geotechnical manual for slope,

THE UNIVERSITY OF HONG KONGLIBRARIES

GEOTECHNICALMANUAL

FOR SLOPES

This manual is published indraft form to promotediscussion among andcomments from those

concerned with constructionin Hong Kong. It will be

amended in the light of thesecomments and of any improve-ments in our knowledge of the

various subjects dealt within the manual.

iPBwa1* £ Published November 1979% * ! . . •

* Reprinted November 1981

In the November 1981 reprint of the manual, the Errataof the original version were corrected in the text* The opportunitywas taken to also correct other typographical errorsf to includesubheadings in the Contents list and to revise and in some casesexpand the references for each Chapter. Figure 8*2 was revised toinclude more recent rainfall data*

CONTENTS

Chapter 1

GEOLOGY OF HONG KONG1,1 The rocks1*2 The ©oils

Chapter 2

SITE INVESTIGATION2.1 Introduction

Page Chapter 3 (cont'd)

12

SURFACE INVESTIGATION2*2 Surface studies

Desk studiesExisting maps and plansDocumentsPhotographyField studiesVisual examination

2*32.42*52*62*72*82*9

55588

1111

Geotechnical investigations 112*10 Joint surveys 122*11 Surface drainage 132*12 Requirements 172*13 Pits and trenches 172.14 Investigation holes- boring 172*15 Investigation holes- drill ing 182*16 Backfilling 202*17 Sampling 202*18 Rock cores 2k2*19 Logging of holes 242*20 Instrumentation 282*21-Field testing 292*22 Standard penetration test 292*23 Static cone test 302*24 Pressuremeter 302*25 Plate bearing test 302*26 Vane test 312*27 Schmidt rebound hammer test 312*28 Point load test 312*29 Robertson shear box 332*30 Permeability tests 332*31 Permeability tests in soil 342*32 Packer or Lugeon test 352*33 Geophysical methods 372*34 Records 38

Chapter 3

LABORATORY TESTING3*1 Introduction 593*2 Selection of samples for

testing 59SOIL TESTING3»3 Classification tests 593«4 Moisture content 593*5 Liquid limit 603.6 Plastic limit 60

Page

3*7 Specific gravity 603*8 Particle size distribution 613*9 Measurement of shear

strength 613*10 Triaxial testing 633.11 Direct shear box test 653»12 Consolidation tests 663»13 Compaction tests 663»14 Direct permeability tests 663*15 Sulphate content 673.16 Acidity 673.17 Critical density 68

ROCK TESTING3.18 Introduction 683.19 Testing of discontinuities 693.20 Tests on intact rock 733*21 Records 7k

Chapter k

GROUNDWATER4*1 Introduction 774*2 Modes of ground water flow 784*3 Negative pore pressure 794.4 Positive pore pressure 794*5 Flow-nets 804*6 Run-off 814*7 The design storm for

infiltration 824*8 Degree of saturation 824*9 Depth of wetting band due

to infiltration 824*10 Effect of rainfall on

groundwater 874*11 Groundwater in rock 884*12 Other factors affecting

groundwater conditions 89

SUBSURFACE DRAINAGE MEASURES4*13 General 894*14 Horizontal drains . 904*15 Drainage galleries 904*16 Vertical wells 914*1? Cut-off drains 914*18 Counterfort drains 924*19 Filters 934*20 Filter Fabrics 96

Chapter 5

DESIGN OF SLOPES5*1 Introduction 99

STABILITY ANALYSIS5*2 Modes of failure 99

CONTENTS

Chapter 5 (cont*d) Page

5.3 Input data 1005.4 Methods of analysis 1055*5 Three dimensional effects 1055*6 Factors of safety 1105*7 Recommended methods of

analysis 1115.8 Reliability of stability

analysis5*9 Sensitivity analysis 115

DESIGN OF CUT SLOPES5*10 Slope profile 1165*11 Improvement of stability 1165*12 Treatment of rock slopes 1185*13 Stability analysis of

rock bolted slopes 1215.14 Design of dowels 1225*15 Boulder and rockfall control 123

EMBANKMENTS5.16 Design 1235.17 Stabilisation of existing

slopes 125

Chapter 6

Chapter 7 (cont!d) ?age

FOUNDATION ON SLOPES6.1 Introduction 129

SHALLOW FOOTINGS6*2 Bearing capacity on slopes 1296*3 Slope stability with

shallow footings 1316.4 Interaction of footings 132

DEEP FOUNDATIONS6*5 Foundation level 1336*6 Lateral loading of deep

foundations 1336*7 Construction of foundations

and slopes 133

Chapter 7

RETAINING STRUCTURES7*1 Introduction 1357*2 Analytical methods of

determining lateral earthpressure acting on walls 135

7*3 Water pressures 1367.4 Surcharge loads 1367*5 Construction loads 1377.6 Base friction 1387*7 Bearing capacity 1387*8 Factors of safety 1387*9 Retaining walls with keys 139

7.10 Propped caisson and pilewalls 139

7.11 Walls with lateral supportat several levels 139

7.12 Settlements outsideexcavations 139

Chapter 8

SURFACE DRAINAGE8.1 Introduction 1438.2 Run-off 1438.3 Area of catchment 1448.4 Time of concentration8.5 Design intensity8.6 Composite catchments8.7 General8.8 Layout of slope drainage8.9 Types of channels

146146146148148152152157

8.10 Channel design8.11 Changes in direction8.12 Junctions of channels

Chapter 9

CONSTRUCTION9.1 Introduction 1599*2 Design of temporary works 1599*3 Programme 1609*4 Methods 1609*5 Effects of vibrations 1629.6 Support 1629*7 Drainage '163

FILL9*8 General 1639*9 Placing and compaction of

fill 1649*10 Control testing 166

SURFACE PROTECTION9.11 Chunam 1699*12 Sprayed mortar 1?09*13 Masonry 1729*14 Planting on impervious

surfaces 1729*15 Planting on pervious

surfaces 173

DRAINAGE SYSTEM9*16 Excavation for drains 179*17 Channels and pits 174

SERVICES9.18 Excavation 1759.19 Ducts and conduits 1769.20 Construction control 176

CONTENTS

Chapter 10

INSTRUMENTATION10*1 Introduction10*2 Beading of instruments10*3 Records10**f Measurement of ground-

water level10*5 Measurement of pore

pressure10*6 Open hydraulic (Gasagmnde)

piezometer10*7 Closed hydraulic

piezometer10*8 Pneumatic piezometer10*9 Electrical piezometer10.10 Measurement of pore

suction

SURFACE MOVEMENT10*11 Significance of movement10.12 Structural cracking10*13 Rock and sdil slopes10*1*f Surveying10*15 Photo gramme try10*16 Vibration measurement

SUBSURFACE MOVEMENT10.17 Inclinometers10*18 Slip indicators10*19 Extensometers10*20 Settlement gauges10.21 Load cells for rockbolts

and anchors10.22 Earth pressure cells10.23 Remote reading and

automatic recording

Chapter 11

MAINTENANCE11*1 Introduction 19911*2 Routine inspections 19911*3 Instruments 20111*4 Slopes and slope surfecing 20111*5 Surface drainage 20611.6 Subsurface drainage 20711.7 Services 20811*8 Access 208

Chapter 12

SOURCES OF INFORMATION12*1 Introduction 20912.2 Technical information

retrieval 20912*3 Locally published

technical papers 210

£age Chapter 12 (cont'd) Page

179179183

183

183

184

187187187

188

188188190190190191

191192192

12. Sources of informationin Hong Kong Government 210

12*5 General local information 21012*6 List of useful addresses

of libraries and learnedbodies 211

12*7 Additional referencesspecific to Hong Kong 212

12*8 Historical references 216

INDEX 217

Foreword

Following disastrous landslides at Po Shan Road and SauMau Ping Estate in June 1972, and the subsequent 'Report of theCommission of Inquiry into 1972 Rainstorm Disasters1, the Governmentcommissioned Binnie & Partners (Hong Kong) to prepare a manual todraw to the attention of practising engineers and architects theparticular geotechnical problems of Hong Kong and to advise themof appropriate practice and standards to prevent the failure ofsoil and rock slopes, cuttings and retaining structures. Thismanual of recommended practice was issued to Authorised Architectsin August 1973 as a circular letter.

The scope of the manual was extended to its present formin October 1975. A Steering Group was set up in March 1976 to adviseon the practice and standards to be recommended in the manual, takinginto account local conditions and current practice. Members of theSteering Group were drawn from the Public Works Department, theUniversity of Hong Kong, consulting engineers and contractors.

By the nature of the geological conditions in Hong Kong,the investigation, design and construction of works connected withslopes will usually require the advice and guidance of an experiencedgeotechnical engineer. The use of this manual in practice requires areasonable knowledge of the basic principles of soil mechanics. Thepresent state-of-the-art with respect to conditions in Hong Kong doesnot allow the manual to be substituted by a simple set of empiricaldesign rules.

A very wide range of subjects is dealt with in the manual. Ithas not been practicable to present in a single volume full and compre-hensive information on all these subjects so as to make it a fullyself-contained treatise and design manual. Reference is made in eachchapter to publications which discuss the subjects in greater detail,or describe the results of relevant research. But it is necessaryto point out that there are several important geotechnical and relatedsubjects in this manual for which current knowledge or experience isinadequate. It is hoped that these will be the subjects of research toprovide a more satisfactory basis for design. When this informationbecomes available, the contents of the manual will be revised.

Chapters 1, 2, 3 and 11 were first published in draft formin May 1978. The present volume comprises the whole of the manual,and is published in draft form to promote discussion among andcomments from, those concerned with construction in Hong Kong. Itwill be amended in the light of these comments and of any improvementsin our knowledge of the various subjects dealt with.

Members of the Steering Group

Chairman : Mr. G.B. O'Rorke

Mr. H.C. Beaton

Mr* S. Rodin

Secretary : Mr* P.B. Keown

Mr. G.D. Wilkinson

Members : Mr. A.A. Beattie

Dr. E.W. Brand

Mr. K.C. Brian-Boys

Mr. H.K. Cheng

Mr. Y.H. Chien

Mr. H.C. Ho

Prof. P. Lumb

Prof. S. Mackey

Mr. R.O. Maher

Dr. A.W. Malone

Mr. P.S. Molyneux

Mr. D.J. Sweeney

Mr. P.J. Thompson

Mr. A.J. Vail

Dr. H.J. Walbancke

Principal Government GeotechnicalEngineer (to January 1978)

Government Geotechnical Engineer(from June to September 1977)

Geotechnical Adviser(from February 1978)

Civil Engineering Office(to May 1976)

Geotechnical Control Office(from May 1976)

Binnie & Partners (Hong Kong)

Government Geotechnical Engineer/Engineering (from June 1978)

Buildings Ordinance Office(to March 1978)

Buildings Ordinance Office(to December 1976)

Geotechnical Control Office(from August 1977)

Buildings Ordinance Office(from December 1976 to December 1977)

University of Hong Kong(from December 1976)

University of Hong Kong andsubsequently as Consulting Engineer

Enpack (HK) Ltd.

Government Geotechnical Engineer/Buildings (from January 1978)

Binnie & Partners (Hong Kong)(to February 1977)

Fugro (HK) Ltd.(from August 1977)

Fugro (HK) Ltd.(to August 1977)

Binnie & Partners (Hong Kong)(from February to November 1977)

Binnie & Partners (Hong Kong)(from February 1977)

CHAPTER 1

THE GEOLOGY OF HONG KONG

1-1 The rocks

Two major rock types of Igneous origin occur in Hong Kong:granitic and volcanic. Coarse grained granite rocks underlie Kowloon,the central part of Hong Kong Island and the lower levels of Victoria,although in places these are covered with recent marine deposits*Generally, finer grained volcanic rocks underlie the middle and upperlevels of Victoria and, with the exception of Castle Peak, Sha Tin,Tsuen Wan and much of the New Territories. Additionally, metamorphosedsedimentary rocks cover a small area of the Territory and in low lyingareas recent marine and estuarine deposits are common.

The granitic rocks are younger than the volcanic and havebeen intruded into them in the form of a batholith, or stock, with anirregular outline. The granites are generally widely jointed with jointspacing varying from 300 mm to 3 m. Sheeting joints are present near thesurface.

Although varying in composition and colour, the granitic rocksare normally composed of feldspar, quartz and hornblende with sheetsof biotite. Towards the contact with the volcanic rocks, the granitesare finer grained and the contact is usually sharp and well defined.

The volcanic rocks, also variable in composition, consist ofrecrystallized and welded tuffs (ignimbrites), and andesite and rhyolitelava flows which are metamorphosed. The welded tuffs are most common.The rocks are generally closely jointed with joint spacings of 50 mm to300 mm. But in the coarser grained varieties, joint spacings up to 3 mmay be found.

Dolerite dykes, varying in width from 150 mm to 1.5 m, havebeen intruded into both the granitic and the volcanic rocks. The dykes inthe volcanic rocks are normally sheared or faulted along one or bothmargins while those in the granitic rocks are rarely faulted. The dykesfollow regional trends and generally dip very steeply.

A granodioritic rock has been intruded into the volcanic rocksof Tsuen Wan and along a NE - SW line passing through Tai Po. Grano-*diorite has also been intruded into the granitic rocks of the StanleyPeninsula.

The rocks have been faulted and sheared in places, the faultsfollowing regional trends. Fault zones vary in width from a few millimetresto 30 m, can be vertical or slightly inclined and weathered to considerabledepths. Adjacent to faults the rocks may be comminuted or very closelyjointed. The faults shown on the geological map of Hong Kong, Kowloon andthe New Territories (Allen & Stephens, 1971) were identified by aerialphotographic interpretation. However many more faults exist than areshown.

1.2 The soils

The soils on the natural slopes of Hong Kong are derived fromdecomposition of the rocks as a result of weathering. They may consistof weathered in-situ rock or may be colluvium (slopewash) materialthat has moved down the slopes during torrential rainstorms. Soilcover over rock can be as deep as 60 m.

The soils derived from granitic rocks are usually sandywhile those derived from volcanic rocks tend to be silty whether theyare formed by in-situ weathering or by colluvial processes. Hillslopes composed of soil derived from the weathering of granitic rocksusually have a slope angle of 30° to 35°, whereas slopes in volcanicsoils are less steep (25° to 30°). Many of the natural soil slopesshow signs of instability at the surface.

Weathered in-situ soil can be recognized by the presenceof relict joint planes and the original crystal fabric of the rock.The depth over which weathered material (soil) changes to fresh rock isextremely variable and is related to the joint pattern, the spacingof the joints and the position of the water table. In the more closelyjointed rock the change from soil to almost fresh rock can occur overa depth of 300 mm to 600 mm. Joint planes in the fresh volcanic rocksare often coated with a clay-type mineral which frequently remainsalong the lines of relict joints in the soil. During the dry seasonthe water table is usually located within the underlying rock and notwithin the soil. Where the rock is moderately weathered (the feldsparshave been altered to clay type minerals but the rock cannot be crumbledin the hand) the joints are usually open and provide a zone which isfrequently more permeable than either the overlying soil or the under-lying fresh rock.

Weathering of both the volcanic and granite rocks produceshalloysitic and kaolinitic clay minerals, the latter particularlyduring advanced stages of decomposition. (Parham, 1969; Lumb andLee, 1975);

The colluvial soils usually consist of angular, sub-angularand rounded blocks of rock (varying in size from a few millimetresto several metres) in a matrix of clayey silty soil resultingfrom the weathering of rock from above. Cut slopes in colluviumcan become unstable during torrential storms especially where thecolluvial material is derived from volcanic rocks.

Alluvium, consisting of gravel, sand, silt and clay isfound in the flatter valley bottoms.

A geological map of Hong Kong appears opposite. A detailedgeological description of the rocks and rock formations is providedby Allen and Stephens (1971). Ruxton and Berry (1957) describethe weathering processes and the resulting geological forms. Lumb(1965) outlines the engineering properties of the residual soils ofHong Kong.

REFERENCES

Allen P.M. and Stephens E.A. (1971). Report on the Geological Surveyof Hong Kong* Government Press, Hong Kongf 107pp.

Lurnb P* (1965)* The residual soils of Hong Kong. Geotechnique Vol. 15ppl80 -

Lurab P» and Lee C*F. (1975)« Clay mineralogy of the Hong Kong soils.Proceedings of the 4th Southeast Asian Conference on SoilEngineer ing , Kuala Lumpur, pp 1 1 - 15Q»

Parham W.E. (1969)* Halloysite rich weathering products of Hong Kong.Proceedings of the International Conference on Clays, Tokyo,Vol. 1f pp

Ruxton B.P. and Berry L, (1957) • Weathering of granite and associatederosional features in Hong Kong. Geological Society ofAmerica Bulletin, Vol. 68, pp 1263 - 1292.

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CHAPTER 2

SITE INVESTIGATION

2,1 Introduction

The general requirements for site investigation forconstruction associated with slopes are given in table 2,1. Thistable relates the height of the slope, the angle at which it standsand the hazard potential of the site to the information and specialistadvice required.

The investigations needed to fulfill the requirements set outin table 2,1 are given in table 2,2. These tables are intended toprovide guidance only.

SURFACE INVESTIGATION

2.2 Surface studies

Much useful information can be obtained from surface studiesand examination of the construction records and performance of existingstructures in the vicinity of the site. The surface studies shouldform the first phase of a site investigation and the sub-surface workshould be planned only after assessing the results. The examination ofsurface features can be separated into two main aspects: desk studiesand field studies,

2.3 Desk studies

Desk studies should be carried out before a detailed fieldstudy but the engineer should visit the site during this first stageof the investigation.

2,*f Existing maps and plans

Topographic maps and plans can be used to identify geo-morphological forms and drainage patterns and can give an indication ofthe materials to be found on the site. Maps and plans covering the wholeterritory of Hong Kong are published by the Crown Lands and Survey Officeat scales of 1:100,000; 1:50,000; 1:20fOOO and 1:1,000,

Geological maps can be used to obtain information on materialsand geological structures which affect the site. Geological maps areextremely useful as part of the site of investigation but they areoften based on isolated exposures and boreholes so that much of theirdetail is conjecture rather than fact. This should be borne in mind bythe user and interpretation should be left to the specialist, A usefulsection on the use and interpretation of geological maps is given inICE (1976), Maps on the 1:50,000 scale are published in Allen andStephens (.1971) and are available separately. These maps supersedeprevious geological surveys.

Hazard Potential Classification '

Category

Low

Signi-ficant

High

a. Loss of lifeb, Economic loss

a. None expected (nooccupied premises).

b. Minimal structuraldamage. Loss ofaccess on minoraccess on minorroads, railway etc.

a, Few (only smalloccupied premisesthreatened)

b. Appreciablestructural damage.Loss of access onmajor roads,railways etc.

a. More than a few,

b. Excessive, Largeresidential andindustrialstructures .Loss of access onprincipal roads,railways etc.

Formed Slope Classification 2) Angle of Natural Hillside in the Vicinity of the Site

Features

Slope-height

angle

Slope-height

angle

Slope-height

angle

SoCut

}7.5m

^

£L5m

Jt60°

>15m

>60°

ilFill

J*5m

>30'

j*10m

/30°

>10m

>30°

Rock

<7,5m

>7 5m

>15m

0° to 20° Type 1

ILAssessment of surrounding geologyand topography for indication ofstability. Visual examination ofsoil and rock forming the site orto be used for the embankment.

Specialist Advice - Requirement (A)

isGeology and topography survey ofsite and surrounding area. Soiland rock joint strength parametersfor foundations and cut slopes.For embankments steeper than 1 on 3recompacted strength parameters o£fill. For cuts information ongroundwater level .

Specialist Advice - Requirement (B)

1HDetailed geology and topographysurvey of site and surrounding area.Soil and rock joint strengthparameters for foundations andcut slopes. Recompacted strengthparameters for fill. For cutsinformation on groundwater level.

Specialist Advice - Requirement (C)

20° to 40° Type 2 >40Q Type 3

Description of Site Investigation (2)

2LAs for type IL, More detailedgeology and topography survey.For the steeper slopes informationon soil and rock joint strengthparameters . Survey of hydrologicalfeatures affecting the site.


2SAs for type IS. Survey of

the site.


2HAs for type 1H, Survey ofhydrological features affectingthe site. Extend investigationlocally outside limits of siteto permit analyses of slopesabove and below the site.


3LAs for type 2L. Area outsideconfines of site to be examinedfor instability of soil, rock andjoulders above the site.


33As for type 2S. Extend outsidelimits of site to permit analysesof slopes above and below the site.


3HAs for type 2H. Extend investi-gation more widely outside limitsof site to permit analyses ofstability of slopes above andbelow the site.


Specialist Advice - Requirements ^

(A) Services of experienced geotechnical engineer probablynot necessary,

(B) Services of experienced geotechnical engineer to depend onlocation relative to developed or developable Land (4) .

(C) Services of experienced geotechnical engineer essential, "'

Footnotes ;—

(1) Hazard potential should be assessed with reference toboth present use and development potential of the area.Use highest loss category obtained under (a) and/or (b).

(2) Formed slope classification to be based upon either slopeheight or angle whichever gives the highest hazard category.

(3) The site should be classified according to the highestcategory obtained under either Clasifieation,

Notes:-

(i) This table is intended to provide guidance only. Each situationmust be assessed on its merits to decide whether or not therecommended investigation procedures are necessary or if peculiarconditions require even more detailed examination,

(ii) Whilst the above gives an indication of the requirements for asite investigation under certain general conditions, Table 2.2gives more precise information on how the above requirementscan be met.

(4) At some sites the services of an Engineering Geologist may be required.

This table is intended to provide guidance only

Category

from table 2.1

Low

Significant

High

Type 1

from table 2.1

l.L

Bl D El

l.S

A Bl Cl D El Fl Gl

C2 E2 G2

G3

l.H

A Bl Cl D El Fl Gl

C2 E2 G2

E3 G3

Type 2

2.L

Bl Cl D El Fl Gl

* *C2 E2

G3

2.S

A Bl Cl D El Fl Gl

B2 C2 E2 F2 G2

G3

2.H

A Bl Cl D El Fl Gl

B2 C2 E2 F2 G2

E3 G3

Type 3

3.L

A Bl Cl D El Fl Gl

C2 E2

G3

3.S

A Bl Cl D El Fl Gl

B2 C2 E2 F2 G2

E3 G3

3.H

A Bl Cl D El Fl Gl

B2 C2 E2 F2 G2

E3 G3

A. Examination of terrestrial photographs, aerial photos and geological maps.

B. Survey of 1. topographical, geological and surface drainage features.2. hydrological features.

C. Geological mapping of 1. surface features.2. structures.

D. Investigation holes, such as trial pits, boreholes or drillholes, as appropriate.

E. Sampling 1. quality class 4 )2. quality class 3 ) see Table 2.53. quality class 2 )

F. Field measurements of 1. groundwater level,(see note i) 2. permeability.

G. Laboratory tests 1. classification tests.(see note ii) 2. density tests for fill materials.

3. strength tests for soils and rock joints.

Notes

(.1) Vane testing may be appropriate in marine silts or other fine grained soils.Installation of instruments for long term monitoring of (a) displacements where movementis suspected and (b) pore pressures, should be considered during the site investigation stage.(Chapter 10)

(ii) Chemical tests will be required if aggressive soil/water is suspected in the vicinity of steelor concrete.

* For steeper slopes only

Table 2.2 Content of site investigation

2.5 Documents

During the preliminary stages of an investigation referenceshould be made to records of development in the area which may containinformation on site formation, site investigation, well boring, piling,foundations and previous instability of slopes* Records are generallyheld by the appropriate office in the Public Works Department for CrownLand and by the consultant architect or engineer for both public andprivate developments, although records for old developments may bescanty or non-existent* Site investigation contractors and the Univer-sity may hold useful information* Old newspaper records are oftenhelpful*

2.6 Photography

In addition to aerial and terrestrial black and white, colourand colour infrared photographs, there are several other types ofremote sensing - including satellite imagery and radar - now availablefor engineering use (ICE 1976). Of these, only black and white aerialphotography is readily available in Hong Kong. Complete black andwhite photographic coverage of Hong Kong at 1:25,000 scale is givenin surveys flown in 1964, 1967 and annually since 1973. There isalso coverage of urban areas at various scales, generally 1:4,000 to1:8,000. Some earlier photographs are available but coverage isincomplete.

Photo mosaics are available at 1:25,000 scale for the wholeterritory and at 1:6,000 for the urban areas. All aerial and someterrestrial photographs are available from the Crown Lands and SurveyOffice.

In addition to details of the topography and geomorpho-logical features, which can be seen on stereo-pairs of aerial photo-graphs, some structural geological forms can also often be inferred*One of the best methods of identifying man-made cut and fill slopescan be achieved by comparing successive aerial photographs and notingany changes in topography. Old slips, drainage patterns and sometimesthe presence of water can be identified by examination of the vegetationalthough colour infrared photographs are generally more effective thanblack and white for this purpose.

Of the the many geomorphological forms which can be identifiedon both maps and aerial photographs, two forms, colluvial fans andlineaments, commonly occur in Hong Kong.

Figure 2,1 is part of a survey sheet of Hong Kong showinga typical colluvial fan, while plate 2.1 is an aerial photograph ofthe same site. A colluvial fan contains loosely packed, poorly sortedresidual material often bouldery at the surface and grading intoresidual soils at depth. Weathering of colluvium can continue afterdeposition in the fan.

tN

Fig. 2.1 (map No. 1 1 - S W - A ) Plate 2.1

100 150 200 250 300 Mel

( p h o t o . N o . 7 4 7 3 )1963

FIG. 2.1 & PLATE 2.1 AERIAL PHOTOGRAPH AND TOPOGRAPHIC MAP SHOWING THE SAME COLLUVIAL FAN.

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HARBOUR

F i g . 2 .2 ( m a p No . 19 ) P l a t e 2 .2 (photo . No .246O)

1964

R e c l a i m e d Land

Al luv ium undif f e ren t ia ted Hong Kong Gran i te

Dominan t l y p y r o c l a s t i c r o c k s w i t h some l a v a sCol luv ium undi f f e r e n t i a t e d

Q u a r t z monzon i t e P robab le f a u l t de te rm ined p h o t o g e o l o g i c a l ly

FIG. 2.2 & PLATE 2.2 AERIAL PHOTOGRAPH SHOWING LINEAMENTS

AND THE INTERPRETATION OF THESE FEATURES

ON THE GEOLOGICAL MAP.

Car tog raphy by Lands & S u r v e y Depar tment . P W . D [ffi] 1 9 7 8

Plate 2.2 is an aerial photograph of part of Hong Kong Islandshowing several lineaments. These are controlled by the structuralgeology and indicate the presence of zones of weak rock which may needintensive investigation and eventually special construction methods.Figure 2.2 shows the same area on a geological map.

Identification of such features will aid the planning andinterpretation of sub-surface investigation and design of the works.Further information on these and other geomorphological features canbe found in Holmes (1965), Cooke and Doornkamp (1974).

2.7 Field studies

The planning of field studies should be based on the findingsof the desk studies with emphasis being placed on the potential problemareas.

2.8 Visual examination

While the site and its immediate environs will be thesubject of detailed studies, the examination of regional patterns oftopography and drainage can provide valuable information about theprobable sub-surface structure of the site.

Land classification, the division of the land form of thesite into land 'facets1 and felements1, is a method of geomorphologicalmapping suitable for use on large virgin sites. However this techniquewill seldom be applicable in Hong Kong. Further information andreferences on land classification are given in ICE (1976).

Unnaturally regular topographic forms often indicate thepresence of a man-made slope. Such features may include truncated valleysand ridges and planar slopes; of the latter, fill slopes willusually be inclined at between 30° and 40° and cut slopes between 50°and 60°. Slumping and settlement are often indications of the pre-sence of fills on site. Old developments on steep hill-sides willhave been formed by excavating on one side of the site and filling onthe other, although the overall affect on the topography may not besufficiently great to allow these features to be identified as man-madeslopes by reference to the topography alone. Vegetation, particularlyon old slopes, can make the visual identification of those which areman-made difficult, if not impossible.

A wall constructed during previous site formation works mayeither be a skin over a cut face or may retain fill. The slope abovethe wall will usually indicate which form the wall takes.

2.9 Geotechnical investigations

For a large or difficult site it is advisable to carry out afull-scale surface investigation and geotechnical mapping. The methodsused when plotting the data collected should be based on the GeologicalSociety of London Working Party Report (1972) on the Preparation of

11

Maps and Plans in Terms of Engineering Geology, modified as necessaryfor Hong Kong conditions. Figure 2.3 is an example of such an engineer-ing geology map for a site in Hong Kong. Further information onengineering geology mapping is given in IAEG (1976) and ICE (1976).

Although soil and rock exposures should be indicated on thegeological map under generic names they should be fully described onfield data sheets for subsequent correlation with the results ofsubsurface investigations. The following features should be recorded:

(a) Colour(b) Grain-size(c) Texture and structure(d) Weathered state (for rocks)(e) Lithological characteristics(f) Name(g) Estimate of strength(h) Estimate of permeability(i) Other engineering characteristics

Descriptions of ground masses will be primarily concerned withthe discontinuities traversing, or dividing, the materials andcan be described by reference to the following features:

(a) Origin of discontinuities(b) Orientation(c) Spacing(d) Nature of discontinuity surfaces(e) Thickness of discontinuity zones(f) Nature of infilling materials(g) Groundwater conditions(h) Weathered state

Colour can be a useful indicator of soil or rock type anddegree of weathering but before noting the colour, the surface layer ofthe exposure should be removed to expose fresh material which has notbeen stained by the atmosphere or by lichen.

The degree of decomposition of a rock exposure should bedescribed using the terms given in the first column of table 2.3. Thesehave been developed for use in Hong Kong and differ from those given inthe Geological Society Working Party Reports (1970, 1972). A diagram-matic section through a complete weathering profile is shown in figure2.4.

2.10 Joint surveys

Discontinuities such as joints can, and often do, exert amajor influence on the engineering properties of a rock mass particularlyby controlling the stability of cuttings formed in it. Where there aresurface exposures of the rock it may be possible to carry out a jointsurvey. The results obtained can be used both to assess the risk of

12

' / LEGEND

HONG KONGGRANITE

REPULSE BAYFORMATION

RECENT ANDPLEISTOCENE

UPPER JURASSIC

MIDDLE JURASSIC

Landslip debris

Alluvium

ColSuvium

Residual soilWith boulders at surface.

Granite

LYJOd Wdded tuff

SYMBOLS

"""" '— Geological boundary- known.

"—~ -—- —... Geological boundary - approximate.

""""""**~~— Geological boundary- inferred.

^U p" Dip of intrusive rock boundary

5QO-j Dip and strike of bedding

^^" Vertical joint set,

"~"CL~ Dip and strike of persistent joint set.

tmm * mmm inferred fault - beneath superficial deposits.

Fine grained quartzitic dyke (0.3 to 2.5m wide)

Centre line of proposed road.

Proposed borehole position.

Pattern of local joint system { ranked in order of predominance following smalljoint survey).

of landslip scar.

Intermittent stream course.

—^-— Contour- 5m intervals

ISLAND TRUNK ROAD

ENGINEERING GEOLOGY MAP

BLANK RIVER CROSSING

Fig. 2.3 Example of engineering geology map.

SCALE

1. : 1200

APPROVED DATE

DEC.,1975PIAN REGISTER No.

instability developing and to indicate those sets of joints which shouldfeature in a detailed analysis of stability. If the form which adevelopment is to take is already defined at the time the survey iscarried out, an alternative approach is to determine the dips and dipdirections of the joints which could cause instability to develop inthe designed faces and to examine the exposures for the presence ofany of these joints. The former should be used prior to carrying outthe design of slopes. However, the statistical method used in analysingthe results of the survey is such that it is possible to overlook arandom joint capable of causing extensive instability (Beattie and Lam1977). To overcome this weakness in the technique, the latter methodshould be adopted during construction to ensure that no joints whichcould cause instability are overlooked. If such joints are foundthen appropriate stabilising measures should be constructed.

A joint survey should only be carried out by an experiencedengineering geologist or geotechnical engineer who will visit thesite, not only to carry out the survey, but also after the surveyand corresponding analyses are completed. The slope or exposure shouldthen be re-examined for the presence of joint sets not identified inthe survey or random joints which could lead to the development ofinstability.

For carrying out a joint survey and the subsequent stabilityanalyses, a knowledge of the mode of formation of the joints isvaluable. From this the probable joint continuity, frequency andchange of orientation can be inferred. For example, sheet joints,which are probably formed by stress relief, are continuous over largedistances, parallel or sub-parallel to the topography and decrease infrequency with depth. The classification of the type of joint systempresent and the inferences which can be drawn from it should be leftto the experienced engineering geologist or geotechnical engineer.

The methods and equipment used to carry out a joint surveyand to analyse the results are described in Hoek and Bray (1974).Examples of joint survey record forms and a sample stereo-plot aregiven in section 2.34.

2.11 Surface drainage

During the surface investigation all existing stream courses,channels, nullahs, ditches, catchpits and culverts should be mapped,and details of size and condition plotted on the geotechnical siteplan. This information will prove useful when assessing surfacedrainage characteristics of the existing site, and how these existingsurface drainage measures will have to be modified or improved toaccommodate the proposed development.

13

Diagnostic features in exposures Appearance in samples or cores Normal recovery method req'd Engineering propertiesto obtain good samples

Fill Heterogenous material which can comprisesoil and/or rock of any grade.(*) Rockfragments may show fresh fracturesurfaces indicative of previousexcavation and disturbance. Densityvariable and the only structure visiblemay be that due to compaction in layers.May contain foreign material e.g. treestumps, building materials etc. Areaof fill may show as a change intopography. Often easily eroded(see plate 2.3)

Structureless heterogeneous materialwhich can contain rock fragments ofany grade (3) and which may showfresh fracture surfaces. Maycontain foreign material e.g.building debris.

Driven sample. Loose fillsnot easily recovered andliable to servere distur-bance by compaction duringrecovery. Can be recoveredwith triple tube corebarrel,

Variable - depends onmaterials and degreeof compaction. Loosefills liable to in-stability, very loosefills liable to formflow slides. Engineeringproperties determinedby laboratory testson samples recoveredfrom loose fills may beoptimistic becauseof disturbance.

Colluvium Loosely packed, poorly sorted materialof variable thickness which can oftenbe great and which may contain boulders.In an excavated face frequently appearsas a random distribution of decomposedboulders set in a soil matrix. Boundariesof deposit often marked by sharp changesin topography (see plate 2.3).

Heterogeneous structureless soiloften with boulders. Allsurfaces of boulders show signsof decomposition. No fresh rocksurfaces or building debris.

Driven samples or tripletube core barrel. Doubletube barrel is requiredif boulders are encountered.Continuous undistrubedsamples are obtainedonly from very carefuldrilling.

Can normally be excavatedby hand and is susceptibleto surface erosion.Boulders, however, maycomplicate excavationand may require splitting.Soil slope stabilitymethods should be usedfor analysis.

A structureless layer of soil ofvariable thickness but normallyless than 2.0 m. The soil-isusually bright red or yellowand shows none of the fabricof the rock from which it isderived.

Structureless soil showingnone of the fabric of theparent rock. The materialwill be grade VI.

Driven samples or tripletube core barrel. Contin-uous undistrubed samplesare obtained only fromvery careful drilling.

Can normally be excavatedby hand and is susceptibleto surface erosion.

A zone of decomposed rock ofvery variable thicknesscontaining rounded and non-interlocking boulders whichmay be much harder thansurrounding material. Theoriginal rock fabric ispreserved throughout. GradesIV or V material will -normallyconstitute more than 50% ofthe exposure.

Sections of grade (3) IV or Vmaterial separated by sectionsof less decomposed material(boulders).

Driven sample or tripletube core barrel. Doubletube barrel is requiredto sample boulders.Continuous undisturbedsamples are obtainedonly from very carefuldrilling.

Can normally be excavatedby hand or machine andis susceptible to surfaceerosion. Boulders,however, may complicateexcavation and mayrequire splitting.Stability of cutsnormally depends onthe strength of thedominant material butoccasionally on thegeometry and nature ofdiscontinuities. Soilslope stability methodsshould normally be usedfor analysis.

A zone of decomposed rock ofvariable thickness containingrectangular blocks, separatedby thin seams of friable material.The original rock fabric is preservedthroughout. Grade IV or V materialwill normally constitute less than50% of the exposure.

Sections of grades (3) I, II, orIII material separated by sectionsof grade IV or V material.

Double tube core barrelwith triple tube barrelin weak seams. Contin-uous undisturbed samplesare obtained only fromvery careful drilling.

Would normally be excavatedby machine supplementedby splitting of largeboulders. Stabilityof cuts depends upongeometry and strengthof discontinuities.Rock slope stabilitymethods should generallybe used for analysis.

A zone of rock which may havesuffered a little decomposition.Friable material, if present, istypically limited to narrow seams.The original rock fabric is preservedthroughout. Grade I or II materialwill normally consitute more than90% of the exposure.

Sections of grades O) j or nmaterial separated by sectionsof grade III, IV or V material.

Double tube core barrel. Normally requiresblasting for excavation.Stability of cuts dependsupon geometry and strengthof discontinuities. Rockslope stability methodsshould be used foranalysis.

Table 2.3 Classification of soils and decomposed rock zones found in Hong Kong

Notes:-(1) The above table relates to the materials likely to be encountered (2)

in those areas of Hong Kong underlain by igneous rocks. Figure 2.4shows a typical section through the various zones described above.Examples of exposures are shown in Plates 2.3 - 2.9 and examples of (3)cores in Plates 2.10 - 2.17.

There may be no well defined boundaries betweenthe zones.

The material grades referred to are given intable 2.4.

14

H«OQ

bo

4>

Zones of decomposition seen in exposures(Based on Ruxton and Berry)

Drillhole Legend

MaterialgradeTable2.4

Probable judgement ofzones based on drillcore

only

CO

iTrt

P-.H

JJ*O

O.«

CQ05

HOHi

Ui

O

H*COO

OHi

OO

CDCDP-

K,O

HOHi

H*!±

Structureless sand silt and clay.Zone A May have boulder concentration at

the surface.

Zone B

predominantly grade IV or V materialwith core boulders of grade I, II orIII material. The boulders consti-tute less than 50% of the mass andare rounded and not interlocked.Would normally fail as a soil withstrength parameters of grade V or IVmaterial but occasionaly relicjoints control stability.

Predominantly core boulders ofgrades I, II and III materialseparated by seams of grades IV andV. The core boulders constitutemore than 50% of the mass and are

Zone C rectangular. The failure mode isnormally controlled by the dis-continuity system and discontinuitystrength parameters may be those ofgrade IV or V material but may belower along smooth joints.

Zone D

Material of grades I or II con-stitutes more than 90% of the mass.The mode of failure is controlled bythe discontinuity system but thediscontinuity strength parameters maybe either those of grade IV or Vmaterial or rock joint strength.

+ 4

4 4 4

4 4 44 -f

VI

III

II

IV

IIIII

TITIV

II

IVI

Tv~

Zone A

Zone B Normally soilfailure mode

Zone CNormal mode offailure con-trolled bydiscontinuities

Zone D

Grade

VI

V

IV

III

II

I

Degree ofDecomposition

Soil

Completelydecomposed

Highlydecomposed

Moderatelydecomposed

Slightlydecomposed

Fresh rock

Diagnostic features in samplesand cores

No recognisable rock texture; surfacelayer contains humus and plant roots.

Rock completely decomposed by weatheringin place, but texture still recognisable.

Rock weakened so that fairly largepieces can be broken and crumbled inthe hands.

Large pieces (e.g. NX drill core)cannot be broken by hand.

Strength approaching that of freshrock - slight staining.

Table 2.4 Classification of degree of decomposition

16

SUBSURFACE INVESTIGATION

2.12 Requirements

The requirements of a subsurface investigation are:

(a) To find the extent of the materials forming and affectingthe site

(b) To obtain information on the relevant properties of thesematerials

(c) To study the groundwater regime of the site

These requirements should be considered when planning the position andinstrumentation of each hole* The investigation should be carried outunder the full-time supervision of an experienced inspector, or othercompetent person, supervised by a geotechnical engineer or engineeringgeologist.

2.13 Pits and trenches

Trial pits and trenches can be either small hand-dug pits orwide trenches excavated, mechanically. They permit the soil to beexamined insitu and allow undisturbed block samples to be obtained.Trial pits should always be supported to prevent collapse and watershould never be allowed to accumulate in them. When the exposed stratahave been logged and the investigation is complete the pits should bebackfilled in properly compacted layers.

2.14 Investigation holes - boring

Boring methods are suitable only for soils and soft rocks;the applications and limitations of the various methods are:

(i) Hand augering is suitable only for shallow holes in loosesoils above the water table and holes in trial pits. It isunsuitable for soils containing coarse gravel, cobbles orboulders.

(ii) Jetting may be used to locate hard strata, however, progresswill be stopped by boulders. The high water pressuresemployed make the method unsuitable for use in places wherean increase in degree of saturation, or in water pressure,could cause slope instability.

(Hi) Wash boring can be used to form holes for the installationof Instruments and to locate hard strata. The water pressuresused are lower than those used for jetting and the hole isgenerally cased. Therefore infiltration of water into theslope will be less pronounced with wash boring and will beless likely to cause instability. However, the methodshould not be used in positions where small changes Indegree of saturation or water pressure could significantlyreduce stability.

17

(iv) Power augers, both bucket and single flight helical, aresuitable for boring shallow holes and are used for resourcessurveys. Continuous flight augers are not very successfulin Hong Kong conditions and are seldom used for site investi-gation.

(v) Percussion boring is a common method whereby the hole isadvanced using cable tools operated by a friction-winch rigor the winch of a rotary drilling rig. Casing, when used,should be kept close to the bottom of the bore hole but notin advance of it. To minimise disturbance when boring belowthe water table in permeable strata, the water level in theborehole should be kept above the natural groundwater levelby adding clean water. Holes should have a minimum diameterof 150 mm.

All investigation holes should be back-filled (section 2,16)as open holes can allow the ingress of water into a slopewith a consequent reduction in stability.

2.15 Investigation holes - drilling

Rotary drilling must be used when cored samples are to berecovered. The drilling rigs should preferably be of the hydraulicfeed type and flushing may generally be effected by means of water,air or mud, although the latter should not be used in holes in whichin situ tests are to be carried out or instruments installed. Thediameter of holes drilled will depend upon the tests, if any, whichare to be carried out on the recovered core and the type and number ofinstruments to be installed in the holes. The sizes of the variouscasing, core barrels and cores are shown in figure 2.5.

It is important to select the correct core barrel as theadoption of the wrong type can cause disturbance or damage to thecores:

(a) A single-tube core barrel rotates against the core which isnot protected from the drilling fluid; core recovery isseldom satisfactory and it should not be used for siteinvestigation.

(b) A double-tube core barrel has an inner tube mounted onbearings so that it does not revolve with the drill string;it can normally be used in the unweathered and the slightlyand moderately weathered rocks.

(c) Triple-tube core barrels may be used where other methodshave been found ineffective and good core recovery isrequired. Triple-tube barrels have detachable liners withinan inner barrel which partially protect the core from drillingfluid and from damage during extrusion and subsequent transit.

18

(a )

Key

10 gp 3gIBI,,,li£0 SO^JSD 7ft JBO 90

mm

Casing Barret type (metric)Core bit T

— K-3

131/U6

116/131

56/66

Core barret sizeCasing size

86/101

(a) British (BS £091), American (DCDMA) and Canadian (SDDA) Standards(b) Metric Standard (Swedish]

Figure 2.5 Core, core bit and casing sizes

19

(d) Non-retractable triple-tube barrels are suitable for usein unweathered to moderately weathered zones of the rockprofile and some of the stronger highly weathered materials.

