any road constructed shall have the following qualities:• Cost effective• Qualitative & Durable• Safe• Maintenance free or with less maintenance• Useful• Social acceptance

Various Components of the Road

• Sub-grade• Sub-base• Base• Surfacing• CD works & Protection works• Shoulders

Execution of Sub Grade: Sub-grade is the foundation to the crust, it should

be properly done. The top level of the sub grade shall be minimum

300mm above the existing ground level. If any water bodies are adjacent to the road, the top

level of sub-grade shall be minimum 600mm above the maximum water level.

The top level of the sub-grade shall not be too high which leads to high construction cost and construction of unsafe roads.

If suitable side earth is available, the sub grade shall be formed with side earth only.

Widening of existing Roads:

• Most of the road works that are being executed by PR department are done by widening the existing roads with some existing crust or without crust.

• While widening the existing roads proper care shall be taken without getting separation of new sub grade from the old sub grade material.

Steps involved in widening of the road formation:• Pre levels are to be taken before widening. • The CBR values of existing sub grade material is to be obtained @ 4 places per KM.

Atleast @ 3 places per KM.• The existing sub grade height and the thickness of sub base and base at different

mileages are to be noted.• Widening is to be done both sides of the road, if the water bodies are not adjacent to

the road.• Widening is to be done only one side i.e., fields side, if any canal or drain or minor

irrigation tank is existing other side. Water bodies side, the stabilized slopes shall not be disturbed.

• As far as possible the formation of road over irrigation bund is to be avoided.• Before widening, vegetation on the side slopes and on berms is to be removed along

with its roots.• Then widening with suitable soils is to be done and it is to be rolled with Vibro or 8 to

10 tonnes roller upto OMC level as per standards and specifications. Then proper sectioning of the side slopes is to be done.

• If the CBR value of sub grade material is less than 2%, sub grade stabilization is to be done or a continuous layer of medium sand with 100mm thick is to be provided across the total width of the road formation to drain off the seepage water.

• If the sub grade material is clayee and have CBR less than 3, then also a continuous layer of medium sand with 100mm thick is to be provided across the total formation width of the road.

• After construction of sub grade to the required height and width, final levels are to be taken to compute the total earth work executed.

Formation of Sub Grade for new roads:

Steps Involved:• Pre levels of the ground shall be taken.• If the side earth is suitable as sub grade material, it shall be used

other wise suitable earth may be carted with minimum possible leads.

• The earth shall be compacted in layers with vibro or 8 to 10 tonne roller upto OMC level as per standards and specifications. Then proper sectioning of the side slopes is to be done.

• If the formation is done on the hillock duly cutting the hillock to obtain smooth gradients, side drains shall be provided invariably.

• If the CBR of the cutting portion is above 15, then base and surfacing (flexible pavement) can be provided directly without sub base. In this case, it is preferable to provide Rigid Pavement on cutting portion without base material.

• After completion of formation, final levels are to be taken to compute the total earthwork executed.

Sub Base:Sub base granular material shall be properly selected with the following properties:• P.I. value shall be less than 6.• CBR value shall be more than 20. Atleast it shall be more than 15.• It shall confirm the MORD specifications clause 401.• If the naturally available gravel is not having the said properties, it shall be mixed with

local sand or quarry dust (whichever is economical) in the required proportion to attain the said qualities.

• In case of sub base over B.C. or clayee sub grade soils, half the thickness of granular sub base or minimum 100mm thick shall be extended throughout the width of the road, if the sand is not provided as the drainage layer.

• In case of sub base over very poor (CBR less than 2) sub grade material, graded chips confirming to MORD specification 401 may be used over modified sub grade material. It is costly material compared to natural gravel. Hence, usage of this material shall be justified.

• The required granular sub base material as per designs shall be properly compacted in layers using 8-10 tonne roller and proper curing is to be done.

• Simultaneously earth work for shoulders to support the sub base material is to be done and it is to be properly compacted in layers using 8-10 tonne roller and OMC is to be attained. It shall be properly cured along with sub base.

• The minimum thickness of sub base shall not be less than 100mm.

