GEOGUIDE 2

GUIDE TO SITE INVESTIGATION

GEOTECHNICAL ENGINEERING OFFICE Civil Engineering Department The Government of the Hong Kong Special Administrative Region

GEOGUIDE 2

GUIDE TO SITE INVESTIGATION

GEOTECHNICAL ENGINEERING OFFICE Civil Engineering Department The Government of the Hong Kong Special Administrative Region

2

The Government of the Hong Kong Special Administrative Region

First published, September 1987 Reprinted, December 1990 Reprinted, December 1993 Reprinted, September 1996 Reprinted, October 2000

Prepared by:

Geotechnical Engineering Office, Civil Engineering Department, Civil Engineering Building, 101 Princess Margaret Road, Homantin, Kowloon, Hong Kong.

This publication is available from:

Government Publications Centre, Ground Floor, Low Block, Queensway Government Offices, 66 Queensway, Hong Kong.

Overseas orders should be placed with:

Publications Sales Section, Information Services Department, Room 402, 4th Floor, Murray Building, Garden Road, Central, Hong Kong.

Price in Hong Kong: HK$80 Price overseas: US$18.5 (including surface postage)

An additional bank charge of HK$50 or US$6.50 is required per cheque made in currencies other than Hong Kong dollars.

Cheques, bank drafts or money orders must be made payable to The Government of the Hong Kong Special Administrative Region.

FOREWORD

This Geoguide p resen t s a recommended s t andard of good practice for s i t e investigation in Hong Kong, t h e need fo r which was formally recognized a s early as July 1983 by t he Subcommittee of t h e Building Authority Working Par ty on Geotechnical Regulations. In i t s format and content , t h e Geoguide follows closely t h e British Standard BS 5930 : 1981. Code of Practice fo r Site Investigations, b u t t h e recommendations in t h e British Standard have been adapted t o su i t local conditions and practices. I t should be used in conjunction with t h e companion document, Guide t o Rock and Soil Descriptions (Geoguide 3). These Geoguides expand upon, and largely replace, Chapter 2 of t h e Geotechnical Manual for Slopes.

This Geoguide covers Sections 1 t o 7 of BS 5930, while Section 8 i s dealt with in Geoguide 3. I t has been prepared in such a way t h a t t h e organization and format of t h e British Standard have generally been preserved. Where portions of BS 5930 have been adopted in t h e text without significant amendment, th is is clearly denoted by t he use of a n i M c typeface.

I t should be noted t h a t th is Geoguide gives guidance on good s i t e investigation practice and, a s such, i t s recommendations a r e not mandatory. I t i s recognized t h a t t h e practitioner will often need t o use al ternat ive methods. There will also be improvements in s i t e investigation practice dur ing t he life of t h e document which will supersede some of i t s recommendations.

The Geoguide was prepared in t h e Geotechnical Control Office (GCO ) unde r t h e general direction of M r J.B. Massey. The main cont r ibutors t o t h e document were D r A. Cipullo, M r K.S. Smith and M r D.R. Greenway, with significant contributions dur ing t he final s t ages of preparat ion from D r P.L.R. Pang and D r R.P. Martin. Mamy o the r members of t h e GCO made valuable suggest ions and contributions.

To en su r e t h a t t h e Geoguide would be considered a consensus document of t h e civil engineering profession in Hong Kong, a d r a f t version was circulated widely for comment in early 1987 t o contractors , consulting engineers and Government Departments. Many organizations and individuals made useful and construct ive comments, which have been t aken in to account in finalizing t he Geoguide, and t he i r contributions a r e grateful ly acknowledged.

Practitioners a r e encouraged t o comment a t any time t o t h e Geotechnical Control Office on t h e contents of th is Geoguide, s o t h a t improvements can be made t o f u t u r e editions.