(e) Retractable barrels where the inner barrel projects ahead ofthe bit when drilling through soft materials and retractswhen the drilling pressure is increased in hard materials,are suitable for weaker highly weathered rocks and for allcompletely weathered rocks and residual soils.

When a hole is required only for the installation ofinstruments or insitu testingf non-coring methods should be adopted.This is most commonly done using a rotary drill with a rock-roller bit,a percussive wagon drill or a hammer drill. The hammer drill, which isdriven by a heavy diesel hammer, is particularly useful in depositscontaining large boulders.

Cobbles set in a matrix of softer material can cause difficultywith all drilling methods as the cobbles tend to rotate with the drill bit,eroding the matrix and reducing the efficiency of the bit. However, thisproblem may be overcome by the use of air/foam as the flushing mediumwith a large diameter triple-tube core barrel as described by Phillipsonand Chipp (198D.

2.16 Backfilling

All investigation holes must be backfilled as open holes allowingress of water into the soil or rock with a consequent reduction instability. Holes in rock should be backfilled with a grout such ascement-bentonite • In large diameter holes fillers can be used with thegrout. Holes in soil should normally be grouted, although under somecircumstances they may be backfilled with well-tamped soil; looselyplaced soil is not acceptable as the permeability of the hole will remainhigh. A few days after completion of backfilling, grouted holes shouldbe checked for settlement of grout and refilled to the surface ifnecessary. Holes backfilled with soil may require long-term maintenance.

2*17 Sampling

In defining the quality of the samples to be recovered duringan investigation the properties which are to be measured must beconsidered (table 2.2) and appropriate sampling methods specified.

The sample-quality classes required for various purposes aredefined in table 2*5t which also summarises the methods which can beused to recover samples of the required quality.

(i) Quality class 1 samples:

These can only be obtained using thin walled samplers,preferably of the fixed piston type, with an area ratio notexceeding 10#, inside clearance not exceeding 1$ and lengthnot exceeding 8 sample diameters. (Area ratio and insideclearance are defined in figure 2.6). Thin walled samplers

20

are seldom robust enough for Hong Kongfs residual soils,therefore class 1 samples will not be obtained. But as thesoils are not very sensitive, class 2 samples will giveacceptable strength and compressibility parameters. Class 1samples can, however, be taken in alluvial and marinedeposits.

QualityClass

1

2

3

4

5

Purpose

Laboratorydata onundisturbedsoils

Laboratorydata onundisturbedinsensitivesoils

Fabricexaminationandlaboratorydata

Laboratorydata onremouldedsoilsSequence ofstrata

Approximatesequence ofstrata only

Soil PropertiesObtainable

Total strengthparametersEffective strengthparametersCompressibilityDensity andporosityWater contentFabricRemouldedproperties

Water contentFabricRemouldedproperties

Remouldedproperties

None

Typical SamplingProcedure

Piston thin walledsampler with waterbalance

Pressed or driventhin or thick walledsampler with waterbalance

Pressed or driventhin or thick walledsamplers* Waterbalance in highlypermeable soils

Bulk and jar samples

Washings

Table 2.5 Sample quality classes

(based on Rowe, 1972)

21

| Ds

!^4— i/////y///

\)

" 1

u)e —

*N

/

/

/

/

/

A

/

/

/

/

Dw

Area ratio -^et projected area of sampler)(Projected area of sample core )

De2

In-idr rk-irince -IID of s3^2 tube)-(Dia.at cutting edge)1! IZ>l(J<6 Cl^cll Cli 1C(C. — — — - : •*-•- • • • ' •• ;" ' " • " ''**" •* -

(Dia.at cutting edge)-Ds - D£

De

QuHdc clearance - toD of shoe) ~(OD of samP(e tube)

(OD of sample tube)

_ D w - D tDt

Figure 2.6 Definition of sampler proportions

(ii) Quality class 2 samples;

These can only be obtained from soils exhibiting some cohesion.Samplers used for the recovery of class 2 samples can beeither thick or thin walled, but the end-area ratio shouldnot exceed 25% and the inside clearance 2%. They shouldhave some overdrive space and should have ports fitted withnon-return valves to permit escape of air and water duringdriving, and to create a vacuum above the sample duringextraction. The minimum internal diameter of the samplershould be 38 mm, but preferably 76 mm, and should be drivenby a down-the-hole hammer, or jarring link, rather than by atrip hammer at the top of the hole. Whenever the hole isbelow-groundwater level, water balance - maintaining thewater level in the hole just above groundwater level -should be used.

22

(ill) Quality class 3 samples:

Class 3 samples are suitable for fabric study/ Samplersused for the recovery of class 3 samples should conform tothe general requirements given for quality class 2 samples.The use of a trip hammer is acceptable and water balance isrequired only in highly permeable soils.

(iv) Quality class 4 samples:

These are badly disturbed samples in which the moisturecontent has been changed by the drilling or boring methods*Samples of this class may be taken from the spoil from holesand trial pits or from the sampler cutting shoes. Smalldisturbed samples should weigh at least 0,5 kg, while bulksamples should comprise at least 10 kg of material. Ifstored in rigid containers the sample should fill thecontainer and if stored in flexible containers as much airas possible should be excluded before sealing,

(v) Quality class 5 samples:

These can be obtained from the flushings, by either air orwater, from any borehole or drillhole.

All samples should be clearly marked both inside and outsidethe container with labels giving the following information, whererelevant:

(a) Name of contract(b) Name or reference numbers of site(c) Reference number, location and angle of hole(d) Reference number of sample(e) Date of sampling(f) Brief description of sample(g) Depth of top and bottom of the sample below ground level(h) Location and orientation of samples from trial pits

Sample containers should be free of air and water tight.The ends of undisturbed samples should be trimmed, the walls of thetube cleaned and dried with a cloth and the sample sealed withseveral thin coats of just-molten wax. Microcrystalline wax does notshrink to the same extent as paraffin wax and therefore gives a betterseal against the walls of the sample tube. Any space between theends of the sample and the end caps should be packed with sawdust,sand or other suitable material. Lids can be sealed using awaterproof tape or wax. Samples of soil and rock which are to go toa laboratory for testing should be carefully transferred as soon aspossible, They should not be subjected to any sudden shocks and shouldbe protected from the weather.

23

2.18 Rock cores

Drilling should be carried out in such a manner , and usingsuch bit sizes, that the maximum amount of core is recovered.This requires close surveillance of, among other things, wash watersupply and return, drilling pressures, lengths of runs. The drillbit should be withdrawn and the core removed as often as necessary tosecure the maximum possible core recovery. Coring runs shouldnormally be limited to lengths of 1.5 m and should not exceed 3 m.When less than 80% of the core is recovered from a run, steps shouldimmediately be taken to increase the core recovery by such methods asreducing the length of run or changing the barrel.

Core should be extruded into split plastic tubing usinga pneumatic or hydraulic core extruder. If a core barrel has to behammered to free wedged pieces, a leather mallet should be used.The core should be placed in the correct order in a corebox made forthat size of core. Core boxes can be made of wood or, preferably, ofmoulded plastic to take the split tubing. The size of core boxesused should be limited to that which can be conveniently handled.Friable core should be sleeved in polythene before being placed In thecore box. Spacer blocks showing the depth from which the core wasrecovered should be placed at the end of runs to prevent movement ofthe core in the box. If a sample is removed from the core box itshould be replaced by a wooden block of an appropriate length.Samples for testing should be selected as the core is extruded,removed and placed in air-tight rigid containers.

Core boxes and each individual run of core If stored inplastic tubing, should be clearly labelled both inside and out with:

(a) Name of contract(b) Name or reference number of site(c) Reference number, location and angle of hole(d) Date and method of drilling(e) Depth of top and bottom of each run

Core boxes should be stored under cover and arranged so they can beeasily located and removed for examination. Cores in split plastictubes and polythene sleeves can be stored, as individual runs, onshelves made of corrugated sheeting,

2.19 Logging of holes

Soils and rocks from investigation holes should be describedas suggested for exposures (section 2,9), Soil samples which are notrequired for testing can be extruded and split open to see the fabric.Fabric shows most clearly when the sample Is partially air-dried.

Rock should be classified as suggested in table 2,4, Thedriller fs log should be taken into account when producing the finallog of the hole, which should be produced by a competent person underthe supervision of an experienced geotechnical engineer or geologist.Examples of trial pit, borehole and drillhole logs are given Infigs. 2.14 to 2.16,

24

Useful field identification methods are described in USER(1974) and draft CP 2001 (1976). Using table 2.6 and 2.7 for guidance,a field assessment of strength and grain size of soils should be made,and the proportions of the various constituents estimated. In thelog the main constituent should be given in capitals and the secondaryconstituents as adjectives (eg clayey, sandy). Other alternativeswhich give an indication of proportion may also be used.

Soil grade Particle size Field identification

Clay >2ym Not visible with x 10 hand lens.Does not dilate on shaking.Adheres to the finger when dry.

Silt 2ym to 60ym Particles >10ym visible with ax 10 hand lens. Dilates onshaking. Does not adhere whendry. Feels gritty on smoothsurfaces.

Sand finemediumcoarse

6Qym to 2QQym200ym to 600ym600ym to 2ymm

Particles >60ymvisible to the naked eye. Finesand feels gritty to the finger,

Gravel finemediumcoarse

2 mm to 6 mm6 mm to 2 mm20 mm to 60 mm

Visual identification.

Cobblesand

boulders

>60 mm Visual identification.

Table 2.6 Field identification of grain size

The geological name should indicate whether or not the soilis insitu or transported, and the description of insitu soils shouldinclude the name of the parent rock. Some examples of typical completedescriptions are:

25

(i) Loose, reddish brown, sandy SILT with occasional graveland cobbles and occasional clayey bands: COLLUVIUM.

(ii) Dense brown, yellowish brown and reddish brown, clayeysilty SAND with occasional rootlets and pieces of brick:FILL.

(iii) Dense yellowish brown silty SAND with relict joints, dip60° main, heavily stained: completely decomposed GRANITE,

These are only examples* It does not mean that all "Loosereddish brown sandy SILT" is colluvium, or that all colluvium will bea "reddish brown sandy SILT". Descriptions should not be too briefor much of the work involved in a site investigation can be wasted*For example, (ii) and (iii) could both be described as "Brownsilty SAND11 although they are very different materials with differentengineering properties,

Soil typesStrength

Term Field identification

Coarse-grained soils Dense

Loose

Stays intact after extrusionfrom sampler; 76 mm core staysintact when split up by hand

Disintegrates or intact cylind-rical shape not maintainedafter extrusion from sampler;76 mm core disintegrates whensplit up by hand

Fine-grained soils Stiff

Firm

Soft

Verysoft

Friable

Cannot be moulded with fingers

Moulded only by strong pressureof fingers

Easily moulded with fingers

Exudes between fingers whensqueezed

Non-plastic, crumbles in fingers

Table 2.7 Field assessment of soil strength

(based on Working Party Report on Core Logging (1970))

26

Rock cores should be described using the terms suggested forrock masses in section 2.9 and the weathering classification givenin tables 2.3 and 2.4. For each core run, measurements of corerecovery, rock quality designation and fracture frequency should bemade. The positions of shear zones and fractures should be noted, andthe reduced levels of these features given on the logs which shouldinclude details of other structural features such as the dip, infillingand presence of joints.

The following terms are used when describing cores:

(i) Total core recovery is defined as ( core recovered ) x 100%length drilled

and any core in which recovery is less than 100% means thatsome material, generally the weakest, has been lost. Areaswhere losses have occurred should be located as closely aspossible and the relevant levels given.

(ii) The length of material which is recovered as solid corepieces at full diameter expressed as a percentage of thelength of core run is defined as solid core recovery,

(lii) Rock quality designation (R.Q.D.) is the length of corerecovered in lengths greater than 100 mm expressed as apercentage of the length of core run. If all the core isrecovered and is in lengths greater than 100 mm, thenRQD% » 100%. Any fractures or deterioration of the corecaused by drilling should be ignored.

(iv) Fracture index is defined as the number of fractures permetre run, measured over any arbitrary length which isgenerally taken as a core run. However, if there is amarked change in fracture frequency during a run, such as ata fault zone, the fracture index should be calculated foreach part of the run separately. Fracture index may alsobe quoted by reference to the maximum, minimum and meanlength of core pieces (Franklin, Broch and Walton, 1971).The definitions for core recovery and fracture indices areshown diagrammatically in figure 2.7.

Methods of showing these indices on a log are given in thesection on records (2.34). Water level, drill water return, casingdata and other information should also be given on the log, thepreparation of which is described in section 2.19.

The description of fractures should include details of thedip angle of the main and subordinate joint sets including clayinfilling, secondary mineral growth in joints, the state of brecciationand the degree of re-cementation of fault zones.

27

Etn

cDi-

2oo

1 [\ 1

e*<r

ai_Qjou0?

2X

• " . * • • ' • * .

V- :/:;

XjS^X

Wjh^w-------K6\

' \

V

« .* • " i ' * '

fe :'•'.:';^V' :

^".*• • • *•^c- • •;T^>s^y

/ Q

,/ Q£

T Core recovery =44S- =93%•23 -18 LbU

Solid core =-l Q|- =71 %recovery ' - 5 0

N.M

1o-»

•33 ^u.

L o•81-07 2

\ Een

.d>-

s*2A

R Q D =M|_=63%1.50

QP" f**5 OT 1 Ifft If^ri/^V-" — C "3 / P ** •* f i««*rraciurc inu^x- ; en " ^ •^ '11 ' run

•71

•""'•J^X' ^.•*^^ ;.'/|x\ ^ -r^•, '•'.*'. 'I PNm -I15 -•17

2.20

Figure 2,7 Core recovery and fracture indices

Instrumentation

During the investigation stage of a project instruments maybe installed to measure pore pressure, stress or relative movement.The various types of instrument, including their applications,limitations and methods of installation, are described in chapter 10.The provision of piezometers in all investigation holes and thesubsequent reading of water levels can yield valuable design informationfor only a small increase in the cost of the investigation; it is re-commended for all investigations.

28

2*21 Field testing

Field tests carried out during the site investigation stagecan be used to assess strength, deformation properties and permeabilityof both soils and rocks. Details of many of the tests considered hereare given in USER (1974). Tests used for construction control arediscussed in chapters 5 and 7,

2.22 Standard penetration test

This test can be used to give a rough relative measure ofthe density of granular soils; the procedure is described in BS 1377(1975). The results can be significantly affected by the testingtechnique so while carrying out the test and interpreting the resultsthe following points should be noted:

(i) The borehole casing should not be ahead of the borehole,and in very permeable soils below the water table waterbalance should be maintained.

(ii) Large diameter rods (BW or equivalent) or smaller rods withrod supports should be used to reduce energy dissipation.

(iii) A trip hammer should be used to drive the sampler as theaccuracy of a monkey and slip winch is too dependent onthe skill of the operator.

The fNf value is defined as the number of blows required todrive the standard split spoon sampler a distance of 300 ram. Thesampler is initially driven 150 mm to penetrate the disturbed materialat the bottom of the borehole before the test is carried out. Theoperator having noted the number of blows required for each 75 mmadvance of the seating then notes the number of blows required foreach 75 mm advance of the test drive.

The following SPT/density relationship has been suggestedby Lumb (1977) for Hong Kong soils. The !Nr values are based ontests carried out using a slip winch and the values may be a littlelower if a trip hammer is used;

!Ny value (number of blows)

<3

3 to 6

7 to 50

51 to 250

>250

29

2.23 Static cone test

The Dutch cone penetrometer comprises a 60° cone, normally10 cm in area, mounted on a sleeved rod. The cone is used to measurepenetration resistance as it is pushed into the soil at a steadyrate while the sleeve or friction mantle measures skin friction.Readings are usually taken at 200 mm intervals, although a continuousreading of both cone resistance and friction can be given by anelectrical cone penetrating continuously. The cone resistance can beused to calculate bearing capacity and density but the results arebadly affected if the penetrometer impinges on particles larger thanthe cone. Therefore the equipment is unsuitable for Hong Kong residualsoils but may be suitable for marine sediments.

2.24 Pressuremeter

The Menard pressuremeter can be used to obtain strengthand deformation characteristics of soils and rocks. The equipmentconsists of a probe which, when placed in a borehole, can be inflated.The volume changes of the probe, the expansion of which is limited tothat in the radial plane, can be measured by means of a surfacevolume meter to which the probe is connected. A pressure versusvolume change graph can be plotted and this is converted into a stressstrain curve. From the test results a "limit pressure" which reflectsthe ultimate bearing capacity is determined. A deformation modulusmay also be determined from which a rapid estimation of settlement maybe made.

Tests are normally carried out at 1 metre intervals in AX,BX or NX holes, and in granular soils a split casing tube is used toprotect the probe from damage. If the seating area for the pressuremeteris oversized, or if the walls of the hole are not smooth, interpretationof results become difficult.

A self-boring pressuremeter for use in soils (the Camkometer)is being developed but at present it is not sufficiently robust foruse in Hong Kong residual soils.

2.25 Plate bearing test

Plate bearing tests may be used to assess strength anddeformation characteristics of soils and soft rocks and can becarried out in trial pits or large diameter boreholes. The resultsof a plate bearing test can be badly affected by the presence ofboulders immediately below the test area, and in any case the accuracyof the application of the results to the prediction of the behaviourof full size structures is dependent upon the size of the plate usedfor the test. Normally a 300 mm diameter plate will be adequate buta larger area may be required to test coarse materials. The loadcan be applied to the plate by kentledge or by jacking against areaction beam. However, the equipment required is cumbersome andvery difficult to use on steep slopes and is, therefore, likely to beused for foundation design in level areas only.

30

2.26 Vane test

The vane is used to measure the undrained shear strengthof soft to firm clays and silts. Erratic results are obtained if thesoil contains gravel or any other large particles, and in Hong Kongthe use of the vane should be limited to the marine sediments.

2.27 Schmidt rebound hammer test

This test is applicable to strong or very strong rocks andis used to assess the hardness of a rock mass. At least 10 readingsare taken of the percentage rebound of a steel hammer against aprepared area of rock face and the mean of these readings is calculated.As the test results are strongly dependent upon the nature of thesurface layer, the preparation of the rock face over the test areamust be consistent. The test, which gives a comparative measure ofboth grain strength and bonding, is influenced by local discontinuities,weathering effects and the presence of large crystals. The test hasnot been correlated with strength for the Hong Kong rocks.

To carry out the rebound hammer test successfully the rockface requires extensive preparation, A simpler way of obtaining acomparative measure of the hardness of a rock exposure is to ring theexposure with a geological hammer; this requires considerable experienceand such a survey should be entrusted only to an engineering geologist,

2.28 Point load test

This test, which is carried out on a rock core, is a rapidand simple field test and the result can be related to uniaxialcompressive strength (Broch and Franklin 1972),

In the test a core of length at least 0,7 D is loaded tofailure between two standard points across a diameter (D), Thefailure loading (P) should be recorded for each sample and a sufficientnumber of tests should be performed to give a representative result.The point load index, Is, is calculated by dividing the force atfailure by the square of the length of the loaded axis, which isthe core diameter.

Is * P/D2

A relationship between point load index, Is, and the uniaxial com-pressive strength is given in figure 2.8. This relationship is basedon the results of tests on several rock types. The test has not yetbeen calibrated specifically for Hong Kong rocks but the indicationsare that this relationship also holds true for them.

31

•20

£"10"fito

oa.

Core diameter D(length <t 1.4D )

Maximum recordingpressure gauge

Hydraulic pump

Point load tester

Point load index Is =

Where P = failure load

o Experimental points

- r02

(Not correlated specificallyfor Hong Kong rocks )

50 100 150 200 290Uniaxial compressive strength (Tc - MPa

300 350

Relationship between point load strength indexand uniaxial compressive strength. (NX core)

(after Broch & Ranklin , 1 9 7 2 )

Figure 2.8 Point load test - equipment and results

32

2.29 Robertson shear box

This equipment which is used in the field for testing theshear strength of rock joints is also used in the laboratory* Thetest is discussed in chapter 3.

2.30 Permeability tests

A knowledge of the coefficient of permeability of a soil orrock mass is required for the design of subsurface drains and, whenassessing slope stability, to estimate the depth of wetting bandresulting from rainfall. To determine where perched water tables mayform, comparative permeabilities rather than absolute values areadequate.

Both the permeability of the intact soil or rock and thatof the discontinuities contribute to the mass permeability of amaterial. The permeability of intact unweathered Hong Kong igneousrocks is negligible and flow is controlled solely by the discontinuities.

The intact residual soil and weathered rock is more permeablethan the unweathered rock and thus discontinuities in these materialsexert less influence on the mass permeability. Laboratory tests(chapter 3) can give accurate values of the permeability of the sampletested but, as these samples are usually of intact material which donot contain major discontinuities, they are unrepresentative of thesoil or rock mass. The coefficient of permeability obtained for soilsin the laboratory can be 10 to 1,000 times lower than that of the massfrom which the sample tested was recovered. The laboratory determinedvalues for the permeability of rock sample usually bear no relationshipto the mass permeability.

Carefully conducted field permeability tests carried outbelow the water table in boreholes give values of about the rightorder of magnitude. Above the water table, where the results have tobe extrapolated to obtain an estimate of steady-state flow values, theaccuracy is questionable. This is because the soil through which thewater is flowing probably never becomes completely saturated. Howeversuch tests may model more accurately the conditions which obtainduring infiltration.

In applying the permeabilities determined by either laboratoryor field tests to the design of subsurface drainage works, or assessmentof depth of wetting, it is normal to assume that the soil or rock massis homogeneous. The presence of a thin impermeable layer, or a thinhighly permeable discontinuity which did not appear in the permeabilitytest area can introduce serious errors into the calculations. Toprevent this, recovered cores should be examined carefully for anysuch features and if these are present their position should beclearly indicated on the borehole log. The designs can then besuitably adjusted to take account of the effect of these features onthe groundwater regime.

33

Lugeon values determined from packer tests, which are definedin 2.32, are used for estimating probable grout takes and whenassessing the consequent reduction in permeability. Variations ofpermeability within the rock mass can be found with the packer testand under certain conditions absolute values of permeability can beobtained.

2.31 Permeability tests in soil

The coefficient of permeability, k, can be calculated fromthe results of rising, falling or constant head tests carried out inboreholes or standpipe piezometers. The way in which the test iscarried out can affect the natural permeability of the material beingtested. In tests involving a flow of test water into the soil, finesfrom the water in the hole can be washed into the soil reducing thepermeability. To minimise this effect only clean water should beused. Less commonly, in tests where water flows into the borehole,fines can be removed from the soil and if the water level is reducedtoo far piping can occur, the derived values of k being too high.

When testing soils below the water table, the followingHvorslev (1951) equations can be used to calculate k:

(i) Rising and falling head tests:

A H,* F(t2 -

(ii) Constant head tests:

where A » cross sectional area of standpipe or casing, (If the holeis not vertical, the horizontal water area should be used).

F * shape factor, which depends on the conditions at the base ofthe hole. The factors are given in figure 2.9.

HI, H2 - water heads above or below the standing groundwater level (H0 )at times t^ and t2 respectively.

HC « water heads above or below the standing groundwater levelmaintained during a constant head test.

The definitions of these terms are shown in figure 2.9 andspecimen test sheets and calculations are given in section 2.34.

34

These are steady-state equations suitable for thecalculation of permeability when the test is carried out below thewater table. In Hong Kong it is often necessary to measurepermeability above the water table where the steady state equationscan only be used, as the time over which the test is conducted becomesvery long (approaches infinity). However, under these circumstancespermeability can be assessed using the constant head test in whichthe water in the borehole is maintained at a constant level and theflow rate required to maintain this level is measured at differenttimes. The flow rate, q, plotted against the reciprocal of JTshould give a straight line which can be extrapolated to find q when

0 (ie t » infinity)St

The Hvorslev constant head equation:

k =

then can be used to calculate k, the head (H) being measured from thecentre of the piezometer filter zone or from the centre of the uncasedborehole length under test.

2.32 Packer or LugeonTest

The results of this test, used to determine the permeabilityof a rock mass, should, where the flow occurs in only a few fissuresor joints, be quoted in Lugeon values. One Lugeon is defined as thewater absorption in litres per metre of test stage per minute at apressure of 1 MPa in an NX hole. One Lugeon is approximately equal toa permeability of 1x10~7 m/sec.

The test is carried out in a drill hole using either asingle or double packer which is inflated to seal off the lengthof drill hole to be tested. To minimise end effects the test lengthgenerally should be at least 10 hole diameters. Water is pumped intothe test section under pressure and after allowing time for saturationof the ground the steady flow rate is recorded* The test is carriedout at a series of pressures but should not exceed overburden pressureor hydraulic fracture may occur. Under some circumstances verticalcracks in the soil or rock can develop at pressures much lower thanthe overburden pressure. This may be indicated by a discontinuity inthe graph of flow against pressure which should be plotted for alltests.

The q/H graph plotted from the results of tests conductedat pressures considerably less than 1 MPa (often 50 to 500 kPa)has to be extrapolated to give the flow at 1 MPa. This extrapolationintroduces errors arising from the differences in energy loss betweenlaminar flow and the turbulent flow which occurs at higher testpressures - low head tests tending to over-estimate the Lugeon values,However in Hong Kong there is often no alternative to using low pressuresand when this is done the test range should be quoted with the Lugeonvalue. A change of hole diameter from NX has little effect on theLugeon value.

35

-*r

-ap-

4=

;J

h

la

1

Rest waterlevel

I II I

Falling HeadTest

He

Rest waterlevel

Constant HeadTest

Falling Head Test

Ho = Head at time zero

Hi = Head at time t = ti

Ha = Head at time t = ta

A = Area of casing

K ~ A vlQnllLK F(t2-ti)

Xl0geH2

Constant Head Test

He = Constant headq = Flow rate to maintain

constant head

k -. q"FHc

Rush bottomin uniformsoil

( A )boundary

(B)

Shape Factors :.-

(A) F = 2.75D(B) F s 2.00D

(C) F = 2 n L

(D) F

Hi

•i'?''-;-;': , ,*j';*;'' ... sssuaS|£ D

LjFlush bottom Well point-filter Well point-fitter WeH point -filter Well pointr filterat impervious in uniform soil at impervious in uniform soil at impervious

(C)

: imperviousboundary

(D) (E)

at imperviousboundary

(F)

(E) Fh =-

(F) Fh =- 2 nt

Where m =(kh/k¥)1/2

Figure 2.9 Permeability tests - definitions and shape factors

36

If the rock jointing or discontinuity pattern is sufficientlyclose for the test section to be representative of the rock mass,a mass permeability can be calculated using the following formulae:

" L > 12 r- k -If 1 0 r > l > r , k-

where k = permeability

H = gradient of the flow v pressure head graph

L = length of test section

r = radius of hole

2.33 Geophysical methods

Geophysical survey methods are inferential. They rely onthe assessment of differences in measurable properties to derive byinference the changes in the sub-surface conditions which are ofinterest to the engineer. An experienced geophysicist is requiredto plan the survey and interpret the results.

All geophysical surveying should have borehole control toavoid errors in the Interpretation of the results. The more boreholecontrol there is available, the better the interpretation of thegeophysical profile between the holes.

Seismic refraction has been used successfully to assess thedepth of weathering over a large area. In detail it may not be verysatisfactory because of core boulders and differential weatheringon joints. The method is most successful when used on large sitesto obtain profiles of soft soil underlain by rock. Where it is usedits accuracy should be checked at an early stage of the survey and iffound to be inadequate the method should be abandoned.

The seismic refraction method relies upon monitoring theearliest time of arrival of seismic waves, refracted at the interfacebetween different strata, at a spread of geophones. These timesare plotted against distance from the seismic source and a series ofstraight lines of varying gradient are thus obtained. The gradientsof the lines are used to determine the depth of each interface.Problems can arise when weak layers are overlain by stronger materialor when layers exist which are thin relative to the overall depthbeing surveyed. Control boreholes can help to overcome some of theseproblems and a check should be made to establish whether a seismicmethod can be used.

37

In some cases it may not be possible to overcome the noiseor background vibrations which result from traffic flows and con-struction operations, but by using a signal enhancement seismographand choosing working times carefully an acceptable flsignal to noise11

ratio in urban areas may be obtained.

Resistivity methods and seismic reflection are sometimesused for engineering purposes but neither are suitable for foundationor slope investigations in Hong Kong. Other geophysical techniques,such as magnetic, electromagnetic, gravity, nuclear and thermalsurveys, are mainly of value in mineral prospecting and have seldomfound engineering application; in their present stage of developmentthey should not be used for slope investigations. Further informationon geophysical methods can be found in ICE (1976),

2.34 Records

The keeping of good records during site investigation worksis essential. This section contains examples of proforma, logs andmaps showing how information should be recorded. These are notintended as standard forms but as examples of good practice. Theseforms have been based on the three Geological Society working partyreports (1970, 1972 and 1977) and CP 2001 (1957), varied where necessary.for application to Hong Kong:

(i) Rock joint surveys:

When carrying out joint surveys, (section 2.10), data shouldbe recorded on forms of the type shown in figures 2.10 and2.11. In addition to surveying dip and dip direction, fieldestimates of strength, joint spacing and, if applicable,grain size should be recorded using the legends printed onthe form. The forms given in figures 2.10 and 2.11 havebeen filled in to show how they should be used. The resultsof a joint survey are plotted on a stereoplot as describedin Hoek and Bray (1977); a typical example is given infigure 2.12.

(ii) Geological maps:

The information collected during a geological survey of anarea can be plotted on a map, as shown in the example(figure 2.3), The method of presentation of informationwill depend upon the nature of the project and the choice ofthe engineer or geologist carrying out the survey. Theinformation should be presented in such a way that the usercan read the map with ease.

(iii) Trial pit, borehole and drillhole logs:

A legend of symbols representing the soils and rocks ofHong Kong, and suitable for use on logs, is given in figure2.13. The symbols can be combined to illustrate mixed soiland rock types. Alternative•symbols may be used providedthe use is consistent and explained on a legend. Examplesof trial pit, borehole and drillhole logs are given in

38

figures 2.14 to 2.16. They show methods of recordingthe information collected during the site investigation.Some notes on drawing up logs are given in table 2.8.

(iv) Permeability tests:

Field and calculation sheets for water absorption (packer)tests and rising and falling head permeability tests areshown in figures 2.17 to 2.19. The calculations used toobtain permeability from the falling head test are shown infigure 2.20.

A. Test pit logs

(1) Normally only one face parallel to the strike of the slopeis logged, however, other faces may also be logged ifuseful information can be obtained.

(2) Water levels recorded on logs should be dated and the timenoted.

B« Borehole logs

(1) Water levels should be read and recorded (i) before boringstarts in the morning (ii) after the lunch break and (iii)at the end of the working day.

(2) The sections of the hole from which the undisturbedsamples are recovered should be shaded in on the log toindicate percentage recovery. The number of blows requiredto drive the sampler should also be recorded.

(3) The standard penetration test result is given as an !Nr

value.

(4) Results of all field tests should be given on the log.

C. Drillhole logs

(1) Quantity and colour of water return are estimated visually.

(.2) Penetration rate of drilling, if required, can be given byany convenient method using numbers or diagrams, providingthe system is explained on the logs.

(3) Core recovery can be shown graphically, numerically or byboth methods.

(4) Various fracture indices which are measured on the core canbe given. Those shown on the logs are RQD and averagefracture spacing. Other indices (not shown) are: fracturesper metre over a given length, usually shown as a diagram;percentage of solid core recovered, usually indicated as anumber and maximum, minimum and mean core lengths, given asa diagram or a number.

(5) Details of instruments can either be shown in diagram formor as a note.

Table 2.8 Notes on preparation of logs

39

GENERAL INFORMATIONSeq.No.

AlN ,Y IVV,H ,E IR ,E

Day Month YearNorthing or

Elevation

Date 1 ,0 0 , 3 7,6 Operator Method of,location [3j{metresl

1. By co-ordinates2. Chainage3. On aftatched map/

drawing/photograph

i l l ! 1 1 1 i i I f I 1

I"" I I ILocality o I Size of I < j No. of supplementary ityp* I I locality | ' | of discontinuity d;

1. Natural exposure J.>!0m22. Construction 2, 5-10 m2

excavation , t c-23. Tunnel 4*.<!n^

5, tine survey

Sketch Photograph

0. No,1. Yes.

Slope dip

Dipdirection*

Remarks 3 2 m high . Signs of instability.

Some wedge failures have occured

ROCK MATERIAL INFORMATION

Colour j

1. Light 1. pinkish2. Dark 2. reddish

3. yellowish4. brownish5. oliveS. greenish7. bluish8. greyish

1. pink2. red3. yeltow4. brown5. oliw6. green7. w*j«8. white

Grain size 2

9-grey j. Verycoarse|>60mmJ0. black 2. Coarse C 2-60 mm)

3. Medium { 60 p - 2mm)4. F ine t2 -OGj j )5. Very frne( < 2p)

Compressive /strength | A [

1. Very weak-can be broken in the hand2. Weak- crumbles under firm blows with a pick3. Mod. strong - indents with a pick4. Strong - breaks with single hammer blow5. Very strong - requires several hammer blows to break

GRANITEQualifying termsto describe rock S>1 KjrTtJ^ wearthered mqde_ratelj£ m

we§thered_pn_ jo[ntsj

ROCK MASS INFORMATION

Fabric Block size State ofweathering

f, Blocky2. Tabular3. Columnar

1. Very large C> 8m3)2. Large t0.2-8m3j3. Medium (0.008-0.2m3)4. Small {0.0002-0.008 m3)5. Very smaM {0.0002m3)

j hto.ofmajorjI discontinuity i

setst. Fresh2. Slightly3. Moderately4. Highly5. Completely6. Residual soil

LINE SURVEYS TO DETERMINE DISCONTINUITY SPACINGSPlunge Trend Length of No. of

Line 1

Line 2

Line 3

0 , 0

0 , 0

8, 0

, ' l °, 1 , 0

1 , 0 , 0

, . 6 - °

, , 4 , 8

, , 3 , 2

,v, 4 , 0

, 5 , 3

\

\

\

Base of slopei i i t i t i i t i i iBerm level

i t ! 1 i 1 1 i 1 1 1 1

Centre line of slopei i i i i r i i i i i iDiscontinuity spacing I. Ext,wrde{< 2m) 4. Mod. wide (60 -200 mm)

2. Very wide 1 600 mm- 2m) 5. Mod. narrow! 20 -60mm)3. Wide (200 -600 mm) 6. Narrow ( 6 - 20 mm)

7. Very narrow (< 6 m m )

Figure 2.10 ROCK MASS DESCRIPTION DATA SHEET

GEN

No,

IERAL INFORMATION

7,

NATURE

Chainageor No.

1,i,1,i,i,V,1,1,

Jj_

9 , 8 Site A , N , Y ,W,H,E,R E Date

Day Month Year

1 , 0 0 , 3 7 , 6

AND ORIENTATION OF DISCONTINUITIES^

, 1 ,3

, 1 , 4

, 1 , 5

,1,7

, 1 , 8, 1 , 9

,2,0

, 2 , 1

1, , 2 , 2

1,

I.

, 2 , 3

, 2 , 4

2

2

2

2

2

2

1

2

2

2

2

1

5 1.6'8 ,6

8,6

6,6

8,4

5 ,5

4 , 4

9,0

8,2

8 ,1

6 ,2

8,0

1

3

2

1

2

1

,7,8.

,1 , 3

, 3 , 5

, 4 , 6

, 4 , 9

1 , 4 , 4

2

2

1

2

, 3 , 4

, 3 ,2

, 4 , 4

,6,3

i i 3 i5

i , 1 i 5

, , j9

, , i?

, , 4 - , 3

, , 2 i 4

. 2 , 5 i O

, 1 3 i 3

, . -,4

, 1 , 8 : 0

/,

5

7

6

/,

5

3

7

4

7

5

3

4

2

2

6

4

2

4

6

2

2

2

4

,2

I ""

j -

,5

,2

1 "

,4

.5

l"

I"

t"

,4

1,5,0

j 5 , 0

,2 ,0

, 4 , 0

1 , 2 , 0

, 3 , 0

2 , 0 , 0

, 2 , 0

1 ,0 ,0

, 2 , 0

, 5 , 0

2 , 0 , 0

Curator A | CDiscontinuity data

sheet No.

* ^V® 4" Remarks

, 2iO

, 1 ;0

I "* !

i *" I

, 2 , -0

,1 [5

. 3 ) 0

, 0 j 5

, 1 ( 5

I ~ J

. , 1 ,0

Type dp , Dip direction Persistence Aperture Nature of infilling0. FauHzofic (Expressed in (Expressed 1. Wide (> 200 mm) 1, Clean1* Fw degrees) in metres J 2, Mod wide {60-200 mm! 2. Surface stainingscSaLam 3, Mod, narrow (20- 60mm) 3. Non- cohesiveiSdS?«5*v 4. Narrow(6-20mm) 4. Cohesive4,Sdi»t0sity 5. Verynarrow(2-6mm) 5. Cementedfr FJffflrr 6. Ext, narrow (< 2mm) 6, Catcite^Tension crack 7- Ti** 7. Chlorite, talc•.Foliation i. Others -specifyf. Bedding

.0 ,5

i O , 4

i ~ ,

, ~ ,

in ,5

il ,0

i O , 8

j O , 5

i O , 3

1 ~ I

i O , 5

. 2 , 0

1

1

1

1

1

1

2

1

1

1

1

3

I 1 l i i ( I l i l i [ I I i

i l i 1 1 1 i l l i i l | i 1

t l 1 t i r i 1 i i l l i i |

i i i 1 i i t i l i I i I lShot hole shows 50 mm movement

i t 1 1 1 ! 1 I t 1 1 I f 1 t

1 t 1 1 1 1 I f t ! 1 i 1 1 1

i I 1 i 1 i ! i i I i 1 E 1 !

i I 1 1 ! 1 I 1 ! 1 1 i i i 1

1 1 1 1 I 1 i 1 t 1 I S 1 I !

1 f I i 1 1 l 1 I I L 1 1 1 I

! i t l f l l r 1 i 1 E ! t i

i f I 1 l i i l 1 i 1 I i i I

Consistency of infilling Roughness WavinessSoil strength Rock strength 1. Polished Express1 Soft 5 Weak 2. Slickensided wavelength &2.' Firm 6. Mod.strong 3. Smooth amplitude in3. Stiff 7. Strong 4. Rough metres4. Hard 8. Very strong 5. Defined ridges

6. Small slope7. Very rough

2 of

[ f

1 I

I I

( i

f t

I I

I 1

1 1

1 \

I i

1 t

1 1

1 . 2

1

I

1

I

I

I

I

1

i..,, „

I

1

,

1

i

1

1

I

1

I

1

Water

1. Dry2. Seepage3. Slight flow <0.1 I/sec4, Mod, flow 0.1-1 I/sec5. High flow > t (/sec

Figure 2.11 DISCONTINUITY SURVEY DATA SHEET

ROCK SLOPE opposite M & H dept. laundry Chai Wan RoadArea A

EQUAL AREA STEREO PLOTN Slope : dip direction : 100°

dip angle : 80°

Unstable zone

30°friction coneNotes

1. Within the contours the density of poles of joints per unit area(expressed as a percentage of the total number of jointssurveyed) is equal to the number shown on the contour.e.g. within the contour marked 8 there are 8 joint polesper unit area for every 100 joints surveyed

2. A j B j C are pole concentrationsIAB is the pole of the line of intersection of the two joint sets A & B

Fig. 2.12 Example of a stereoplot

42

SOILS

gggggj BOULDERS .COBBLES

iSt-VJ GRAVEL

SAND

IXX»XI SILT

Rffifl PEAT

METAMORPHIC ROCKS

^^ SLATE ,PHYLLITE

^^g SCHIST

IGNEOUS ROCKS(a) for general use

|;'V.;.V.-'i Weathered GRANITE

11 * t I GRANITE

(b) for detailed use

!::::! GRANITE

|»XI GRANODIORITE

I S S S i l l DIORITE, SYENITE

I T I QUARTZ MONZONITEAdemellite

SEDIMENTARY ROCKS

\ili til BRECCIA

CONGLOMERATE

SANDSTONE

I I II I I SILTSTONE

MUDSTONE

SHALE

QUARTZITE

H y '•>'•! Weathered VOLCANIC

; v y I VOLCANIC

riStfel MICROGRANITEGranite porphyry t Felsite

;£'iSJll| MICROOIORITE- SYENITEPorphyrite , Porphyry

g$ l MICRpGABBRODolerite

RHYOLITE

ANDESITE, TRACHYTE

AGGLOMERATE

VOLCANIC BRECCIA

TI ii— i—TUi i

(based on . Gcol.Soc.,,1972)

Fig. 2.13 Legend for use on logs

43

TEST PIT NoSheet 1 of 1

.AW1

OitaMtaltf taceA «J» * 1 -5m N 20°E Feature INY^STIQATJON .OF FJU, $\,QP£ . . .Tvnt of excavate1 Hand dug Location . . .ANYWHERE; . . .Type of pump(if used) Sykes Ground level on i . . . .9.1 7. m.

Timbering . Yes Coordinates E . .3713.5-.00. N

WateQondittons

i

r•"-——:

•L y

-3/3/77-13.00

^2/3/77-5LOO

r

Depth(Samph& tests

Reduce>. level) m

f

o2

|

90.9

go 3

Depth

metres

0.5-

\ 0 —

1.5 —

2.0-

-

2.5 —

3.0-

3.5-

4.0-

4.5-

-5.0 —Samples f

Smalt disturbed sample 1Large disturbed sampleUndisturbed sample

E3 Block sampleA Water sample; Plate bearing test

m Moisture content */•

Profile of face A

— (Width =1.5 m) -

y^VV N

^^AA^yVV\

/*"\1 * *-vL 4. V\

I* + */

"^v* * * * * A

KXXX

/TT" -,/•*• * * * \

/+ * + * 4 * A

,

P,Q,

1.5275.00

Description

Stone pitching t cement pointed .

Greyish brown and pinkish brown silty

medium & coarse SAND , cobbles and

occasional boulders of weathered granite,grade II1/IV. Some domestic refuse

and pieces of wood.

.

(Weathered granite FILL )

End of trial pit

Remarks: p(an. *•Density tests at 0.5m, 1.0m ' 1Si 1.5m. Pit pumped out X il5r - HIbefore back filling. >/O^ ^^ |

/ ^^ ^ IContractor.Dri.il. Co XDate dug )/. 3/77. .Date backfilled .3/.3/77 *^z

-

-

-

.1

~

I

I

Scale 1 : 2 5

Checked by. . . .,Date A/?/ 77.

Fig.No.

Fig. 2.14 Test pit log

44

Type of boring . . PercussionType of rig. . .BqylcsDia of boring . J5Q m.nr)Casing details . .150 mm . . .

Date

(Time)

-10.30120/2

.

l_-

-

r-

:i-

:

:"•

—

:

118.00120/2—

"_.

I"

Depth &diam ofboring &depthofcasing )

150mm

to 4.5m

GroundWater

Nil at13.00hr

6.2m18.00 hr

Samples & Tests

Sample Depth pest&lnstr.

I,1• 2

• 3

.4

1• 6

• 7

1• 9

•10

I

89 blows

m = 7 */•

|N*12*

30 blows

mr17V.

JNrU

33 blows

m = 19V.

JN=17

70blowsOk=7.35

XlfJ6

m/secm=17V.

LegendJ Small disturbed sample

Large disturbed sampleUndisturbed samplt(102mm)

* Water sample| Field permeability testm Moisture content (*/•)

Depth

metres

H-1

-1 7

— 3

:-

r"~:s

:•

—7

BOREHOLE No.Sheet 1 of A

3

Feature SLOPE pfEHIND SCHOOL,Locatbn ANYWHEREGround level 122-5 m PD.Coordinates E . 37125;QQ. . . N .