Clause 402 of MORD SPECIFICATION• Construction of granular sub base/shoulders by

providing well graded material (i.e., stone aggregates (Gravel) of 9.50mm to 4.75mm size - 66% and sand of size <2.36mm - 34%), spreading in uniform layers with motor grader on prepared surface, mixing by mix in place method with rotavator at OMC and compacting with three wheel 80-100 KN static roller capacity to achieve the desired density, complete as per technical specification clause 402.

Base:

The base material as per the designs shall be provided in layers usually in two layers of different graded materials (Grade-II & Grade-III of 75mm compacted thickness of each layer).

The base material should have the following properties:• Impact value shall be less than 20.• Well graded material as per the specifications shall be used.• WBM shall confirm to MORD specifications clause 405. The

screening and binding materials of required quantities with required specifications shall be used. Wet Mix Mecadam (clause 406) and Crusher Run Macadam (clause 411) can be used.

Grade- II material, screening and binding material shall be collected of required quantities. Then it is to be spreaded over granular sub base and is to be properly compacted and watered to push the screening material into the voids.

Later Grade-III material, screening and binding material shall be collected of required quantities. Then it is to be spreaded over Grade-II material and it is to be properly compacted and watered to push the screening material into the voids.

Simultaneously Gravel Shoulders are to be provided in layers and is to be rolled and watered along with grade-II material and grade-III material.

The minimum thickness of base shall not be less than 150mm.

Clause 405 of MORD specification

Grade-II• Providing, Laying, Spreading for a compacted thickness of 75 mm,

using stone aggregates of Grade - II as per Table 400.80 of MORD specification with 60-63mm @ 50%, 40-45mm @ 46% and 19-22mm at 4% HBG Metal for Water Bound Macadam specification including cost, conveyance and seignorage charges and spreading in uniform thickness, hand packing, rolling with Power Road Roller 8- 10 Tonnes in stages to proper grade and camber applying and brooming requisite type ' B ' screenings and binding materials such as Stone Dust so as to fill up the interstices of coarse aggregates, watering and compacting to the required density for finished item of work as per MORD specification clause 405 (2) (A) by manual means and as directed by the Engineer-in-Charge.

Grade-III• Providing, Laying, Spreading for a compacted thickness

of 75 mm, using stone aggregates of Grade - III as per Table 400.80 of MORD specification with 40-45 mm @ 82%, 19-22.4 mm @ 18% HBG Metal for Water Bound Macadam specification including cost, conveyance and seignorage charges and spreading in uniform thickness, hand packing, rolling with Power Road Roller 8- 10 Tonnes in stages to proper grade and camber applying and brooming requisite type ' B ' screenings and binding materials such as Stone Dust so as to fill up the interstices of coarse aggregates, watering and compacting to the required density for finished item of work as per MORD specification clause 405 (3) (A) by manual means and as directed by the Engineer-in-Charge.


Wet Mix MacadamProviding, laying, spreading and compacting graded stone aggregate to Wet Mix Macadam specification with 45mm to 22.40 mm @ 30%, 22.40mm to 2.36mm @ 40% and 2.36mm to 75 micron @ 30% including premixing the material with water at OMC in mechanical mixer (Pug Mill), carriage of mixed material by tipper to site, laying in uniform layers in sub base/ base course on a well prepared sub-base and compacting with smooth wheel roller of 80 to 100 KN weight to achieve desired density including lighting, barricading and maintenance of diversion of traffic etc., as per table 400.11 & 400.12 and technical specification Clause 406 of MORD.


Crushed Run Macadam Base

Providing crushed run stone aggregate grading 63mm to 45mm @33%, 22.50 mm to 5.60 mm @ 32% and below 5.60 mm @ 35% confirming to table 400.20 depositing on a prepared surface by hauling vehicles, spreading and mixing with a motor grade, watering and compacting with a three wheeled 80-100 KN static roller as per technical specification Clause 411 to form a layer of sub base/base.