ReducedLevel

Strata

Legend Thicknesi

(122.5)

i? fI O

(116.0)

• . 0 -*x • . .

• • - o ." O 1 ,

0:*o• 0

.X •

^ ,'^

x '.

i * **i

* • . •

x "

i r<

x "

t • "

.'ri

« • V

^ .

=•;

x':

, •

^•/

^ '

x

1.7

penetrat

Remarks ; Falling head permeability test 4

ContractorDate startiDate finis!

Drill Co.

Ned. ?*/ .2/76

.15I75-.00. . .

Description

Light greyish brown siltyftne -

SAND with some light grey Igravel and cobbles -

COLLUVIUM ^

Z-

JI

Greyish brown and yellowish -

brown silty fine SAND z

and clayey sandy SILT with -

occasional

gravels

rootlets and I

ICOLLUVIUM I

dI

—

—

I

II

Boring operation stopped -

at 6.5m ;_

Hole penetration continued I

by rotary drilling -

5.- 6.5m Scale :.1 -50

Logged by.^?? , ,CheckedDate. 27/.2.W . .

Fig. No.

Fig. 2.15 Borehole log

45

4S ON

Fig 16 Drillhole log

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-

FIELD DATA FROM WATER ABSORPTION TESTDRILLHOLE No. T3 TEST No. 4

Date of test24/11./.75....Packer type (delete as necessary)

*w§te/ doublepneumatic /hydraul ic/mcehanical

Test section from. 19.J81.ro...to .22,86.m.Depth of hole at time of test..3.3,8.4 .mDetails of casing at time of test -

Packer pressure Gauge height above ground level.. 1..3 2. .IDDepth to centre of test section (measured down line of drillhole ).2t3.4.171Depth to groundwater level (measured down line of drillhole)

FIRST PERIOD Gauge pressure. .1.24. k.N / m2

Time (minutes)

Rowmeter readJnQ ( { ,U IpjrtrcTt

Water take ( I )

0

218.6

5

229.3

10.7

10

239.9

10.6

15

250.7

10.8

AverageFlow

( I Anin)

2.14

SECOND PERIOD Gauge pressure.248 kN/rp2

Time (minutes)

g™£fer reading ( 1 )

Water take ( I )

0

281.8

5

296.4

14.6

10

311

14.8

.2

15

326.3

15.1

AverageFlow

( I /min)

2.96THIRD PERIOD Gauge pressure.3.72 kN/m2

Time (minutes)

Rowmeter readinq{ , ,TTtpSrtrctt*

Water take ( I )

0

255.9

5

276.6

20.7

10

297.5

20.9

15

318.5

21.0

AverageFlow

( I /min)

4.17

FOURTH PERIOD Gauge pressure. .248. kN/m2

Time (minutes)

SEE*" readina( > >Water take ( 1 )

0

54.5

5

69

15.4

.9

10

85.4

15.5

15

101.1

15.7

AverageFlow

( 1 /min)

3.10

FIFTH PERIOD Gauge pressure.124.kN/m2

Time (minutes)

Rowmeter readjng (OlpatlCtT

Water take (

Tested by :.MK.F.

1 )

1 )

0

377.3

5

388.6

11.3

Date :24/11/75

10

400.0

11.4

15

411.5

11.5

AverageFlow

( I /min)

2.28

Fig. 2.17 Water absorption test results

47

WATER ABSORPTION TESTDRILLHOLE No. . 1 3 TEST No. . .4

Date of tePacker tyf

t*'mr*\s* 1 f\^jn«\r\^* / \j

pneumatiPacker pr<

st 24/11/75)e(dekte as necessary):oubleft /,Ku/4«"*»i i!i /i rt/-ir»W'-»p\i«^C/Ttyv

assure

LEGEND OFTEST SECTION

1}VERTICAL DEPTHTO GROUND -WATER FROMG.l".21..34m(2)HEIGHT OFPRESSUREGAUGE ABOVEGI- 1,3.2. m(3)

6

5

riri

0^^^

FLOW

qlitres/min .

U)2.142.964.17

3.10

2.28

&&^(B-***

10

FROM GRAPH :

TestDep1DianDrillCasiRocJ-

section fth of hole cneter of hehole inclinng details( type. . . .

GAUGE PRESSURE

units :.kN/m2

(5)

248372

248124

*****

20

head of

(6)

12.625.237.8

25,212. 6

*^

[ met re;

j&&*

o 3 0

rom. . 19-81it time of tes)le in test aration from h

. .m to.. 22,86 . .m> t . . . . 33,8.4 mea. . . 1,02 mmorizontal 90° . . . .

.ORANJTE.. Grade.]II..:

FRICTION HEADLOSSIn basicpipework

(7)

N^

/"

in extrarods or pipes

m(8)

f

^.^^**^

TOTAL HEAD

(2+3+6-7-8)m(9)

35.2647.8660.46

47.8635.26

^<"£g^^

40 50 60

\=. 3,4/54L* lh q =.2,0where I =lengtl"

'NOTE : If ground water leveluse depth to centre

6v of tes

lug

t s

eon units

ection inunknown or belowof test section.

metrestest section

Tested by Calculatedby

M K F

Fig. 2.18 Water absorption test calculations

48

RISING and FALLING HEAD PERMEABILITY TEST

1.00.90.80.7

0.6

0.5

(U0.37

03

0.25

0.2

0.15

i

v

K

5

Lh

BoreholDrillhole

ise only CLEAN water 1las water been added d

Timeon

clock

Timeelapsed

min sec0

1 02 03 04 05 06 07 08 09 0

10 015 020 0

dt =Depthof waterbelowtop. ofcasing

9.601 m9.854

10.1091030010.48410.66810.82610.98511.10011 .22711.36611.82412.065

2

: . . . P.?

for the testuring boring?

ht

3.129m2.8762.6212.4302.2462.0621.9041.7451.6301.5031.3640.9060.665

X

1.0000.9190.8380.7770.7180.6590.6080.5580.5210.4800.4350.2900.212

\

\

ho

.

\V

ti

V

N\

min)

r/\

.0 m

I\

n

V^

^•

2 £ 6 8 10 12 U 16 18 20

Fit

CasiG.L

Deothbe

DeWE

G.

Ml. I

c

rL

D(

1

i

DC

Ptetlo

Date . . ?:Observe

n the boxcInter naldof casing50.8mm --\

j above ]

1.07m

hs of' cashgswG.L.—

mmt

pth ofiter below.. before

56m j

lepth ofmallestasingbelcwi.L.10.67m

>pthofholplow G.L.12.9 m

^

luOmmL_

Diameter ofbelow casin

\

hM

Vater table

low this deeasured piezonm

3/2 /74 . .r ABC

5s in the diagram"am.

-H

i

Y

..„

T?< |

~a

>mm

£

y

Water table at)time of test-^d^ =12.73 m

7-holeg

:lev

duertric U

>[ 11.66m

ed?tvcl.

Fig. 2.19 Falling head test results

49

\

d=50.8mm

D=UOmm

Drillhole D7Falling (read test

for L /D>4Permeability k= d , T

LTfrom testj time factor T = 12 min

8x1.524x12x60k= 9 . 0 5 x 10'7 m/sec

Figure 2.20 Falling head test calculations

50

Granitic Fill showinglayering parallel tothe slope.

Volcanic Colluvium showing boulders contained in astructureless soil matrix.

Plate 2.3

Granite Soil - Zone A

Volcanic Soil - Zone A

Plate 2,4

Decomposed granite in Zone B showing well defined relictjoint.

Decomposed volcanics in Zoneshowing well defined relictjointing

Plate 2.5

Decomposed granite Zone B exposure showing relictjointing and core boulders. Zone A is absent.

Junction between Zone B and Zone D volcanics,Zone C is missing.

Plate 2.6

Zone C Decomposed graniteshowing onion skin weatheringin the joints.

Decomposed volcanic rockface showing Zone B overlyingZone D. Zones A and C areabsent.

Plate 2.7

Zone D Granite showingstaining on joints.Blasting fractures canbe seen at centre right.

Zone D Volcanics showingstaining along joints.

Plate 2.8

Fresh Granite - Zone D

Fresh Volcanics - Zone D

Plate 2.9

HONG KONG HOUSING/AP LEI CHAU SIT

DRILL HOLE NO ALC/^ DEPTHCORE BOX NO OF * DATE

Residual soil 0 to 0.24mCompletely decomposed 0.24 to 4.20m

HONG KONG HOUSING AUTHORITYAP LEI CHAU SITE B

DRILL HOLE NOALC/^DEPTBOX NO * OF * DATE

Completely decomposed 4.20 to 9.21mHighly decomposed 9.21 to 10.05 mModerately decomposed 10.05 to 11.04m

Plate 2.10 Decomposed volcanic core

. j_ct uc; ^ . '

HONG KONG HOUSING AUTHOR!TTAP LEI CHAU SITE B

DRILL HOLE NO ALC/ DEPTH;ORE BOX NO » OF * DATE

Slightly decomposed 11.04 to 17.33m



HONG-KONG HOUSINGJUTHOfllTlfAP'LEI CHAU SITii

DRILL HOLE WOALC/'*DEPTICORE BOX NO ^ O F - * DATE


Zones of decomposition(based on drillcore only)

A. 0 to 0.24m

B. 0.24 to 11.04m

C. 11.04 to 15.01m

D. 15.01 to 25.08m


Detail A Residual volcanic soil

Detail B Completely decomposed volcanic rock

Plate 2.13

Detail C Completely decomposed volcanic rock

Detail D Completely decomposed volcanicrock with stained discontinuities

Plate 2.14

Detail E Moderately decomposedvolcanic rock

Detail F Completely decomposed seamin slightly decomposedvolcanic rock

Plate 2.15

Detail G Slightly decomposed volcanic rock

Detail H Moderately decomposed seam inslightly decomposed volcanicrock

Plate 2.16

LANDSLIDE: STUDY PHASE II PWT). CONTRACT Nfi A3G/77 *CONSULTING ENGINEER : CONTRACTORHOLE NO SITE 6 AREA CORE BOX It T

Completely decomposed 0 to 1.00mHighly decomposed 1.00 to 3.59mModerately decomposed 3.59 to 4.65m

c c DCONSULTING ENGINEER =HOLE NO ' SITfc 6 AKbA

CONTRACTOR' 'X NO 2

Moderately decomposed 4.65 to 5.55mHighly decomposed 5.55 to 10.71m

•LANDSLIDE STUDY PHASF Tf PWH. CONTRACT NQ 4-3G/77CONSULTING ENGINEER'- CONTRACTORSNil NO SITE & AREA CORE BOX m 3


ANDSLlDt STUDY PHASE I P.W.D. CONTRACT Nfi 43G/77JONSULTING ENGINEER « • « CONTRACTOR-

I HOLE N2 SITE 6AREV1*"" "*»» !ll>M CQRfc BOX N£» -f

Highly decomposed 13.71 to 16.45mModerately decomposed 16.45 to 17.95m

Plate 2.17 Decomposed granite core

:nr •• mi

Highly decomposed 17.95 to 20.17mSlightly decomposed 20.17 to 21.23mModerately decomposed 21.23 to 21.48m

il^ blUUTING ENGINE

P W D CONTRAf T

CORE

Moderately decomposed 21.48 to 22.00mSlightly decomposed 22.00 to 24.48m

9IUU1

jy.


Zones of decomposition(based on drillcore only)

Zone

A Absent

B 0 to 16.45m, 17.45 to 20.17

C 16.45 to 17.45m, 20.17 to 22.50

D 22.50 to 25.95m

Plate 2.18 Decomposed granite core

Detail A Highly decomposed granite

Detail B Moderately decomposed granite

Detail C Highly decomposed granite

Detail D Slightly decomposed becomingmoderately decomposed granite

Detail E Highly decomposed seam inslightly decomposed granite

Plate 2.19

DRILLHOLE No: ALC/USheet 2 of 3

Typdril

rigbit:

Dril

ling

|pr

ogre

ss

e Of Coordinates:

infl ' Rotary

E 34071M 11453

D-l Angle from horizontal __9J)°_

T.C. & Diamond Bearing: N __—"" E — T

fi ii |j_j

15/3

15/3

-.6/3

16/3

~ 7/3

7

NXCat

11.5m

NXCat

11.55m

1.26mat

8.00hrs.

7.65mat

19.00

hrs.

11.48n

at8.00hrs.

L4.80mat

L9.00hrs.

L5.96mat

8.00hrs.

Notes e.g. Colourwater return caving

instrumentation

n

o

No water returnfrom 10.05m to20.00m

Depth&

diameter

Reducedlevel

m.P.D. «{• Rat

e of

Pene

tratio

n

Q

C3

a

FLGtt

llLL

m*tf*S 76.50 50

- 1I. H^

' t fA

-12

I Tl

~ 13

1 U

>

k

,C

fL

IW

c~ SPT

i 5- A

1 16

1 17' T

-18

- -2tf

k

sw

r

76.45

75.46

74.5S

73.15

72.10

,46 '

%

II^/^/

I/V

0

//

II\I

//

/ s

/ s/ s

/ s

*/'

/ ' /

^/

~7"f/ /

/ s/ f

/ ' // /f /

/ V

/ '//// V//-X ^^ ,/x/x//!x^/v//////////

f /////1/Vy

0

12

73

0

90

72

93

-

79

99

64

22

77

39

>»

4

>10

3

7

0

-

3

0

5

6

4

9

eature: HKHA - Geotechnical AdivceDCQtion: Ap Lei Chau "Site 'B 1

/Qter table level: _see J>iezpmeter._shee_ts

Legend

W V V Vw v v vw v v vwvvvwvvvw v v vv v v y vV11.04m

w v v vw v v vw v v vwvvvv v v v Vw v v v

wvvvw v v vwvvvwvvvw v v vw v v vw v v vw v v vw v v vw v v vw v v v

w v v vw v v vw v v vw v v vw v v vw v v vw v v vV14.40mvy v v vwvvvv v v vv v v vv v v vv v v vwvvV V V Vv v v vV V VVVwvvvwvvvwvvvwvvvw v v vwvvvwvvvwvvvwvvvwvvvwvvvwvvvwvvvwvvvwvvvwvvvwvvvwvvvwvvvV V V V VwvvvwvvvwvvvwvvvwvvvV.WVVwvvvwvvvwvvvwvvvwvvvwvvvwvvvv v v v vwvvvwvvvV VVV VwvvvwvvvwvvvV VVV Vw v v vY vy v v

Description 10

Light grey hard moderately

decomposed medium grained

porphyritic TUFF.Discontinuities closelyspaced.Light to dark grey hardslightly to moderately de-composed fine grainedslightly porphyritic TUFF.Discontinuities moderatelywidely spaced.

Dark grey hard moderately

to slightly decomposed

porphyritic TUFF.

Discontinuities closely to

moderately spaced.

Light grey to grey green

hard slightly decomposed

fine TUFF.

Discontinuities closelyspaced.

Dark grey hard slightly

decomposed porphyritic

TUFF with occasional

inclusions.

Discontinuities moderately

widely spaced.

Moderately to slightly

decomposed.

III-

IIIZ

II 2

ni7iii

™ii -

ii-v I

II-

IIIIII-

Legend: Remarks:

W R. Water return| Large disturbed sample See sheet 3 for standplpe piezometer installation.1 Undisturbed sci Standard penet

Piezometer tipD Mazier sample5 Permeability t<rn Moisture conte

mpleration test

Contractor :'St Date started: 2=r3;rl9...,•nt Date finished: 3,0-3-79

Scale: 1:50

Logged by Jkp

Checked by JLP

Fig, No. 2.21

52

Type of Oxxdincdrilling: Rotary

r ig :bit:

Dril

ling

prog

ress

D-l Angle fT.C. & Diamond Ronrinn

0)

(S'S.G>

S! Is

- 7/2

I 8/3

18/3

1

weat11.55

m

14 . 85nat

19.00hrs.

15.07nat

19.00hrs.

Legend :W R Water return| Large disturbed• Undisturbed sci Standard penet

Piezometer tipQ Mazier sampte5 Permeability t<m Moisture contu


instrumentation

^ ^ Standpipetip at24.00 m

No waterreturn from20.00 m to

; 25.08 m

J

ites: E 34071

N .11453rom horiz• N

ontal 90°c

Depth&

diameter

Reducedlevel

m.P.D.

™2* 66-50

12 1

1 TNW

r2 3

r"

126

~r27

128

-29

65.31

64.50

63.11

61.42

If-50

j

z/

I\1

/

/ ^//

^

^//

//

Rate

of

Pene

tratio

n

0

dcc

DRILLHOLE No: ALc/12Sheet 3 of 3

Feature; gj2*A_ -Geotechnical AdviceLocation: Ap Lei Chau Site 'B f

Ground leveWater table

Frac

ture

Inde

x

53

54

97

74

42

0

7

3

5

8

>»

Legend

v v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v v21.19mV V V V V1

v v v v vv v v v vv v v v vv v v v vv v v v v

v v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vV V V V Vv v v v v23.39m

_y v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v vv v v v v

25.08m

Remarks • Sta^pip*3 piezometer installation :

1. Tip at 24.00m; perforated at 21.00m - 24.00msomole 2- Sand/ gravel filters at 18.00m - 25.08 m.

i 3. Bentonlte ball seals at 15.00 - 18.00 m.p , 4. Hole grouted from 15.00 m to surface,ration test °

Contractst Date stmt 0°fc ^

tor '

nished: 10-3-79

level; _ge.e j>iezpme_te_r. .sheets

Description

Dark grey slightly decom-posed porphyritic TUFF withoccasional inclusions.

Discontinuities moderatelyspaced.

Dark grey hard slightlydecomposed fine grainedTUFF.

Light to dark grey hardslightly decomposed porphyri-tic TUFF.

Discontinuities closelyspaced.

Light grey green hardslightly decomposed finegrained TUFF.

Discontinuities very closelyspaced.

Drillhole completed.

10

II -

II -

II I

III

i

Scale : 1 :50

Logged by JLPChecked bv JLPDatt 13/8/79

Fig. Ma 2.21

53

Type of Cc

drilling: Rotary, waterflushrig:

bit:

c £

DR5 ArT.C. & Diamond Be

o

°t ii H128/2

'-

: 1/3

r

r

12/3

:

r

0.50104nm

2.50127

mm

3.00127

mm

3.59127mm

5.50L27mm

b.O127 (mm

7.50127mm

LOO104mm

J.OO104*mm (

0.3518.00)

1.4508.00)

2.0012.00)3.5013.00)

2.ol18.00)5.5008.00)

1.5012.00)2.5113.00)

Legend:

W. R. Water return4 Lar9e d"SturbeI Undisturbed sj Standard penei Piezometer tiLI Mazier samp5 Permeability 1m Moisture cont

ordinates: E 35448

igle f

Kiring


instrumentation

4(80% water ^returns ^

i

^1 "

1

ii(

4

1

i<1A

4

2 -

^t

3

^

fk

f

k

irk.

|T

L.

f

k

^h.

I

r

i!iri

f

F

d sompteample

tration test

e «est Cent

Nrom he

21332

rizontal.__?0

Depth&

diameter

metres

NX

1

Reducedlevel

m.P.D.

& §

Rate

of

[ Pe

netra

tion

ciCJ

cd

FeLcGrW

Fra

ctu

re1

inde

x

0 50 min/m

f

I 101 mm

2

NX

2

101 mm

- IJ

- 4

- 5

r

NX

101 mm

- 17

- NX

• 101 mm

1 j|0

Remarks :

69.00

66.41

64.45

40

%^"/

%2

1^\// /

78/X

$

1

|:

j

20

18

12

/ 16

14

20

51

70

24

14

17

28

24

' 15/

DRILLHOLE No: PPF?Sheet i of 3

•ature: sio_pe 11-NW-B/C39cation: Police Playing Fieldsound levelater table

Legend

'.*'. "."'. I. * . " . "x " .

A*.'."..:.:."*t:.1,00»t : : :• • X •

".'.'. I•?*.*,•••

t : 5<:':":":":""v- : ::Y:V:

Si*4,- +

4, + 4,4- 4-

4 - 4 - 4 -+ 4-

4 - 4 - 4 -4- 4-

4 - 4 - 4 -4- 4-

4, 4. +4- 4-

4- 4-

4- 4*+ 4- •5.55

*x*. *.*.*.

:Y:Y:

:V:Y:

*.*.*„"„*

.:.v*.• • i *

•«*.".•„*

.*.*.*,*.

"X*.*.*.*

: + 70.0 m.P.D.level' See piezometer plots

Description iLoose brown silty coarseSAND.

Completely decomposedGRANITE.

Loose to dense orange brownsilty coarse SAND.

Highly decomposed GRANITE.

Moderately weak yellowishbrown coarse grained mediumjointed moderately decom-posed GRANITE.

Loose to medium dense yellowbrown silty coarse SAND.


v -

iv :

i

:

in—

IV

":

•:

Contractor : „ Z"-.«Date started :M_ 2Date finished:

a/z/jja—873/79

Scale; 1:50

Logged by H<? _Checked by JTU?

Fig, No, 2,22

54

Typdril

bit:

Dri

lling

prog

ress

a Of Coordinates : Eing: Rotary, waterflush fv

DR5 ArT.C. and Diamond g€

Cas

ing

dept

h.si

ze ii n12/3

13/3

;5/3

r

;6/3

L

"7/3

r

"-

-

0.73104mm

.4 . 71104

mm

.6.45

104mm

.7.95104

.8.25104

mm

2.7118.00)10.6508.00)

2.78

(12. oo;L0.34(13. 00]

5.76(18.00Dry

(08.00

6.30(12.0013.50

(13.0012.16

(18.00Dry

(08.00

1.25(12.0012.90

(13.00

12.10(18.0015.25

(08.00

2.75(12.0015.18

(13.00

Legend :

W R, Water return4 Large disturbe• Undisturbed si Standard pene

Piezometer tif0 Mazier sampt$ Permeability tm Moisture cent

^gle f

»aring


instrumentation

80% water ^returns

i

4 ~

11

^

6

L

>

f

r

i

Tn

ir

F

i sampteampletration test>e iest«nt

35448t 21332

rom horiz

• Nontal 90°

c —

Depth8,

diameter

metres

- 101

^ |

- 13

^- 14

- NX

1 51101

-1 7

Reduced(eve!

m.P.D.

0

nm

r

ran

r

— 1 8

1 101

: ir 19

mm

f

59.29

56.29

53.55

52.05

Remarks :

^ontracDate 5Date f

torarted:nished :

0 §0 §B>0 2

50

^

\

%

\<?<

S D / y l

f/s\L/

%

%

1///

Rat

e of

Pene

tratio

n dcia;

DRILLHOLE No: PPF7

Sheet 2 of 3

Feature: H-NW-B/C39LcGW

a>

|l(JL

min/m

A 23

/ 88/

/

//

/

^ 86/

/

29

/ 36

; 22

53

^ 85

/ 63

28

38

JCQtion: Police Playing Fieldsound levelater table

Legend

:V:V:10.71"

f f f4 4

4 +4 4 4

4 44 4 4

4 44 4 4

4 44 4 4

4 44 4 4

4 44 4 4

4 44 4 4

4 44 4 4

4 44 4 4

4 44 4 4

4 4+ + 4.

V:":Y: : :*

.*.*•. :.:.*.*.** "x"«: ; i i

••£0!:!:.:.:.v-:.i.:.:.:.; ; • • ;>*.•.'.*.*

lo.45 '

4 4"4 4 4

4 44. + +

4 44 4 4

4 44 4 4

4 44 4 4

4 .4+ 4 417,95

!*::Y:

. •*•.'.'.

.:.:.:>e.

:V:V:

*.".':V"

":*:":":"

*:": :*x

28/27.8/3/

7A-—7^..__

: 470.0 m.P.D.level: See piezometer _plots

Description 10

Highly decomposed GRANITE,

Strong, pale pink and greycoarse grained widelyjointed slightly decomposedGRANITE.

Loose to medium denseyellowish brown silty coarseSAND.


Moderately strong, grey andpink coarse grained mediumjointed moderately decom-posed GRANITE.

Loose to medium denseyellowish brown silty coarseSAND.


IV -

II ^

«

v —

•:

:

:EII I

IV

~

;Scale; l :50

Logged byJK—Checked by JLP -

Date,12/J_/2i-

Fig. No. 2.22

55

Typedritlirig:bit:

Dril

ling

prog

ress

I

Z 7 / 3

:s/3

r

1

of Coordina

nQ" Rotary wat"pi'r*'f:"'T'|e!^11

PR5T.C. and Diai

Angle frmond Reorina

tes: E 35448N 21332

om horizN._ ~-

jntal 90°

E

Casin

o,de

pth,

size

j

o.i:104mm

*^ll

3.2518.007.3708.00

9.60(12.0014.93

(13.00

L3.40(18.00

1?

Legend :

W R. Water return| Large disturbeI Undisturbed si S t a n c t a r d pene

Piezometer ti0 Mazier samp5 Permeabilitym Moisture conl


instrumentation

A .60% water •returns

Piezometer tipat 20.17m

Depth&

diameter

metres-20- -r

101 "mm

:|-21

"> *>

— 23

124

1-25

-26

J-27

-28

129

-30 —

Reducedlevel

m.P.D.

0

49 83

48.77

4ft. 00

44.05

6> <5

S|-

50

8*HT

«00/>

^Y/','//',fa///'/A97V

%fam//y//v//y'///'w////W//n//4////////yY/y///:93V///fc

~° i

min/m_ _.

///

^105/

/

/

/

/

/

/

/ 9S

/

/

/

/

/

/

/

/ 87

/ 93

dciod

95

81

96

88

FeLaGrW

Frac

ture

Inde

x

—

3

1

DRILLHOLE No: *»PF7Sheet 3 of 3

ature: i;cation: pc

ound levelater table

Legend

"20.17*+ + +

+ ++ + +

+ ++ + +

+ ++ + +4- -<-

2jU2,3+ + +

-I- +4- + +

4- ++ 4- -f,22,00.,

+ ++ + +

4- ++ + +

+ ++ + +

4- ++ + +

+ ++ + +

+ ++ + +

+ • ++ + +

+ ++ + +

+ ++ + +

+ ++ + ++ +

+ 4 - 4 -4 +

4 4- ++' 4-+ + +

.+ +4. + 4.+ +

4 - 4 - 4 -+ 4.4. 4. +

+ 4-+ 4. ++ +

25.95

Remarks c Piezometer installation:1. Piezometer tip at 20.17m.

d sampte 2- Saild filter from 20.32 to 19.32m.ample 3- Bentonite ball seal from 22.32 to 20.32m and 19tration test *' Grouted from 25.95 to 22.32m and 17.32 to surf a

P '1* Contractor: -- „ _ _test Date started; 2ent Date finished:

8/2/JJL9..8/3/79

.-NW-E/C3.9LUce^ Playing .Fields: +70.0 m.P.D.level: See_£iezp_meter _Elo_t@.

Description

See sheet above

Strong, pale pink and greymedium jointed slightlydecomposed GRANITE.

Moderately strong grey andpink coarse grained mediumjointed moderately decom-posed GRANITE.

Strong, pale pink and greymedium jointed slightlydecomposed GRANITE.

Hole completed at 25.95m

!o

IV"!

III

III -

ii :

J

|Scale; 1:50

.32 to 17.32m. , . . HrLogged by HC_.,Checked by -JLPDak_12/3£79-

Fig. No. 2.22

56

REFERENCES

Allen P.M. and Stephens E.A. (1971). Report on the geological survey ofHong Kong. Government Press, Hong Kong, 107pp.

Beattie A.A. and Lam C.L. (1977)* Bock slope failures - their predictionand prevention. Hong Kong Engineer, Vol. 5 No. 7, pp 27- 0.Discussion Vol. 5 No* 9, pp 27-29.

Broch E. and Franklin J.A. (1972). The point-load strength test.Transactions of the Institution of Mining & Metallurgy.

CP 2001 (1976). Draft Code of Practice for Site Investigation. BritishStandards Institution - Revision of CP 2001.

Cooke R.U. and Doornkamp J.C. (197*0. Geomorphlogy in EnvironmentalManagement . Clarendon Press, Oxford,

Franklin J.A., Broch E. and Walton G. (1971)* Logging the mechanicalcharacter of rock. Transactions of the Institution of Miningand Metallurgy, Vol. So, pp A1-A9.

Geological Society of London* (1970). The logging of rock cores forengineering purposes. Geological Society Engineering GroupWorking Party Report, Quarterly Journal of Engineering Geology.Vol. 3, PP 1-2*f.

Geological Society of London. (1972). The preparation of maps and plansin terms of engineering geology. Geological Society EngineeringGroup Working Party Report, Quarterly Journal of EngineeringGeology. Vol. 5* PP 295-381.

Geological Society of London. (1977). The description of rock masses forengineering purposes. Geological Society Engineering GroupWorking Party Report. Quarterly Journal of Engineering Geology.Vol. 10, pp 355-389.

Hoek E. and Bray J.W. (1977). Rock slope engineering, The Institution ofMining and Metallurgy, London, 309pp»

Holmes A. (1965). Principles of physical geology, Nelson, London, 1288pp.

Hvorslev M.J. (195D. Time lag and permeability in groundwater observations,Waterways Experiment Station (Vicksburg, Miss.) Bull. 36.

International Association of Engineering Geology. (1976). Engineeringgeological maps. The UNESCO Press, Paris.

Institution of Civil Engineers. (1976). Manual of applied geology forengineers, Institution of Civil Engineers, London, 375pp*

Lumb P. (1977). Private communication.

Phillipson H.B. and Chipp P.N. (1981). High quality core sampling - recentdevelopments in Hong Kong. Hong Kong Engineer , Vol. 9 No. k,PP 9-15.

57

Howe P.W. (1972). The relevance of soil fabric to site investigationpracticef Geotechnique, Vol. 22, pp 135-300.

U.S. Bureau of Reclamation (197*0 * Earth Manual U.S. Government PrintingOffice, Washington.

58

CHAPTER 3

LABORATORY TESTING

3.1 Introduction

chapter discusses the testing of soils and rocks inthe laboratory* Where detailed descriptions of tests are given inother texts, to which reference has been made, these descriptionsare not reproduced and the discussion is limited to the problemspeculiar to carrying out the tests on Hong Kong soils and rocks,Where suitable references are not readily available, tests aredescribed in more detail. The choice of appropriate designparameters, based on the test results, is discussed in chapter 5.

3.2 Selection of samples for testing

Samples chosen for testing should be as representativeas possible of the materials encountered in the investigation,Because of handling and testing difficulties with the weaker samplesand those containing discontinuities, testing is often carried outon the stronger, intact samples only. As a result, laboratory teststend to over-estimate strength and underestimate compressibilityand permeability. Difficulties experienced in selecting samples fortesting should be recorded in the test report so that the designercan judge the extent to which laboratory derived parameters mayover-estimate the mass strength of the material.

SOILS TESTING

3.3 Clajssification tests

These tests, which include the determination of moisturecontent, liquid and plastic limits, specific gravity and particlesize analyses, are so called because of their use in identifying asoil as belonging to a group which exhibits similar behaviour(USSR 1974), In the case of Hong Kong residual soils, liquid andplastic limits and specific gravity have limited application for soilclassification and more Information can be obtained from the particlesize analysis. Moisture content and specific gravity are also usedin the calculation of other soil properties, such as compressibility,dry density and degree of saturation. Particle size analyses arerequired for the design of filters. Special precautions, describedunder the Individual tests, are required for soils containingcertain minerals. In Hong Kong residual soils the most common ofthese minerals is halloysite, but significant amounts are unlikely tooccur,

3.4 Moisture content

The standard method is described in BS 1377 (1975) test 1(A),The subsidiary methods, tests 1(B) and 1(C) are suitable only forsite testing (chapter 9),

59

Soils containing halloysitic clays, gypsum or calcite,dehydrate or lose water of crystallisation if dried at the standardtemperature of 105° - 110°C. If the presence of a significant amountof these minerals is suspected, the effect on the determination ofmoisture content can be assessed by drying at various temperatures.

In a humid atmosphere, oven-dried samples re-absorb watervery easily* It is therefore important that they are cooled in adesiccator before weighing,

3.5 Liquid limit

Two methods of determining the liquid limit of the soilfraction passing a 425um test sieve are described in BS 1377, tests2(A) and 2(B). The cone penetrometer method, test 2(A), is preferableto the Casagrande method, test 2(B), because it is easier to performand less prone to operator error; but either method is acceptable*

Very few correlation tests have been carried out for thetwo methods on residual soils. The one point method, test 2(C),should not be used unless there is insufficient material availableto carry out tests 2(A) or 2(B).

Soils containing significant proportions of halloysiticclays must be tested without previous air drying and rewetting as theresult obtained from a dried sample differs from that obtained fromthe sample in its natural condition. Testing of samples without airdrying is preferable for all soils and the test report must state ifthe sample was dried.

3'6 Plastic limit

The plastic limit test is described in BS 1377 (1975)test 3. The comments on air drying in the liquid limit test (section3.5) also hold true for the plastic limit test,

3.7 Specific gravity

Test 6 of BS 1377 (1975) describes the standard method fordetermining specific gravity. The removal of air under a vacuumfrom an oven-dried sample is difficult when the soil contains siltand clay sized particles. Three variations to the standard methodhave been found to improve de-airing:

(a) The test is carried out on a sample at natural moisturecontent

(b) The soil and water mixture is boiled in a pressure cookerinstead of under vacuum

(c) Kerosene is used instead of distilled water. This methodshould not be combined with (a) or (b)

60

When variation (a) is used, or when it is suspected thatsoil has been lost from the sample during de-airing, the remainingsoil sample must be carefully collected, oven-dried and accuratelyweighed at the end of the test*

3.8 Particle size distribution

The standard method of wet sieving coarse-grained soilsis described in test 7A of BS 1377 (1975). This test requirespreparation of the sample by wet sieving on a 63um BS test sieveto remove silt and clay sized particles, followed by dry sieving ofthe remaining coarse material. Some residual soils have a largeamount of clay-sized particles in the interstices of the largeparticles and these need additional dispersion.

The method of dry sieving, test 7(B), is not recommended forHong Kong soils because clay particles may adhere to larger sizedparticles.

The pipette and hydrometer methods of particle size analysisfor fine grained soils are described in BS 1377(1975) tests 7(C) and7(D). Whiles in BS 1377 the pipette method is preferred, the hydro-meter method, test 7(D), is suitable for most Hong Kong soils.

3.9 Measurement of shear strength

The values of shear strength parameters obtained from samplestested in the laboratory are affected by the method of testing,disturbance, orientation and sample size.

The shear strength parameters of Hong Kong residual soilsshould be determined either in terms of effective stress by tests onsaturated specimens or by drained tests on unsaturated specimens.Unconsolidated undrained tests on unsaturated specimens should not beused. The strength parameters obtained from the two types of testsdiffer and the soil conditions appropriate to each test must be takeninto account when choosing the method of analysis to be adopted(chapters 5 and 7).

The stress range used for strength tests should coverthe range which exists in the field and samples from the correctlevel should be used, Undisturbed samples should normally betested at the stress corresponding to the level from which they wererecovered. Extrapolation outside this range can give unreliableresults for materials with curved failure envelopes (figure 3.1).The stresses which exist on the shallow potential slip surfaces inHong Kong are very low (0-50 kPa) and to test at these stresses,testing equipment should be as free from friction errors as possible.The effects of sample disturbance are more marked at low stressso good sampling techniques must also be used (chapter 2). Theeffects of sampling and testing errors should be taken into accountwhen assessing results for use in stability analyses.

61

[Range where c'= 36 kPa4flf'= 30

table lean safely be applied

20 40 60 80 100 120 UO 160 180 200 220

p' kPa

-|~ Test result

Best fit straight line

Possible failure envelopes

Figure 3,1 Effect of curved failure envelopes on strength parameters

Sample disturbance resulting in distortion or change ofdensity has a significant but variable effect on strength.

Sample orientation can be important when the sample containsdiscontinuities. In laboratory strength tests, the soil is constrainedto fail on planes which are not necessarily those along which failurewould occur in the field. Where failure in the field occurs alongdiscontinuities such as relict joints, which do not coincide with theplanes on which failure occurred in the laboratory, the strengthmeasured in the laboratory may differ from that which can be mobilisedin the field. In Hong Kong residual soils the strength of the massis not much greater than the strength of relict joints, unless thejoints are infilled with clay, so that the effects of varying theorientation of samples is unlikely to be significant.

The effects of sample size, if conforming with the recom-mendation of section 3.10, are unlikely to have a significant effecton the shear strength parameters obtained by laboratory testing ofHong Kong residual soils.

62

3.10 Triaxial testing

Triaxial testing of soils in general is described in detailby Bishop and Henkel (1962)* To improve accuracy when testing at lowstresses, triaxial cells should have either internal load cells orrotating bushes which eliminate the effects of ram friction.

Samples should be large enough to represent the materialbeing tested. The minimum sample dimension, for intact materials,should be at least four times the size of the largest particlecontained in the sample. Samples must be of at least 38 mm diameter,but in some coarse grained Hong Kong soils, 76 mm or 100 mm sampleswill be required. Small (38 mm) samples for testing should not beobtained by recoring larger diameter samples.

180

160

140

120

100

80

60

20

'= 20kPa

20 40 60 80 100 120 UO 160 180 200 220 240 260 280

a kPa

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280

Figure 3.2 Methods of plotting triaxial test results

63

Samples of soils which are unsaturated in the field caneither be fully saturated prior to testing, which will give lowerbound values for shear strength parameters, or tested unsaturated,which will model the strength of the soil at the degree of saturationat which the test is conducted. Shear strength will, however, beover estimated if the degree of saturation in the field exceedsthat at which the test was carried out. The method of testingmust, therefore, be taken into account when using laboratoryderived strength parameters for analysis and design (chapter 5).

The results of triaxial tests can be plotted either asMohf circles or as :

. fagainst

which is the pf q plot (figure 3.2). The latter is preferable becauseit is clearer and simpler to construct and also can be used to plotstress paths (Lambe and Whitman 1969).

The mode of failure of a triaxial sample should be observedand, if the sample fails on a distinct plane, the angle of inclinationof the plane of failure should be measured. Theoretically, allsamples should fail on planes at (45 + <f>f/2) to the minor principal ,stress, the horizontal in the triaxial cell. However, samplescontaining discontinuities may fail on other planes and, under thesecircumstances, the results of the test cannot be included in ananalysis using the Mohr circle. By assuming cf =0, an assessment ofthe value of cj)1 can be made for saturated samples by resolving forcesalong and normal to the failure plane. Alternatively a multi-stagetest can be carried out on the samples, but this requires that thestrain at the end of the first stage is strictly limited. Thismethod is equally applicable to saturated and unsaturated samples.

In multi-stage tests, samples arfe consolidated under lowcell pressures, sheared until peak ( - a3) is reached, then recon-solidated under a higher pressure and sheared again. Usually threeshear stages are used. It is important that at the end of theintermediate stages the samples should not be allowed to shear beyondpeak (°i - Q3 ), otherwise lower shear strengths will be obtained inthe following stages. Multi-stage tests are not suitable for sampleswhich show indications of an appreciable reduction in strength afterfailure.

(i) Saturated samples:

Samples should be saturated by back pressure, maintaininga constant, small, effective stress on the sample. Sampleswhich start with a very low degree of saturation can bedifficult to saturate. In those cases saturation can becarried out by first percolating de-aired water under asmall hydraulic gradient, through the sample until airstops bubbling from it and then, if necessary, by applyinga back pressure.

64

After consolidation, shearing may be carried out with thesample either fully drained or undrained with.pore pressuremeasurement. The drained test is preferable for routinetesting. It is simpler to perform and less liable to errorthan the undrained test but the rate of shearing must beseveral times slower to ensure full pore pressure dissipation.Additional information on pre-failure and post-failurebehaviour can be obtained from the undrained test resultsby plotting the stress path (Lambe and Whitman 1969). Thisrelies, however, upon the measurement of pore pressurewhich is subject to more error than other measurements madein triaxial testing.

(ii) Unsaturated samples:

The strength parameters obtained from tests on unsaturatedsamples are applicable to analyses (chapter 5) only whenthe degree of saturation in the field is the same as thatin the sample during the test. Triaxial tests on unsaturatedsamples should be carried out fully drained. Undrainedtests are misleading and should not be used (Smith 1974).

When testing unsaturated samples, bubble traps should beused on drainage lines so that the volume of air draining .from the sample can be measured and the degree of saturationat failure calculated.

Soil strength parameters can also be measured in testssimulating infiltration condition. In these tests, de-airedwater is allowed to percolate through the sample under asmall hydraulic gradient, flushing much of the air from thesample, and the sample is then sheared fully drained. Thepercolation time can be related to the duration of aparticular rainstorm and the effect on shear strengthof varying the percolation time then can be assessed.

3.11 Direct shear box test

Drained direct shear tests can be carried out on soilsusing a 60 mm or 100 mm square shear box. The maximum particle sizeof the soil under test must be limited so that there are at leasttwo and preferably three particles on either side of the shear, plane.Trimming undisturbed samples for the shear box can be difficult andthis equipment is better suited for testing remoulded soils. Withcare, a sample can be trimmed to align a relict joint with the shearplane of the box, Unsaturated samples cannot be saturated in thestandard commercial shear box, and they are generally tested at theirnatural moisture content. However, boxes may be modified to allowpercolation of water through the sample.

Residual shear strength can be measured by using a reversingshear box and continuing the test to large strains.

65

3.12 Consolidation tests

Consolidation tests are used to determine the compressibilityand the rate of consolidation of fine grained soils, including fills,under applied load. The tests are usually carried out on undisturbedsamples but recompacted samples of fill may be used.

The one dimensional consolidation test, BS 1377 (1975) test17, is suitable only for fine grained materials because of the verysmall thickness of the specimen.

Hong Kong residual soils, which contain sands and gravels,can be tested in the triaxial cell. The bulk modulus of the soil,obtained from the triaxial consolidation test or consolidation stageof the triaxial test, can be related satisfactorily to the one-dimensional coefficient of compressibility (mv), as the Hong Kongresidual soils have low drained Poisson's ratio values. The calculationof the coefficients of compressibility and consolidation from thisinformation is described in Akroyd (1969) and Bishop and Henkel (1962).

3.13 Compaction tests

Ttm dry density, moisture content relationship of a soilused for fill is determined by the standard compaction test, BS 1377(1975) test 12.

Many Hong Kong soils are susceptible to crushing and theextent to which this occurs can be checked by carrying out particlesize distribution tests on a sample before and after the compactiontest. The description of the test gives an alternative method forsoils susceptible to crushing which should be followed if there isany possibility that the soil may crush. Good practice would normallyrequire the use of this method at all times.

3.14 Direct permeability tests

The permeability of a soil sample can be measured in fallingor constant head tests. Details of the methods are given in Akroyd(1969).

Samples of undisturbed soil used in laboratory tests areusually small and tend to be intact samples rather than samplescontaining relict joints and other discontinuities. As a result, themeasured values of permeability may be lower than the actual fieldvalues, sometimes by two or three orders of magnitude. The resultsobtained from field tests, if correctly conducted (chapter 2), willgenerally be closer than the results of laboratory tests to the truepermeability of the insitu material.

66

3.15 Sulphate content

Hong Kong residual soils contain negligible amounts ofnaturally occurring sulphate* Sulphate tests are, therefore,necessary only where pollution of groundwater from industrialeffluent or other contaminants occurs and where such pollutioncould result in sulphates attacking cement in either cement-stabilisedsoils or concrete. BS 1377 (1975) test 9 describes the deter-mination of total sulphate content of soil, and test 10 the sulphatecontent of groundwater and aqueous soil extracts. The BuildingResearch Station Digest (BRSD 90, 1968) which is referred to inBS 1377 under test 10 has been superseded by BRSD 174 (1975) andreference should therefore be made to this more recent publicationparticularly when determining water soluble sulphate concentration.