Sieve Analysis of Grade-II, Grade-III Metal

GRADE-II Metal

GRADING REQUIREMENT OF COARSE AGGREGATES

Grading No. Size Range IS Sieve Designation

Percent by weight

passing

2 63mm to 45mm 90mm 100

63mm 90-100

53mm 25-75

45mm 0-15

22.40mm 0-5

Grade-IIIGRADING REQUIREMENT OF

COARSE AGGREGATESGrading No. Size Range IS Sieve

DesignationPercent by

weight passing

3 53mm to 22.40mm 63mm 100

53mm 95-100

45mm 65-90

22.40mm 0-10

11.20mm 0-5

TABLE 400.12 : GRADING REQUIREMENTS OF AGGREGATES FOR WET MIX MACADAM

IS Sieve DesignationMinimum per cent by Weight Passing the

IS Sieve

53.00 mm 100

45.00 mm 95-100

26.50 mm --

22.40 mm 60-80

11.20 mm 40-60

4.75 mm 25-40

2.36 mm 15-30

600 mm 8-22

75 mm 0-8

TABLE 400.20 : AGGREGATE GRADING REQUIREMENTS FOR CRUSHED RUN MACADAM BASE

Sieve SizePer cent Passing by weight

53mm Max. Size 37.5 mm Max. Size

63 mm 100

45 mm 87-100 100

22.4 mm 50-85 90-100

5.6 mm 25-45 35-55

600 micron 10-25 10-30

75 micron 2-9 2-9

Specifications and sieve analysis of screening material• Screenings to fill voids in the coarse aggregate shall

generally consist of the same material as the coarse aggregate. However, where economic considerations so warrant, predominantly non plastic material (other than rounded river borne material) may be used for this purpose provided liquid limit and plasticity index of such material are below 20 and 6 respectively and fraction passing 75 micron sieve does not exceed 10 percent. The screenings shall not contain any of the undesirable constituents lists in clause 301.2.3 which would render it unsuitable as a fill material.

• Screenings shall conform to the grading set forth in table 400.9. The consolidated details of quantity of screenings required for various grades of stone aggregates are given in table 400.10. The table also gives the quantities of materials (loose) required for 10m2 for sub base compacted thickness of 100/75mm.

GRADING FOR SCREENINGS

Grading Classification

Size of Screenings

IS Sieve Designation

Percent by Weight

passing the IS Sieve

A 13.2mm 13.20mm 100

11.20mm 95-100

5.6mm 15-35

180 micron 0-10

B 11.2mm 11.2mm 100

5.6mm 90-100

180micron 15-35

APPROXIMATE QUANTITIES OF COARSE AGGREGATES AND SCREENINGS REQUIRED FOR 100/75MM COMPACTED THICKNESS OF

WATER BOUND MACADAM (WBM) SUB BASE/BASE/SURFACING COURSE FOR 10m2 AREA

Classifi- cation

Size Range

Mm

Compacted thickness

Mm

Loose Quanti

tym3

Stone Screenings Crushable screenings such as moorum or gravel

Grading

Classification & Size

For WBM sub-base/ Base course (Loose qty)

m3

Properties Loose Quantity

m3

Grading 1

90 to 45 100 1.21 to 1.43

Type A 0.27 to 0.30 LL<20;PI<6 0.30 to 0.32

13.2 Percent passing 0.075mm <10

Grading 2

63 to 45 75 0.91 to 1.07

Type A 0.12 to 0.15 -do- 0.22 to 0.24

13.2

Grading 2

63 to 45 75 0.91 to 1.07

Type B 0.20 to 0.22 -do- -do-

11.2

Grading 3

53 to 22.4

75 0.91 to 1.07

Type B 0.18 to 0.21 -do- -do-

11.2

4. Bituminous Surfacing:

It serves the following purposes:• Improves the riding quality.• Seals the surface, thus preventing the entry of water

which would otherwise weaken the pavement structure.• Protects the granular base from the damaging effects of

traffic.• 20mm thick open graded premix carpet as per clause

508 is to be provided and it is to be properly compacted. Care shall be taken to maintain required temperature while laying of the carpet in case of hot mix. Also to ascertain that sufficient quantity of bitumen is mixed with graded chips of standard sizes.