Total sulphate contents of more than 0.2% by weight insoil and 300 ppm in groundwater, are potentially aggressive (BRS 174,1951). It is important to note that sulphate content is subject toseasonal and other variations, and the results of the tests areapplicable only for the particular time and conditions of sampling.

Water used for flushing during wash boring and rotarydrilling may cause alteration in the chemical properties of ground-water. Sulphate tests on samples of groundwater taken when boring bysuch methods may therefore give results which are not representativeof ground conditions.

3.16 Acidity

The acidity of soil and groundwater affects the rate ofcorrosion of metals and deterioration of concrete.

The standard electrometric method of determining acidityusing a pH meter is described in test 11A of BS 1377 (1975) andcan be applied either to the sampled soil in suspension in water orto samples of groundwater. The subsidiary calorimetric method oftest 11B also gives acceptable results. Either method may be used.The pH can alter if there is a delay between sampling and testing,so field measurement should be used.

The acidity of residual soils is negligible and as in thecase of sulphates tests are most likely to be carried out on contaminatedsoils.

67

3.17 Critical density

The critical density of a soil is defined as the densityat which it will shear without volume change. Soils with densitylower than critical can collapse on shearing, and if saturated ornearly saturated, liquefy. The density of uncompacted or poorlycompacted fill material is often below critical. Critical densitycan be determined by shear box or triaxial tests which are describedbelow:

A series of shear tests is carried out on 60 mm squareshear box or 100 mm diameter triaxial samples of the fillmaterial which have been compacted to a range of densities.Each sample of a series is saturated and tested, fullydrained, at the same cell pressure or normal load, the volumechange being carefully measured. For each sample, therate of volume change at failure:

[5^/ 6e]f

is plotted against dry density Yd* ^e density at which therate of volume change at failure is zero is the criticaldensity at that pressure. Critical density varies withpressure, so the test pressure should correspond to theoverburden pressure in the field.

The shear strength at constant volume, cj>'cv, can also beobtained from this series of tests by plotting the rate ofvolume change at failure against tan 4>ff. The value oftan <|>!cv is then read directly from the graph for zerorate of volume change.

ROCK TESTING

3.18 Introduction

The property most relevant to the assessment of the stabilityof rock slopes is the shear strength of the rock mass and this sectiondescribes shear strength tests on discontinuities and intact rock.Tests to obtain deformation properties of rock are discussed inCoates (1970) and Jaeger and Cook (1976).

The stability of rock slopes usually depends on thestrength along discontinuities and emphasis has, therefore, beenplaced on the testing of discontinuities. Testing of intact rockcan be important where shear surfaces are constrained by thegeometry of the slope to pass through intact rock.

Groundwater flow through rock is generally alongdiscontinuities and permeability tests in the laboratory willnormally be carried out on intact rock. The results will bear norelation to the permeability of the rock mass. Field testing isthe only method of obtaining a reasonable assessment of rock masspermeability (chapter 2).

68

The rock samples tested in the laboratory are smallrelative to the scale of the rock structure and therefore arenot representative. This must be considered when using the testresults for the analysis of stability and design of rock slopes(chapter 5).

3.19 Testing of discontinuities

Various methods have been developed to determine the shearstrength of discontinuities in the rock mass. The failure envelopefor rock is non-linear and it is therefore important that the shearstrength parameters be determined within the appropriate stress range(Barton 1974), (Hoek and Bray 1974) (figure 3.1),

The laboratory shear box can be used to determine shearstrength parameters of rock discontinuities and is suitable for thelow normal loads likely to be used for testing in Hong Kong. Thestandard shear box is capable of taking normal loads of up to 3 kN5which, on a circular sample 60 mm in diameter, is approximatelyequivalent to the overburden pressure induced by 45 m of overlyingrock. Notes by Richards (1976) on the modification of a 60 mmshear box and on sample preparation for testing rock joints aregiven below; larger samples can be tested using a 100 mm box.

(i) Modification of equipment:

To steady the top half of the box two PTFE strips arefixed to the top half on either side of the box and the gooseneck is extended to maintain correct alignment (figure 3.3).This prevents both rocking of the box when high points ofthe joint are in contact and catching of the specimen onthe edge of the box.

/Top half of box

IT•5mm PTFE strip

Goose neck extended downwards 5 mm to'keep correct alignment with proving ring.

Figure 3.3 Modification of shear box69

(ii) Sample preparation:

The sample may be machine cut to 60 mm square with thejoint horizontal or core samples suitably cut down tosize, being set in plaster of paris or cement. Coresamples are prepared in a mould (figure 3.4), each halfmade in two L-shaped pieces to aid removal of the sample,

Locating pin

60 mm

©crm:

©

_ 2 Q m 20mm

Dismantling screws

Figure 3.4 Mould for rock core

Method using core samples:

(a) Place bottom mould on flat surface(b) Place one half of sample centrally in mould with joint

on flat surface(c) Place 3 mm fine sand around sample and smooth out,(d) Pour plaster of paris or cement around and over sample(e) Strike the top off smoothly with a trowel (figure 3.5),

70

(f) When the plaster or cement has taken its initial set,turn the bottom mould over and place the top mould onit; insert locating pins

(g) Position the other half of the core to fit the jointexactly

Plaster of Paris or cement

Sand BX core (41 mm dia)

Figure 3.5 Preparation of sample I

(h) Pour 6 mm of fine sand around the sides of the coresmoothing it off carefully

(i) Pour the plaster or cement round the core striking thetop off smoothly (figure 3.6)

Figure 3.6 Preparation of sample II

71

(j) The plaster of paris or cement should be allowed toharden

(k) In the 60 mm box, NX core can only be set up in this wayif the joint is at almost 90°, BX and AX cores can bebe set up at more oblique angles

Multi-stage tests, where a sample is sheared under a seriesof increasing normal loads, can be carried out. This will avoid thedifficulty of obtaining mean shear strength parameters from tests onseveral individual samples caused by differences in joint conditions(for examples, degree of weathering or roughness). The parametersobtained from multi-stage tests, however, refer only to the jointtested.

Upper shear box

Rope load equaliser

Concrete or plastercast specimen mount

.Gauge for measurement ofshear displacement

hear surface

Shear load jack

Lower shear box

From Hoek & Bray (1977)

Figure 3.7 Robertson shear box

The Robertson Shear Box, figure 3.7 (Ross-Brown and Walton1975) was developed to test the shear strength of discontinuities inrocks forming the slopes of deep open cast mines. At very lowloads the accuracy of the pressure gauges used to monitor shear andnormal loads can be of the same magnitude as the imposed load. Thismakes the value of the results obtained questionable. Testing at thestress range obtaining on the predicted failure surface is recommended,

72

and special gauges may be required on the shear box to match the testload-range. In this equipment the ram contains a self-return springwhich can, if incorrectly calibrated, contribute to systematic errors.Prior to testing, the magnitude of this error, and that due to frictionof the ram, may be determined by calibration of the equipment. Thetest procedure is described in Hoek and Bray (1977).

The inclined plane, described by Cawsey and Farrar (1976),is still only in the research stage and should not be used todetermine parameters for design.

3.20 Tests on intact rock

Unconfined compression (uniaxial) and triaxial compressiontests are the most suitable for standard testing and only these twoare discussed here. Information on these and other tests is given inJaeger and Cook (1976).

(i) Unconfined Compression Test:

The testing machine used should be as rigid as possible toreduce problems associated with violent failure of thesample.

The sample should have a length to diameter ratio of 2 to3, and the ends should be machined flat and normal to thelong axis of the sample.

Flat end platens are used for standard testing whichindicate a uniaxial strength that is higher than it shouldbe. Elaborate methods have been tried to eliminate theproblem, such as dumb-bell shaped specimens with a centralgauge length, platens with the same rigidity as the rockand conical and lubricated platens. None of these iscompletely satisfactory.

A stress-strain curve should be plotted for the test andthe uniaxial compressive strength is the maximum stressrecorded during the test.

(ii) Triaxial Compression Test:

This is based on the triaxial test used for soils; theprinciples are the same but the equipment is more rigid.Hydraulic oil is used as the cell fluid for low pressuretesting. The specimens to be tested must be smooth cylinderswith end faces accurately machined flat and normal to thelong axis, Hoek triaxial cells (figure 3.8) are availablefor standard core sizes from E (21.5 mm) to N (60.8 mm).

73

Hardened and groundsteel spherical seats

|- • • "V Clearance gap

Mild steel cell body

Rock specimen

Oil inlet

Strain gauge

Rubber sealing sleeve

From Hoek & Franklin (1968)

Figure 3.8 Hoek triaxial cell

3.21 Records

It is most important to maintain good records of both soilsand rock tests. Appendix B of BS 1377 (1975) and Akroyd (1969)give typical data and calculation forms for the tests they describe.One method of presenting the data from a triaxial test is shown infigure 3.9 and two methods of presenting the results in figure 3.2.

74

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EEFERENCES

Akroyd T.N.W. (1969). Laboratory Testing in Soil Engineering* SoilMechanics Ltd, London.

Building Research Station (195D. Concrete in sulphate-bearing clays &ground waters. Building Research Station Digest No. 31*

BS 1377 (1975) Methods of testing soils for civil engineering purposes.British Standards Institution, London.

Barton N. (197*0. Estimating the shear strength of rock joints. Proceedingsof the 3rd International Society for Bock Mechanics CongressDenver, Vol. II-A, pp 219-220.

Bishop A.W. and Henkel D.J. (1962) The Measurement of Soil Properties inthe Triaxial Test. Edward Arnold, London* 227pp.

Cawsey D.C. and Farrar N.S. (1976). A simple sliding apparatus for themeasurement of rock joint friction. Geotechnique, Vol. 26,pp 382-386.

Coates D.F. (1970) Rock mechanics principles, Mines Branch Monograph 87 ,Department of Energy Mines & Resources, Ottawa.

Hoek E. and Bray W.J* (1977) Rock Slope Engineering, The Institution ofMining & Metallurgy, London 309pp*

Jaeger J.C. and Cook N.G.W* Fundamentals of Rock Mechanics, Chapman andHall, London, 585pp*

Lambe T.W. and Whitman R.V. (1969) Soil Mechanics. Wiley, New York. 533pp«

Richards L.A. (1976). Personal Communication.

Ross-Brown D.M. and Walton G. (1975) A portable shear box for testing rockjoints. Rock Mechanics , Vol. 7 No. 3, pp 129-153.

Smith G.N. (197*0 Elements of Soil and Mechanics for Civil and MiningEngineers. Crosby Lockwood Staples, London, M8pp.

U.S. Bureau of Reclamation (197 ). Earth Manual . U.S. Government PrintingOffice, Washington.

76

CHAPTER 4

GROUNDWATER

4.1 Introduction

The incidence of slope failure during periods of intenserainfall indicates the degree to which rainfall and subsequent movementof groundwater affects slope stability. A knowledge of groundwaterconditions is needed for the analysis and design of slopes. Thegroundwater regime is often the only natural parameter which can beeconomically changed to increase the stability of slopes. Thischapter describes the methods whereby the influence of rainfall ongroundwater can be assessed. Most of the chapter deals with flow insoils. Groundwater flow in rock is discussed in section 4.11.

table:The divisions of sub-surface water are given in the following

Zones Water Process Division Pressure

TJQ

23

a

•o

"(5i—3

OS

Hygroscopic

Pedicular

Capillary

Water

Ground water( Phreatic zone)

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0

13oo

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Discontinuouscapillary saturation

Semi- continuouscapillary saturation

Continuouscapillary saturation

Unconfinedground water

G as phase= atmospheric

Liquid phase< atmospheric

< atmospheric

> atmospheric

Table 4*1 - Divisions of sub-surface water. ( after ICE 1976)

ways:

(a)

(b)

(c)

(d)

Water affects the stability of slopes in the following

By changing the mineral constituents of the materialsforming the slopesBy changing the bulk density of the material forming theslopeBy generating pore pressures, both positive and negative,which alter stress conditionsBy both internal and external erosion

77

4.2 Modes of groundwater flow

Rain falling on the ground surface will infiltrate. Theamount of infiltration resulting from rainfall will depend uponthe intensity of the rainfall, the nature of the surface vegetation,the topography, the permeability of the surface and subsurfacematerials, and the degree of saturation of the materials into whichinfiltration occurs.

Infiltration through an unsaturated zone is vertical andcauses no positive pore pressures (section 4.4). If the infiltratingrainfall, during its descent, encounters a material of lower permeabi-lity than that through which it is descending, flow will be impededif the permeability of this lower zone is less than the rate ofinfiltration (section 4.7). Under these conditions a perched watertable will form on the surface of the impermeable zone and lateralflow will take place along the upper surface of the less permeablezone (figure 4.1). Below the less permeable zone the infiltrationrate will be reduced to the value of the permeability of the zone.When the infiltrating rainfall meets the groundwater table (phreaticsurface) most of the vertical component of flow is destroyed andlateral flow in the general direction of groundwater flow takesplace. Under these circumstances the groundwater table rises by anamount equal to the depth of saturation caused by the descendingzone of infiltration and will be less than the depth of this zone.In the zone in which lateral flow takes place positive pore pressuresexist.

i I Rainfall

Rainfall retentionon vegetation

When flow verticalno positive porepressures exist

Infiltratioft rate=I mm/sec

Perched water table formswith lateral flow along uppersurface if kjmp<Imm/sec

Less permeable

Infiltration rate = I mm/sec

Ground water tablebefore rainfallifter rainfall

-Positive pore pressures wherelateral flow takes place

Figure 4.1 Modes of groundwater flow

78

These two modes of groundwater flow affect slope stabilityby different mechanisms. Above the phreatic surface the infiltratingrainfall raises the degree of saturation of the soil, which reducesthe negative pore pressure, and, consequently, the shear strength.As lateral flow develops, pore pressures increase, and as a resulteffective stresses and shear strength are reduced. These two mechanismsdo not occur simultaneously. The reduction of negative pore pressureoccurs during infiltration and may not become fully re-establishedfor some time after infiltration ceases. The increase of positivepore pressure occurs when the infiltrating rainfall forms a perchedwater table or has caused a rise in the groundwater table.

4.3 Negative pore pressure

Above the phreatic surface, in the zone where verticalinfiltration takes place, lie the zones of capillary, pellicularand hygroscopic water. In the first two zones the pore water iscontinuous or semicontinuous and the pore water pressures are belowatmospheric. The magnitude of the negative pore pressure is controlledby surface tension at the air-water boundaries within the pores and islimited by grain size. Within the hygroscopic zone the pore air iscontinuous and at atmospheric pressure. Negative pore pressureswhich increase the effective stresses within a soil mass and improvethe stability exist in Hong Kong soils, and high values have beenmeasured in undisturbed samples of residual soils in the laboratory(Wong 1970). Negative pore pressures reduce when the degree ofsaturation increases.

4.4 Positive pore pressures

At the phreatic surface pore water is at atmosphericpressure, while below the phreatic surface the pore pressure will beabove atmospheric. If no flow is taking place, the pore pressuresare hydrostatic and the water level measured in a piezometer withinthe saturation zone will coincide with the phreatic surface. Porepressures are no longer hydrostatic if there is any flow (figure 4.2)Positive pore pressure, by reducing effective stress, reduces theavailable shear strength within a soil mass, thereby decreasing thestability. Analysis of slopes in terms of effective stress isdiscussed in chapter 5.

Positive pore pressure can be measured by using piezometersinstalled in boreholes (see chapter 10) and the position of thephreatic surface can thus be estimated. Observation of seepage outof the face of slopes gives additional Information on the position ofthe phreatic surface, and its rise due to rainfall. Care must betaken to distinguish between seepage of groundwater and run-offotherwise erroneous conclusions can be drawn as to the response ofthe phreatic surface to rainfall.

79

4.5

Figure 4.2 The effect of flow on piezometric level

Flow-nets

The pore pressures which occur below the water table can beassessed using analytical, numerical or graphical methods. When thepermeability is isotropic, flow lines and equipotentials can besketched on true-to-scale drawings. Under anisotropic conditionstransformed sections can be used. Flow-net sketching is explained inmany text books, such as Cedergren (1977), but these apply to two-dimensional homogenous soil conditions and are not suited for takingaccount of the localised variations found in Hong Kong soils. Harr(1962) describes several analytical approaches to the solution ofthese problems.

The technique of constructing flow-nets for steady stateflow conditions is correct only for coarse grained materials wherethe capillary rise is insignificant and flow above the phreaticsurface can be ignored,

The groundwater conditions which are critical for slopestability in Hong Kong are generally transient. Cedergren (1977)gives a section on transient flow-nets but these depend on capillaritybeing small enough to be ignored.

80

4.6 Run-off

Hydrologists define the $ index of a catchment for aparticular storm to be the average rate of rainfall above which alladditional rainfall appears as run-off in streams. For short durationstorms the value of the $ index will be affected by catchment retention,depression storage and infiltration; but as the duration of the stormincreases the effects of infiltration will predominate.

The Water Supplies Department of PWD determined the $ indexfor 43 storms in eight natural catchments for which a limiting $ indexof 1 x 10~6 m/sec for long duration storms was determined (figure 4.3).Various curve fitting techniques were applied to the data, and from thesethe run-off corresponding to that $ index was found to lie between 50%and 70%. Therefore assuming that infiltration takes place at the limitingrate, and that 50% of rainfall runs off, a rainfall intensity equal totwice the saturated permeability is necessary to create these conditions.

100

75

50

25

-Estimated upper limit of $ index for 43storms occuring in 8 catchment areas.

This result does not folbwgeneral pattern for thecatchment concerned

3 4 5 6 ' 7 8 9 10 20

Duration of Rainfall in hrs.30 40

Figure 4*3 $ Index for Hong Kong catchments

For long duration rainfall the '4 index drops to approximately3 mm/hr compared with short duration rainfall intensities of 80 mmper hour which are not uncommon in Hong Kong.

81

4*7 The design storm for infiltration

The extent to which rainfall will affect slope stability,by either reducing negative or increasing positive pore pressure, isdependent upon the depth of the wetting band which forms as a con-sequence of the rainfall. Assuming the intensity of rainfall to beat least sufficient to cause infiltration at the limiting rate, thedepth of the wetting band will be dependent upon the duration of thestorm, which is inversely proportional to the intensity of rainfall.

The intensity adopted for the design storm should be equalto the saturated permeability of the soil forming the slope surface(Lumb 1962) multiplied by 2 to take account of run-off (as explainedin section 4.6). The duration of the storm of that average intensity,and of a selected return period, can then be obtained by reference tocurves of probable maximum rainfall prepared by Bell and Chin (1968)(for example; the 1 in 10 year storm, and the 1 in 1,000 year storm,as discussed in chapter 5).

4*8 Degree of saturation

The wetting band thickness which forms as a result ofrainfall is inversely proportional to the difference between theinitial and final degree of saturation of the soil mass. Thickerwetting bands are, therefore, more likely to occur after a series ofheavy storms when the initial degree of saturation will be higherthan after dry spells.

The initial degree of saturation may be determined in thefield. This will, however, depend upon the amount of rainfall whichhas occurred prior to sampling, and also upon the method adopted forobtain-ing the samples. Preferably these should be obtained fromtrial pits. They should be tested for moisture content immediatelythey are recovered, although evaporation during excavation andsampling could still be significant and disturbance can affect thedensity measurements necessary for obtaining degree of saturation.An added complication is the variation in moisture content which willoccur with depth. Degree of saturation should, therefore, be determinedat frequent intervals over the first few metres of the soil profile.

4.9 Depth of wetting band due to infiltration

The wetting band caused by a rainstorm will extend downwardsfrom the ground surface under the effects of gravity. The relationshipbetween rainfall on unprotected slopes, infiltration and the thicknessof the wetting band has been studied by Lumb (1962, 1975) who uses thefollowing equation for the advance of a wetting front:

where h » depth of wetting front

D » diffusion parameter

82

k * coefficient of permeability

n = porosity

S0= initial degree of saturation

Sf» final degree of saturation

t = duration of rainfall

Lumb (1975) has proposed that this expression be modified toa more approximate formula:

ktn(Sf - S0)

This simplification assumes that following heavy and prolongedrainfall the diffusion term can be ignored. The parameters requiredfor estimating the thickness of the wetting band are rainfall intensityand duration, run-off, permeability and degree of saturation.Figures 4.4 and 4.5 give examples of graphical solutions of thisequation and show the effects of variation in the value (Sf ~ S0 ),the increase in degree of saturation.

The value of k for the surface may be determined by carryingout infiltration tests within a U100 sample tube driven into the soilsurface. Water is fed to the exposed surface at a carefully controlledrate so that a minimum of ponding occurs on the surface. The totalvolume of water infiltrating is noted at various time intervals andthis is plotted against time. The flow under steady conditions maybe used to determine permeability. A suitable field infiltrometer isshown in figure 4.6 and typical results are given in figure 4.7.

The assumptions made when applying the above expressionresult in an extremely simplified model for infiltration. Forexample, if in the soil profile there exists a very thin band ofmaterial of permeability lower than the overlying and underlyingmaterials, this will act as a throttle on infiltration. Above theband positive pore pressures will develop, while below the band fullsaturation is unlikely to be achieved. If the surface permeabilityis lower than that of the underlying material the surface then actsas a throttle and no band of saturation can develop. Similar analyticaldifficulties are Introduced as a result of the degree of saturationchanging throughout the soil profile.

83

1x10

2 3

Depth of wetting band (him

Figure 4.4

The effect of permeability and degree of saturation on wettingband thickness ( i ) 1:10 year storm

84

1x10

I* 5 6 7 8 9 10 20

Depth of wetting band (h ) m

30 40 50 60 70 8090100

Figure 4.5

The effect of permeability and degree of saturation on wettingband thickness ( i i ) 1:1000 year storm

85

Water -

Control Tube-

-Polythene Feeder Bottle

- Supporting Frame

U 100 Tube

Feeder Tube

Soil surface

Figure 4.6 Diagram .of field infiltrometer.(after Lam 1974)

TEST 2

RESULTS OF TWO (N SITU

CONSTANT HEAD

PERMEABILITY TEST

3 L 5

TIME MINUTES

Figure 4.7 Typical results from field infiltration

86

4.10 Effect of rainfall on groundwater

The groundwater and pore water pressure conditions in aparticular slope are dependent upon the geology, topography and landuse of the area* Because these vary considerably between slopes itis not possible to draw general relationships between rainfall andgroundwater response with any accuracy. Ideally the groundwaterconditions assumed in any stability analysis should be those observedin the field. However, for very rare rainfall events this is seldompossible.

As an alternative, piezometers may be observed closely overat least one wet season and the groundwater response at the sitecorrelated to rainfall. This is most cases will require the instal-lation of a rain gauge at, or near, the site and frequent observationof piezometers. The analysis of the data obtained may however bedifficult as the groundwater response may be both seasonal and stormspecific. The extrapolation of the data to such rare events as the 1in 1000 year storm, or even to the 1 in 10 year storm, may be corres-pondingly difficult. Where the possibility of a deep-seated slide inan existing slope is being considered it may be appropriate toobserve piezometers for extended periods to see if critical levelsare approached, and to analyse the data by reference to the extremevalues of groundwater level.

If sufficient piezometer readings are not available analternative method, which for deep soil mantles is conservative,may be used. This assumes that under the influence of gravity thewetting band, the thickness of which has been calculated by themethods described in section 4.9, descends vertically even after thecessation of rain until it reaches a zone of lower permeability,which may be very thin, or until it reaches the phreatic surface.

Under the former conditions a perched water table will formabove the zone of lower permeability and pore pressure will becomepositive. When the descending wetting band reaches the phreaticsurface the phreatic surface will rise with a consequent increase inpore pressure. The thickness of the perched water table or theincrease in ground-water level will be approximately equal to thethickness of the descending wetting band, reduced to allow for theextent, if any, to which the soil within the wetting band is assumedto be not fully saturated. The groundwater level upon which thedescending wetting band is superimposed should be the highest groundwater level observed after an average wet season, or, alternativelythe level corresponding to the highest seepage traces or stainsobserved on adjacent slopes or retaining structures.

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4.11 Groundwater in Rock

Sections 4.1 to 4.10 have dealt almost exclusively withgroundwater flow in soil and how it is affected by rainfall. Themethods discussed do not apply to groundwater flow in jointed rocksunless the joints are infilled with material which has a permeabilityas great as the intact rock. Normally in Hong Kong permeability ofrock masses will be controlled by the joint geometry, which includesspacing, direction, joint width and form, as well as the degree ofinfilling and the roughness of the joint surfaces. These factors areall variable over small distances and make both observation andanalysis difficult.

The normal observational technique for soils requires apiezometer to be installed in a short pocket of free-draining filtermaterial. If such a technique is adopted for rock, the jointsintercepted by the piezometer pocket may be either dry or of such lowpermea-bility that the instrument cannot respond sufficiently quicklyto register maximum piezometric pressures. There is no easy way ofdealing with this problem. Hence, it is better to monitor changes inthe phreatic surface rather than piezometric pressure. This is doneby setting a piezometer tip in a very long filter pack, which mayextend over the full length of hole drilled in rock, with the piezometeracting as a standpipe (see chapter 10). By sealing the filter pack,as described in chapter 10, observations should not be affected byrainfall infiltrating the backfilled hole.

There are, however, also problems with this method ofobservation in that the standpipe may cut across joints with differentflow and pressure characteristics. Examination of cut faces may dolittle to help interpret the results of observation of standpipesas the rate of flow from relatively tight joints, in which there arehigh hydraulic pressures, may be so low that all seepage is removedby evaporation giving the impression that the face is dry (Coates1970, Hoek and Bray 1977). As the pressure response in the rock maybe quite different to that in the overlying soil separate piezometersand standpipes should be inserted in the soil and rock layers.

Despite these shortcomings the standpipe system can yielduseful information on the distribution of groundwater in a rock mass,although the Interpretation in terms of fissure water pressure, asdescribed above, is difficult.

Open joints at shallow depths in rock slopes present adifferent problem: very Intense rainfall over a short period maycause the joint to fill up with water. For exposed rock faces, whereopen joints exist, it is reasonable to assume that these joints couldbecome full of water to ground surface and therefore to analyse thestability of individual blocks accordingly.

88

4.12 Other factors affecting groundwater conditions

Locally, groundwater conditions can be significantlyaffected by factors other than rainfall and these other influencesshould be considered when interpreting piezometer data or estimatingmaximum groundwater levels for inclusion in stability analyses.While the effects on groundwater regime of leakage from sewers andwater mains are obvious, less obvious are the effects of groundwaterpumping. Slope designs based upon groundwater levels determined Inareas of active groundwater extraction can subsequently becomeinadequate should pumping cease and the groundwater level rise, ascan happen during site redevelopment. When determining maximumgroundwater levels to be considered for stability analyses an assessmentshould be made of the likely draw down effects of nearby wells, andthe phreatic surface used for analysis should be based on the assump-tion of no draw-down.

Groundwater levels can also be affected by construction,both by temporary dewatering and by the construction of sheet pilewalls which can redistribute groundwater flow. Allowances should bemade for these factors when interpreting piezometer observation data.

SUBSURFACE DRAINAGE MEASURES

4.13 General

The factor of safety against failure on any potential slipsurface which passes below the phreatic surface can be improved bysub-surface drainage which reduces groundwater levels. This sectiondiscusses horizontal drains, drainage galleries, vertical drainagewells, cut-off and counterfort drains - all of which will lower waterlevels.

In any subsurface drainage system monitoring is important.Piezometers should be installed to measure the pre-construction porepressure, and should be read during and after construction to observethe effect of the drains. In the long term, piezometer readings canindicate impaired efficiency of the drainage system caused by slltation,deterioration of seals or breakdown of pumps.

The volume of water flowing from any drain is directlyproportional to permeability and to hydraulic gradient. When a drainis Installed the groundwater level will be lowered reducing thehydraulic gradient. The seepage will, therefore, progressivelyreduce from its Initial value to a steady state value. This reductionin flow is not an indication of drain deterioration. In low permeabi-lity materials there may be no visible seepage from drains; theyare nonetheless operating successfully.

In rock masses the groundwater flow will generally beconfined to the joints and therefore any drainage system mustintersect the discontinuities.

89

Where the drain is above the phreatic surface the liningshould be impermeable. This is important as drains do not rechargethe soil or rock. Where an unlined drain passes through partlysaturated zones seepage from the drain can decrease the pore suctionin the vicinity, thereby decreasing stability.

In all gravity drains the outlet to the collecting chambersshould be lower than the invert of the drain to prevent water backing-up the drain.

4.14 Horizontal drains

The main advantages of horizontal drains are that they arerelatively quick an3 simple to install and rely on gravity drainage.Holes are usually 75 mm to 100 mm diameter, drilled at a gradient of3% to 5% uphill using a slotted casing. A filter should be introducedto prevent erosion or clogging.

Spacing is often decided on a trial and error basis usingthe piezometer installation to monitor the effect of widely spacedholes, additional holes being drilled if the draw-down obtainedwith those already installed is insufficient. The performance of thedrainage system should be monitored to properly assess its continuingadequacy.

Tong and Maher (1975) describe the design of horizontaldrain installations which are grouted to obtain impermeable inverts.They suggest that the spacing of the drains in each row should be ofthe same order as the likely thickness of the aquifer and that, wherethe impermeable bed of the aquifer is steep, the spacing should beclosed up as much as possible. Choi (1977) gives an example of athree dimensional method for the analysis of the performance ofdrains in slopes of homogeneous soil overlying rock.

4.15 Drainage galleries

Drainage galleries excavated behind the slope face areoften as economically justifiable as a drainage measure for largeslopes. Owing to the larger cross-sectional area, drainage gallerieshave better hydraulic connexion with the material being drained, andare more reliable for long term operation because the galleries canbe easily cleared of silt.

The optimum location and size of galleries has beeninvestigated and the results published by Sharp (1970) and Sharp etal (1972).

90

4.16 Vertical wells

Vertical wells are useful during construction as they donot interfere with work on a cut face or retaining wall. To maintaindrawdown they depend on pumps which can be put out of action by powerfailures although standby generating equipment may be provided.However, in the long term, and even with stand-by facilities, pumpingis not considered a satisfactory solution. Where possible, permanentwells should be drained under gravity through a system such as agallery. Horizontal drains can be used, but drilling to interceptvertical wells is difficult. The design of wells, well screens andpumps has been described by Johnson (1972).

4.17 Cut-off drains

Cut-off drains may be used to intercept shallow groundwaterflow above slopes which have been protected against infiltration. Atypical layout is shown in figure 4.8. These drains are most effectivewhere an impermeable layer is present at shallow depth. An impermeablezone or membrane should be used as a cut-off downslope of the drainand the top zone of the trench should be backfilled with impermeablematerial. Run-off from the upper slopes should be collected insurface water channels designed to the requirements given in chapter8. The free draining material used to backfill the trench should bedesigned to conform with the filter criteria given in section 4,19.The size of perforations in perforated pipes, slots in slotted pipeand width of joints in open jointed pipe should be based upon thegrain size of the filter material used as backfill to the trench.Spalding (1970) quotes:~

Maximum diameter of circular holes « D85F or 5 mm, whicheveris smaller.

Maximum width of slots » 0.83 x D85F or 5 mm, whichever issmaller.

(D85F is the sieve size which allows 85% of the filtermaterial to pass.)

Based upon Spalding1 s analysis of existing data it is re-commended that the criteria for width of slots also be applied to thewidth of open joints. Pipes which are perforated or slotted overonly part of the circumference should be placed with the perforationsor slots down on a bed of filter material. This will reduce theamount of water flowing in the filter material below the pipe. Ifporous pipes are used these should be placed as tightly together aspossible to prevent fine filter material being washed into the pipe.As a precaution the stability of the slope should be checked, assumingthe drain forms a tension crack.

91

Impermeable back fill%

Chunam

Filter

Critical slip surface

Figure 4.8 Cut off drain

4.18 Counterfort drains

The use of counterfort drains may be considered for soilslopes where the phreatic surface is close to ground level. Thespacing and depth may be estimated using flow nets. The trenchshould be backfilled with free draining material which conforms tothe criteria given in section 4.19, and this trench should be sealedat the surface with impermeable material to prevent the drain collectingsurface run-off. Porous, perforated or slatted pipes incorporated inthe drains should conform to the criteria given in section 4.17.

92

4.19 Filters

A graded filter is a material which, when placed against asoil, is so graded that in water flowing across the soil/filter inter-face migration of finer particles from the soil is prevented.Filters should be more permeable than the protected soil and shouldbe so graded that segregation does not occur during placing. Filtersare used to prevent fine grained particles from entering subsurfacedrains, the most erosion susceptible materials being coarse silts andfine sands.

Table 4.2 gives details of the normal filter design criteriaapplicable to soils in Hong Kong. The base soil particle size dis-tribution used for the design of filters should be obtained as des-cribed in chapter 3, but without the use of dispersants. Where thebase soil contains a large percentage of gravel or larger sizedparticles the finer fraction should be used for the filter design.

While the USWES (1953) criteria are applicable to siltysoils it has been found by experiment under relatively low heads thatthe U.S. Corps of Engineers concrete sand is suitable for use as afilter for all silts and finer soils. The grading of BS 882 zone 2natural sand is very similar to that of the US Corps of Engineersconcrete sand. This is shown in figure 4.9 which also shows thegrading of a free draining material which may be used in conjunctionwith this sand. The grading for BS 882 zone 2 sand produced fromcrushed stone can be unacceptable as this standard permits 20% of thematerial to be finer than 150 ym sieve (see Rule 8 in table 4.2).

Where filter materials are used in conjunction with acoarser free draining material, such as crushed rock, the grading ofthe coarser material should conform to the filter design criteria(given in table 4.2) to protect the filter from erosion. Wherefilters are to be provided between material of widely differing grainsizes, such as between decomposed volcanic material and rockfill, twoor more filter zones may be required.

A common practice in Hong Kong is to use no-fines concreteor hand-packed rubble as a drainage layer behind retaining walls.These materials should be protected with a transition zone of gravelor crushed rock which will not migrate into the voids of the rubbleor no-fines concrete, but which conforms to the filter design rulesapplied to the soil-protecting filter. However, when this is done itis probable that the hand-packed rubble or no-fines concrete can beomitted and that the transition zone can be used as the drainagelayer.

93

Rule Number Filter Design Rule

(1)

(2)*

(3)

(4)

(5)

(6)

(7)

(8)

D15F < 5 x D85S.c f

D15F < 20 x D15S-c f

D15F- > 5 x D15Sc

D50F < 25 x D50S.c t

Uniformity Coefficient 4 <

Should not be gap graded

D60FD10F < 20

Maximum size of particle should not begreater than 75 mm

Not more than 5% to pass 63 jim sieve andthis fraction to be cohesionless

* For well graded base soil this criterion can be extended to5f

In this table, D15F is used to designate the 15% size of thefilter material (i.e. the size of the sieve that allows 15%by weight of the filter material to pass through it).Similarly, D85S designates the size of sieve that allows85% by weight of the base soil to pass through it. D60Findicates the D60 size on the coarse side of the filter c

envelope. D10Ff indicates the D10 size on the fine sideof the filter envelope.

Table 4.2 Filter design criteria to be used in Hong Kong

94

BRmSH STANDARD SIEVE SfZES

I 100 60 36 1t 10 71' 3-1" 3'T 1" ,TgO | 72 ,52 , • g,. If .1.' I itr t a g 1-4 r *j!

mm 0.002 0.006 002 Q06 Q2 600mm

CLAYFine | Medium I Coarse

SILT

Fine I Medium! Coarse

SANDFine ] Medium I Coarse

GRAVELCOBBLES BOULDERS

Figure 4.9 Grading of sand filter (A) for all siltsand finer soils. The associated drainagematerial (B) is also shown. Sand A will actas a filter for all materials with gradingcurves passing through the hatched area.The rules shown on the figure are those givenin table 4.2.

Problems also arise when designing filters to protect gap-graded soils. Where some particle sizes of a soil gradation arescarce or missing the filter material should be designed on thebasis of the finer soil particles only. This is also true forlayered soils. Figure 4.10 shows how this is done.

Calculations for the design of filter gradings are con-veniently carried out in tabular form. An example of such a cal-culation is given in figure 4.11; it was used to define the gradingenvelope of a drainage material suitable for use with the BS 882zone 2 (figure 4.9).

Material used for filters should consist of hard durablestone which, when placed, should be well compacted.

95

D85S = 0.095mm

D6QS=0.075 mm

D50S=0.068mm Redrawn soil gradingused in calculation

/Calculated filter

limits/p10S=a035mm

0.01 2 3 A 5 67890.1 2 3 4 5 1.0

Grain size (nun)

(1) Replot grading curve for finer portion of soil.(2) Determine D85S, D60S, D50S, D15S, BIOS.(3) Determine value Dj50S,

(4) Select appropriate filter criteria.(5) Determine D50F, D15F for filter criteria and min. D15F

for permeability criteria.

Figure 4.10 Filter design for gap graded soilsusing criteria given in Table 4.2

The minimum width of a zone which is to act purely as afilter should be that which can be constructed without segregation ofthe material or "holes" being left in the zone where the protectedsoil boundary is directly in contact with free draining material. Aminimum thickness of 450 mm is recommended for machine placed material,Where a thickness less than 450 mm is to be placed it may be necessaryto hand place the zone to ensure its integrity. Where the filterzone is also to act as a drain the thickness of the zone should besufficient to carry the maximum expected groundwater flow whileallowing free drainage of the protected material.

4.20 Filter fabrics

Several proprietary filter fabrics are now available.These have not been widely used in Hong Kong, so little evidenceexists at present on their durability under local conditions.

96

Filter Design Rule

(1) Dl5Fc < 5 x D85Sf

(2) Dl5Fc < 20 x D15Sf

(3) D15Ff > 5 x D15Sc

(4) D50Fc < 25 D50Sf

Uniformity Coefficients

(5) D60 F,t > 4

D10 Ff

D60 Fc > 4

D10 Fc

(6) D60 FC < 20

D10 Ff

Soil to be protected

size in mm

D85Sf = 1.0

D15Sf - 0.18

D15S = 0.35c

D50Sf = 0.48

D10

1.3

3.2

1.3

D60

6.0

15

15

Filter

size in mm

D15F < 5c

D15F < 3.6c

D15Ff > 1.75

D50F < 12.00c

Uniformity Coefficient

4.6

4.7

11.5

O.K.

O.K.

O.K.

Figure 4.11 Typical calculations for the design of filters.The criteria used are those given in Table 4.2and the material designed is the drainage materialshown on Fig. 4.9.

97

INFERENCES

Bell G.J* and Chin P.O. (1968). The probable maximum rainfall in Hong Kong.Royal Observatory Hong Kong Technical Memoir No, 10 1

Cedergren H.R. (1977). Seepage, drainage and flow nets. Wiley, New York,

Choy E.C.C. (1977). Seepage around horizontal drains- two-and three-dimensionalfinite element analysis. Hong Kong Engineer, Vol.5 No. 9$ PP35-39.

Coates D.F. (1970)0 Rock mechanics principles. Mines Branch Monograph 8?Department of Energy Mines and Resources, Ottawa, Canada.

Harr M.E. (1962). Groundwater and seepage. McGraw-Hill, New York, 315p*

Institution of Civil Engineers (1976). Manual of applied geology for engineers.Institution of Civil Engineers, 378p.

Johnson E.E. (1972). Groundwater and wells. Johnson Division, Universal OilProduct Company, St. Paul, Minn.

Lam K. C. (197 ). Some aspects of fluvial erosion in three small catchments -New Territories, Hong Kong. M. Phil. Thesis, Department ofGeography and Geology, University of Hong Kong.

Lumb P. (1962). The Effect of rainstorms on slope stability. Symposium onHong Kong soils, Hong Kong Inst. Engrs. pp 73-78.

Lumb P. (1975). Slope failures in Hong Kong. The Quarterly Journal ofEngineering Geology, Vol. 8, pp 31-65*

Sharp J.C. (197QX Drainage characteristics of subsurface galleries.Proceedings of the 2nd Congress of the International Society forRock Mechanic. Beograd, Vol. 3t Paper 6.

Sharp J.C., Hoek E. & Brawner C.O. (1972). Influence of groundwater on thestability of rock masses; 2 - drainage systems for increasing thestability slopes. Proceedings of the Institution of Mining &Metallurgy, pp A113-A120.

Spalding R. (1970). Selection of materials for sub-surface drains. RRL ReportLR 3 6, Road Research Laboratory, Crowthorne Bucks, 28p.

Tong P.Y.L. & Maher E.G. (1975)* Horizontal drains as a stabilizing measure.Journal of the Engineering Society of Hong Konff, Vol. 3, No. 1,PP 15-27.

U.S. Army Engineer (1953). Filter experiments and design criteria.Technical Memoir No. 3-360, U.S. Army Engineer Waterways ExperimentStation.

U.S. Bureau of Reclamation (197*0. Earth manual, 2nd Edition, U.S. GovernmentPrinter, Washington.

Wong K.K. (1970). Pore water suction in Hong Kong soil by pgychrometrictechnique. M. Sc. Thesis, University of Hong Kong. Unpublished(available from the University Library). 177p.

98

CHAPTER 5

DESIGN OF SLOPES

5.1 Introduction

This chapter deals with the types of failure which occur inHong Kong residual soils and igneous rocks, and describes suitablemethods for their analysis. The design of cuttings and fill slopesand the treatment of unstable slopes is discussed. Constructionmethods, where they influence slope stability, are considered inchapter 9.

STABILITY ANALYSES

5.2 Modes of failure

When choosing a method of stability analysis for designthe probable mode of failure of the slope must be considered. Themethod chosen should model the failure mode.

The most common failures in Hong Kong residual soils, fillsand colluvium are very shallow, being controlled by the depth ofinfiltration during rainstorms. The failure surfaces are oftenplanar or only slightly concave over a considerable proportion of thesurface.

The structure of loose fill material can, on shearing,collapse and if the material is saturated, or nearly saturated, highpositive pore pressures can develop very rapidly. This phenomenonis known as liquefaction because the soil then behaves as a fluid.The resulting debris from a slope which has suffered a liquefactionfailure can travel very large distances at high speed, even onrelatively flat surfaces.

Where the soil surface is protected against infiltration orthe soil has a sufficiently high effective cohesion, cf, deeperfailures controlled by the depth of weathering, the presence ofjoints or by rising groundwater levels can occur. The stability ofslopes in highly and completely weathered rock and soils should beassessed by analysing a wide range of potential failure surfacespassing through these zones.

Failure in the weathered rock zones is often guided by thepattern of relict joints which may be observed in the face ofexposed excavations.

Failures in unweathered, slightly weathered and moderatelyweathered rock are controlled by the joint system.

Typical failure profiles in soils and rock are given infigure 5.1, and records of failures in Hong Kong are listed in thebibliography (5.19).

99

5.3 Input data

Details of topography, geology, shear strength, groundwaterconditions and external loadings are required for the analysis ofslope stability:

(i) Topography:

An accurate site plan is required showing the positions ofsite investigation holes, joint survey areas and thelocation of cross-sections to be analysed. Cross-sectionsmust be drawn to a natural scale large enough to read offdimensions to an accuracy of about 0.1 m. A scale of 1 to100 is usually suitable. A larger scale, 1 to 50 or 1 to20, may be required to obtain accurate dimensions for thestability analysis of slopes which are less than 10 m high.

(ii) Geology:

The depth of weathering, presence of colluvium or fill andthe structure of the fresh and weathered rock should beassessed from the results of the surface and subsurfaceinvestigation (chapter 2). The details of site geologyavailable for analysis are usually based on a small amountof data which is often open to more than one interpretation,and a range of possibilities must be considered whencarrying out stability analyses. Designs may have to bemodified if geological conditions found during constructiondiffer from those assumed in the design calculations. Thegeological structure assumed for the design should be shownon the slope section.

(iii) Shear strength:

Shear strength parameters for the slope forming materialsshould be obtained by testing samples of soil and rockjoints obtained from the site (chapter 3). The samplesshould be tested at stresses comparable to those in thefield.

The shear strength parameters, cf and (j)1, derived from theresults of tests on fully saturated samples will be lowerbound values which should be used for soils below the watertable and for soils which can become saturated by infiltration.In practice, full saturation by infiltration is unlikely tooccur and a small suction will probably be maintained inthe zone affected by infiltration. No values of themagnitude of this residual suction are, at present, availablefor Hong Kong soils.

100

m80

70

60

50

40

Residual soil

m300

290

280

270

260

250

240

230

m100

90

80

70

60

50

40

30

Rock

After Lumb 1975

Figure 5.1(a)Examples of slope failures inHong Kong soils and rocks

101

Intersecting joints whichform unstable

Rock

Movement along line ofintersection of two joint planes

Figure 5.1 (b)Examples of slope failures inHong Kong soils and rocks

102

In the partly saturated zones, the shear strength of thesoil will be increased by the soil suction. Shear strengthparameters in the unsaturated zone may be determined by twomethods, both of which require major assumptions to bemade. The first approach is to use the fully saturatedshear strength parameters and include in the stabilityanalysis a value of suction as a negative pore pressure.The magnitude of the suction which may be relied upon isuncertain and research, to obtain reliable values, is inprogress. Pending the results of this research beingpublished the value of suction to be used in analysisshould be obtained by back analysis of nearby existingslopes which have, for a number of years, successfullywithstood the effects of periods of heavy rain. Observedgroundwater levels should be used in the back analysis andthese will give a conservative value of suction unless anextreme groundwater condition has been recorded during theobservation period.

The second approach is to measure the fully drained strengthparameters of unsaturated samples, and to choose designvalues of drained cohesion corresponding to the expecteddegree of saturation that would develop on the site.Sufficient tests must be performed to provide a reliableestimate of the variation in drained cohesion with degreeof saturation for each soil type on the site. This willgenerally imply that some tests will be carried out at thenatural water content of the samples and other tests athigher water contents achieved by allowing water to percolatethrough the specimen before testing. If the natural degreeof saturation of the samples is about 90%, then it may beacceptable to make the conservative assumption that drainedcohesion decreases linearly, with increasing degree ofsaturation, from the measured values to a value of zero atfull saturation.

The assessment of expected degree of saturation in theslope being designed is the main difficulty when using thismethod; it is a matter of engineering judgement and experience.There is no specific value which can be recommended assuitable for all occasions and each site must be consideredindividually. The changes in degree of saturation followinginfiltration of rainwater are critically dependent on theratio of the permeability of the protecting surfacing,on and above the slope face, relative to the permeabilityof the soil being protected. Unless the surfacing can berelied upon to limit the amount of Infiltration duringheavy rains then full saturation could develop.

In doubtful situations and when the designer lacks therelevant experience it may be necessary to conduct sitetrials of various protective surfaclngs. These can be

103

carried out by taking samples at shallow depths to measurethe degree of saturation developed immediately after heavyrains or after artificial rainfall tests using sprinklers.Samples may be recovered from adjacent slopes protected bychunam to give an indication of the natural soil moisturecontent under these conditions* Due regard must, however,be paid to possible seasonal variations«

(iv) Groundwater:

Groundwater conditions can be assessed during and followingthe site investigation by installing and reading piezometers,and by observing traces of seepage. The levels obtainedduring the observation period are unlikely to represent thedesign storm levels. Therefore, an estimate must be madeof the extent to which water levels in the slope willincrease in response to rainfall and other factors (seechapter 4). The estimated groundwater levels for 1 in 10and 1 in 1,000 year storms which are to be used in stabilityanalysis should be shown on the slope cross-section.Wetting bands, which form by infiltration (see chapter 4),need not be considered for slopes protected from infil-tration by properly maintained chunam, sprayed concrete orother very low permeability surfacing. However, thepossibility of infiltration further up slope must beconsidered.

The concept of descending wetting bands has little relevanceto rock slopes in which the maximum water pressure maydevelop during any heavy storm which causes a tension crackor open joint to become full of water. The fissure waterpressure acting on the joint should be assumed to be amaximum at the base of the tension crack, reducing nearlyto zero where the joint daylights in the slope face. Waterpressures can vary from joint to joint within a rock massand the pressures measured by piezometers are only relevantto the joints which intersect the filter surrounding thepiezometer tip. They can be relied upon only if the filterintersects a single joint.

Leakage from services, such as sewers, stormwater drainsand water-mains, can cause both saturation of slopes andrising groundwater levels. Service trenches should normallybe constructed as suggested in chapter 9 but, where thereare details of proposed services available, and where thereis any possibility of leakage into the slope, this shouldbe considered in the design. If water carrying servicesare to be laid in an existing slope the stability of thatslope should be checked to determine how it will be affectedby leakage from the proposed services*

104

(v) External loadings:

Loadings from building foundations, retaining walls or spoilheaps which can influence the stability of a slope must beincluded in the analysis. If external loadings are to beconsidered the method of analysis must allow for theirinclusion.

Earthquake forces need not be included in slope stabilityanalyses in Hong Kong, but, the effect of blasting and piledriving should be considered (chapter 9).

5.4 Methods of analysis

Many methods of stability analysis are available for thedesign of soil slopes; the majority are based on limit equilibrium,some on plastic limit theory and some on deformation. Fewer methodsare available for the analysis of rock slopes and almost all arelimit equilibrium methods. The better known methods which can handlec, <J> soils or rocks and water pressures are listed in table 5*1. Theadvantages and limitations are given and recommendations are madeas to their application. The references given in the table are notalways the original references but are easily accessible in books orpapers where the methods are given in enough detail for design officeuse.

5.5 Three dimensional effects

The majority of stability analyses assume a slip of infinitewidth in a planar slope. This assumption is reasonable in the middleof a slip but the ends of the slip are affected by shear on the endwalls. The resulting increase in the overall factor of safety isdifficult to quantify. Hovland (1977) takes the cases of a cylindricalslip with conical ends and of a wedge with two sliding planes. Inthe first case he shows the three dimensional effect to increase thefactor of safety by 10% to 50% depending on the ratio of the width ofcylinder to height of slope. The maximum difference occurs when thecylindrical section is non-existent. In the case of the wedge theincrease is again about 50% for soils with c/yH greater than 1.However, for small values of c the difference reduces and for co-hesionless soils there is a reduction of factor of safety by about5%. Hutchinson et al (1973), by considering the earth pressures onthe sides of a slide in Etruria Marl, showed an increase in factor ofsafety of 16% for the three dimensional case.

Plan curvature of the slope can also affect the overallstability of a slope. Slopes with concave plans are theoreticallymore stable than those with convex plans. However, drainage ofconvex slopes is better and the lower pore pressures can reduce thedifference in factor of safety. The effect is most marked in jointedrocks. Open-cast mining experience (Piteau and Jennings, 1970; Hoekand Bray, 1977) has shown that for concave slopes with radii of

105

Table 5.1(A) - Stability Analysis Methods (Soils)

Method

InfiniteSlope

SlidingBlock

Bishop

FailureSurface

Straightline

Two or morestraightlines

Circular

As sump t ions

Any vertical slice isrepresentative of thewhole slope

The sliding mass can bedivided into two ormore blocks, theequilibrium of eachblock is consideredindependently usinginterblock forces

Considers force andmoment equilibrium foreach slice, Rigorousmethod assumes valuesfor the vertical forceson the sides of eachslice until allequations are satisfied.Simplified methodassumes the resultantof the vertical forcesis. zero on each slice.

Advantages

Simple hand calcul-ation method

Suitable for handcalculation when2 or 3 blocks areused.

Simplified methodcompares well withfinite elementdeformationmethods (averageF within 8%) .Computer programsreadily available.

Limitations

Failure surface assumpt-ions always an approx-imation. Method mayonly be used for slipsurfaces where the leng:hto depth ratio is largeand end effects can beneglected.

Does not consider thedeformation of blocks.Result sensitive to theangle to the horizontalchosen for the inter-block forces and theinclination of thesurface between theblocks.

Circular failuresurface not alwayssuitable for Hong Kongslopes but large radiuscircles can sometimesbe used.

Reference

Lambe & Whitman(1969) pp 352-356.

Lambe & Whitman(1969) pp 366-369.

Bishop (1955)

Recommendat ion

Suitable for long slopes,especially those with athin layer of weatheredsoil over rock.

Useful where there is aweak stratum within orbelow the slope and whenthe slope rests upon avery strong stratum.

Useful where circularfailure surfaces can beassumed.

Table 5.1(A) (Soils) - cont'd.

Method

Hoek'sCharts

Bishop &MorgensternsCharts

Janbu

Mor gens tern& Price

FailureSurface

Circular

Circular

Non-circular

Non-Circular

Assumptions

Sliding mass consideredas a whole. Lower boundsolution, assumingnormal stresses areconcentrated at onepoint ,

Uses Bishop's simplifiedmethod with an averageru value,

Generalised procedureconsiders force andmoment equilibrium oneach slice. Assumptionson line of action ofinter slice forces mustbe made. Verticalinterslice forces notincluded in Routineprocedure and calcul-ated F then correctedto allow for verticalforces .

Considers forces andmoments on eachslice, similar toJanbu Generalisedprocedure.

Advantages

Slope angles from10° to 90° given.Very simple to use.

Simple to use.More accurate thanHoek!s charts.

Realistic shearsurfaces can beused. Routineanalysis can beeasily handled bya programablecalculator or byhand.

Considered moreaccurate thanJanbu. Computerprograms readilyavailable.

Limitations

Limited to homogeneoussoils and fivespecified groundwaterconditions.

Limited to homogeneoussoils and slopesflatter than 27°.

Published fo factorsare for homogeneousmaterials and routineprocedure can givelarge errors in slopescomposed of more thanone material. Factorof safety is usuallyunderestimated in thesecases. Generalisedmethod does not havethe same limitations.

No simplified method.Computer solutionnecessary, often verytime consuming.

Reference

Hoek & Bray(1977)

Bishop &Mor gens tern(1960)

Janbu (1973)

Routine methodgiven in Hoek &Bray (1977)pp. 247-253.

Mor gens tern &Price (1965)

Recommendat ion

Very useful forpreliminary calculationsor for small low riskslopes.

Limited usefulness.

Very useful for themajority of soil slopesin Hong Kong. Limitationsof routine method mustbe considered.

Usually unnecessarilydetailed for Hong Kongsoils where strength andpore pressure are notknown with accuracy.Most useful for backanalysis of landslides.

Table 5.1(A) (Soils) - cont'd.

Method

Sarma

Chen &Giger(Plasticitylimitanalysis)tables

Deformationmethods

FailureSurface

Non-circular

Log-spiral

Non-circular

Assumptions

A modification ofMorgenstern & Pricewhich reduces theiterations required byusing earthquake forces.

Soil assumed to beperfectly plastic,obeying Coulombyield criterion and itsassociated flow rule.

Linear and non-linearelastic finite elementor finite differencemethods .

Advantages

Considerablereduction incomputing timewithout loss ofaccuracy.

Slope angles from15° to 90° given.

More realistic thaneither limitequilibrium or limitanalysis .

Limitations

Computer programs notyet readily available

but can be used with acalculator.

Pore pressures cannot beincluded. Criticalheight not factor ofsafety found fromcalculations .

Requires high qualitylab. and field testingto obtain stress-strainrelationships andelastic parameters.Considerable computertime required.

Reference

Sarma (1973)

Chen & Glger(1971) Chen,Giger & Fang(1969)

Recommendation

Can be used as analternative toMorgenstern & Price.

Useful for preliminarycalculations for dryslopes .

These methods may beused by specialistgeotechnical engineersbut are beyond thescope of this manual.

o00

Methods not recommended for the analyjsis_ of soil slopes

£ - circleFelleniusSpencer & Lowe

Table 5.1(B) - Stability Analysis Methods (Rock)

Method

Planefailure

Wedgefailure

Topplingfailure

FailureSurface

Single planewith tensioncrack

Two jointplanes form3 dimension-al wedge.

Steppedcrossjoints

Assumptions

Both sliding surfaceand tension crackstrike parallel to theslope surface, Releasesurfaces are present sothere is no resistanceon lateral boundaries.

Line of intersectionof joints dips lesssteeply than rockface and daylightswithin the face.Both joint planesremain in contactduring sliding.

Analysis assumes thatsome blocks will slideand some topple.Water pressuresnot included.

Advantages

Water prssures intension crack andon sliding planecan be includedSimple analysismethod .

Tension crack andwater pressures canbe included inanalysis. Charts,which considerfriction only,are available-

Limitations

Momemts not consideredin analysis. Can giveover estimate offactor of safety onsteep slopes wheretoppling could occur.

Moments not considered

Limited to a fewsimple cases withsuitable geometry.

Reference

Hoek & Bray(1977) pp. 150-198.

Hoek & Bray(1977) pp. 199-224 and 333-395.

Hoek & Bray(1977) pp. 257-270.

Recommendation

Useful where planefailure can be assumedsuch as on sheet joints.

Useful. Charts canbe used for apreliminaryassessment.

Not yet a rock slopedesign tool but mayoccasionally beuseful.

OVO

curvature less than the slope height, the slope angle can be 10°steeper than that predicted by conventional stability analysis, whilethe convex slope should be 10° flatter. The influence of curvaturebecomes negligible when the radius is greater than twice the slopeheight.

Boulders in a soil slope can increase both the strength anddensity of the sliding mass. The effect on the factor of safetycannot be quantified for routine stability analyses.

5.6 Factors of safety

The minimum factor of safety against failure which can beaccepted for a slope will depend on the risk to life associated withthe failure of that slope. If slope failure puts no lives at risk,it is often more economic to design to a low factor of safety,accepting that some slopes will occasionally fail and that regularremedial works may be necessary.

Where lives would be endangered by failures, a low factorof safety cannot be accepted.

Table 5.2 gives, for slopes in the three risk categoriesquoted in table 2.1, minimum acceptable factors of safety for thegroundwater conditions associated with a 10-year return periodrainfall. The factor of safety for a slope should also be checkedfor the water levels associated with the 1,000-year return periodstorm (see chapter 4). This factor of safety must not be less than80% of the 10-year value. The reduction in factor of safety for arise in groundwater level can be estimated rapidly using the curvesgiven in figure 5.2a and the associated notes.

Risk Category

L Low

S Significant

H High

Minimum Factor of Safety*1:10 yr storm +

1.2

1,3

1.4

'* These factors of safety against slope failureare the minimum values acceptable. Higher factorsof safety will be required where deformation mustbe limited to prevent distress of structures(see chapters 6 and 7),

4- The factor of safety for a 1:1000 year stormshould be at least 80% of the 1:10 year value.

Table 5.2 Factors of safety

110

A lower factor of safety may be appropriate for thosetemporary works where the risk to the general public is much lowerthan would be the case for permanent works (for example, wherebuildings are unoccupied.) In some cases a low factor of safetyand the consequent high risks may have to be accepted during certainstages of construction. Under these circumstances the constructionperiod should be kept as short as possible and the designer shouldinspect the site frequently to ensure that the conditions assumed inthe analysis are being realised on site (chapter 9).

The reliability of the factor of safety obtained for anyslip surface can be checked using the methods suggested in section5.8.

5.7 Recommended methods of analysis

(i) Preliminary design and low risk slopes:

At this stage of a project a time consuming complex analysisis seldom justified as the input data are often scanty.Hoek's charts, Chen & Gigerfs tables, infinite slope andsliding block analyses are most useful for a rapid assessmentof the stability of soil slopes; Hoek!s wedge failurecharts can be used for rock slopes.

(ii) Significant and high risk slopes:

Non-circular analytical methods such as those by Janbu orMor gens tern and Price are recommended for most soil slopesin Hong Kong. However, occasionally a sliding block orBishop circular analysis may be more appropriate. Wedge orplane failure analyses of rock slopes should take accountof cohesion and maximum fissure water pressures.

The assumptions made in obtaining shear strength parametersand pore pressures for use in the calculations are oftensuch that a more rigorous but complex method, such as thatof Morgenstern and Price, is not justified for the analysis.The Janbu routine method is sufficiently accurate for mostpurposes and is recommended for general use, with theproviso that there are circumstances (see table 5.1)under which the method does not work well.

Morgenstern and Price's method is useful for special cases;for example, (a) where the shear surface passes through twoor more very different materials, (b) where high qualityinput data is available and (c) when carrying out a backanalysis of a failure to obtain shear strength parameters.It is necessary, however, to assume one of the shearstrength parameters in the back analysis; as the variationsin<j>f are much smaller than the variations in c, <|>f isusually assumed based upon previous experience of thematerial.

Ill

0.1 0.1

0.2 0.3 (U 0.5

A.F. rr tan ?

0.6

0.6

0,5

0.4

0.3

0.2

0.1

B=50°

39°

30°

0.6

0.1 0.2 0.3 0.4 0.5 Q6

A F Jr ., tan f'

0.2 0.3 O.A asAF. JC_ tan *'

0.6

LegGnd

Y= bulk density of soilYW« density of water

Figure 5.2(a) Reduction in factor of safety as a result of risingground water level. For use of these charts seethe associated notes in figure 5.2(b).

112

Rock

Slope angle = g°

Slope height * H (m)

Rock angle (or water table angle if steeper than rock angle) « R°

Water level above rock = h (m)

AF = F - F r . where F is the factor of safety and(h) (h = 0) AF is the Decrease in factor of safety

for a rise (h) in groundwater level.

Notes: (i) Line AF represents either the base water or rocksurface on which a groundwater rise is to besuperimposed t

(ii) When the base rock or water surface does not passthrough the toe, A (e.g. as shown by A!F!) the heightof the slope used in figure 5.2a is Hf.

(ill) 'The values of h/H against AF vary slightly with H. Thecurves are calculated for values of H up to 30 m andthe results should be used with caution outside thatrange »

Figure 5.2(b) Notes on the use of figure 5. 2 (a)

113

5.8 Reliability of stability analyses

The uncertainty associated with the design values of soilstrength, groundwater conditions and other input data, is not takeninto account directly in the usual stability analysis* Because ofthe innate variability of all the parameters used in the analysis,and because of the sparcity of test data, the reliability of thefinal calculated safety factor may be low, and the actual safety of adesign could be much less or much greater than that calculated.

In principle it would be possible to estimate the probabilityof failure of a design by regarding all the parameters as randomvariables, and to assess the reliability in terms of this failureprobability. In practice there will rarely be sufficient data tojustify a full probabilistic analysis but it will always be possibleto estimate the mean value F of the safety factor and its standarddeviation Sj. From these it is possible to determine a standardisedReliability Index Rp, which is the difference between mean safetyfactor and a safety factor of unity divided by the standard deviation:

The significance of the reliability index in terms of theprobability of failure is shown in figure 5.3.

1 - 0 2 . 0

Reliability Index

3.0 4.0

Figure 5.3 Probability of failure versusreliability index (Normal distribution)

114

Even without interpretation as a probability of failurethe Reliability Index itself is a useful measure particularly whencomparing alternative designs. If two designs have the same meansafety factor but different reliability indexes, then the designwith the larger RF value will be the safer of the two. Conversely,two designs with different mean safety factors but an equal reliabilityindex will be equally safe.

There are several ways of calculating the mean and standarddeviation of a function of random variables (Lumb 1974), but thesimplest and most convenient for routine work is the 'Taylor series1.

When using unsaturated strengths to assess the reliabilityof a slope design (with zero pore-water pressures) it will be foundthat the influence of the variability of the unsaturated cohesion, c,will predominate since the cohesion variability is far greater thanthe variability of the other parameters. In this case the coefficientof variation of the safety factor Vp is approximately equal to thecoefficient of variation of the mean cohesion, Vs, and the reliabilityindex can be estimated from:

v-

vc

where N is the sample size.

When using saturated strengths the cohesion will normallybe very small. Under these circumstances the effects of variationsin pore water pressure will dominate the assessment of reliability.Normally there is insufficient observational data to determine therange of possible pore water pressures. It is therefore preferableto determine the reliability index for a number of fixed values ofpore water pressure and to examine these in the light of engineeringjudgement to decide if the factors of safety determined for eachwater condition are adequate.

5.9 Sensitivity analysis

As an alternative to accounting for variability of the soilforming the slope by the statistical methods described in 5.8, theslope stability analyses may be repeated using differing values forcf and <j>f and different groundwater conditions. The effect on factorof safety as a result of variations in these parameters can then beassessed and, if considered necessary, more or less conservativecriteria may be adopted for design based upon the frequency distri-bution of the values of the parameters obtained during testing andobservation.

115

DESIGN OF CUT SLOPES

5.10 Slope profile

A slope may be cut at one angle for its full depth, oralternatively, at angles which vary according to the material throughwhich the slope is cut. In the case of soil overlying rock the slopecan be steeper through the rock than through the soil. Berms may beformed at regular intervals up the slope. If berms are provided, thestability of both the overall slope and the slope between bermsshould be checked* Where there are wide berms, slopes between themcan be less stable than the overall slope. In Hong Kong failuresoccur most frequently in the slope above the top bench where theexcavation is formed in the most heavily weathered material and wherethe slope is most susceptible to infiltration through the naturalground above the crest of the slope.

If slopes are to be formed in fresh or weathered rock sothat joints which dip into the excavation are exposed it may be saferand more economic to form these slopes on a continuous profileparallel to the joints, than to provide berms on a slope which hasintermediate steeper slopes (see figure 5.4),

Berms, where used, must be at least 1,5 m wide and shouldgenerally be spaced at not more than 7.5 m vertical intervals. Oneof the main advantages of providing intermediate berms with drainagechannels is the reduction in volume and velocity of run-off on theslope surface and the consequent reduction of erosion and infil-tration. Wide berms can also catch debris from slips occurringhigher up the slope, reducing the damage to structures at the toe ofthe slope. They also improve access for maintenance but can causeincreased infiltration unless drains and surface protection are wellmaintained. Benches can also reduce stability in adversely jointedsoils and rocks.

Excavation of soil and rock cuttings is discussed inchapter 9,

5.11 Improvement of stability

If the initial analysis shows that the stability of aproposed cut is inadequate, the designer should first establishwhether a change in the geometry of the excavation is feasible toreduce the height or angle of the cutting. At an early stage indesign this may be the cheapest solution. Otherwise stability may beimproved by the provision of retaining structures, internal drainageto lower the permanent water table, anchors and rock bolts, oralternatively, by a combination of all these methods.

The design of retaining structures and the various factorsof safety required of them are discussed In chapter 7, The constructionmethods and factors of safety acceptable during construction arediscussed in chapter 9,

116

Potentially unstable wedgefailures on benches due todaylighting joints

Rssurmg parallel tosheet joints.

Standard slopeprofile

a) Potentially unstable slope aggravated by cutting benches into rockwith parallel sheet jointing,

Modified slope-profile

Standard slopeprofile

b) More stable slope is formed by laying back at the angle at which the sheet joint isdipping . For a reasonable range of orientations of the sheet joints, there may belittle extra volume of excavation required.

Figure 5.4 Effect of benches in adversely jointed rock

117

Horizontal drains or drainage adits are effective onlywhere the original groundwater level is high relative to possiblefailure surfaces and any installation on which the safety of a slopedepends will require monitoring throughout the life of the slope.The design of horizontal drains and adits is discussed in chapter 4,and their maintenance in chapter 11.

Anchors and rock bolts can be used on their own in rock orin conjunction with retaining structures in both soils and rock. Thedesign of anchors is discussed in chapter 7, that of rock bolts insections 5*12 and 5.14.

5.12 Treatment of rock slopes

Most rock slopes, after bulk excavation, need some form oftreatment to ensure continued stability. Table 5.3 gives the rangeof applications of various stabilisation measures, while figure 5.5shows typical situations in which these methods may be used.

(i) Scaling:

Immediately after excavation loose blocks or bouldersshould be removed from exposed rock faces. Potentiallyunstable blocks should be removed carefully, withoutblasting, to prevent further loosening of the face.

STABILISATION MEASURES

Excavation

Failure Type

m

Structural support

faP ( oc

al s

truc

tura

l"d

entit

ion

*B

utt

Drainage

edSc

reed su

r

Roc kf all control

Rock

trap

fenc

wal

1

Scal

ing

bloc

ksFailure

Failure

Rock orbjtsfot

Rock slope failure types and some appropriate stabilization measures.

( Modified from Fookes & Sweeney 1976)

Table 5 .3 Rock slope stabilisation measures

118

Gunite and meshdowelted into rock"

Weepholes^Inaccessible block incritical state strapped,to dowel f!5) bychains and cables

bedding seamsor Fault Zones

Anchor

Weathered rock

Dowel

"Dentitionwith drainage to clay seamsand to overbreak and cavities

(see detail)

Anchored retaining wallwith drainage

Reinforcing barsor mesh lappedto dowels bars

Weep-pipes at intervalsfor intermittent seepageonly Longitudinal drainfor steady seepage

1 Filter layer

Weak material trimmed back andreplaced by underpinning in rein forcedconcrete

Seepage inargilaceousstratum

Filter layer formed ofsandbags or graded filter

Structural facing keyedor dowetled at base

DETAIL OF4DENTITION1

Figure 5.5 Various methods of stabilising rock slopes(After Fookes and Sweeney 1976)

119

(ii) Buttresses:

Buttresses to support unstable rock masses may be concreteor masonry gravity structures which can be anchored toimprove stability* Drainage should be provided behindbuttresses to prevent a build-up of water pressure incovered fissures.

(iii) Dentition:

Bands of soft material which are exposed in a rock faceshould be trimmed back from the face; the resulting slotsshould then be filled with a suitable filter materialand the face with masonry or reinforced concrete toprevent erosion of the soft material. In Hong Kong rocksthese soft seams will normally only occur where weatheringhas taken place along a joint, a fault or a dyke; thispenetrative weathering is indicative of water flow.Weepholes should, therefore, be provided in the facing toensure that the soft seam is adequately drained and thathigh water pressures do not develop. Cavities, overhangsand open joints can be treated in the same way as exposuresof bands of soft material. If required, concrete ormasonry facing should be dowelled into the harder rock inwhich the soft seam occurs.

(iv) Sprayed concrete:

Sprayed concrete (details are given in chapter 9) can beused to provide surface protection for zones of weak orhighly fractured rock* Where concrete is required tospan between rock bolts or other supports, it should besuitably reinforced with steel fabric which can beattached to the rock surface with dowels, bolts oranchors before spraying. Sufficient weepholes should beprovided where necessary to prevent a build up of waterpressure behind the surfacef

(v) Dowels and rock bolts:

Dowels are untensioned steel bars usually 15 mm to 30 mmdiameter and 1 m to 2 m long, grouted over their fulllength into holes drilled in the rock. They are used forreinforcing closely jointed rock and for anchoringreinforcement, concrete or masonry and small blocks ofrock.

Rock bolts are post-tensioned steel bars which comprise ashort anchorage zone in sound rock and an unbonded zonein which tension is developed. The head of the. bolt isthreaded and fitted with a torque nut used to developtension in the bolt. The load developed Is applied to

120

the face by means of a steel plate bearing onto the rocksurface, although for weak or badly fractured rocks aconcrete pad may also be required. Typical rock boltsare 25 mm to 40 mm in diameter, 1.25 m to 8m long andhave a tensile working load of up to 100 kN. Singleshafts are used for short bolts, but for longer onesmultiple short lengths are joined by screwed couplings.Coupled shafts are useful when working space is limited.

The bolt anchorage in sound rock may be formed by eithera mechanical device such as the torque set bolt or bybonding. Drive- set anchorage (slot and wedge) bolts areunreliable and are not suitable for permanent use.Grouting carried out to form the bonded anchorage isknown as primary grouting. Bonded anchorages may beformed with a cement, polyester or epoxy resin primarygrout introduced into the hole either by pumping or in acartridge. Torque-set bolts also are often primarygrouted to improve their strength and reliability. It isrecommended that all permanent bolts should, aftertensioning, also be secondary grouted to protect theinitially ungrouted zone from corrosion.

Where persistent joints or faults occur ties may berequired to prevent failure between rock bolts. Theseties may be either structural channel sections withslotted holes cut to accommodate variations in thespacing of the bolts, or alternatively, they may becast in situ concrete beams.

5*13 Stability analysis of rock bolted slopes

The load required in a rock bolt to prevent sliding of ablock on an inclined plane can be calculated from the expression:

cA + (W cos ty'- U - V sin ifr -f T cos 6) tan <frW sin ty + V cos i|; - T sin 9

where c « cohesion on joint

A « area of block in contact with joint plane

T « bolt load

W - weight of block

V » horizontal force from water in tension crack

U « uplift force

0 - angle between bolt and the normal to the sliding plane.

121

ip » inclination of sliding plane

<|> « angle of friction on the plane

F a factor of safety

The bolt load required is a minimum when 0 = <j>

The shear strength used in the calculation of bolt loadshould be the shear strength measured in direct shear tests onundisturbed samples of rock joints similar to those on whichinstability may develop. Testing should be carried out at stressesequivalent to those which will occur after tensioning of bolts.

Factors of safety of 2 on ultimate strength should beused for the load carrying capacity of permanent rock bolts. Fortemporary works a factor of safety of 1.5 may be used.

5.14 Design of dowels

The deformation of the rock required to induce stress indowels can vary from very small movements to about 30 mm, and willdepend upon the quality of the primary grouting of the dowel in thevicinity of the joint on which movement is taking place. Verysmall stresses In the dowel can prevent dilation of a rock joint,thus increasing the shear strength on that joint.

An assessment of the effectiveness of dowels can be madeusing the formulae developed by Bjurstrom (1974) from the resultsof laboratory shear tests on dowelled joints In granite blocks:

(i) Where dowels cross a joint at an angle greater than 45°,they act mainly in shear and the dowel effect can be estimatedfrom the expression;

Tdowel - 2/3 d2 (°s * ere)*

where Tdowel "* shiear resistance due to the dowel effect (MN)

d - diameter of bar (m)

crs « yield stress of dowel steel (MPa)

GC "• uniaxial compressive strength of the rock (MPa)

The measurement of uniaxial compressive strength Is discussed inchapter 3.

122

(ii) Where the dowel crosses a joint at an angle smaller than45% or if it spans a wide gap, the bar will act more in tensionthan in shear and in the same manner as a rock bolt. The deformationrequired before the dowel becomes effective depends on the freelength in the vicinity of the joint. Bjurstrom (1974) suggests therelationship:-

Tdowel ** T (cos a * sin a tan $)

where Tdowel * shear resistance due to the dowel effect

T « tensile force in the dowel due to sheardisplacement (limited to either the yieldforce in the dowel bar or the steel/grout/rockbond)

ot a bar angle relative to the joint

4> » angle of friction of the joint

5.15 Boulder and rockfall control

It may not be economically practical to eliminate allrockf alls from a cut face or to remove all boulders from steepnatural slopes* In these circumstances precautions should be takento reduce the danger which such falls present to life and property.Figure 5.6 shows some rockfall control methods suggested by Fookesand Sweeney (1976).

A rock trap ditch and fence can be used at the foot of aslope (Ritchie 1963). The effect of the ditch must be consideredin the analysis of slope stability.•

EMBANKMENTS

5.16 Design

Embankment slopes should be designed using shear strengthparameters obtained from tests on samples of the proposed earthfillmaterial compacted to the design density.

If embankments are provided with berms these should be atleast 1.5 m wide and not more than 7.5 m apart vertically.

Surface protection and drainage should be designed toprevent erosion (chapters 8 and 9).

123

Hanging nets or chainsfor blocks tumbling,from above.

Free hanging mesh netssuspended from above

Supports stayedby rock anchorsor deadmen

Warningsign

Bench as rockfallcollector

Move |structure jto safe Idistance '

loose blocks to bescaled from anyface without nets

Rock trap ditchFence or wall Gravel bed

(from Fookes and Sweeney 1976)

Figure 5.6 Rockfall control measures

A free draining layer, conforming to the filter criteriagiven in chapter 4, may be required between the fill and naturalground to eliminate the possibility of high pore pressures developingwhere they could cause slope instability. Where springs or seepagetraces are found in the formation of an embankment, suitable drainsshould be provided to collect the flow from them and to dischargeit outside the limits of the embankment. Care should be taken toprevent drains acting as a source of infiltration*

124

5.17 Stabilisation of existing slopes

Where a slope of loose fill is to be stabilised toeliminate the possibility of a flow-slide, the surface layersshould be stripped to a vertical depth of not less than 3 m andreplaced with fill compacted to a density of not less than 95% ofBritish standard maximum dry density, A drainage system may berequired between old and recompacted fill to prevent the developmentof water pressure behind the compacted face.

When designing works to stabilise a failed cut or fillslope, the stability analysis must take account of the fact thatonly the residual strength will be available on the shear surfaceto resist further movements. This will not be the case if all thefailed material is removed.

While the simplest method of restoring a failed slope isto remove the slipped material and reform the slope at a safe angledetermined by stability analysis, this is not always possiblebecause of the constraint of land availability.

Where a slope must be returned to its original profile,the failed material should be excavated and any steep failuresurfaces benched prior to receiving fill. Drainage measures shouldbe incorporated to deal with any groundwater flow. Slopes rebuiltwith compacted earthfill should not be steeper than 1 on 1.5.Steeper slopes should be reconstructed with cement-stabilised soil,lean-mix concrete or masonry. Retaining structures can be incorporatedin the design of the remedial works to permit the adoption offlatter slopes and to improve overall stability without the slopeencroaching onto surrounding land.

REFERENCES

Bishop A.W. (1955) The use of the slip circle in the stabilityanalysis of slopes, Geotechnique, Vol. V, pp 7-1 ?•

Bishop A.W. and Mbrgenstern N.R. (1960) Stability coefficients forearth slopes, Geotechnique, Vol. Xf pp 129-150.

Bjurstrom S. (1974) Shear strength of hard rock joints reinforcedby grouted untensioned bolts. Proceedings of the 3rdCongress of the International Society for Rock MechanicsDenver, Vol. IIB, pp 119 -1199*

BS 1377 (1975) Soil testing for civil engineering purposes,British Standards Institution, Londonf 1 3 pp.

125

Fang H.S.Y. (1975). Stability of earth slopes. Foundation EngineeringHandbook, Ed. H.F* Winterkorn and H.Y* Fang. Van NostrandReinhold, New York, pp 35 -372.

Fookes P.G. and Sweeney M. (1976). Stabilisation and control of localfalls and degrading rock slopes. The Quarterly Journal ofEngineering Geology, Vol. 9* PP 37-55*

Hoek E. and Bray W.J. (197 )- Rock Slope Engineering, The Institutionof Mining and Metallurgy, London, 309 PP*

Hovland H.J. (1977). Three dimensional slope stability analysis method.Journal of the Geotechnical Engineering Division, Proceedingsof the American Society of Civil Engineer. Vol. 103, PP 971-986.

Hutchinson J.W., Somerville S.H. and Petley D.J. (1973)* A landslide inperiglacially disturbed Etruria Marl at Bury Hill, Staffordshire.The Quarterly Journal of Engineering Geology, Vol. 6, pp

Janbu N. (1972). Slope stability computations, Embankment Dam Engineering,Casagrande Volume, Ed. E.G. Hirschfield and S.J. Poulos, Wiley,New York, pp V7~86.

Lambe T*W. and Whitman R.V. (1969). Soil Mechanics , Wiley, New York, 553 PP-

Lumb P. (1970). Safety factors and the probability distribution of soilstrength. Canadian Geotechnical Journal, Vol. 7* pp 225-2 2.

Lumb P. (197*0. Applications of statistics in soil mechanics. SoilMechanics «• New Horizons, Ed. I.K. Lee, Newnes-Butterworth,pp Mf-111.

Lumb P. (1975). Soil variability and engineering design. Soil Mechancis,Recent Developments, University of New South Wales, Sydney,Australia, pp 383-397.

Lumb P. (1975). Slope failures in Hong Kong. The Quarterly Journal ofEngineering Geology, Vol. 8, pp 31-65.

Morgenstern N.R. and Price V.E. (1965). The analysis of the stabilityof general slip surfaces. Geotechnique, Vol. 15, pp 79-93.

Piteau D.R. and Jennings J.E. (1970)* The effects of plan geometryon the stability of nature slopes in rock in the Kiraberleyarea of South Africa. Proceedings of the 2nd Congress ofInternational Society of Rock Mechanics, Belgrade, Vol. 3,Paper 7-4. ' ! '

Ritchie A.M. (1963) Evaluation of rockfall and its control. HighwayResearch Board, Highway Research Record » No» 17, pp 13-28.

Sarma (1973). Stability analysis of embankments and slopes*Geotechnique, Vol. 23, pp 423-433.

126

5.19 Bibliography cf landslides in Hong Kong

Beattie A.A. and Lam C.L. (1977). Rock slope failures - their predictionand prevention* Hong Kong Engineer» Vol. 5 No. 1f pp 27-4-0.

Government of Hong Kong (1972). Final report of the commission of inquiryinto the rainstorm disasters of 1972. Government Printer, HongKongf 91 PP*

Lumb P. (1975)* Slope failures in Hong Kong. The Quarterly Journal ofEngineering Geology, Vol. 8, pp 31-65•

O'Borke G.B* (1972). A cutting failure in Hong Kong granite. Proceedingsof the 3rd Southeast Asian Conference on Soil Engineering, HongKong, pp 161-169*

Slinn M.A., Greig G.L. and Butler D.E. (1976). The design and someconstructional aspects of Tuen Mun Boad, Hong Kong Engineer,Vol. k No. 2f pp 37-50. Discussion, Vol. 4 No. 37 pp 69-72.

So G.L. (1971). Mass movements associated with the rainstorm of June 1966in Hong Kong. The Institute of British Geographers, Transactions,No. 53, PP 55-65-

Vail A.J. and Attewill L.J.S. (1976). The remedial works at Po Shan Road*Hong Kong Engineer, Vol. k No. 1, pp 19-2?.

127

CHAPTER 6

FOUNDATION ON SLOPES

6.1 Introduction

The design of foundations for structures on, or closeto, soil and rock slopes must take into account the interactionbetween the structure and the slope. Two criteria must be considered:(a) the stability of the slope and (b) the bearing capacity orsettlement requirements of the foundation. The stability of aslope can be affected by excavations carried out for the constructionof an adjacent foundation below the slope, the load imposed by afoundation on or above the slope, or the temporary or permanentchange in the groundwater regime caused by construction of thefoundation. The design of foundations in Hong Kong residual soilsis often controlled by settlement requirements rather than bearingcapacity and large factors of safety are used on bearing capacityto reduce deformations. The presence of an adjacent slope canreduce the allowable bearing capacity for a foundation.

SHALLOW FOOTINGS

6.2 Bearing capacity on slopes

The ultimate bearing capacity of a footing on a soilslope is less than that for the same footing on level ground.Several modifications of the basic Terzaghi bearing capacityequation have been suggested to allow for the effects of adjacentslopes. Of these methods, those by Meyerhof (1957), Brinch Hansen(1968) and Giroud (1973) and a further modification of BrinchHansen1s method by Vesic (1975), are the most important. BrinchHansen or Giroud fs methods are preferred as they give more conservativeresults. Table 6.1 shows the effect of slope angle on the ultimatebearing capacity of a square footing.

Bearing capacity calculations in general do not allowfor the fact that the soil on the slope is already under stress,and therefore, the overall stability of a slope, under the influenceof the loaded footing, must also be checked. On shallow soilslopes, where the slope angle is less than <j>!/2, bearing capacityor settlement will usually control the allowable load on a footing.

If a footing on level ground is closer than six timesits width from the crest of a slope, the permissible bearingcapacity may be lower than that for a similar footing where noslope is present. The effect of the slope on the permissible

129

Dimensionof

footing

1 m x 1 m0.5 m deep

1,5 m x 1.5 m0.5 m deep

2 m x 2 m0.5 m deep

2.5 m x 2.5 m0.5 m deep

\c' kPa6° N. $°Slope N.angle ° \

010203040

010203040

010203040

010203040

Ultimate Bearing Capacity WS

0

30

0.400.250.140.070.02

1.010.630.350.160.06

1.911.190.670.320.11

3.242.021.140.540.19

0

35

0.790.490.280.130.04

1.971.230.700.830.11

4.102.561.450.690.24

6.484.042.301.100.38

0

40

2.221.380.770.360.12

4.152.591.460.690.23

8.265.162.701.400.48

13.828.654.922.360.80

10

30

0.830.660.520.430.36

1.911.491.170.930.78

3.502.672.081.641.35

5.654.313.302.572.08

10

35

1.441.110.860.680.56

3.352.541.941.501.21

6.504.843.602.712.13

10.177.545.604.203.28

10

40

3.272.391.731.260.97

6.414.743.492.612.03

12.188.886.224.703.57

19.8414.3610.317.425.54

Table 6.1 Examples of the effect of an adjacent slope on Bearing Capacity

(based on Brinch Hansen, 1968)

Footing depthD

Slope angle 9

i>earin capacity will depend on both <j>' of the soil forming theslope and the depth of footing. Meyerhof (1957) has studied thisproblem and his curves, reproduced in Navfac DM7 (1971), can beused to calculate bearing capacity. Alternatively, Brinch Hansenor Giroud's methods, which are based upon footings on slopingground, can be used by assuming an increased surcharge and areduced slope angle equal to that between the toe of the slope andground surface at the footing, (figure 6.1)

Foundation load,P

Consider as surcharge

Reduced slope Dangle *>

6.3

Figure 6.1 Assumptions used to extend Brinch Hansen orGiroud's method for use with a footing adjacentto a slope

Slope stability with shallow footings

For soil slopes steeper than <j>f /2 it is seldom necessaryto calculate bearing capacity because the stability of the slopewill be the controlling factor. For the purposes of stabilityanalysis, the footing load can be considered either as a line loador as a surcharge over a defined width of the soil (chapter 5).The backfilling above a footing may be poor and the stabilityanalysis should, therefore, consider the possibility of a tensioncrack forming on the up slope edge of the footing (figure 6.2).

131

Foundation load

,PBack fill

Tensioncrack

Shear surface analysed

Figure 6,2 Position of tension crack

An acceptable factor of safety against slope failure(chapter 5) obtained from a stability analysis which includes thefoundation loads does not necessarily mean that the foundation Isacceptable. To limit deformations of the foundation, a muchhigher factor of safety (often >2) may be required.It will often be more economic to use deep foundations which donot affect slope stability, than to design slopes with the highfactors of safety required to satisfy deformation criteria*

The analysis of the stability of foundations on or aboverock slopes should take Into account potential instability onadversely orientated joints* The rock-wedge analytical methods ofHoek and Bray (1977) can take account of the external loadingsimposed on the slope by foundations.

6.4 Interaction of footings

Shallow footings constructed at more than one level on aslope may interact and additional loadings may thus be imposed onthe lower footings. The additional loading must be allowed for inthe footing design.

132

DEEP FOUNDATIONS

6.5 Foundation level

Deep foundations constructed through soil slopes shouldbe formed in suitable bearing strata below any potential shearsurface, where the foundation load will not affect the stabilityof the slope.

6.6 Lateral loading of deep foundations

The horizontal stresses in a soil slope will varythroughout the slope and the horizontal loading will be greater onthe upslope side of a foundation than on the downslope side.Providing that the slope does not fail, this load difference willbe negligible and need not be considered during the design of mostfoundations. If slope failure does occur, high lateral loads willbe transferred onto the foundations and, to prevent this happening,potentially unstable slopes should be stabilised before foundationsare constructed. If soil creep and small movements occur on anotherwise stable slope and, as a consequence, the soil moves awayfrom the downslope side of a pile or caisson, lateral loading willresult.