• Then 6 mm thick seal coat is to be provided over the OGPC and ascertain proper grade of the materials, required quantity of BT, compaction, completion of wearing coat over OGPC every day, day end.

• Gravel Shoulders shall be done before BT surfacing only.

• After laying of wearing coat, 20mm level difference of BT and shoulders is to be leveled with gravel on shoulders.

Design Speed in KM/hour

RoadClassific

ation

Plain Terrain Rolling TerrainMountaineous

TerrainSteep Terrain

RulingMinimu

mRuling

Minimum

RulingMinimu

mRuling

Minimum

Rural Roads 50 40 40 35 25 20 25 20

Camber (in percentage) for different surface types

SurfaceLow Rainfall

(Annual Rainfall < 1000mm)

High Rainfall (Annual Rainfall >

1000mm)

Earthen 4 5

WBM & Gravel 3.5 4

Thin Bituminous pavement

3 3.5

Rigid Pavement 2 2.5

Super elevation: e = V2

225Rwhere e = Super elevation in meters

V = Design speed in km/hourR = Radius of curvature in

meters

Limiting value of super elevation in percentage• Plain and Rolling Terrain 7• Snow bound area 7• Hilly area but not snow bound 10

Widening of pavement at curve

Radius of curvature (in meters) Upto 20 21 - 60 Above 60

Extra widening for 3.75 m wide (single lane carriage way) 0.9m 0.6m Nil

Gradient of the road

i) Ruling gradient: It is a gradient which in the normal course must never be exceeded in any part of the road.

ii) Limiting gradient: It is a gradient steeper than ruling gradient which may be used in restricted lengths where keeping within the ruling gradient is not feasible.

iii) Exceptional gradient: It is a gradient steeper than the limiting gradient, which may be used in short stretches only in extraordinary situation.

Recommended gradients for different Terrain conditions

Terrain Type Ruling gradient Limiting gradient Exceptional gradient

Plain & Rolling

3.30% 5% 6%

(1 in 30) (1 in 20) (1 in 16.7)

MountainousTerrain andSteep terrainhaving elevationless than 3000mabove the MSL

5% 6% 7%

(1 in 20) (1 in 16.7) (1 in 14.3)

Steep terrainhaving elevationmore than3000m above theMSL

6% 7% 8%

(1 in 16.7) (1 in 14.3) (1 in 12.5)

Quantity of material required for 10sqm of road surface for 20mmthick pre mix carpet using Penetration Bitumen or Cut back

Requirement of Aggregates:

a) Nominal metal size 13.2 mm (passing 22.4 mm sieve 0.18m3and retained on 11.2 mm sieve)

b) Nominal metal size 11.2 mm (passing 13.2 mm sieve 0.09m3and retained on 5.6 mm sieve)

Total : 0.27m3

Requirement of Bitumen: a) For 0.18 m3 of 13.2mm nominal size metal chips 9.5 Kgs

@ 52 kgs.bitumen per m3. b) For 0.09 m3 of 11.2mm nominal size metal chips 5.1 Kgs

@ 56 kgs per m3

Total : 14.60kgs

Required quantities of Binder for Seal Coat

Type of Seal Coat Bitumen (Kgs)Bituminous

Emulsion (Kgs)

Type “A” Liquid Seal Coat 9.8 12 to 14

Type “B” Sand Seal Coat 6.8 10 to 12

Type “C” Premixed Seal Coatusing metal chips of 6 - 7mm

size

4.5% by weight of total mixture

9 to 11

Bituminous Material Requirement for Tack coat

Type of Surface Quantity per 10 sqm (kgs)

Normal Bituminous Surface 2 - 2.5

Dry and Hungry Bituminous surface

2.5 - 3

Granular surface tread with primer

2.5 - 3

Non Bituminous surface

i. Granular base (Not primed)

3.5 - 4

ii. Cement Concrete pavement

3 - 4.5


OPEN GRADED PREMIX CARPET (OGPC)

Providing and laying and rolling of Open Graded Premix Carpet (OGPC) surfacing material of 20 mm thick composed of 13.20 mm to 5.60 mm size machine crushed hard blasted granite chips using bitumen of 60/70 grade at the rate of 14.60 kgs per 10 sqm area of road surface over tack coat to required line, level, grade to serve as wearing course on previously prepared base including mixing in a Mixer of 6 to 10 T capacity, transporting the mixer to point of use and rolling with three wheeled 8-10 Tonnes capacity power roller and finishing to required level and grades including cost and conveyance of all materials from approved sources, seignorage, labour charges and hire and operational charges of power road roller and all other machinery, tools and plants, providing barricading for diversion of traffic, erecting red flags other incidental and operational charges etc., as per technical specification Clause 508 of MORD complete for finished item of work.