Where foundations downslope of retaining structures arewithin the passive zone (see chapter 7), lateral loadings must beconsidered in the design. The lateral load on the foundation willbe the difference between the passive and at-rest earth pressures.Where piles or caissons are widely spaced, the loading will betransferred onto the piles only from the strip of soil of the samewidth as the diameter of the pile. When closely spaced, archingmay occur and the loading from the soil between piles may betransferred onto the piles (Wang and Yen, 1974).

To prevent lateral loading, annular sleeves may beconstructed around isolated foundations such as bridge piers.These sleeves, which have an internal diameter considerably largerthan the piers, are installed eccentrically with the centre of theannulus upslope of the centre of the pier. The sleeves can thenmove with the soil without transferring loads onto the piers.This technique has been adopted in both South America and HongKong.

6.7 Construction of foundations and slopes

During construction of foundations precautions must betaken to minimise short term instability of the slopes. Constructionand temporary works are discussed in chapter 9. The effects ofseepage into foundation excavations are discussed in chapter 4.

133

EEFEEENCES

Brinch Hansen J. (1970). A revised and extended formula for bearingcapacity. Danish Geotechnical Institute, Bull. No« 28,21 pp.

CP 200k (1972) Foundations. British Standards Institution, London,158 pp.

Giroud J.P. (1973). Tables pour le Calcul des Fondations, Vol. 3,Section 8.3, Dunod, Paris.

Meyerhof G.G. (1957). The ultimate bearing capacity of foundationson slopes. Proceedings of the 4th International Conferenceon Soil Machanics and Foundation Engineering, London, Vol. 1,PP 33 -336.

Navfac DM? (197D. Design manual - Soil Mechanics, foundations andEarth Structures. Department of the Navy, Washington.

Vesic A*S. (1975) • Bearing capacity of shallow foundations, FoundationEngineering Handbook, Ed. ELF. Winterkorn & H.Y. Fang, VanNostrand Reinhold, New York, pp 121-1

Wang W.L. and Yen B.C. (197*0. Soil arching in slopes. Journal ofthe Geotechnical Engineering Division, Proceedings of theAmerican Society of Civil Engineers, Vol. 100, pp 61-78.

134

CHAPTER 7

RETAINING STRUCTURES

7.1 Introduction

The forces exerted on retaining walls by soils (zones Aand B in table 2.3) are considered in this chapter. The forcesexerted on retaining walls by rock (zones C & D in table 2.3) arenot considered. The design of ground anchors is not included inthis manual.

?«2 Analytical methods of determining lateral earth pressureacting on walls

Methods of calculating earth pressures are given in mosttext books, for example Terzaghi and Peck, (1967). Very large wallmovements are required to mobilise the ultimate passive pressureand for design purposes the passive pressure should be limited tonot more than 50% of the calculated ultimate passive.

If relative movement can occur between a wall and thesupported soil, the effects of wall friction may be taken intoaccount when determining the pressure acting on the wall.

The magnitude of the forces acting on retaining structuresis affected by the soil and structure interaction. Movements of wallsnecessary to generate different pressure conditions are given byWu (1975).

Design curves for active and passive earth pressures aregiven in several publications, notably Navfac £>M? (1971)*

The adoption of active conditions for retaining walldesign assumes movement of the supported material which may beunacceptable because of possible damage to adjacent existingstructures or services (NRCC 1975). Under these circumstanceswalls must be designed to prevent movement of the supported soilmass (see section 7.12).

When taking account of passive pressure in retaining walldesign, the possibility of excavation subsequently being carried outin the passive zone should be considered. If this can happen thenthe wall should be designed ignoring passive pressures.

There is no information available on the magnitude of thecoefficient of earth pressure at rest for the insitu residual soilsof Hong Kong.

135

7.3 Water pressures

The water pressure acting on a retaining wall behind whichthere is inadequate drainage can be considerably greater than theactive pressure. Although in practice a drain is normally usedagainst the backface of the wall, the most effective way of preventingthe development of these pressures is to provide an inclined drainbetween the backfill and the insitu soil, (Cedergren 1977)

If an inclined drain is provided which is capable ofcarrying all the flow (see chapter 4), rainfall infiltrating fromthe surface will flow vertically into the drain without producingpore pressures in the soil or water pressure on the wall. Incontrast, for rain infiltrating the platform above a wall to reacha vertical drain there must be a horizontal component of flow andtherefore positive pore pressures. Under these conditions, thepore pressure is zero in the drain but positive in the soil massthus reducing the shear strength and increasing the active pressureor reducing the passive pressure.

Allowance for the effects of these pore pressures shouldbe made by including the total water pressure as another forceacting on the trial wedge. A simplified method of doing this isdescribed in Navfac DM? (197D*

Compacted backfill of low permeability between the drainand the insitu soil may have serious damming effects on groundwaterflow and as a consequence render a particular analysis and designof a wall inapplicable. If adequate drains are not to be providedbehind walls, for example diaphragm, basement and sheet pilewalls, the walls must be designed to resist any water pressureswhich can develop subsequent to construction. Water pressuresacting on the back of a wall are accompanied by uplift pressures,which act on the base and these must also be included in theanalysis of stability. They reduce the normal stress and hencethe resistance to sliding.

Figure 7.1 shows a typical arrangement for a drainbehind a wall. The long term effectiveness of the drainagematerials should be carefully considered when preparing thedetailed design. A channel at the top of the wall will preventthe ingress of run-off into cracks between the wall and thebackfill and the impervious base will prevent infiltration fromthe drain into the foundation.

Services upslope of a wall should be installed asdescribed in chapter 9 as leakage entering the backfill canincrease water pressure and this has been known to cause retainingwall failures.

7.4 Surcharge loads

The magnitude of surcharge loads due to traffic onadjacent highways is given in BS 5400 (1978). Design methods forincluding these loads and those due to buildings, stock piles andconstruction plant are given in Spangler (I960), Navfac DM? (1971)Kezdi (1975) and Wu (1975).

136

Surface water channel

Weephole

Surface waterchannel

rain designed tofilter requirements

Preferred positionof drainage layer

Excavation line

Impervious baseto drain

Concrete backfill

7.5

Figure 7.1 Drainage to retaining wall (schematic)

Construction loads

Backfill to retaining walls should normally be compacted inthin layers using light compaction plant. This will avoid imposing onthe wall the high loads which are generated by heavy compaction plant.However if heavy compaction plant is to be used this must be takeninto account when determining the loads for which the wall is to bedesigned. Broms (1971), Kezdi (1974) and Aggour and Brown (1974)discuss the effects of backfill compaction on lateral pressuresacting on retaining walls. Heavy compaction of backfill can causethese to be much greater than at-rest pressures.

Wall designs should also take account of abnormal loadswhich may be imposed during construction. These may be caused byearth moving plant, batching plants or cranes stationed above a wall.

137

7.6 Base friction

The value of the base friction angle will depend upon thenature of the materials used to construct the wall and upon the methodsadopted for construction. It is commonly taken as the effective angleof shearing resistance, <j>!, for cast in situ concrete and as theangle of wall friction, <$, for precast concrete (CP 2, 1951 andHuntington, 1957)* These values are only applicable to a foundationthat has been properly prepared.

7.7 Bearing capacity

Closely related to the problem of overturning of retainingwalls is the determination of bearing capacity. The forces actingon the back of a retaining wall produce a non-rectangular stressdistribution below the base and the maximum pressure should notexceed the allowable soil pressure derived from consideration of thebearing capacity and settlement. The bearing capacity may bedetermined using the methods of Meyerhof (1957), Brinch Hansen(1970) or Vesic (1975) which take account of slope angle, shape anddepth of foundation and eccentricity of resultant load. A simplifiedand abreviated version of the Brinch Hansen method is included inthe Danish code of practice for foundation engineering which hasbeen translated into English (1978).

7.8 Factors of safety

Retaining structures can fail by sliding or rotation, as aresult of bearing capacity failure or as part of a larger scaleslope failure. Failure of walls, particularly masonry walls, mayalso result from structural failure within the wall itself. Thefactors of safety adopted should be applicable to the mode offailure being considered and should conform with the recommendationsgiven below:

(a) against sliding: (i) F not less than 1.5 on frictionalresistance

(ii) F not less than 1.5 on any includedpermitted passive

(b) against rotation: (i) F not less than 2.0(ii) For masonry structures resultant must

remain within the middle third.

(c) against bearing F not less than 3 on ultimate bearingcapacity failure: capacity calculated by the methods

given in section 7*7.

(d) against slope failure: With shear surface passing under thewall, the factors of safety as givenin table 5.2.

These factors of safety should apply to the 1 in 10 yeargroundwater condition as discussed in chapter 4.

138

7»9 Retaining walls with keys

Where possible, deep keys should be avoided because theprocess of constructing a shear key frequently loosens and softensthe material on which reliance is placed for passive and shearresistance. Complex section walls which take longer to construct,although requiring marginally less material, are often more expensivethan simple walls with base widths increased to provide sliding re-sistance.

When walls with shallow keys are used, Huntington (1957)suggests that they be analysed, using effective stress parameters,assuming sliding occurs on a horizontal plane through the soilunder the key and that both active and passive forces be increasedby the increased wall depth. When checking rotational stability,the assumption should be made that the wall will tend to rotateabout the toe and that the key will be below the centre of rotation.During rotation the key will be moving into the soil whilst thestem moves away. Under these conditions passive pressure shouldbe assumed to act on the back of the key and the active pressure onthe front.

7.10 Pro pp ed caisson and p lie walls

Several design methods are available and have beencompared in CIRIA Report (1974) in which the Rowe design method ispresented in the form of a design manual.

7.11 Walls with lateral support at several levels

The design conditions assumed for such walls are dependentupon the way in which loads are induced at the support. Walls whichimpose loads on their supports, whether they be floor slabs, struts,or unstressed anchors, as a result of wall deformation can be designedusing the pressure distribution rules proposed by Peck (1969) forthe design of braced excavations. These rules were based upon measure-ments of loads in struts during construction, and supersede rulespreviously published by Terzaghi and Peck (1967). They were nothowever determined for residual soils.

If load is induced at the supports, by prestressing anchorsor struts, and the wall is forced against the retained material, thepressure on the walls is dependent upon the imposed support loads andcan have a lower limit of active pressure and an upper limit of passivepressure. Normally the load applied to such walls is designed toprevent distress of adjacent structures. Brinch Hansen (1953),Tschebotarloff (1962), James and Jack (1975) and the NRCC (1975)describe methods of analysis of these walls.

7.12 Settlements outside Excavations

The design of permanent retaining walls to prevent settlementof adjacent properties is mentioned in section 7.2 but unless thetemporary works are designed and carried out with equal care serioussoil movements can still take place. Peck (1969) gives details ofsettlements which have occurred around excavations in various

139

materials supported by systems using both unstressed and prestressedanchors and raking struts. Peck makes the point that movementsdepend to a large extent on the soil supported and that if goodworkmanship is used in installing and removing a well-designed,well-constructed temporary bracing system, ground movements in densesands and relatively stiff cohesive granular materials can benegligible. Poor workmanship can make even the best design ineffectiveand this requires that construction of temporary bracing works beclosely supervised. O'Rourke, Cording, Boscardin (1976) haveextended the work of Peck.

REFERENCES

Aggour M.S. and Brown C.B. (197 )* The prediction of earth pressureon retaining walls due to compaction. Geotechnique, Vol. 2*f,pp 89-502.

BS 5 00 (1978). Steel Concreteand Composite Bridges - Part IISpecifications for Loads. British Standard Institution,London, Vj PP*

Brinch Hansen J. (1953)« Earth pressure calculation. Danish TechnicalPress, Copenhagen, 271 pp*

Brinch Hansen J. (1970). A revised and extended formula for bearingCapacity. Danish Geotechnical Institute, Bulletin No. 28,21 pp.

Broms B. (197D» Lateral earth pressures due to compaction of cohesionlesssoils. Proceedings of the 4-th Conference on Soil Mechanics,Budapest, pp 373-3S**.

Cedergren H.E. (1977). Seepage Drainage and Flow nets. Wiley, New York,53 PP.

CIRIA (197*0* A comparison of Quay Wall Design Methods. CIRIA ReportNo. 5k.

CP 2 (1951)* Earth Retaining Structures. Institution of StructuralEngineers, London.

Danish Geotechnical Institute (1978). Code of practice for foundationengineering. Danish Geotechnical Institute, Bulletin 32,Copenhagen.

Huntington W.C. (1957)* Earth Pressures and Retaining Walls. Wiley,New York.

James E.L. and Jack B.J. (1975). A design study of diaphragm walls.DiPPDiaphragm Walls & Anchorages, Institution of Civil Engineers,j—^ , ~— ^ ^ _

Kezdi A. (1975)* Lateral earth pressure. Foundation Engineering Handbook.Ed. H.F. tfinterkom & H.Y. Fang. Van Nostrand Reinhold, NewYork, pp 197-218.

Meyerhof G.G. (1957). The ultimate bearing capacity of foundation onslopes. Proceedings of the *fth International Conference onSoil Mechanics and Foundation Engineering, London, pp 38*f-386.

National Research Council of Canada (1975). Canadian Manual oa FoundationEngineering. Associate Committee on the National Building Code,National Research Council of Canada, Ottawa.

Navfac DM7 (197D* Design Manual - Soil Mechanics, Foundations and EarthStructures, Department of the Navy, Washington.

Q'Rourke T.W., Cording E.J. and Boscardin H. (1976). Ground movementsrelated to braced excavations and their influence on adjacentbuildings. Report DOT - TST 76T-23 U.S. Department ofTransportation, Washington.

Peck R.B. (1968). Deep excavations and tunnelling in soft ground.Proceedings of the ?th International Conference on SoilMechanics and Foundation Engineering* Mexico, pp 1V7~150«

Spangler H.G. (1960). Soil Engineering. International Text Book Co.,Scranton, Pennsylvania, USA.

Terziaghi K. and Peck R.B. (19&7) Soil Mechanics in Engineering Practice.Wiley, New York, 729 pp.

Tschebotarioff G.P. (1962). Retaining structures. Foundation EngineeringEd. G.A* Leonards, McGraw-Hill, New York, pp 438-52 .

Vesic A.S. (1975). Bearing capacity of shallow foundations. FoundationEngineering Handbook Ed. H.F. Winterkorn & H.Y* Fang, VanNostrand Reinhold, New York, pp 121-1 5*

Wu T.H. (1975), Retaining walls. Foundation Engineering HandbookEd. H.F. Winterkorn & H.Y. Fang, Van Nostrand Reinhold, NewYork, pp

CHAPTER 8

SURFACE DRAINAGE

8.1 Introduction

The association between rainfall and slope failures in HongKong has been reported by Lumb (1975). The design and detailing ofslope drainage is of paramount importance in ensuring continuedstability,

The slope drainage system should collect and safely conveyrun-off from the catchment above the slope and from the slope itselfto convenient points of discharge. While the methods of design arewell established, they are empirical and should therefore be usedwith care. Flow in an existing water course should not be divertedinto another channel without first checking its adequacy downstreamof the confluence. Where pipes or culverts discharge upstream ofwater courses affected by a proposed development, due allowance mustbe made for them in the design of the drainage works associated withthe development.

8.2 Run-off

Run-off from a catchment depends upon many factors whichinclude :

(a) rainfall intensity(b) the area and shape of the catchment(c) the steepness and length of the slopes being drained(d) the nature and extent of vegetation or cultivation(e) the condition of the surface and nature of the subsurface

soils

The determination of run-off by reference to unit hydrographs(Linsly, Kohler, Paulus 1949) has been found successful in manycountries. In Hong Kong the method was used for the design ofdrainage systems for small catchments but was not found to offer anysubstantial advantage over design methods using empirical equationsto represent the complex relationship between rainfall and peakrun-off. The "Rational Method" is commonly adopted as it is bothsimple and straightforward to use and, for the relatively smallcatchments In Hong Kong, yields satisfactory results (Tin 1969),The formula is:

where Q « maximum run-off in litres/sec

1 m design mean intensity of rainfall in mm/hr which isdependent upon the time of concentration(see section 8.3 and 8.4)

143

A = area of catchment in m

K - run-off coefficient

The run-off coefficient cannot be determined precisely. Therecommended value of K for slope drainage is 1.0. While some allowanceis made for silting by using this value, the drainage system should bedesigned to minimise siltation and prevent debris from causingblockages.

8.3 Area of catchment

The catchment area is determined by reference to contouredplans and is defined as all that area draining to the point in thedrainage system under design. Where natural catchments are consideredthe boundaries will be defined by the topographic contours, overlandflow taking place at right angles to the contours. Where the hydrologyof a catchment has been affected by the construction of catchwaters orpipe discharges the effects of these constructions must be considered.In the case of a catchwater, the catchment area draining to a streamdownstream of the catchwater will normally be reduced, althoughallowance should be made for the flow from any spillways to thecatchwater provided on that stream. The construction of drainagedischarges on the stream, by diverting flows from adjacent areas,may lead to substantial increases in the size of the catchment andthis should be allowed for when determining catchment area,

8.4 Time of concentration

The time of concentration may be defined as the timerequired for run-off from the most remote part of the catchment toreach the point in the drainage system under design. For naturalcatchments, it is calculated using the Bransby Williams equation:

115.86A2

106 H

L,D

where t = time of concentration, minutes

A ~ area of catchment, m^

H = average fall in metres per 100 metres from thesummit of catchment to the point cf design

L = distance in metres measured on the line of naturalflow between the design section and that point of thecatchment from which water would take the longest timeto reach the design section.

D a diameter, in metres, of a circle of same area as thecatchment.

144

BSEND

t s Time of concentration ( min)H = Slope (m/100m )

A s Catchment area ( m2)

L s Distance of most remote poirrt to the design section (m)

Figure 8.1 Nomogram for the rapid solution ofthe Bransby Williams equation

145

The nomogram in figure 8.1 may be used for rapid solutionof the above equation, an example of the use of the nomogram beinggiven on the figure, A minimum time of concentration of one minuteshould be used. The time of concentration is required for thedetermination of the design intensity of rainfall.

Special attention should be paid to catchments where thestream course has been channelled and straightened. In such casesthe time of concentration will be much shorter than that indicatedby the Bransby Williams equation, and should be calculated by addingthe time of travel within the drainage channel during peak flow tothe time of concentration calculated from the Bransby Williamsequation for the most remote subcatchment to the drainage channel.

For slopes constructed in urban areas, the determinationof the peak flows from areas above the slope may be complex when theeffects of existing drainage systems within the natural catchmentsare, as they must be, considered.

8.5 Design intensity

As mean rainfall intensity reduces with duration, thegreatest run-off occurs when the duration of the storm is equal tothe time of concentration. The maximum intensity for any givenreturn period, which is a measure of the frequency of occurrence,may be determined by reference to intensity duration rainfall curvesfor the area considered. Figure 8.2 gives the intensity durationcurves for Hong Kong based upon work by Cheng and Kwok (1966).

The design of all drainage works on steep slopes, thestability of which could be affected if the drains cannot carry therunoff, should be based upon a 1 in 200 year storm. Temporarydrainage for use during construction should be designed on the basisof a 1 in 10 year storm, although if the construction period extendsover several wet seasons the design parameters should be greater.

8.6 Composite catchments

Where two drains join, the designer should consider theflows from both the individual catchments and the combined catchment.

DETAILED DESIGN OF DRAINAGE SYSTEM

8.7 General

Drainage systems should be designed to minimise routinemaintenance and, wherever possible, to avoid blockages by debris.Covered channels or pipes should be avoided as Inspection andmaintenance is difficult.

146

O.

3*

401.5- 4 5 6 7 8 9 10 15

Duration , Time of concentration ; t(min)

20 30 50 60

Notes :-The Intensity-Duration rainfall curves are based on Cheng & Kwok (1966) — A statistical study of heavy rainfall in Hong Kong1947-65 i technical note No. 24/Royal Observatory , Hong Kong. Revised table from tilting siphon records 1947-1977 ( RoyalObservatory ) and instantaneous rate-of-rain fail records , 1952 -1977 ( King's Park).

Figure 8.2 Curves showing duration and intensity of rainfallin Hong Kong for various return periods

Trash grills and sand traps should not be provided aboveor on slopes. At the toe of slopes and in locations convenient forinspection and maintenance, run-off should pass over a sand trap andthrough a trash grill to prevent material from entering and blockingany storm drains into which channels discharge. As trash grills canrapidly become choked by twigs and floating debris, grilled openingsshould be large enough to pass the maximum flow when partiallyblocked. For normal situations, 50% of the opening should beassumed to be blocked, and for slopes below areas covered with densevegetation 75% is probably more appropriate. Figure 8.3 shows atypical sand trap arrangement.

Streams can carry large rocks or boulders during heavyrain which may block or damage the slope drainage system. If thestream course is strewn with rocks and boulders, a rock trap (anexample is shown in figure 8.4) should be provided together withsuitable access for maintenance.

Instructions regarding the inspection and maintenance ofsilt and rock traps and grills should be included in the designerfshandingover notes (see chapter 11).

8.8 Layout of slope drainage

Run-off should be conveyed by the most direct route awayfrom vulnerable areas of the slope, particularly from beyond the topof the slope. Run-off should be led down the larger slopes inseveral stepped channels and should not be concentrated into onlyone or two. Streams intercepted by a slope should be conveyeddirectly down the slope. Any change in direction needed to rejointhe stream course should occur at the toe of the slope.

On slopes susceptible to erosion, a system of chevrondrains as shown on figure 8,5 is recommended. On rock, chunamed,stone- pitched or other slopes not susceptible to erosion, a systemof berm and stepped channels should be used.

8.9 Types of channels

Channels for slope drainage should be open concrete-linedU or half-round channel. Pipes should not be incorporated in slopedrainage systems.

Concrete lined channels backfilled with free drainingstone, or unlined trenches containing porous pipes surrounded withfree draining stone, known as French drains, should not be used. Inthe former case the carrying capacity of the channel is seriouslyreduced by the presence of the stone. In the latter the stonesurround in the unlined trench creates a source of infiltration, theporous pipe not conveying any water until the ground in which thetrench is excavated is incapable of accepting any more infiltratingwater. In both cases the surface of the stone eventually becomesblocked with organic debris and topsoll In which grass and othervegetation can take root. The system then becomes ineffective as adrain.

148

Cover slabs if required

stone filterlower layer size150 mm .upper layer

LONGITUDINAL SECTIONAL

SECTION A ~ A

Notes1. All dimensions in millimetres

2. Normally for drains of 900 mm dia.and below.For bigger drains and steep terrain, sand trapshould be specially designed.

3. SizeDepth : 0 4 750Width : W > 3BLength: L s^67 h°-5 X-°5* 48

*• Graded stone filter shall be crusher rungranite aggregate.

5- Capacity DWL to be according to size andnature of catchment, providing detention timenot less than 5 minutes for max. design flowof inlet.

Figure 8.3 Typical sand trap arrangement

149

To suit natural channel

Section A - A

Sized todesign

IX)

Natural streamcourse—,

-co

Grill spaced to catch boulders innatural stream bed

Plan

Figure 8.4 Typical rock trap detail

150

Slope line^

Top soil and turfx

U-channel

SECTION A -A

Chevron drains at 7500c/cdown slope

Concrete lined channels

Baffle wall (see fig.S.B)

Chevron drain : Plan

Figure 8.5 Plait showing junction of tributary channelswith main channels

151

8.10 Channel design

The minimum gradient for channels is determined by thevelocity of flow sufficient to remove silt. The velocity should notbe less than 1 m/sec for the peak flow occurring with a frequencyof at least twice per year.

The channel size, which depends upon the gradient adopted,may be determined from figure 8.6. Channels larger than 600 mm maybe designed using the charts developed by Ackers (Hydraulics ResearchStation 1969) or, alternatively, by using Manning1s formula assuminga maximum permissible velocity of 4 m/sec and a roughness factor of0.013.

Stepped channels are not particularly effective as energydissipators; however, there would seem to be no practicable alternative,The flow in stepped channels is turbulent, and sufficient freeboardmust be allowed for splashing and aeration. The step channel detailsshown on figure 8.7 make some allowance for splashing and will bemore effective in reducing the velocity of flow than the step channelsused in Hong Kong to date. In the absence of any experimental data,the size of the stepped channel and gradient of the invert may bedetermined using figure 8.6 by assuming a velocity of 5 m/sec throughthe minimum section (at the top of each step). At the top of slopes,the velocity will be lower and the cross-sectional area of flowgreater, but splashing and aeration will be less. Therefore, thesection adopted for the stepped channel may also be used to crossnarrow berms.

The design of channels carrying major streams down theslope will need special attention. A simple stepped channel will notbe as effective as an irregular boulder strewn channel in dissipatingenergy. A proper hydraulic design will be needed for the largerstreams and, in certain circumstances, a hydraulic model may berequired to demonstrate the adequacy of the design.

8.11 Changes in direction

At any change in direction, the pattern of flow in thechannels will be affected. Channels in which the velocity isapproximately 2 m/sec should change direction through bends of radiusnot less than three times the width of the channel. This radiusshould be increased where the velocity is greater than 2 m/sec oralternatively, sufficient freeboard should be provided to contain thesuper-elevation of the water surface (Chow 1959).

Where a stepped channel crosses a berm, a hydraulic jumpmay form which must be contained within the channel. The splashallowance provided for the stepped channel may, therefore, be extendedacross the berm.

152

CA

PA

CIT

IES

OF

CH

AN

NE

LS

(litres

pe

r m

inute

]

-GrassGrass x

-Apron

Section A- A

This dimension variesto suit the slope butnot to exceed 1 m

Longitudinal section

Dimensions of stepped channel

Nominal size of channel H(mm)

225 To 300

375 To 675

750 To 900

Thicknesst (mm)

100

100

125

Thicknessb ( mm)

100

150

200

Splash allowances( nnrp)

200

350

400

Figure 8.7 Typical details of stepped channel

ISA

Mind+ASQmmmeasured fromlowest invertof the steppedchannel

O-':k 7 _ rr t3 -:iSWr^-ffr Jr ^^^rTy,^Q{^.'^

Xro

Section C-C

Plan

Figure 8.8 Typical details of junctions of steppedchannel and U-channel at toe of slope

155

^Impervioussurface

This dimension varies to suitfall on channel

^^r^~^r^S^ '*&%

Dimensions of U - channel

Nominal size of channel h(mm)

225 To 600

675 To 1200

Thickness t (mm)

150

175

Thickness b (mm)

150

225

Figure 8.9 Typical U-channel details

156

8.12 Junctions of channels

Junctions of channels pose the greatest problem whendesigning slope drainage. They inevitably cause turbulence andsplashing and any chamber constructed to contain this is vulnerableto blockage by debris. Avoidance of such chambers is recommended,except, possibly, at the base of the slope where the deep channelsrequired to contain the splashing would cause a hazard.

At junctions the smaller channel or channels should bebrought in at half the width of the main channel above the invert ofthe main channel. The channels should be deepened with an addedfreeboard allowance to contain the turbulence, splashing and backwatereffects.

Where channels are to discharge into a step channel crossinga berm, they should be curved into the step channel.

8.13 General comments

If excessive splashing and turbulence is expected at aparticular junction or change in direction consideration should begiven to providing a baffle wall as shown on figure 8.8.

The tops of all channels should be flush with the slopesurface (figure 8.9)* Where possible, an apron which drains towardsthe channel should be provided to return any splashing to the channel;this applies particularly to stepped channels (figure 8.?).

8.14 Subsurface drainage outfalls

All surface channels, into which subsurface drains discharge,should be designed to prevent the subsurface drainage outlets becomingdrowned. If this cannot be done a separate system should be designedfor the subsurface drainage outfalls*

REFERENCES

Ackers P. (1969)* Charts for the hydraulic design of channels andpipes. Hydraulics Research Paper No. 2, H.M.S.O. London.

Chow V.T. (1959). Open Channel Hydraulics. McGraw-Hill, New York,680 pp.

Cheng S. and Kwok W.H. (1966). A Statistical Study of Heavy Rainfallin Hong Konff 1 9 7-65 • Royal Observatory, Hong Kong.

Linsley R.K., Kohler M.A. and Paulhus J.L.H. (19 9). Applied Hydrology.McGraw-Hill, New York.

Lumb P. (1975). Slope failures in Hong Kong. The Quarterly Journal ofEngineering Geology, Vol. 8, pp 31-65*

Tin Y.K. (1969). Stormwater Drainage Design in Hong Kong. Sewage &Drainage Advisory Unit, Technical Report No. £>7 Public WorksDepartment.

157

CHAPTER 9

CONSTRUCTION

9.1 Introduction

This chapter deals with those aspects of constructionwhich influence the safety of slopes, or which are peculiar to andaffect the safety of structures which maintain slope stability.

9.2 Design of temporary works

The Po Shan Road failure (Government of Hong Kong 1972)demonstrated the critical effect which constructional activity canhave on slope stability. This requires that temporary works bedesigned to minimise these effects on adjacent slopes and structures.In designing temporary works, due regard must be paid to conditionswhich may arise during their life. If works extend into the wetseason, the design must include facilities for adequate drainageof the site and must make allowance for changes in groundwaterlevel (for example; the effects which changes in groundwater levelhave on the loads on strutting to excavations)»

The design and construction of temporary works must notsignificantly alter the conditions assumed in the design of thepermanent works. If conditions are changed, the design of thepermanent works must be amended to take account of the new circum-stances.

Factors of safety adopted in the design of temporary worksshould reflect the risk posed to the public, the work force and thestructure. If, for example, an occupied building could be threatenedby failure of a temporary embankment, that embankment should be designedwith the same factor of safety against slope failure as a permanentembankment. Where failure of a temporary embankment involves norisk to the public, the embankment may be designed with a lowerfactor of safety (see table 2, chapter 5). But, the consequentcosts of failure should be weighed against the savings made inworking to the lower factor of safety. A greater responsibilityfor the regular monitoring of the performance of the works and,when required, for their maintenance is placed on those supervisingthe project when lower factors of safety are adopted.

Throughout the construction period all temporary worksshould be subject to regular inspections by an experienced engineeror technician. Signs of distress in any structure or slope shouldbe recorded and steps taken to alleviate the distress. Whereconditions are more severe than those assumed for the design of thetemporary works, the design should be reviewed and the works modifiedaccordingly.

159

EXCAVATIONS

9*3 Programme

The programming of works involving excavations must takeinto account Hong Kong's heavy seasonal rainfall which can begin asearly as April and normally ends in late September. Excavationswhich result in high slopes and the associated drainage and surfaceprotection should preferably start in September or October and becompleted by April. If excavations are carried out during the wetseason the surfaces of the slopes formed must be protected anddrained as excavation proceeds, drainage around the crest of theproposed excavation having been provided as a preliminary operation.Where necessary, temporary conduits should be provided to carry thedischarge from those drains already completed. If trenches on orabove slopes have to be excavated during the wet season this shouldbe done with extreme care. Ideally they should be excavated andbackfilled in short sections. Precautions should always be taken toprevent water collecting in the trench.

9.4 Methods

Excavation in the residual soil and completely weatheredrock zones may be carried out using conventional bulk and handexcavation methods.

In highly weathered granite core boulders will be encountered.If these have to be removed this should be done as excavation proceeds;they should not be left protruding from the exposed face until bulkexcavation is completed when access will be difficult and when theirremoval may threaten any development in progress below the formedslope. Boulders may be trimmed using plugs and feathers or hydraulicsplitters. If blasting is to be used to trim them to the requiredslope profile, the amount of explosive used should not be excessiveas over blasting can loosen boulders sufficiently to allow them tobe subsequently dislodged from the face of the excavation.

Where boulders threatening existing developments have tobe trimmed, precautions must be taken to protect these developmentsduring the trimming operations. This may be accomplished by meansof wire nets or bamboo protective barriers but their design musttake account of the trajectories of falling debris.

Excavation in the slightly weathered to unweathered zoneswill normally require blasting which, if not properly controlled,can lead to shattering of the rock mass and to a general loosening andopening up of joints. For the effects on slope stability of vibration,caused by blasting and piling, see section 9.5. To avoid blast damage,rock may be pre-split (Langefors and Kihlstrom 1967, Hoek and Bray1977). Pre-splitting is accomplished by drilling a row of closelyspaced and usually small diameter holes along the line of the final

160

face. These holes are lightly charged with the explosive centred inthe blast hole by means of a spacer so that an air space is leftbetween the charge and the wall of the hole. This row of chargedholes is then fired before the main charge and if the burden infront of the pre-splitting row is sufficiently large a clean fracturewill form between adjacent holes. The presplit fracture acts as avent path for the explosion gases of subsequent blasting, but doesnot protect the rock behind the presplit row from vibration.Presplit blasting is not usually successful in well jointed hardrocks where the joints are open and are inclined to the presplitline.

Smooth wall blasting or post-split blasting in which theline of holes is fired after the main blast is an alternative methodto presplit blasting. The method is often used to clean up faceswhich have been affected by heavy blasting.

When excavation is complete all rock faces should be scaledof loose blocks, even if blasted by either of the two methods describedabove.

During excavation the designer must visit the site frequentlyto examine the exposed faces for signs of conditions which are moresevere than those assumed in the design. Relict joints may bepresent in the completely weathered rock zones. Where these areseen in excavations, particularly if they are major joints dippingout of the excavated face at angles greater than 20° to the horizontal,they should be drawn to the attention of the person responsible forthe design of the works. The shear strength of material infillingrelict joints is often considerably lower than that of the weatheredmass and instability can develop very quickly along these surfaces.Where rock is being excavated such examinations should include acomparison between the exposed joint system and that assumed in theslope stability analysis and design. When a difference exists thedesign should be checked and if necessary amended. Random joints,or combinations of joints, which do not appear in the analysisshould.be surveyed and if they form potentially unstable blocks,should be stabilised. Signs of seepage should also be noted duringexcavation and should be compared with assumptions on the locationof the groundwater table made for the purpose of design.

Areas of over-excavation on slopes flatter than 1 on 1.5may be made good with suitable fill compacted to 95% of BritishStandard maximum dry density (BS 1377 test 15 (B)). The surface onwhich fill is to be placed should be benched and fill should beplaced in horizontal layers, with care being taken to ensure thatthe compaction of the fill at the surface of the slope meets therequired standard. Where it is necessary to reinstate over-excavatedslopes which are steeper than 1 on 1.5, cement stabilised soil orconcrete should be used.

161

9*5 Effects of vibrations

(i) Due to blasting:

When explosives are to be used for excavation, the effectsof the blasting on adjacent slopes should be consideredwhen planning the work.

In Hong Kong any proposal for blasting must be submittedto the Mines Department for approval* An essentialrequirement is that blasting operations should have noadverse effects upon adjacent slopes and buildings. Priorto blasting, all structures such as buildings or retainingwalls and slopes which could be affected by vibration,should be the subject of a detailed dilapidations survey.This should be repeated at regular intervals during theperiod of blasting and, finally, when all blasting iscompleted.

During blasting operations ground vibrations should berecorded near structures or slopes which could be affectedby the work. The result of these observations should berecorded on standard record sheets. In the case of aslope, the vibrational forces should be compared withthose assumed when the effects of blasting on the stabilityof the slope were assessed.

(ii) Due to construction operations:

The ground vibrations which result from constructionoperations, such as pile driving and the movement of heavyearth moving machinery, can have a detrimental effect onslope stability, as well as causing discomfort to peopleliving nearby. Wiss (1974) has examined and compared theeffects of vibrations which arise from various constructionactivities and has related these to damage and humanevaluation criteria.

9.6 Support

In the case of multi-strutted excavations Terzaghi andPeck (1962) have described methods of determining the loads imposedupon supports to both deep and shallow excavations. These methodsof analysis of the forces acting upon the support system assume somedegree of yielding and, as a consequence, movement of the supportedmaterial. They may not, therefore, be acceptable for designing thesupports of excavations where movement could endanger the stabilityof adjacent buildings or slopes and cause damage. The supportingsystem must be designed and constructed to prevent significantmovements of the soil mass (see sections 7.11 and

The support of excavations by various methods is discussedin detail by Dismuke (1975) who presents typical design calculations*

162

9*7 Drainage

The effect of rainfall in reducing the stability of slopesin Hong Kong has been well documented (Lumb 1962 and Lumb 1975). Theprovision of adequate drainage on any major excavation is, therefore,of prime importance. Before excavation commences, concrete lineddrainage channels, designed as described in chapter 8, should beconstructed around the crest of the proposed excavation to collectrun-off from above the excavation. If construction works are scheduledto extend into the wet season temporary drains should incorporatesilt traps suitably constructed to prevent seepage of water from thetraps into the ground. Where silt traps are provided they must becleared regularly and the material removed from the trap depositedwhere it will not be washed into the drainage system during subsequentrainfall.

Ponding on the surface of the excavation should be prevented*In the case of excavations for foundations, basements and servicetrenches this will be accomplished best by pumping from small,preferably concrete lined, sumps, which will limit the amount ofinfiltration through the base of the excavation. The excavatedsurface of the general site formation and all temporary drainsshould fall to concrete lined surface channels discharging to eithera stream course or a stormwater drain* Drainage channels should bedesigned to the standards given in chapter 8,

The construction of temporary works may cause a loweringof the groundwater table. As this can result in the settlement ofadjacent structures the possible effects of groundwater lowering,which will be more significant in the fine grained marine depositsthan the residual soils, should be considered.

The siting of stockpiles of material which will eventuallybe placed in the excavations must not interfere with pre-existingsurface water channels which maintain the stability of adjacentslopes. Although stockpiles may not affect adjacent works whenfirst formed, the effects of erosion, due to rainfall and generalconstruction traffic, can lead to the migration of material and thisshould be borne in mind when they are sited initially.

FILL

9.8 General

The disastrous landslides at Sau Mau Ping in 1972 (Governmentof Hong Kong 1972b) and 1976 illustrate the dangers inherent inslopes formed from loose fills being subjected to prolonged andheavy rainfall* On liquefaction, the material in the failing masscan travel great distances at high speed. Thus where failure couldendanger lives, temporary earthfill structures, even if for accessroads and spoil heaps, should be designed and constructed to thesame standard as that required for permanent works (see chapter 5).

163

Wherever possible, construction programmes should be arrangedso fill is placed during the dry season when the placement moisturecontent of the fill can be controlled more easily*

9.9 Placing and compaction of fill

The basic requirements for all fills to be placed in or onslopes in Hong Kong is that they should, in general, be compactedto at least 95% of standard maximum dry density (see chapter 3). Insome exceptional cases, such as fills forming platforms of largeareas not supporting structures, the degree of compaction specifiedfor some of the fill may be reduced to 90% of standard, providingthat the fill forming the peripheral slopes is compacted to 95%, seefigure 9.1.

General fill compacted to90%

Layer 1.5m thick may be compacted t o -95% max."}fH to reduce infiltration throughsurface

Face zone compacted to95% max. #

Slope to depend uponmaterial used

-Drainage to be provided in allabandoned water courses.

Note :-Maximum dry density

Min. 300 mm drainage layerto comply with filter criteriafor til!

B.-S. 1377 (1975) test 12.(max.X.) from

Figure 9.1 Typical detail of general fill areaemploying dual compaction standards

Surfaces on which fill is to be placed should normally bestripped of all topsoil and vegetation and the stripped surface shouldbe rolled before fill is placed on it. Where there is very soft levelground it is sometimes better to leave topsoil and small vegetationin place to act as a mattress to support the fill being placed.

164

On large scale filling operations it is good practice toconstruct a test fill to determine the depth to which the fill canbe placed and compacted to the required standard with the intendedequipment. The results should be plotted as shown in figure 9.2 andthe appropriate depth and number of passes of the roller can then beestimated. This allows for easy construction control by a foremanor inspector of works. But, control field density tests (seesection 9.9) must be carried out at frequent intervals to check thatdesign densities are, in fact, being achieved. If the type orcondition of the fill or the compaction equipment changes, additionaltests should be carried out to ensure that the number of passes ofthe roller required to achieve the specified dry density has notchanged from that defined as the result of the initial compactiontrial.

Compaction Trial - Any site.1 ton vibrating roller-No.731

2.0

1.9

1.8

roe 1.7

.£ 1.6

g 15TJ

Q U

1.3

1.2

3rd Feb. 1978

150mm uncompacted layer



£ 6 B

Number of passts of roller

10 12

Figure 9.2 Typical compaction trial results

165

The results and location of each test carried out as partof the routine quality control should be recorded on standard sheets(an example of a suitable record sheet is given in figure 9.3).

Earth fill should not contain large quantities of bouldersor rocks. No isolated boulder in earthfill should have a minimumdimension larger than the compacted thickness of the layer beingplaced. Lumps of fill should be broken up when the layer is beinggraded prior to compaction. Fill should not contain unsuitablematerial such as topsoil, roots, tree stumps, or rubbish in general.Fill should be placed so that the rolled surface sheds water and doesnot pond. Fill placed on the surface of the embankment should becompacted before closing the site for the night, or on the onset ofrain, to limit delays caused by rainfall. Fill adversely affected byrain should be allowed to dry out, or should be removed from thesurface of the embankment, before filling recommences.

At embankment edges spreading of the fill under theinfluence of the placing and compacting equipment makes compactiondifficult. Embankments should, therefore, be over-built and shouldthen be trimmed back to the desired profile. The overbuilt sectionshould be compacted to the same standard as the remainder of theembankment to ensure that the surface exposed by subsequent trimminghas been adequately compacted. Density tests should be carried outin the exposed surface to check that the density of the surfacematerial is as designed.

9'10 Controltesting

(i) Measurement of insitu density:

(a) Sand replacement method. This method is described inBS 1377 (1975) Test 15(A) and 15(B). The two tests differonly in the size of equipment, the second being moresuitable for coarse grained soils. The method is accuratebut requires considerable care. The calibration of thesand is sensitive to humidity and should be checked daily.The sand should be oven dried and stored for about a weekfor the moisture content to reach equilibrium with theatmospheric humidity. If the sand is to be re-used, itshould be dried and sieved to remove any fill materialbefore further use.

(b) The hand scoop method BS 1377 (1975) test 15(C). A pouringcylinder is not used and so the test is not accurateenough for good compaction control.

(c) The densometer (ASTM D-2167 63T). The densometer allows amuch simpler and more rapid determination of density. Theequipment consists of a rubber balloon attached to ameasuring cylinder which can be pressurised. The equipmentis partially filled with water. A zero reading is determinedby setting up the equipment on a smooth flat surface andpressurising the system until a minimum volume is recorded

166

from ../.../... to .../.../.

FILL COMPACTION TEST RECORD

Job ..........

Fill location Specification: Relative Compaction

Relative Moisture Content +. ..% to ~....J

Layer Thickness Compaction Plant No. of passes

Date

19,.,

In-situTestRef.No.

Type offillmaterial

Source offill

CompactionTestlef .Nos,

Location ofIn-SituDensityTest

Levelm P.D.

Density Mg/m^

In-SituBulk

In-SituDry

Max DDRelCom%

Moist ui

In-Situ

re Content %

Opt. Dlff.-

Acceptor

Reject

Remarks

ON

Figure 9.3 Record sheet for density tests in compacted fill

in the cylinder. A suitable size hole with walls assmooth as possible and with a hemispherical base is dug,care being taken to collect all the soil removed from thehole. The soil from the hole is weighed and a sample fromwhich the moisture content can be obtained is taken. Theequipment is then placed over the hole and pressurised,the minimum volume in the cylinder being recorded. Thedifference between the two recorded volumes gives thevolume of the hole. From this volume, the moisturecontent and the weight of material removed from the hole,the density can be calculated.

(d) Core-cutter method, A sample of the fill is taken using athin walled core-cutter, BS 1377, test 15(D), which can beeither driven or jacked into the fill. The weight of thesample is measured and with the known volume of the corecutter the density is determined. The sampling processcan change the density of the fill, and the presence ofgravel in the fill can cause disturbance of the sampleand give rise to errors in determining density.

(e) Nuclear method. The soil density can be measured indirectlyby back scatter from an artificial Y-ray source. Thismethod is affected by large particles such as gravels andcobbles arid is usually only successful on very consistentmaterials such as asphalt. The method is not suitable forcompaction control in Hong Kong.

(f) Large scale replacement. The density of rock fill can bemeasured insitu by excavating a 2 cubic metre hole in therock fill which is lined with polythene and filled withwater. The weight of material excavated, and the volumeof water used to fill the hole, can be measured and thedensity thus calculated.

(ii) Measurement of moisture content:

The best method of determining moisture content is by ovendrying (BS 1377 test 1(A)) but this has the disadvantageof taking 24 hours before a result is available, Adecision on whether fill requires more compaction to meetthe specification is, therefore, also delayed for up to 24hours. Moisture content can be determined using a microwaveoven, although the soil temperature cannot be carefullycontrolled. Samples may also be dried by spreading thesoil over the bottom of a large metal tray which is heateddirectly by gas burner. The result is obtained morerapidly than if the rapid methods described in BS 1377 areused. However, heating is uneven. When using these twomethods care should be taken to ensure that the sample isdried before the soil temperature reaches 110°C,

168

Two rapid methods for site use are given in BS 1377 tests1(B) and 1(C)* They are not as accurate as the standardmethod and cannot be used for soils containing largeproportions of halloysites, gypsum, calcareous or organicmatter; although they are, in general, suitable for HongKong soils* Soil moisture content may also be rapidlydetermined using the "Speedy Moisture Content Tester"which measures the pressure of acetylene gas released whencarbide reacts with the soil moisture. However, any rapidmethod used for construction control should be checkedoccasionally against the standard oven drying method, anda calibration curve should be drawn up showing the relation-ship existing between the moisture content determined bythe two methods. The moisture content used to determinethe dry density is the equivalent oven determined moisturecontent obtained by reference to the calibration curve.

Rapid compaction control can be achieved using Hilffsmethod (USER 1974) without measurement of moisture content.The result is good enough for compaction control butshould be checked against densities obtained using theresults of oven-dried moisture content tests.

SURFACE PROTECTION

9.11 Chunam

Chunam is a cement-lime stabilised soil used as a plasterto protect the surfaces of excavations from erosion and infiltration.The recommended mix for chunam plaster, the proportions beingmeasured by weight, is 1 part Portland cement, 3 parts hydrated limeand 20 parts clayey decomposed granite or volcanic soil.

The cement and lime should be mixed dry before adding thesoil. The minimum amount of water consistent with the required work-ability should then be added to the mix. If the water cement ratiois too high severe cracking can result, see plate 9.1(a). Plates9.1(b) and 9.1(c) show a chunam covered slope which has been properlyconstructed and provides a relatively impervious erosion-resistantsurface.

Before placing chunam on an existing slope all vegetation,topsoil and roots should be removed and the slope graded. To holdthe chunam in position during placing 25 mm diameter bamboo dowels300 mm long should be driven into the surface at 1*5 m centres on astaggered pitch until only 25 mm of the stake projects from thesurface. The chunam should then be applied to the surface in twolayers, each not less than 20 mm thick. The surface of the baselayer should be suitably scored with a trowel or left with a rough

169

surface to provide a key for the second coat. This should be placedafter the first coat has taken its initial set, but without anundue delay; 24 hours between coats would normally be consideredsuitable. As the chunam is intended to provide an impermeablesurface it should be placed in as compact a condition as possible,and the final surface should be trowelled smooth to improve run-off.

Trowelled chunam can cause excessive reflection of sunlight,and as a consequence of the resulting glare discomfort to anyonein the area. To reduce this effect colourants may be added to thechunam in the top layer. As suitable colourants, manganese dioxideor ferrous oxide powder may be added to the mix for the top coatat a rate of 3% by weight of the cement content. The extent to whichsunlight is reflected is dependent upon the smoothness of the finishedsurface and final trowelling with a wooden float rather than a steelfloat may reduce the amount of reflection.

Chunam is normally placed with no regularly formed con-struction joints, although care should be taken to ensure that thejoints which do occur in the top and bottom layers do not coincide.When regular bays are formed, sealed joints should be formed betweenthe bays.

Chunam used to protect temporary excavation surfacesshould be placed in one layer not less than 20 mm thick.

Where water is observed to be seeping from the surface tobe protected or where water seepage may develop as a result of heavyrainfall (for example: below valleys truncated by the cut slope)appropriate drainage measures should be provided.

9.12 Sprayed mortar

The general principles of the method are discussed in theConcrete Manual (USER 1963) and a draft Code of Practice for thespraying of concrete has been published by The Association of GuniteContractors (1978).

Slope protection applied by spraying mortar onto thesurface of the slope is an alternative to chunam plaster. Thespecifications for the materials used are identical to those adoptedfor conventional concreting, although the aggregates are speciallyselected to meet not only the requirements of the finished surfacebut also to prevent segregation while the mortar is being pumped.In general, the maximum grain size of the aggregate for slopeprotection should not exceed 10 mm. The sand should be well gradedas shown in figure 9.4.

170

Plate 9.1(a)

ChunamCracking caused

,by high water-cement ratio.

Plate 9.l(b)

Chunam coveredslope, properlyconstructed.

Plate 9.1(c)

Showing detailof joint inchunam.

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Figure 9.4 Grading of sand suitable for sprayed mortar

Sprayed mortar mixes must be designed to resist segregationas well as meet strength durability and impermeability requirements.Kempster (1968) has suggested a maximum aggregate cement ratio of3.1 for wet mixes. In Hong Kong careful consideration must also begiven to the problems of drying and consequent shrinkage crackingwhich can occur when sprayed mortar is used for slope surfacing.Having designed a mix to meet the requirements for segregation andstrength it should be used in a test panel prepared and constructedunder the conditions which will obtain during slope surfacing. Theperformance of the panel with respect to durability impermeabilityand shrinkage should then be assessed and if necessary the mixshould be redesigned to meet all the requirements. The mortarshould be cured by water spraying. The surface obtained by mortarspraying is rougher and therefore less reflective than chunam, andin practice creates fewer problems with glare. Rebound materialshould be removed from the surface after completion of spraying.

Where necessary, drainage facilities (such as weepholes)should be included in the sprayed surface.

171

9.13 Masonry

Various methods of slope protection incorporating masonryblocks which are aesthetically pleasing have been used in Hong Kong.

Masonry blocks should be bedded on a minimum 75 mm thicklayer of free draining, crushed stone or gravel conforming to thecriteria for the design of filters (see chapter 4). Joints betweenadjacent blocks should be filled with a 1:3 cement sand mortar toprevent infiltration between the blocks and the establishment in thejoints of grass and other vegetation which would impede run-off.Weepholes draining the bedding material should be provided at thetoe of the masonry wall.

9.14 Planting on impervious surfaces

Large areas of chunam and concrete are aestheticallyunpleasant and to improve the appearance of the surfaces smallareas may be specially prepared to permit planting. Such beds shouldbe formed in the surface as shown in figure 9.5. The area of plantingbeds provided should not exceed 5% of the total plan area of theslope and this should be evenly distributed over the slope. A HongKong Government Working Party is currently preparing a document whichwill give guidance on the plants which can be cultivated on slopes inHong Kong.

Planting bed filled withsoil —

.Impervious surface

Chunam raised locally around opening

600 mm dia, or, if beingformed around an establishedtree, the diameter of thetree* 600 mm

Figure 9.5 Typical arrangement for planting bedin impervious slope surface

172

9.15 Planting on pervious surfaces

A dense cover of shrubs or low trees can improve the stabilityof a slope surface. There is no published information on the effect ofvegetation cover on the rate of infiltration in Hong Kong. Apart frombeing aesthetically pleasing, the main value of providing vegetationcover, including grass, for slopes comes from the resistance which itprovides to erosion.

The resistance of grass to erosion may be improved by in-corporating into the surface layer of the slope one of the proprietaryprecast concrete slab revetments designed to support grass growth. Atypical system is shown in figure 9.6. Alternatively, polypropylene netmay be pinned to the surface over the grass to further increase itserosion resistance.

Approx.• 600mm

in<N

1

Approx.-400mm-

mSide Elevation

(a) GENERAL DETAILS

Section A-A

•When used on slopingsurfaces, every secondor third slab should bestaked for extra stability

(b) PERSPECTIVE OF BLOCK

Trimed slope

[c) APPLICATION ON SLOPES

Figure 9.6 Proprietary concrete revetment system whichlimits erosion but permits the growth of grass

173

Grass may be planted as seed or as turf. Grass seed may besown by either the conventional manual method or alternatively byhydroseeding in which the grass is sprayed on to the slope alongwith a mulch (an organic dressing intended to retain moisture) anda binder which may be bitumen based. This method has been widely usedin Japan, Europe and North America but has not to any great extent inHong Kong. Turf should be placed on a layer of topsoil 40 mm thick,on slopes steeper than 1 on 1.5 individual turves should be peggedwith bamboo pegs.

Various grasses grow in Hong Kong but not all are suitablefor erosion protection or for growing on slopes; grasses for thispurpose should grow evenly over the surface and should be relativelyshort. The sowing of only one type of grass seed is not recommended,and in general the seed mixture should contain three types of seed.The grass most suitable for that particular location being allowed todominate after the initial development of cover.

If grass covered slopes are not properly maintained weedsand wild tall grasses will become established and in time cause thecultivated grass to die. To prevent those weeds and naturally seededgrasses which do grow from maturing and seeding the grass must be cutregularly,

DRAINAGE SYSTEM

9.16 Excavation for drains

If possible, excavations for drainage works should not beopened during the wet season. If this is unavoidable the excavationshould proceed from the lowest to the highest point in the drainagesystem and should be carried out in sections. Each completed sectionshould be lined before the adjacent section of excavations is openedup. This will prevent erosion and infiltration through the bed ofthe channel. Spoil from the excavation should be removed to aposition where it cannot affect the drainage system.

9.17 Channels and pits

All surface channels and catchpits should be formed instructural-grade concrete. Chunam should not be used. Drainagechannels should, if possible, be built to regular falls with nosudden changes of direction. If these are unavoidable baffle wallsshould be provided to prevent water splashing over the top of channels,Slope protection should be carried across the top of the walls ofdrainage channels as shown in figure 9.?» Concrete channels shouldbe cas,t in sections not more than 5 m long to accommodate thermalmovement, and joints should be sealed with a good quality jointsealer. Joints should also be provided at any changes of direction.Channels built partially in fill and partially in original materialshould incorporate construction joints at the boundaries of the fill.

174

Impervious surf ace carriedover top of channel

Design depthof channel

Concrete surfacewater channel

Figure 9.7 Detail of slope surfacing at surface water channel

SERVICES

9.18 Excavation

Trenches excavated on or above slopes provide a locationwhere infiltration of water into the hillside can eventually lead toslope instability. Trenches loosely backfilled with soil will permitalmost as much infiltration from the surface as an open trench andwill permit the lateral flow of water along the trench through thebackfilled material. Excavations for services above slopes should,therefore, not be opened up during the wet season unless unavoidable.When such excavations are carried out In the wet season the trenchshould be protected against the ingress of run-off from the surfacein which the trench is excavated by means of sand bags, concretekerbs or small compacted earthfill bunds along each side of thetrench. Pumps should be provided at all low points on the trench tomaintain the bottom in a dry state and a watchman should supervisethe maintenance and functioning of pumps at all times when work isnot proceeding. On completion of work the trench should be backfilledin layers not greater than 150 mm deep and each layer should becompacted to not less than 95% of British Standard maximum drydensity (see sections 9.9 and 9.10).

175

9.19 Ducts and conduits

Ducts are defined here as pipes which contain cables(for example: telephone ducts) while conduits are defined as pipesdesigned to convey water. The danger inherent in siting conduitsabove steep slopes is obvious. But, a less obvious danger is thatof ducts conveying water over considerable distances, from leakingconduits to places where leakage from the essentially open-jointedduct can cause slope instability. The positioning and detailing ofducts and conduits in the vicinity of steep slopes therefore requirescareful consideration.

Ducts and conduits should not be positioned close to thecrests of slopes unless the effects of leakage are taken into con-sideration in the design of the slope. If leakage will result in thereduction of factors of safety to below those defined in chapter 5,steps must be taken to reduce the effect of leakage by positioningthe conduit further away from the crest of the slope, or alternativelyto prevent the leakage from the conduit, by constructing it in aconcrete trough or sleeve drained to a suitable surface channel.Flow along ducts should be prevented by casting polystyrene plugsaround the cable, within the duct, at the nearest inspection box tothe slope whose stability could be affected by the presence of theduct. If possible the inspection boxes should be drained to asuitable surface channel. Flow along trenches for services can bestopped by forming a collar of concrete, or excavated ground mixedwith bentonite, around the duct or conduit and keying it into thesides and bottom of the trench.

Where conduits pass from fill to original ground or whereconduits are laid in fill, differential settlement can cause cracking.Under these circumstances pipes incorporating flexible joints shouldbe used.

9.20 Construction control

The importance of the designer being adequately representedon site throughout the construction period cannot be overstated.This is particularly true for geotechnical engineering, the success ofwhich depends largely upon the experience of the engineer or geologistsupervising the works on site. The designer is failing in his duty ifhe does not advise his client that it is inadequate to rely purelyupon the obligations placed upon the contractor, under the terms ofthe contract, to guarantee that the quality of the permanent works andsafety of the temporary works meet the intent of the designer. Theextent to which the designer is represented on site will depend uponthe size and complexity of the particular project: site staff mayrange from one inspector of works on a small project to a ResidentEngineer aided by several assistant engineers and other supervisorystaff on a larger project. Where fill is to be placed the site staffshould include persons experienced in carrying out the necessary fieldand laboratory testing.

176

. , 4 ^ L8hould be famili** with the conditions assumedin the design. Should conditions vary markedly from the conditionsassumed for design then the attention of the designer must be drawnimmediately to the changes. The designer should also arrange to visitthe site frequently to ensure that conditions remain constant.

Site staff should keep detailed records of progress and of theconditions encountered when carrying out the work. Weekly progressphotographs, from positions agreed with the designer before work commences,should be taken and should be supplement by additional photographs takenof salient points as the need arises. Photographs should be dated anddescriptions of weather conditions, time of taking photographs and points ofinterest (particularly if not part of a series of routine progressphotographs) should be attached. All tests which are carried out shouldbe given a serial number, and details of the location, weather conditions,date and time of testing should be recorded. As-built drawings should beprepared as the work proceeds.

Under certain circumstances site staff will be responsible forrecording the performance of temporary structures. Records of suchreadings should be maintained on record sheets similar to those proposedin chapter 11. Similarly, where temporary slopes are to be examinedduring the construction period, records of these inspections should bekept on forms of the type recommended in Chapter 11 for the inspections ofpermanent slopes.

REFERENCES

American Society for Testing and Materials (1963). Method of test forin-placesoil density using rubber balloon method.AmericanSociety for Testing and Materials, Designation D-216? 633?*

Association of Qunite Contractors, (1978). Draft code of practice forthe spraying of concrete otherwise known as gunite or shotcrete.Association of Qunite Contractors.2k Osmond Road, Richmond,Surrey TW10 6TH, U.K.

B.S. 1199, (1976). Building Sands from Natural Sources, British StandardsInstitute, London.

B.S. 1377, (1975). Methods of Testing Soils for Civil Engineering Purposes.British Standards Institution, London, 1 3 pp.

Dismuke T.D. (1975). Cellular structures and braced excavations FoundationEngineering Handbook, Ed. H.F. Winterkorn & H.Y. Fang, VanNostrand Reinhold Company, New York, pp 4 5-480.

Government of Hong Kong (1972). Final Report of the Commission of Inquiryinto the Rainstorm Disasters 1972, Government Printer, Hong Kong,

177

Government of Hong Kong (1972)* Interim Report of the Commission ofInquiry into the Rainstorm Disasters 1972, Government PrinterHong Kong, 20 pp.

Hoek E. and Bray J.W. (1977)• Bock slope engineering, The Institutionof Mining & Metallurgy, London, 309 pp.

Kempster E. (1968). Pumpability of mortars, Building Research Station,Current Paper 10/68.

178

CHAPTER 10

INSTRUMENTATION

10.1 Introduction

Instruments are installed in the field to provide informationon parameters for design, for construction control and for long termmonitoring of performance.

When designing an instrumentation scheme the magnitude anddistribution of the properties to be measured, the constructionprogramme and the expected operational life of the scheme should beconsidered. These factors are important when selecting the bestinstruments for the purpose, and for locating the instruments andreading equipment where remote reading methods are used. Otherfactors to be considered are the accessibility of the instrument forreading and the effects of its presence on construction.

Instruments which measure pore water pressure, movement orstress are those which are used most commonly when dealing withconstruction on slopes in Hong Kong.

10.2 Reading of instruments

To yield the maximum amount of information, instrumentsshould be read systematically by a competent person (such as a trainedtechnician) who has an understanding of the purpose of each instrument.The frequency of reading will depend upon the situation and the natureof the changes which the instruments are being used to monitor. Forexample, piezometers in decomposed granite may require reading severaltimes a day after heavy rain. Readings should be recorded on standardfield sheets (figure 11.1) which include details of the probable rangeof readings from the instruments being observed. Any readings whichindicate a marked change in conditions should be checked immediately.The functioning of the instruments should also be checked when unexpectedreadings occur, although in some cases a complete check may not bepossible. The calibration of some instruments can change with timeand re-calibration may become necessary. Those instruments whosecalibration cannot be checked after installation should, if possible,not be used. Instrument readings which indicate that a structure isnot functioning as designed have often been dismissed as instrumentmalfunction until backed up by more tangible evidence. Duplication ofinstruments in critical areas will aid the interpretation of readings.

When instruments are used to monitor the continuing stabilityof a slope or structure the limits for the readings, beyond which theslope or structure is no longer safe, should be shown on both therecord sheet used by the reader in the field and the time plots(fig.10.1).

179

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180

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182

10.3 Records

All instrument readings should be plotted on a time baseso the significance of variations can be assessed more easily.Examples of such plots are given in figures 10.1 to 10.3.

10.4 Measurement of groundwater level

Groundwater levels taken in investigation holes duringdrilling (chapter 2) are not reliable due to the effects of theflushing water on the groundwater regime. For reliable observation,the groundwater level should be allowed to stabilise and this maytake several days after completion of drilling. The level of thephreatlc surface can then be measured in an open borehole, but theresponse time is very slow and infiltration of surface water maycause the hole to act as a recharge well. As an alternative to leavingthe casing in position, a smaller diameter standpipe may be installedin the hole around which the borehole may be backfilled with materialremoved from the hole. Observations of the groundwater level in sucha standpipe are more significant than those obtained during drillingand standpipes should be installed in any hole which is not to be usedfor any other form of instrumentation. The standpipe installation canbe improved by using a perforated pipe over the lowest 1-2 m, and bybackfilling the hole with filter material. The top 0.5 - 1 m of theborehole should be filled with bentonite and cement which will stopinfiltration. The smaller diameter standpipe will give a shorterresponse time than will a standard borehole casing. However, if thehole penetrates more than one water-bearing zone, flow between zonescan occur and water level measurements obtained from the standpipewill be meaningless. If the length of filter surrounding the standpipeis limited and is sealed to connect with one particular zone of thesoil or rock, the installation acts as a crude open-hydraulic piezometer(section 10.6) with a very slow response time. The installation of asuitably placed piezometer is however preferable as the informationobtained is more easily interpreted.

10.5 Measurement of pore pressure

Piezometers are used to measure pore pressure; they consistof a cavity separated from the soil or rock by a porous element - thetip - and a method of measuring the water pressure in the cavity (Hanna,1973; Vaughan, 1974),

The measurement of pore pressure requires that the piezometertip be sealed into the ground, in the specific zone in which aknowledge of pore pressure is required, by placing it in a sandpocket in the borehole. The pocket is sealed above, and sometimesalso below, with bentonite and the hole is grouted. The length ofthe sand pocket should be at least four hole diameters long, preferably600 mm to 1 m, and the grout used to seal the piezometer in theborehole should have the same or lower permeability than the surroundingsoil. A grout mix suitable for soils is one part cement, one part

183

bentonite to six parts water. Where large water losses have beenrecorded when drilling in rock, this mix may require modification toavoid grouting large volumes of the rock. Grout volumes used should-,be checked and compared with the volume of the hole over the lengthintended to be grouted. Poor sealing of the piezometer in the holewill permit water from different levels to migrate, and may permitsurface water to infiltrate to the piezometer. Readings taken fromsuch an installation will, at best, indicate the level of the groundwatertable. But more probably, the borehole will act as a sump and thereadings obtained will be of no significance.

Bentonite balls formed from wet powdered bentonite, althoughdifficult to handle and place, make a satisfactory seal in a borehole.Compressed bentonite pellets, which swell to many times their originalsize, are much easier to place than bentonite balls. However, theycan artifically reduce the pore pressure for long periods and should,therefore, only be used with caution.

The choice of piezometer type depends on the predictedwater pressures, access for reading, the installation life and theresponse time required. Hong Kong residual soils are normally sufficientlypermeable that response time need not be considered when selectingpiezometers. The advantages and disadvantages of the different typesof piezometers are given in table 10,1.

10.6 Open hydraulic (Casagrande) piezometer

This consists of a porous tip, generally about 40 mm O.D.and 300 mm to 600 mm long, with a standpipe between 12 mm and 20 mminternal diameter (figure 10.4). The top of the standpipe shouldfinish above ground level and should be capped and protected fromdamage by a drained, lockable cover box. Beneath road surfaces andpavements, the piezometer must finish below ground level, and drainageof the cover box is especially important to stop surface waterentering the piezometer.

Access to the top of the standpipe is generally requiredfor plumbing the water level, although for remote reading an airbubbler system can be installed (Penman, 1972). An electricaldip-meter is the most commonly used method of measuring the waterlevel. The dip-meter consists of a weighted coaxial cable with thetwo wires terminating a short distance apart with an insulator betweenthem. When both terminals are submerged the circuit is closed andthe current activates a voltmeter or buzzer* When the water level isfairly close to the surface, a simple method is to lower a smalldiameter polythene tube down the standpipe and blow down the tube.The change of resistance and bubbling which occurs when the end ofthe tube is submerged can be felt and heard. The rubber system forautomatic recording is an extension of the second method. A smalldiameter air line is taken to the piezometer tip and a very small gasflow is passed down to it to produce several bubbles per minute. Thepressure of the gas measured is equal to the height of water in thestandpipe above the end of the bubbler tube. A chart recorder can beused to record the gas pressure automatically.

184

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ClosedHydraulicsLow airentrypressure

ClosedHydraulic:high airentrypressure

Pneumatic

Electricvibratingwire type

Electricresistancetype

Psychrometer

Gypsumblock

Range

Atmosphericto top ofstandpipeLevel

Anypositivepressure

-1 atmosphereto anypositivepressure

Any positivepressure



Below-1 atmosphere

-0,1 to -10atmospheres

lesponse

Slow

federate

Moderate

Rapid

Rapid

Rapid

Variable

Very slow

De-airing

Self de-airing

Can bede-aired

Can bede-aired

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As above

As above

Not relevant

Not relevant

demote Reading

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Yes

Yes

YesSome head lossover longdistance

Yes butspecial cablerequired

Yes but withcare because oftransmissionlosses

Short distancesonly

No

Long Term Reliability

Very good

Depends on pressuremeasuring system1) Mercury manometer -

very good2) Bourdon gauge -

poor in humidatmosphere

3) Pressure transducer -moderate but easilyreplaced

As above

Appears moderate topoor but very littlelong term experienceavailable .

Signal qualitydegenerates with time.Instrument life about10 years but reliabilityof instrument thatcannot be checked isalways suspect

Poor

Instrument life 1-2 yearsVery little long termexperience available

Probably moderate topoor

Other

Advantages Disadvantages

Cheap, simpleto read & main-tain. Insitupermeabilitymeasurementpossible

Fairly cheap.Insitupermeabilitymeasurementpossible

Fairly cheap.Insitupermeabilitymeasurements inlow permeabilitysoil

Fairly cheap.No gauge houserequired

Vandal damageoftenirreparable

Gauge houseusually required.Regular de-airingnecessary.Uncovered tubingliable to rodentattack

As above.Very regularde-airingrequired whenmeasuringsuctions

No method ofchecking if pore-water or pore-air pressure ismeasured

As above.Expensive. Zeroreading liable todrift and cannotbe checked

As above

Not accuratebetween 0 and -1atmosphere

Errors because ofhysteresis andelec tr o-chemicaleffect

Recommendation

1st choice formeasurement withinpressure rangeunless very rapidresponse or remotereading required

Useful when remotereading requiredand for artesianpressures

Useful formeasuring smallsuctions

Only suitable whentip almost alwaysbelow groundwaterlevel and no largesuctions occur

Not generallyrecommended

Not recommended

Research stage atpresent

Not suitable fortransient measure-ments

The response time of this type of piezometer is comparativelyslow, but the effect does not become serious until the soil or rockpermeability is less than about 10~"7m/sec, when the response time isabout 1.5 hours for a piezometer with a 150 mm x 600 mm sand pocketand a 20 ram diameter standpipe, the response time being indirectlyproportional to the permeability. If the pore pressure temporarilydrops below atmospheric, the piezometer discontinues reading but,being self de-airing, resumes satisfactory operation without main-tenance. Standpipes of less than 12 mm diameter may not be selfde-airing (Vaughan 1974). The long term reliability of these piezo-meters is very good except that they are rather more susceptible tovandalism than some of the other types.

Where artesian pressures occur they may be measured usinga Bourdon gauge or mercury manometer fitted directly to the standpipeor alternatively indirectly, using a cap fitted with twin tubes thusforming a de-airable closed system.

Where necessary, more than one piezometer can be installedin a single borehole, but it is important to obtain a good seal inthe borehole, isolating the instruments. More than one piezometercannot easily be installed in holes smaller than H size.

Lockable cover

Cap

Concrete cover box

Drain

Stand pipe

Cement- bentonite grout

Bentonite seal

Sand

Piezometer tip

Figure 10.4 Standpipe (Casagrande) piezometer

186

10.7 Closed hydraulic piezometer

A twin-tube closed hydraulic piezometer with a low air-entry-pressure ceramic filter can be installed in a sand pocket in thesame manner as a standpipe piezometer* Because head losses arenegligible in a well installed system, hydraulic leads can be takenfairly long distances for remote reading but a permanent gauge houseis required if a Bourdon gauge or mercury manometer is to be used forpressure measurement. Portable pressure measuring systems usingpressure transducers and portable de-airing units, which eliminate theneed for a gauge house, are available. The gauge house, or measuringpoint, should be at approximately the same level or lower than thepiezometer tips, as the lowest pressure that can be recorded is about10 m of water head below the highest point in the system. On slopes,piezometer tips can be installed in holes drilled at an angle of 10°to 15° to the horizontal, but packers will then be required duringgrouting to ensure complete sealing of the hole.

The response time is quicker than that of an open standpipe,but is dependent on the method of pressure measurement. The systemis not self de-airing and regular manual or automatic de-airing isrequired to maintain satisfactory operation.

A high air-entry pressure ceramic filter considerablyreduces the flow of air into the piezometer cavity, even in unsaturatedsoils, and suctions (negative pore water pressures of up to oneatmosphere at the highest point in the system) can be measured. To dothis, the piezometer ceramic must be installed in direct contact withthe soil. Very careful and regular de-airing is required for theinstrument to continue reading suctions. These piezometers should beused In the fills of especially Important structures such as earthdams.

10*8 Pneumatic piezometer

This piezometer comprises a porous tip capped with a flexiblediaphragm which controls a pneumatic valve. The valve operates whenthe pressure in the pneumatic system equals the pressure in thepiezometer cavity. The piezometer tip can be installed in a sandpocket in the same way as a standpipe piezometer. As they cannot bede-aired, pneumatic piezometers are of limited use in Hong Kongresidual soils and should not be used where the soil is unsaturated.The long term reliability of these instruments is not yet proven.

!0'9 Electrical piezometer

Electrical piezometers are unsuitable for partly saturatedsoils, and therefore have a limited use in Hong Kong residual soils.They comprise a porous tip isolated by a flexible diaphragm, thedeflection of which is measured by vibrating wire or resistancestrain gauges. Response to pore pressure change is rapid; They havethe same de-airing problems as the pneumatic piezometer and thecalibration, which is liable to drift (change with time), cannot bechecked after installation.

187

Vibrating wire systems are more reliable than resistancesystems but because they are read with an oscilloscope they arecumbersome. The major disadvantage of electrical systems is drift*Instrument life of vibrating wire systems is moderate and of resistancesystems is often very short*

10*10 Measurement of pore suction

In the range 0 to -1 atmosphere, direct measurement of poresuction can be made using a closed hydraulic system (section 10*7).Cavitation of water in the measuring system precludes direct measure-ment of greater suctions*

An indirect method uses the relationship between soil suction,relative humidity and temperature. Relative humidity can be measuredwith a psychrometer in direct contact with the soil, using the Peltiercooling effect* The psychrometer can be used to measure suctions ofup to 1,000 atmospheres but, as the humidity is very close to 10Q#, theaccuracy of measurement at suctions of less than 1 atmosphere is poor.The response time can vary from a few hours at 100 atmospheres, toas much as 1*t days, at 1 atmosphere. The readings are affected bytemperature and atmospheric pressure changes (Richards 197*0*

Another indirect method using gypsum blocks has been triedelsewhere but abandoned for transient conditions because of the veryslow response time (Richards 197*0.

SURFACE MOVEMENT

10*11 Si gni ficance o f Movement

Landslides in Hong Kong residual soils often occur withoutwarning and, for these, movement measurement is seldom useful.However, measurement of movement and deformation of and behindretaining structures, anchored rock or soil and rock slopes can beimportant. Methods of measuring surface movement are discussed insections 10.12 to 10*16 and sub-surface movement in sections 10*17and 10.18*

10.12 Structural cracking

A telltale placed across a crack should be designed toindicate if further movement of the crack has occurred since instal*-lation and the magnitude and direction of the movement* All telltalesshould be marked with the date of installation.

The simplest form of telltale, suitable for monitoringcontinuing movement of walls, comprises one or more straight linesscribed across a crack* Two lines at right angles will ehow anymovement in the plane of the face on which the lines are scribed.

188

Glass plates, mounted across a crack using mortar pats orepoxy resin, are common and easily installed but they do not indicateclearly the direction of relative movement. The glass often breakseither through shrinkage of the mortar pats on which it is set orvandalism, glass plates should not be used.

A more robust telltale, which can be used to measure pro-gressive movement is shown in figure 10.5. Movement of the overlyingmetal strip can be measured relative to datum lines scribed on theunderlying plate, the datum lines being offset by recorded distanceswhen the telltale is installed. Movement normal to the face candamage the metal strip and prevent continued reading. To overcomethis problem, a second telltale may be placed a short distance fromthe first, with the overlying strip bonded to the opposite side ofthe crack. By measuring the separation of the two plates with afeeler gauge, movement normal to the face can be determined.

Mechanical deformation gauges such as "demec" gauges can beused for accurate measurements. Two gauge points are attached to thestructure on either side of a crack, their spacing being fixed usinga special locating bar. The gauge comprises an invar steel beam withtwo conical gauge points, one fixed at one end of the beam and theother pivoting on a knife-edge. The deformation is recorded by adial gauge, the movement being magnified by a simple lever system.Temperature compensation is achieved using an invar steel referencebar. Very high accuracies, which are probably higher than requiredfor monitoring crack movement, can be achieved with this instrument.

Offset dated-scribed line

Recording baseplate

Non corroding metalcover strip

Crack in structure

Figure 10.5 Telltale

189

10.13 Rock and soil slopes

The methods discussed in section 10.12 are also applicableto the measurement of movement on joints in fresh or slightly weatheredrock* Surface mounted gauges are, however, unsuitable for moderatelyweathered rock and weaker materials.

Steel pegs, grouted with epoxy resin into holes drilled inthe rock on either side of a joint, can be used to monitor relativemovement. Usually three pegs are used, two on one side of the jointand one on the other. On soil slopes where movement across a crackis to be monitored, the pegs should be set in concrete beacons oneither side of the crack. The top of the pegs should be centrepunched, or preferably have conical seatings, for precise positioningof measuring instruments. The distances from one peg to the two pegson the other side of the crack or joint are measured using a verniercaliper or a mechanical extensometer. The change in relative levelof the pegs can be determined using a spirit level on a straight edgebetween the pegs.

fDemecf gauge seatings can be used as an alternative tosteel pegs.

10.14 Surveying

Relative movements can be assessed using precise surveyingtechniques and measurements of absolute movements can be obtained ifa fixed datum located outside the zone of influence of the movingmasses is used. The movements measured are often very small, so thereference points used should allow accurate positioning of measuringinstruments. A selection of suitable reference points is describedin Burland and Moore (1974), Cheney (1974) and Hanna (1973).

10.15 Pho t o grammet ry

Measurements of movement can be made using stereographicpairs of photographs taken with either a phototheodolite or someother form of precise camera. A series of photographs is taken fromtwo fixed stations on a base line parallel to the face or slope beingstudied. The camera axes should be parallel and normal to the baseline. Where possible, fixed ground control points at the top andbottom of the slope should be installed and at least two shouldappear on both photographs of each stereopair. If it is not possibleto set up fixed ground control points, the points used for controlmust be accurately surveyed for every pair of photographs. Theaccuracy of the measurements of movement is dependent on the precisionwith which the ground controls are surveyed. Steep slopes are moresuited to photogrammetric survey than shallow ones because theaccuracy varies with distance from the focal plane of the camera.Descriptions of both the technique and the equipment and its use ingeological mapping are given by Ross~»Brown and Atkinson (1972) andRoss-Brown, Wickens and Markland (1973). Photogrammetry has beenused to monitor movement of earth dams (Moore 1974a) and to mapjoint movements on which were subsequently made using Inclinometers,extensometers and precise survey techniques (Mbore 1974b and Burlandet al 1977).

190

10.16 Vibration measurement

Specifications for velocity seismographs suitable formonitoring the vibrations due to blasting are given in table 10.2.

Property

Resonant frequencyUseful frequency rangeSensitivity

Particle velocity rangeAttenuation rangeAllowable tilt angleInternal impedance of coilMaximum acceleration withoutinstrument damage

Maximum displacement withoutdistortion

Record paper speedPower requirement*Total weight of instrument

Unit

cycles/seccycles/secinches deflectionper in/secin/sec

degreesohms

in

in/secvolt x ampIb

0.1

5-10<100<50

The power requirement is based upon the assumption that theinstrument will be operated from an automobile storage battery.

Table 10.2 Specifications for velocity seismographs by Duvall

The more simple instruments will record peak particlevelocity in one plane only. Some are limited to working in thevertical plane only. Suitable combinations of these instruments mustbe used to monitor vibrations in three orthogonal directions.

Instruments which measure all three components are moreuseful and several types are available, some of which also recorddisplacement and acceleration. A list of suitable instruments isgiven in Hoek and Bray (1977). Skip? and Marriot (1974) discussinstruments for a wider range of vibration monitoring and indicatethose suitable for blast monitoring.

SUBSURFACE MOVEMENT

10.17 Inclinometers

Inclinometers measure horizontal movements both of andbehind retaining structures, and sometimes P«faijure/° m^ of

fills and cut slopes. It comprises a tilt measuring instrumentcontained within a torpedo which travels in grooved or square casing

191

to maintain horizontal alignment. The most successful type of inclino-meter uses a Wheatstone bridge to measure the angle of tilt, whileothers use photographic methods, vibrating wire or resistance straingauges* Inclinometers based on servo-accelerometers are now beingused for high precision work.

Where the inclinometer casing is installed in soft material,or where the backfill is less rigid than the surrounding soil orrock, deformation may only occur at the joints in the casing, givingan incorrect deformation profile.

10.18 Slip indicators

The position of a slip surface can only be obtained usingquite simple instruments. One of the most commonly used slip surfaceindicators comprises two identical, relatively short lengths of rodwithin a flexible tube installed in a borehole, one length of rodbeing kept at the bottom of the hole and one at the top. The zone inwhich shear is occurring is found by raising the rod from the bottomof the casing and lowering the other from the top until the rodscannot pass through the casing. The same instrument can also become acrude inclinometer by using a series of rods of various lengths andassessing the curvature of the tube by finding the length of rod thatcan just pass down the casing.

Among other methods which are available is the shear stripwhich comprise a series of parallel resistances. The depth at whichthe shear strip fractures is determined by measurement of the resistance,A second type comprises a bubbler tube device connected to a thinglass tube within a larger diameter metal pipe. The air pressurerequired to continually pass bubbles into the ground decreases whenshearing breaks the glass tube. The Japanese pipe strain gauge hasfoil strain gauges mounted in a PVC pipe, and deformation or shear canbe monitored by recording the change of strain in these gauges.

10.19 Extensometers

An extensometer comprises tensioned wires or rods anchoredat different points in the borehole, and measures the movement of theanchor points, relative to a surface datum, along the line of thehole in which it is installed, Extensometers are most suited tomeasuring deformation of and behind retaining structures and in soilsand rock stressed by anchoring or affected by excavation.

Relative movements between the face and the end of eachextensometer rod may be measured with a dial gauge, or with a lineardisplacement transducer for remote reading. The rod gauge (fig. 10.6)can be used at any angle.

192

Measuring head Steel rod Grout

Figure 10.6 Rod extensometer

The wire of the more simple tensioned wire extensometer(fig, 10.7) is taken over pulleys and a constant tension is maintainedwith weights. Alternatively, the tension may be maintained by springcantilevers in the measuring head and the deflections of the cantile-vers measured by dial gauges or transducers.

Steel plate

Anchorage

/Telescopic tubing

Weights

Figure 10,7 Tensioned wire extensometer

193

In confined spaces, the protection of the tensioning systemof wire extensometers is difficult. Rod extensometers with removablemeasuring heads are less subject to damage. Readings can be affectedby thermal expansion of the metal and, for precise measurements,temperature corrections should be made.

10.20 Settlement gauges

Settlement gauges, for installation in either fill duringconstruction or boreholes, remote reading hydraulic gauges andprofile gauges are all described in Hanna (1973).

10.21 Load cells for rockbolts and anchors

Only load cells suitable for long term monitoring ofrockbolt and anchor loads are considered in this section.

The anchor load can either be determined at intervals, bymeasuring the force required to jack the anchor head away from itsseating, or it may be monitored continuously with a compression loadcell between the anchor head and bearing plate. The types of loadcell available and their advantages and disadvantages are given intable 10.3.

10«22 Earth pressure cells

Measurement of earth pressure is unlikely to be required inHong Kong residual soil slopes. In large excavations, cells may bespecified occasionally to measure contact pressure between the soiland a retaining structure. The type and position of a cell should bechosen with great care, because the introduction of the cell into thesoil causes a redistribution of the stresses around it, and theerrors depend on the geometry of the instrument. Details of thetypes of cell available and the problems which may be encounteredwhen using them, are given in Hanna (1973).

10,23 Remote reading and automatic recording

The simpler and more reliable instruments are usually thosewhich, to be read, require access. This is often dangerous and timeconsuming, and is sometimes impossible. In bad weather reading tendsto be postponed or missed and it is often on these occasions that thereadings are most critical.

A remote reading system can be developed for most instruments.Hydraulic and vibrating wire systems can be used over long distances(500 m if necessary), but head losses in pneumatic systems begin toget significant at distances over about 100 m. Transmission distancesfor instruments which rely on a resistance change must be strictlylimited; 10 m has been known to have a significant effect.

194

Type

Disc loadcell

Hydraulic

Photoelastic

Electrical:vibratingwire type

Electrical:resistancetype

Long Term Reliability

Good

Goodexcept pressuregauges subject tocorrosion

Good

Moderate(up to 10 years)

Poor

Remote Reading

Not with dialgauge, possiblewith lineardisplacementtransducer

$0, accessrequired tomeasure movement

No

Yes

No(a few metresis possible)

Other

Advantages Disadvantages

Simple tooperate.Dial gauge canbe removed forprotection

Simplehydraulicsystem

Cheap ,simple to installeasily replaced

Simple,easy to read

Calibration non-linear. Loading andunloading curvesdiffer because ofhysteresis

Not continuousreading. Jack mustbe pressurised untilanchor head beginsto move. Difficultto just lift head,large movementsoften occur.

Reading requirespractice.Limited to low loads.

Fairly expensive.Calibration maydrift butrecalibrationpossible.

As above.Stain gauges verysusceptible tocorrosion.

Recommendation

Most suitable for long termmonitoring unless remotereading is required

Not very suitable for longterm regular monitoring.Removable jack suitable foroccasional testing.

Suitable for rock bolts andsmall anchors.

Most suitable where remotereading required.

Not suitable for long term use.

VDUi

Table 10.3 Load Cells

When the distance between the instrument and a safe accessiblereading point is very great, or if frequent readings are required, theuse of a data logger should be considered* A bubbler system(section 10.6) can be used for automatic recording in open hydraulic(Casagrande) piezometers.

Land line links to instruments are quite common for watersupply and have occasionally been used for geotechnical instruments(Sherwood and Currey, 197*0» A computer can be used to initiate areading scan and to record and process the date. Telemetric instrumentsare beginning to be used in soil mechanics (Prange, 1971) and maybecome useful in overcoming signal transmission difficulties*

REFERENCES

Burland J.B., Longworth T.I* and Moore J.F.A. (1977)- A study ofground movement and progressive failure caused by a deepexcavation in Oxford Clay. Geotechnique , Vol. 27*PP 557-592.

Burland J.B. and Moore J.F.A. (197 )» The measurement of grounddisplacement around deep excavation. Field Instrumentationin Geotechnical Engineering, Butterworths, London, pp 52-69»

Cheney J.E. (197 )- Techniques & equipment using the surveyor's levelfor accurate measurement of building movement. FieldInstrumentation in Geotechnical Engineering, Butterworths,London, pp 85-99 .

Duvall W.I. (1965). Design requirements for instrumentation to recordvibrations produced by blasting. U.S. Bureau of Mines Reportof Investigations 648?.

Hanna T.H. (1973)- Foundation Instrumentation. Trans Tech Publications,Clausthal, 372 pp.

Hoek E. and Bray J.W. (1977). Rock Slope Engineering. The Institutionof Mining and Metallurgy, London, 309 pp«

Moore J.F.A. (197*0. The photogrammetric measurement of constructionaldisplacements of a rockfill dam. Building Research 'StationCurrent Paper No

Moore J.F.A. (197*0* Mapping major joints in the Lower Oxford Clayusing terrestrial photogrammetry. Quarterly Journal ofEngineering Geology, Vol. 75t PP 57-5T

Penman A.D.M. (1972). Instrumentation for embankment dams subjected torapid drawdown. Building Research Station Current PaperCP 1/72, 21 pp.

196

Prange B. (197D. The state of telemetry in soil mechanics. Proceedingsof the Roscoe Memorial Symposium, Foulis, Henley on Thames ,pp *f 76-483.

Richards E.G. (197*0. Behaviour of unsaturated soils. Soil Mechanics -New Horizons , Ed* I.K. Lee, Newnes - Butterworths, London,pp 112-157-

Ross-Brown D.M, and Atkinson K.B. (1972). Terrestrial photogramraetry inopen-pits: I - description and use of the phototheodolitein mine surveying. Transactions of the Institution of Mining8c Metallurgy, Vol. 81 , pp A205-A213.

Ross-Brown P.M., Wickens E.H., and Markland J.T. (1973). Terrestrialphotogrammetry in open -pits: II - an aid to geological mapping.Transactions of the Institution of Mining & Metallurgy, Vol. 82,pp A115-A130.