Clause 510 of MORD specification for seal Coat

Providing and laying Seal coat sealing the voids in a bituminous surface laid to specified levels, grade and cross fall using Type –A, Type-B and Type C chips of 6-7 mm size at 100% as per technical specification clause 510 of MORD.

Drainage:

• Scientifically worked out drainage design is a vital part of design of a pavement system.

• Proper cross slopes are to be provided over the carriageway and road side shoulders to shed off the rain water quickly.

• Proper camber plates are to be used while construction of Grade-II and Grade-III layers.

• The surface of the road shall not be wavy or there shall not be any ditches over the BT surface. If any water stagnates over BT surface, the BT surface looses adhesive and cohesive forces and the BT particles will come out.

• The BT surface shall not be provided over (i) where there is proper drainage of water is not possible i.e., village limits (ii) where there are steep gradients and (iii) in water logging areas.

• Where 1000mm dia vent culverts are required, it is preferable to provide Slab Culvert.

• In case of field channels, it is enough to provide 450mm dia or 600mm dia pipes without body walls.

• High level causeways and High level bridges are preferable to low level causeways. Low level causeways may be provided on unimportant roads crossing shallow depth and less discharge streams.

• Construction of road dams are to be avoided. Slab culverts are to be constructed in place of road dams.

• The body walls of pipe culverts shall be of minimum possible length and minimum possible height.

• Construction of retaining walls are to be avoided as far as possible. Revetment may be provided in such cases.

Design of CulvertsCalculation of mean velocity (v) (Mannings

Formula)V = 1 R2/3 S1/2

nwhereV = Velocity in meters/secR = Hydraulic mean depth = A = Cross Section area

P Wetted perimeterS = Slope of Energy gradient (bed slope)n = Co efficient of Rugosity of the river

Q = A X V

Sl. No. Surface of Natural streams Value of n

Perfect Good Fair Bad

1 Clean, straight bank, full stage, no rifts or deep pools

0.025 0.0275 0.030 0.033

2Same as (1) but some weeds and stones

0.030 0.033 0.035 0.040

3 Winding, some pools and shoals clean

0.035 0.040 0.045 0.050

4 Same as (3) lower stages more ineffective slope and sections

0.040 0.045 0.050 0.055

5 Same as (3) some weeds and stones

0.033 0.035 0.040 0.045

6 Same as (4) stony sections 0.045 0.050 0.055 0.060

7 Sluggish river reaches rather weedy or with very deep pools

0.050 0.060 0.070 0.080

8 Very weedy reaches 0.075 0.100 0.125 0.150

Calculations of Discharge

Dicken’s Formula (applicable to Rayalaseema andTelangana Areas)

Q = CM3/4

whereQ = The peak run-off in cum/sec.M = The catchment area in sq. km.C = 11 to 14 where annual rainfall is 60 -120 cm.

Ryve’s Formula (applicable to Coastal Area)Q = CM2/3

whereC = 6.8 for areas within 25 Km of coast.C = 8.5 for areas between 25 and 160 KM of coast.C = 10 for limited areas near the hills.

Linear Water Way

LW = K Q1/2

whereLw = Linear waterway in metres.Q = Design maximum discharge in M3/sec.K = usually taken as 4.8, but may vary between 4.5 to 6.3 according to local conditions.

‘LW’ for canals = B + dwhere “B” is bed width of canal and ‘d’ is the depth of flow of water at FSL.

For the channels in alluvial beds with well defined bankLW = Distance between banks at the water surface elevation (MFL

for high level bridges and OFL for submersible bridges).