Sherwood D.E. , Currey B. ( 197*0 • Experience in using electrical tiltmeters. Field Instrumentation in Geo technical Engineer ing ,Butterworths, London , pp 396-410.

U.S. Bureau of Reclamation (197 ) * Earth manual. U.S. GovernmentPrinting Office, Washington.

Vaughan P.R. (1969). A note on sealing piezometers in boreholes.Geotechnique, Vol. 19* PP

Vaughan P.R. (197*0. The measurement of pore pressures with piezometers.Field Instrumentation in Geotechnical Engineering, Butterworths,London, pp

197

Chapter 11

MAINTENANCE

11.1 Introduction

Regular inspections and maintenance are essential for thecontinued stability of well designed and well constructed slopes.While those responisble for design may not be able to do anythingafter completion of construction to ensure that recommendations formaintenance are followed, careful design and detailing can reduceboth the amount of maintenance required and the physical labourinvolved.

When handing a development over, the designer shouldrecommend and provide details of a regular inspection and maintenanceprogramme. Such a programme should include guidance on maintainingand reading installed instruments and should indicate the probablerange of readings which will be obtained if the structure is functioningas designed. If, during the lifetime of the development, the readingsobtained from instruments indicate conditions more severe than thoseallowed for in the design, the owner should be advised to obtainspecialist advice. Instrument readings should be kept on standardrecord sheets, an example of which is shown in figure 11.1.

11.2 Routine inspections

It is important that only suitably qualified staff shouldcarry out routine inspections. Normally these may be entrusted to anengineering technician who has had experience of constructing thevarious features to be examined but where the slopes have a history offailure and the consequences of further failure are severe or wherethe instrumentation is complex, the inspections should be carried outby a geotechnical engineer or an engineering geologist. If an inspectioncarried out by a technician reveals potential instability of a slopeor structure, the feature causing concern should be further examinedby either an experienced engineer or engineering geologist, dependingupon the form the potential instability takes. The findings ofroutine inspections should be used to plan maintenance operations toensure that no features of the development essential for the continuedstability of slopes or structures are overlooked.

Reports of routine inspections, including recommendationsfor remedial and preventive works, should be kept on standard sheetswhich, when filed, will become a record of the performance of thestructure or slope during its lifetime. The standard record sheetsshould be sufficiently detailed for those features of the design whichrequire recurrent and extensive maintenance to be identified andimproved. Records should include detailed descriptions of signs ofpossible distress and should quantify observations of these signs,such as the width of a crack in a retaining wall. The inclusion ofcolour photographs on record sheets will assist an engineer or architect

199

Piezometer Readings

Development at :

13 Blank Street

Piezometer No. P 3

Tip level : Ground level :2 5 4 - 8 m P.O. 2 7 0 . 3 6 m P.O.

Date

26 / 8 / 7 6

2 8 / 8 7 76

Depth from topof standpipetowater surfacefrri

5.36

0.95

Weather

Heavy rain

Heavy rain

Critical waterlevel Depth of critical W.L.below top of standpipe

2 6 9 - 5 5 m P.O. 0.75 m

Level of top of standpipe (m)270.30

Depth of Tip below top of stand pipe.(m!1 5 . 5 0

Comments

Cover box clear of water -drain free.

Cover box flooded - drainblocked. Cleared beforeremoving cap.

Recorded by

C L M

C L M

Figure 11.1 Piezometer record sheet

200

to determine the time over which the structure has shown signs ofdistress and to decide what has caused that distress. -Routine in-spections should include an examination of the records of readingsobtained from installed instruments, if any, and the person carryingout the routine inspection should sign the record sheet stating thatthese readings have been examined.

When handing a development over the designer should supply,along with the programme described in 11.1, sample record sheets whichthe owner should be advised to use for recording all inspections andinstrument readings. An example of a suitable record sheet is givenin figure 11,2.

11.3 Instruments

The instruments which are most likely to be installed andused for continued monitoring are piezometers (chapter 10), Theaccuracy and serviceability of these instruments relies, among otherthings, upon preventing the ingress of water and foreign matter intothe standpipe. The surface boxes for the instruments should beexamined during each inspection. All standpipes should have tightfitting caps and all surface boxes should be drained. If necessary,caps should be renewed and drains cleared.

Other instrument installations should be checked to ensurethat they are functioning under the conditions specified by themanufacturers«

11.4 Slopes and slope surfacing

All slopes should be examined for signs of movement in-dicative of slope failure. However, in other aspects the inspectionand subsequent maintenance required for a colluvium, fill or decomposedrock slope will differ from that required for a rock slope. As soilslopes rarely show signs of progressive failure, slips occur veryquickly when slopes become saturated, inspection and subsequentmaintenance should be principally directed towards preventing theinfiltration of water. Forms of surface protection, other than grass,are generally brittle and therefore are susceptible to cracking.Inspection records should give details of crack positions, lengths,widths and relative movement. Telltales of one of the types describedin chapter 10 should be installed on new cracks. During inspectionsof grass covered slopes, the positions, depth and extent of erosionscars should be noted.

Rock slopes can show signs of progressive failure bymovement along joints. Where joints appear to be opening, telltalesshould be installed to monitor progressive movement. Closely jointedrock is likely to deteriorate generally and not show signs of movementalong any one joint or series of joints. Colour photographs of suchfaces taken during each inspection will assist in assessing the extentto which the slope has deteriorated. Erosion around isolated blocksor boulders should be recorded.

201

Record of inspection of slope

Page : 1

Location of slope

Behind No. 74 Blank Street

Dates

Of last inspection1 5 / 4 / 7 7

Of this inspection2 3 / 7 / 7 7

Carried out by (name)

Shum W . B .

Position

Engineer

Weather condition

Dry, sunny

Have works recommendedpreviously been carried out?

Sonne

Observations

( i ) Instrumentation

( T i c k items observed )

Installation

Checked v-Record of readings

examined ^All readings within

design range

YesX^

/^H*

^ Delete as appropriate.

Comments >

Drains in piezometer boxes P4 and P6 must be cleared and standpipe caps must

be replaced on piezometers P3 and P5.

(ii) Slope and slope protection( Tick items observed)

Erosion

^

Slumpingof slope

Cracks attop of slope

Movementon rock joints

Cracks inslope

protection

^

height of $

Spading ofsurface

Weepholesblocked

^

Unplannedvegetation

^

slope. 30m

Condition of plannedvegetation

Good Poor

Comments :-

Chunam above 270 m berm cracked 25m from west end. Water seepage showing

at crack and vegetation developing along crack. Chunam 5m from west end of

2 77*5 m berm has been undermined and sounds hollow when rung with a hammer.

Some weepholes blocked by vegetation, Condition otherwise good.

202

Inspection of slope

Page : 2 Location. .7A. .Blank Street Date. .2.3/7/77.

(iii) Retaining walls

(Tick items observed)

height of retaining wall

Signs of movement

Cracking Tilting Settlement

Weephoies blocked General deterioration

Concretespalling

Pointingmissing

Cracking

Comments >

(iv) Drainage system


Cracking

^

Subsidence Silting

^

Blockage

^

Generaldeterioration

^

Seepage notpreviously noted

Comments >Invert of step channel down centre of face deteriorating. Catch pits on 285 m berm

full of grit and channel partially blocked.

(v) Services


Water mains leaking

u-*.

Seepage from slopeapparently sewage

Other which could affectstability of slope

Service trench excavatedwhich could affect slope

Comments :-Water main 10m from crest of slope slows signs of leakage from valve box 20m

east of property boundary.Water Authority informed at 3-00 p.m. 23/7/77.

Figure 11.2 (continued)203

Inspection of slope

Page : 3 Location 74. Blank..Street Date. 23/7/7.7

(vi) Access

Access gate- hinges broken and no padlock.

(vii) General

No significant deterioration since last inspection.

(viii) Recommendations

(a) Chunam should be repaired where cracked or undermined.

(b) Vegetation should be removed from cracks and weepholes.

(c) Step channel down centre of face requires reconstruction.

(d) Silt and sand must be cleared from catchpits and channels.

(e) Gate should be repaired.

( f ) Check that Water Authority have repaired leaking main.

Signature ?.W.B

Figure 11,2 (continued)204

During inspections, seepage traces on and adjacent to allformed slopes should be recorded. Flow from seepage sources, weepholesand horizontal drains should be recorded and, where possible, should beexamined for signs of the migration of solid material indicatinginternal erosion. For the effects of cracked services on flow fromthese sources see section 11,6.

The development of vegetation on slopes, if allowed for inthe design of the surfacing, can be beneficial both aestheticallyand structurally. Surfacing in the vicinity of trees should beexamined for signs of deterioration as a result of root action. Thedevelopment of vegetation which is not planned can be detrimental toslope surfacing in the case of soil slopes, and to basic stabilityin the case or rock slopes (Beattie and Lam 1977).

Routine maintenance of all slopes should include the removalof undesirable vegetation. Cracked rigid surfaces should be repairedby cutting a chase along the line of the crack and filling it withmaterial of a suitable mix. On inclined surfaces a good method ofrepair is as shown in figure 11.3, This prevents ingress of waterthrough any shrinkage cracks which may develop between the originalsurface and the repair.

Original surfacingundercut

Cut slope

Repair to originalsurfacing

Patch overlyingoriginal surfacing

Figure 11.3 Repair of cracked chunam surfacing

205

Cracks in surfaces comprising masonry blocks set in andpointed with cement mortar will tend to follow joints between blocks.The affected joints should be cleaned and repointed.

Rigid slope surfaces which have been undermined by groundwaterflow should be removed, the source of the flow identified, the flowstopped or taken to the surface by means of horizontal drains, and thesurface made good.

Eroded grass slopes should be regraded, if necessary withfill compacted to a density not less than that required in chapter 5of this manual. The fill should be compacted in horizontal layers andnot in layers parallel to the slope. If necessary the eroded areashould be benched and graded so that fill is not placed againstextensive vertical surfaces. This treatment will be required if thevertical eroded surfaces against which fill is to be placed are greaterthan 600 mm high. Before placing fill all concentrations of freedraining material formed in the eroded area should be removed. Therepaired surface should be protected with turf staked down with bamboopegs or held in place with staked polypropylene net.

Rock slopes are not usually completely surfaced. They mayhowever require local surfacing to prevent water from entering openjoints. Care must be taken to ensure that while the surfacing orcapping prevents ingress of water it does not prevent seepage. Such"dental work" should include the provision of weepholes where necessary.

Where there are traces of seepage from a slope in areas whereweepholes are not provided, the source of the seepage should beinvestigated and adequate drainage provided.

11.5 Surface drainage

Inspections should be carried out regularly and the frequencyof inspection should be increased during and immediately before theonset of the wet season. Records should be kept of those features ofthe drainage system which require modification to reduce routinemaintenance. Inspection records should include (a) the position andextent of broken and cracked channels and catchpits (b) the positionand extent of silted up sections of channels and catchpits (c) theposition and extent of deteriorating channels and catchpits (d)details of construction works, possibly outside the boundaries ofthe development, from which mud and debris could migrate to blockdrains of the system being inspected. All channels should be inspectedfollowing the issue of heavy rainfall warnings by the Royal Observatory.

If it appears possible that a drainage system can be blockedby soil washed from works on an adjacent site, preventive action shouldbe taken.

Material removed from channels and catchpits should bedisposed of where it cannot be washed back into the channels duringsubsequent storms* Catchpits to intercept grit can only functionproperly if they are cleared regularly*

206

Cracks in channels should be repaired with cement mortaror with a suitable plastic sealing compound. If channels crack fromsettlement, the settled section should be removed and reconstructedin such a way that it is not susceptible to damage from furthersettlement. A mortar repair to a channel which is settling is onlya temporary measure. Where channels are found to be settling plasticsealers can be used to repair cracks, but major repair works shouldnot be carried out during the wet season. Most plastic sealers,whether bitumen or epoxy based, require the material on which theyare to be placed to be dry. The efficiency of the sealer will beseverely limited, unless the manufacturers1 instructions are followed.

Where sections of channels have to be rebuilt, this workshould only be done in the dry season when the existing channelsmay be safely removed. All reconstructed sections of channels shouldbe built in accordance with the recommendations contained in chapter 8.The reconstruction of channels to an increased capacity may requirereconstruction of all channels downstream of the repair.

Pipes should not generally be used for surface drainage onslopes. Where pipes have been used, and are found to leak or have tobe replaced, they should, if possible, be replaced with channelsconstructed as described in chapter 8.

11.6 Subsurface drainage

The efficiency of horizontal subsoil drains will, in general,decrease with time. When interpreting the records of flow fromhorizontal drains it should be remembered that any initially highdischarges should decline as the drains lower the groundwater regimeand a steady state is achieved. It is therefore important to recordflows from individual drains during each inspection, and to correlatethese with rainfall records for that area and the readings of thepiezometers which should be installed as an integral part of anysubsurface drainage system. Where increased flows are recorded thedischarge should be examined for any signs that the water originatesfrom leaking services. If these exist the appropriate utility shouldbe notified and requested to trace and repair the source of leakage.The discharge from drains should be examined for signs of solidmigrating material.

If piezometer readings indicate a rise in groundwater level,and, at the same time the discharge from the horizontal drains decreases,it should be concluded that the efficiency of the drainage system isdecreasing. It may be possible to partially reinstate the system byflushing the drains with a suitably designed compressed air and waterjet. In general, horizontal drains should not be pressurised with waterin an attempt to clear the filter material surrounding the drains as theconsequent infiltration of water into the slope may cause slope failure.If flushing of the drains fails to raise the effectiveness of the systemto an acceptably level, additional drains should be installed. Routinemaintenance of a horizontal drainage system should include the removal ofobstructions at the outlets.

207

The safety precautions recommended in a Practical SafetyGuide to Working in Confined Spaces (Government of Hong Kong) shouldbe observed during the inspection of drainage adits which should beexamined for signs of structural distress* The rates and locationsof inflow should also be recorded and this should be compared withthe total discharge* When increases in flow occur which are not adirect result of rainfall, the discharge from the adit should bechecked for signs which indicate leakage from sewers and watermains. A comparison of previous records of the locations and ratesof inflow may indicate the probable location of the leakage givingrise to the increased flow. Radial drain holes installed within theadit should be inspected in the same way as horizontal drains. Flowmeasuring devices within the adit should be checked to ensure thatthey are functioning properly and the measuring edge of V-notchesshould be examined for, and subsequently cleaned of, adhering debrisof slime. Stilling pools behind V-notches should be cleared of settledsand and silt.

Should the adit show signs of distress, expert adviceshould be sought. No remedial measures should be carried out untilthis advice has been obtained.

11.7 Services

Stormwater drains, sewers and water mains are the servicesmost likely to affect slope stability, but other conduits such astelephone ducts, electric cable ducts and disused pipes can alsotransmit water into slopes and reduce their stability. Duringroutine inspections all services should be examined for signs ofleakage or water flow. Where this is detected, the service ductshould be treated as described in chapter 5. The inspection recordsshould include a drawing showing the position and nature of all servicesin the vicinity of the slope. The appropriate utility should be requestedto test water mains and sewers where leakage could lead to instabilityof the slopes being examined.

11.8 Access

Access should be provided to berms, channels and adits topermit inspection and maintenance* Lockable gates should be providedto prevent unauthorised entry and vandalism and the access should bekept clear of rubbish and debris.

REFERENCES

Seattle A.A. and Lam C.-L. (1977). Rock slope failures - their prediction& prevention. Hong KOBE Engineer. Vol. 5 No* 7f pp 27-*fQ»Discussion Vol. ? No. 9t pp 27-29.

Government of Hong Kong, Practical Safety Guide to Working in ConfinedSpaces» The Industrial Safety Training Centre, LabourDepartment, Hong Kong.

208

Chapter 12

SOURCES OF INFORMATION

12.1 Introduction

Throughout this manual reference has been made to manyother texts. The lists of references given are not exhaustiveand many of these references will, in time, be superseded asresearch and careful recording improve our understanding of slopestability and the behaviour foundations and retaining walls*

General local information is available from a number ofsources, mainly governmental. In this chapter these sources aredescribed and details are given of the information which they cansupply.

This chapter concludes with a list of technical papersreferring specifically to Hong Kong.

12.2 Technical Information Retrieval

Some of the more important information retrieval systems are:

(i) Asian Geotechnical Engineering Digest : Publishedby the Asian Institute of Technology this digest isvaluable because it lists papers published in Asia,which may not be listed elsewhere, and papers publishedin many of the larger journals* Copies of papers listedin the digest may be obtained from the Asian InformationCentre for Geotechnical Engineering whose address isgiven in section 12.6, The Centre also providesbibliographic and reference services for members ofAGE.

(ii) Geodex Retrieval System : This is a commerciallypublished punched card system designed to be usedwith the Geotechnical Abstracts which are producedby the German National Society of the ISSMFE. Thereare 347 cards each representing a keyword and bysuperimposing different cards, compounds of severalkeywords can be used. This saves considerable timein literature reviews. The system has been endorsedby the International Society for Soil Mechanics andFoundation Engineering and is available from GeodexInternational Inc.

(Hi). American Society of Civil Engineers Transactions:Annually the American Society of Civil Engineerspublish transactions which contain abstracts of allpapers and articles published by the Society.

209

12*3 Locally Published Technical Papers

A full set of technical papers published by the Hong KongSociety of Engineers, and subsequently by the Hong Kong Institution ofEngineers, is kept in the Institution Office. While these publicationsare not available for loan the Institution will supply copies ofparticular papers.

A number of conferences have been held at Hong KongUniversity and their proceedings contain interesting papers. TheUniversity Library operates an inter-library loan service and willalso provide copies of articles to graduates and students.

12.4 Sources of Information in Hong Kong Government

Government Departments retain record drawings of projectsdesigned and constructed on their behalf. These records, which ofteninclude details of site investigations, may be viewed by arrangementwith the appropriate department.

Records of private development are retained by the BuildingsOrdinance Office for about seven years, after which the files aretransferred to the Public Records Office. The information containedin these files cannot be copied, except by hand, and is not lentout. Permission to view records held by the BOO is obtained fromthe Secretary of the BOO, who will require the address of theproperty and the lot number to locate the appropriate files.

Maps, aerial photographs and plans of lot boundaries areheld by the Crown Lands and Survey Office and these may be viewed atthat office by appointment. Copies of maps and aerial photographsmay also be purchased from the office*

Government publications are available from the GovernmentPublications Office; those which are out of print may be viewed atone of the libraries, at the appropriate Government department orby appointment with the Government Printer.

Rainfall records are published monthly and annually by theRoyal Observatory, The Royal Observatory has published a seriesof papers on the statistical analysis of climatic and seismologicalconditions in Hong Kong. These may be obtained from the Observatoryor from the Government Publications Office.

12.5 General Local Information

The archives in City Hall contain many documents which givea valuable record of the history of development in Hong Kong. Oldnewspapers are kept on microfilm and these can be viewed by arrange-ment with the librarian. A series of maps, the oldest of which waspublished in 1845, may be viewed at the Public Records Office.

A useful ready reference on the history of development inHong Kong is given by Endacott (1973).

210

12.6 List of Useful Addresses of Libraries and Learned Bodies

Asian Information Centre for Geotechnical Engineering,Asian Institute of Technology,PO Box 2754,Bangkok,Thailand,

American Society of Civil Engineers,General Office,345 East 47th Street,New York,NY 10017,U S A

Institution of Civil Engineers Library,5 Great George Street,Westminster,London SW1P 3AA,U K

British Geotechnical Society,c/o Institution of Civil Engineers,5 Great George Street,Westminster,London SW1P 3AAU K

The British Library,Lending Division,Boston Spa,Wetherby,West Yorkshire,LS23 7BO,U K

Danish Geotechnical Institute1 Maglebjergvej,DK-2800 LyngbyDenmark.

GeodexGeodex International Inc.P 0 Box 279SonomaCalifornia 95476U S A

211

Geological Society of London,Burlington House,London WIV OJU,U.K.

Geotechnical Abstracts,Deutsche Gesselschaft Erd Und Grundbau,Essen,Germany (F.R.)

Hong Kong Institution of Engineers,1513 Hang Lung Centre,Paterson Street,Causeway Bay,Hong Kong. Tel.: 5-779 6?

South East Asian Society of Soil Engineering,c/o Asian Institute of Technology,P 0 Box 275 »Bangkok,Thailand.

12.7 References Specific to Hong Kong

Allen P.M. and Stephens E.A. (197D* Report on the Geological Surveyof Hong Kong* Government Press, Hong Kong, 107 pp*

Beattie A.A. and Lam C.L. (1977)* Eock slope failures - theirprediction and prevention• Hong KonR Engineer, Vol. 5No. 7i PP 27-40. Discussion, Vol. 5 No. 9t PP 27-29.

Bell C.J. and Chiu P.C. (1968). The probable maximum rainfall inHong Kong. Royal Observatory» Hong Kong, Technical MemoirNo. 10,Hf5 pp.

Berry L. (1961). Erosion surfaces and emerged beaches in Hong Kong.Geological Societyof America Bulletin, Vol. 72, pp 1383-139 -

Berry L. and Euxton B.P. (1960). The evolution of Hong Kong Harbourbasin. Annals of Geomorphology Vol. k No. 2, pp 97-115*

Brenner R.P., and Phillipson H.B. (1979)* Sampling of.residual soilsin Hong Kong. International Symposium on Soil Sampling,Singapore, pp 109-120.

Brock R.W. (19 3)* Weathering of igneous rocks near Hong Kong.Geological Society of America Bulletint No. 5 1 PP 7*17-738*

Carlyle W.J. (1965)* Shek Pik Dam. Proceedings of the Institutionof Civil Engineers, Vol. 30, pp 5 5 7 - 5 8 8 . '

212

Cheng S. and Kwok ¥.H. (1966). A statistical study of heavy rainfall inHong Kong 19 7-65. Royal Observatory, Hong Kong.

Dunnicliff J.D. (1968). Instrumentation of the Plover Cove main dam.Geotechnique« Vol. 18, pp 283-300.

Fanshawe H. (1962). Soils in the Shek Pik Valley. Symposium on HongKong Soils, Hong Kong, pp 53-56.

Government of Hong Kong (1972). Interim Report of the Commission ofInquiry into the Rainstorm Disasters 1972. Government Printer,Hong Kong, 20 pp.

Government of Hong Kong (1972). Final Report of the Commission ofInquiry into the Rainstorm Disasters 1972. Government PrinterfHong Kong, 91 pp.

Grace H. and Henry J.K.M. (19 8). The incorporation of decomposedgranite in the design and construction of pavements in HongKong. Proceedings of the 2nd International Conference onSoil Mechanics and Foundation Engineering, Rotherdam, Vol. IV,pp 190-196.

Grace H. and Henry J.K.M. (1957)* The planning and design of the newHong Kong airport. Proceedings of the Institution of CivilEngineers, Vol» 7i pp 275-325. (and discussion)

Guilford C.M. and Chan H.C. (1969) . Some soils aspects of the PloverCove marine dam. Proceedings of the 7th InternationalConference on Soil Mechanics and Foundation Engineering,Mexico. Vol. 2 pp 291-299.

Haswell C.K. and Umney A.R. (1978). Trial tunnels for the Hong KongTransit Railway. Hong Kong Engineer* Vol. 6 No. 2, pp 15-23.Discussion, Vol. 6 No. 3i pp 5- 6.

Henry J.K.M. and Grace H. (19 8). The investigation of decomposedgranite in Hong Kong for use as a stabilised base coursematerial. Proceedings of the 2nd International Conference onSoil Mechanics and Foundation Engineering Rotherdam, Vol. Ill,pp 187-192.

Ho C. (1965)* Lower Shing Mun Main Dam. Proceeding of the EngineeringSociety of Hong Kong, Vol. 19t PP 1.1-1*9

Holt J.K. (1962). The soils of Hong Kongfs coastal waters. Symposiumon Hong Kong Soils, pp 33-51.

Holt J.K. (1968). The permeability of decomposed rock fill depositedbelow water in the Plover Cove Main Dam. Proceedings of the1st SoutheasVAsian Regional Conference on Soil Engineering,Bangkok, pp 169-178.

213

Lam K.C. (197*0. Some aspects of fluvial erosion in three smallcatchments, New Territories, Hong Kong* M. Phil. Thesis.

Lamb D.W. (1962). Decomposed granite as fill material with particularreference to earth dam construction. Symposium on Hong KongSoilsf Hong Kong, pp 57-72.

Lau E. (1972). Seismicity of Hong Kong. Royal Observatoryt Hong Kong,Technical Note No, 33§ Government Printer, Hong Kong.

Leonard M.W. and Nixon I.K. (1963)- The influence of ground conditionson the choice of foundations for high buildings in Hong Kong.Symposium on The Design of High Buildings, Hong Kong, pp 70-84.

Liao G.C. (1962). Soil exploration and pile driving in the seabed ofVictoria Harbour, Hong Kong, Symposium on Hong Kong Soils,pp 89-108.

Lumb P. (1962). General nature of the soils of Hong Kongf Symposiumon Hong Kong Soils, pp 19-32.

Lumb P. (1962). Effect of rainstorms on slope stability. Symposiumon Hong Kong Soils, pp 73-78.

Lumb P. (196*0. Report on the Settlement of Buildings in the Mong KokDistrict of Kowloon, Hong Kong. Hong Kong Government Press,18 pp.

Lumb P. (1965)* The residual soils of Hong Kong. Geotechnique. Vol. 15tpp 180-19**.

Lumb P. (1972). Building settlements in Hong Kong. Proceedings of the3rd Southeast Asian Conference on Soil Engineering, pp 115-121*

Lumb P. (1975)» Slope failures in Hong Kong* The Quarterly Journalof Engineering Geology, Vol. 8, pp 31-65*

Lumb P. (1976). Land reclamation in Hong Kong. Workshop on Materialsand Methods for Low Cost Road, Rail and Reclamation Work,Leura, Australia, pp 299-314.

Lumb P. (1977). The marine soils of Hong Kong and Macau Proceedingsof the International Symposium on Soft Clay, Bangkok, pp 45-58.

Lumb P. (1979). Statistics of natural disasters in Hong Kong 1884-1976.Proceedings of the International Conference on the Applicationsof Statistics & Probability to Soil & Structural Engineering,Sydney, Vol. 1f pp 9-22.

Lumb P. and Holt J.K. (1968). The undrained shear strength of a softmarine clay from Hong Kong. Geotechnique, Vol. 18, pp 25-36.

Lumb P and Lee C.F. (1975)* Clay mineralogy of the Hong Kong soils.Proceedings of the 4th Southeast Asian Conference on SoilEngineering! Kuala Lumpur, pp 1.41-1.50.

214

Mackey S. (1961). Factors affecting pile loading capacity in Hong Kong.Proceeding Syposium on the Design of High Building, Hong Kong,pp 51-69*

Milburn R. (1960). The Kowloon Foothills road investigation design.Proceedings of the Engineering Society of Hong Kong, Vol. 14,PP 3*1-3*2/4

O'Rorke G.B. (1972). A cutting failure in Hong Kong granite. Proceedingsof the 3rd Southeast Asian Conference on Soil Engineering, Hong;Kong, pp 161~169»

Parham W.E. (1969)* Halloysite rich weathering products of Hong Kong.Proceedings of the International Conference on Clays, Tokyo,Vol. 1, pp 403-416.

Payne J.C. Walker, D.W. and Armstrong-Wright A.T. (1962). The Lion RockTunnel. Proceedings of the Engineering Society of Hong Kong,Vol. 10, pp 6.1-6.33t pp 66.1-66.34

Phillips K.A. (1972). Some aspects of the Cross Harbour Tunnel of Hong Kong.Proceeding of the 3rd Southeast Asian Conference on Soil Engineeringtpp 123-12&.

Ruxton B.P. (1960). The geology of Hong Kong. Quarterly Journal of theGeological Society, Vol. 115» pp 2 3 3 - 2 6 0 1 ~

Ruxton B.P. and Berry L. (1957)* Weathering of granite and associatederosional features in Hong Kong. Geological Society of AmericaBulletin, Vol. 68, pp 1263-1292.

Slinn M.A., Greig G.L. and Butler D.R. (1976). The design and someconstructional aspects of Tuen Mun Road. Hong Kong Engineer,Vol. 4 No. 2, pp 37-50. Discussion, Vol. 4 No. 3, PP 69-72.

So C.L. (1971). Mass movements associated with the rainstorm of June 1966in Hong Kong. The Institute of British Geographers, Transactions,No. 531 PP 55-65*

Tin Y.K. (1969). Stormwater Drainage Design in Hong Kong. Sewage & DrainageAdvisory Unit. Technical Report No* 6, Public Works Department.

Tomlinson M.J. and Holt J.B. (1953)* The foundation of the Bank of ChinaBuilding, Hong Kong. Proceeding of the 3rd International Conferenceon Soil Mechanics and Foundation Engineering, Switzerland, Vol. 1,pp 466-472.

Tong P.Y.L. and Maher R.O. (1975). Horizontal drains as a stabilizing measure.Journal of the Engineering Society of Hong Kong, Vol. 3 No. 1,pp 15-27*

Tregear T.R. and Berry L. (1958). The development of Hong Kong. South ChinaMorning Post, Hong Kong, 25 PP*

Twiat D.W.L* and Tonge WJU (1979)* Planning and design of the AberdeenTtannel. Hong Kong Engineer, Vol. 7 No. 3$ PP 13-30.

215

Vail A*J. and Attewill L.J.S. (1976). The remedial works at Po ShanRoad. Hong Kong Engineer, Vol. 4 No. 1f pp 19-27.

Wong H.Y. (1978). Soil strength parameter determination. Hong KongEngineer, Vol. 6 No. 3» PP 33-39* Discussion, Vol. 6 No. 6fPP 37-53* Vol. 6 No. 7, pp 31**

Wong K.K. (1970). Pore water suction in Hong Kong by psychrometrictechnique. M.Sc. Thesis. University of Hong Kong, 177 pp.

12.8 Historical References

Endacott G.B. (1973)$ A History of Hong Kong Oxford University Press,323 pp.

216

INDEX

Access on slopes 11,8Acidity of soil 3,16Active pressure 7,2Addresses of libraries and learned bodies 12,6Aerial photographs 2,6Alluvium 1.2American Society of Civil Engineers1 transactions 12,2Asian Geotechnical Engineering digests 12,2

1 " *

Backfill - to investigation holes 2,14; 2,16- to retaining walls 7,3

Base friction 7.6Bearing capacity - from static cone tests 2.23; 6,1

- on slopes 6.2- under retaining walls 7,7

Berms 5.10Blasting 5.12; 9.5Borehole logs 2.19; 2.30; 2.34Boring 2.14Boulders 5.15; 9.4Bransby Williams equation 8.4Bulk modulus 3.12Buttress 5.12

Calcite 3.4Camkometer 2.24Catchment - area 8,3

- composite 8,6Channels - changes of direction 8.11

- construction of 9.17- design of 8.10- detailing of 8.13- freeboard 8.11; 8.12- junctions of 8.12- stepped 8.10- types of 8.9

Chunam 9.11Classification tests 3.3Clay 2,19

- filter for 4,19

217

Colluvial fan 2.6Colluvium 1.2Compaction 3*13

- control of 9.9- Hilf method 9.10

Compaction load on retaining wall 7.5Compressibility tests 3.12Conduits - design of on slopes 9.19

- leakage from 9.19Consolidation 3.12Constant head tests 2.31Construction control - field density tests 9.9

- field testing 9.10- site supervision 9.20

Construction programmes 9.8Core barrels 2.15Core penetrometer - liquid limit determination 3.5Core recovery - total 2.19

- solid 2.19Core runs 2.18Cored samples - boxes for 2.18

- examination of 2.30- extrusion of 2.15

Critical density 3.17Cut off drains 4.17Cut slopes 2.6

Decomposition - degree of 2.9Deep foundation - lateral loading of 6.6

- level of 6.5- loading from retaining walls 6.6

Demec gauges 10.12Density - insitu testing 9.10Densometer 9.10Depth of wetting 2.30Designers - handing over notes on completion 8.7; 11.1Desk studies for site investigation 2.3Discontinuities - description of 2.9

- influence on rock properties 2.10- influence on triaxial tests 3.9- testing of 3.18; 3.19

Disturbed samples 2.17Diversion of streams 8.1Dolerite dykes 1.1Dowels 5.12

- design of 5.14Drainage— behind retaining walls 7.3

- gallery 4.15

218

Drains - changes of direction 8.8- counterfort 4.18- cut off 4.17- excavation for 9.16- galleries 4.15- horizontal 4.14- in rock masses 4.13- layout in slopes 8.8- pipes for 4.17- subsurface 4.13- temporary 8.5- types of 8.9- vertical wells 4.16

Drillhole logs 2.19; 2.34Drilling 2.15Ducts - drainage of 2.15

- designing on slopes 9.19

Earth pressure at rest 7.2Earth pressure cells 10.22Earthquake forces 5,3Effective stress 3.9; 4.3Embankments - construction of 9.9

- design of 5.16Engineering geology mapping 2.9Erosion protection 9.15Estuarine deposits 1.1Excavation - braced 9.6

- drainage of 9.7- effect on groundwater level 9.7- examination of by designer 9.4- for trenches 9.18- methods 9.4- programming 9.3- settlement outside 7.12- support of 9.6

Extensometer 10.19

Fabric 2.17Factors of safety - effect of groundwater level 5.6

- for rock bolt design 5.13- of slopes carrying foundations 6.3- retaining walls 7.8- slope stability 5.6- temporary works 5.6

219

Falling head tests 2.31Fill - compaction of 9.9

- loose 5.2- loose slopes 9.8- placing of 9.9- slopes 2.6- slopes, stabilisation of 5.17- soil for 9.9

Filter fabrics 4.20Filters 4.19Fissure water pressure 5.3Flow nets 4.5Flushing of drillholes 2.15Formed slope - classification 2.1

- indications of 2.8Fracture index 2.19

Gap graded soils - filter design 4.19Geodex 12.2Geological maps 1.1; 2.4

- preparation of 2.9- presentation 2.34

Geological Society Working Party reports 2.9Geology - control over slope failure 5.3Geomorphology 2.4; 2.6

- mapping 2.8Geophysical surveys 2.33Granodiorite 1.1Grass slopes - hydroseeding 9.15

- protection 9.15- repair of 11.4

Gravel 2.19Groundwater 3.16; 4.1

- condition for stability analysis 5.3- effect of basements on 4.12- effect of leakage on 4.12- effect of rainfall on 4.10- effect of wells on 4.12- in rock 4.11- level 10.4- lowering 9.7- modes of flow 4.2- observations 4.12- standpipes 10.4- time to stabilise after drilling 10.4

Grout - backfill to investigation holes 2.16- estimation of takes 2.30

Gypsum 3.4

220

H

Halloysite 3.3Hazard potential 2.1Hoek triaxial cells 3.20Hong Kong sources of information 12.4Horizontal drains 4.14; 5.11Hydraulic fracture 2.32

Inclined drains 7.3Inclinometers 10.17Infiltration 2.14; 2.30; 4.2; 4.6

- depth of wetting band due to 4.9- design storm for 4.2- test 4.9

Ins trumentation 2.20- design of 10.1- long term 11.2- long term records 11.1- reading of 10.2- remote reading of 10.23

Instruments - holes for installation of 2.15Intensity - for drainage design 8.5Internal load cells 3.10Investigation holes - logging 2.19

Joints - continuity 2.10- description of in cores 2.19- frequency 2.10- sheet 2.10- spacing 1.1- surveys 2.10

K

Keys - under retaining walls 7.9

Land classification 2.8Lateral earth pressure 7.2

221

Leakage from conduits 9*19Lineaments 2.6Liquefaction 5,2; 9.8Liquid limit 3.3; 3.5Load cells 10.21Local information - general 12.5

- references specific to Hong Kong 12.7- technical paper 12.3

Lugeon value 2.30- definition of 2.32

M

Marine deposits 1.1Masonry - slope surfacing 9.13Mass permeability of rock 2.32Mode of failure - in stability analysis 5.2

- of triaxial samples 3.10Mohr circle 3.10Moisture content 3.3; 3.4Monitoring to assess drain effectiveness 4.13Multi-stage triaxial test 3.10

N

Negative pore pressure 4.2; 4.3; 5.3No-fines concrete 4.19

Over excavation - making good 9.4

Packer test 2.32Particle size distribution 3.3; 3.8Passive pressure 7.2Permeability 4.9

- of rock 4.11- of surface protection 5.3

Permeability tests 2.30- in soils 2.31- laboratory determination 3.14- of surface protection 5,3- records 2.34

222

Photography 2.6Photo-mosaics 2.6Phreatic surface 4.1; 4.2; 4.3; 4.13Piezometers - Casagrande 10.6

- closed hydraulic 10.7- electrical 10.9- installation 10.5; 11.3- installation in rock 4.11- interpretation of observations 4.12- maintenance 11.3- observation 4,10- open hydraulic 10.6- pneumatic 10.8- reading artesian pressures 10.6

Pipes - design of routes on slopes 9.19- effect of leakage on stability 5.3- leakage from 9.19- open jointed 4.17- perforated 4.17- porous 4.17- slotted 4.17

Planting - on impervious surfaces 9.14- on pervious surfaces 9.15

Plastic limit 3.3; 3.5Plate bearing tests 2.25Point load test 2.28Pore pressure 4.2; 4.9; 10.5Pore suction 10.10Post-split blasting 9.4Pre-splitting 9.4Pressuremeter 2.24Probability of failure 5.8Progress photographs 9.20Propped caisson wall 7.10Propped pile wall 7.10

Rainfall - design intensity for drainage works 8.5- design return periods 8.5- effect on groundwater 4.10- effect on rock slope stability 4.11- infiltration of 4.2; 4.9- intensity 8.5

Ram friction 3.10Rational method - run off determination 8.2Records - compaction control 9.9

- instrument readings 10.2; 10.3- laboratory testing 3.21- of pre-existing development 2.5- rock joint surveys 2.34- site investigation 2.34- site supervision 9.20

223

Reliability index 5.8Relict joints 3.9; 9.4

- control of failure plane 5.2Remote sensing for site investigation 2.6Residual shear strength 3.11Residual strength 5.17Resistivity survey 2.33Retaining walls - movement of 7.2Rising head tests 2.31Robertson shear box 2.29; 3.19Rock bolts 5.12

- factors of safety 5.13Rock cores 2.19Rock fall control 5.15Rock slope - dentition 5.12

- design 9.4- joint system 9.4- scaling 5.12

Rock testing 3.18Rock trap 8.7Rocks - classification of 2.18

- degree of weathering 2.9- of Hong Kong 1.1- quality designation 2.19

Routine Inspection - of slopes 11.2- of surface drainage 11.5- records of 11.1; 11.2

Run-off 4.6; 8.2- from slopes 5.10

Samplers - thick walled 2.17- thin walled 2.17

Samples - block 2.13- containers for 2.17- description of 2.19- disturbance at low stress 3.9- disturbed 2.17- indentification and protection 2.17- quality of 2.17- selection of for testing 2.18; 3.2- size for triaxial test 3.10

Sand 2.19- BS 882 4.19- from crushed rock 4,19

Sand replacement test 9.10Sand traps 8.7 '

- construction of 9.17

22k

Saturation - assessment of degree of 5.3- degree of 3.3; 4.2; 4.8; 4.9- effect of surface protection on 5.3- of samples 3.10

Schmidt rebound hammer 2.27Seepage 4.4

- from slopes 9.4Seismic - reflection 2.33

- refraction 2.33Sensitivity analyses 5.9Services - drainage of ducts and conduits 9.19

- excavation for 9.18- inspection of 11.7- leakage from 11.6; 11.7- water carrying 5.3

Settlement gauges 10.20Shallow footage 6.2

-. interaction of 6.4Shear box 3.11

- modifications for rock testing 3.19Shear strength 3.9

- for slope stability 5.3- from testing saturated samples 5.3- from testing unsaturated samples 3.10- residual 3.11

Shear strip 10.18Sheet joints 2.10Silt 2.19

- filter for 4.19Site investigations - content 2.1

- field studies 2.7- photographic 2.6- remote sensing 2.6- requirements 2.1

Site supervision 9.20Slip indicators 10.18Slope - classification 2.1

- cut 2.6- fill 2.6- profile 5.10- stabilisation 5.17

Slope stability - improvement of 5,11Slope surfacing 11.4Slopes - inspection of 11.1; 11.4

- maintenance of 11.1Smooth-wall blasting 9.4Soil suction 5.3Soils - classification 2.8

- description of 2.19- fabric 2.17- of Hong Kong 1.2- weathered insitu 1.2

Specific gravity 3.3; 3.7Sprayed concrete 5.12Sprayed mortar 9.11

225

Stability analyses - factors of safety 5,6- input data for 5.3- methods of 5.4- mode of failure 5.2- reliability 5.8- rock bolted slopes 5.13- three dimensional effects 5.5; 5.7

Stability of slopes carrying shallow footings 6.3Standard penetration test 2.22Standpipes 10.4Static cone test 2.23Statistical analyses of rock joints 2.10Stereo plot 2.10Storms - design for infiltration, duration, intensity 4.7Stress path 3.10Stress range for triaxial test 2.9Subsurface drain outfalls 8.14Subsurface drainage 4.13; 11.6

"• of rock masses 4.13- pipes for 4.17

Subsurface investigation 2.12Subsurface movements 10.17; 10.18; 10.19; 10.20Subsurface water 4.1Sulphate content test 3.15Surcharge loads - due to construction 7.5

- on retaining wall 7.4Surface drainage 11.5

- mapping of 2.11Surface movement - measurement by photogrammetry 10.15

- measurement by surveying 10.14- measurement on rock and soil slopes 10.13- significance in landslide precautions 10.11

Surface protection 5.3; 9.11; 9.12; 9.13

Telltales 10.12Temporary works 7.12

- chunam surfacing 9.11- design 9.2- drainage 9.3; 9.7- embankment 9.8- factors of safety 5.6; 9.2- inspection of 9.2- protection of buildings below works 9.4- records 9.20

Testing ~ field 2.21Tests - acidity 3.16

- classification 3.3- compaction 3.13- consolidation 3.12- constant head 2.31- critical density 3.17

226

- direct shear 3.11- drained triaxial 3.9- falling head 2.31- field density 9.10- liquid limit 3.5- moisture content 3.4; 9.10- multistage triaxial 3.10- packer 2.30; 2.32- particle size distribution 3.8- permeability 2.30; 3.14- plastic limit 3.6- plate bearing 2.25- point load 2.28- rising head 2.31- shear strength of rock joints 3.19- standard penetration 2.22- static cone 2.23- sulphate content 3.15- triaxial 3.10- unconsolidated undrained triaxial 3.9- vane 2.26

Time of concentration 8.4Transition zone 4.19Trash grills 8.7Trench excavation 9.3; 9.18Trial pits 2.13

- logs for 2.34Triaxial tests 3.9; 3.10; 3.20; 4.9

U

Unconfined compression test, rock 3.20Uniaxial compression test, rock 3.20Unit hydrographs 8.2Uplift pressure on retaining walls 7.3

Vane test 2.26Vegetation 11.4Vertical drainage wells 4.16Vibration 9.4

- effects of blasting 9.5— effects of construction 9.5•* measurement of 10.6

227

w

Walls propped at several levelsWater balance 2.17Water level 2.19Water pressure on retaining wallsWater return 2.19Wax for sealing samples 2.17Weathering 2.9Wetting band 4.7; 4.8

- depth of 4.9

7.11

7.3

228

["Binding^ 1

2 4 FES-2005

geotechnical manual for slope,

Documents

vane test

lugeon test

consolidation tests

compaction tests

direct permeability

direct shear box test

point load test

standard penetration