E.W. Brand Principal Government Geotechnical Engineer

September 1987

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5

CONTENTS

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T ITLE PAGE

FOREWORD

CONTENTS

PART I : INTRODUCTION

1, SCOPE

2. TERMINOLOGY

PART I 1 : GENERAL CONSIDERATIONS

3, PRIMARY OBJECTIVES OF S ITE INVESTIGATION

4, GENERAL PROCEDURES 4.1 EXTENT AND SEQUENCE OF INVESTIGATION

4.1.1 G e n e r a l 4 . 1 . 2 A d j a c e n t P r o p e r t y

4 . 2 DESK STUDY

4 . 3 S I T E RECONNAISSANCE

4 . 4 DETAILED EXAMINATION AND SPECIAL S T U D I E S

4 . 5 CONSTRUCTION AND PERFORMANCE APPRAISAL

5. EARLIER USES OF THE S ITE 5.1 GENERAL

5.2 TUNNELS

5.3 MINES AND QUARRIES

5 . 4 WASTE T I P S

5 . 5 OTHER EARLIER U S E S

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5.6 A N C I E N T M O N U M E N T S

6, AERIAL PHOTOGRAPHS 6.1 G E N E R A L

6.2 T O P O G R A P H I C M A P S AND A E R I A L P H O T O G R A P H I C IMAGERY

6.2.1 M a p and Plan Scales

6.2.2 A e r i a l Photographic I m a g e r y

6.2.3 O r t h o p h o t o M a p s and Plans

6.3 A E R I A L PHOTOGRAPH I N T E R P R E T A T I O N

6.3.1 Identification and Interpretation of

G r o u n d F e a t u r e s

6.3.2 E x a m p l e s o f A P I i n H o n g K o n g

PART I 1 1 : PLANNING THE GROUND INVESTIGATION

7, INTRODUCTION TO GROUND INVESTIGATION 7.1 O B J E C T I V E S

7.2 PLANNING AND CONTROL

8, TYPES OF GROUND INVESTIGATION 8.1 S I T E S FOR NEW WORKS

8.2 D E F E C T S OR F A I L U R E S O F E X I S T I N G F E A T U R E S OR WORKS

8.3 S A F E T Y O F E X I S T I N G F E A T U R E S AND WORKS

8.3.1 E f f e c t o f N e w W o r k s upon E x i s t i n g

F e a t u r e s and W o r k s

8.3.2 T y p e s of E f f e c t s

8.3.3 P r o c e d u r e

8.4 M A T E R I A L S FOR C O N S T R U C T I O N P U R P O S E S

9, GEOLOGICAL MAPPING FOR GROUND INVESTIGATION

10, EXTENT OF THE GROUND INVESTIGATION 10.1 G E N E R A L

10.2 CHARACTER AND V A R I A B I L I T Y O F T H E GROUND

10.3 N A T U R E O F T H E P R O J E C T

10.3.1 G e n e r a l

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10.3.2 Slope a n d R e t a i n i n g W a l l C o n s t r u c t i o n

10.3.3 Foundations for Structures

10.4 P R E L I M I N A R Y I N V E S T I G A T I O N

10.5 LOCATION

10.6 S P A C I N G

10.7 D E P T H O F E X P L O R A T I O N

10.7.1 G e n e r a l

10.7.2 F o u n d a t i o n s for Structures

10.7.3 E m b a n k m e n t s

10.7.4 C u t Slopes

10.7.5 P a v e m e n t s

10.7.6 P i p e l i n e s

10.7.7 M a r i n e W o r k s

10.7.8 T u n n e l s

11, SELECTION OF GROUND INVESTIGATION METHODS 11.1 G E N E R A L

11.2 S I T E C O N S I D E R A T I O N S

12, EFFECT OF GROUND CONDITIONS ON INVESTIGATION METHODS 12.1 G E N E R A L

12.2 GRANULAR S O I L S CONTAINING B O U L D E R S , C O B B L E S

OR GRAVEL

12.3 GRANULAR S O I L S

12.4 I N T E R M E D I A T E S O I L S

12.5 VERY S O F T T O S O F T C O H E S I V E S O I L S

12.6 F I R M T O S T I F F C O H E S I V E S O I L S

12.7 C O H E S I V E S O I L S CONTAINING B O U L D E R S . C O B B L E S

OR GRAVEL

12.8 F I L L

12.9 ROCK

12.10 S O I L S D E R I V E D FROM I N S I T U ROCK WEATHERING

12.11 D I S C O N T I N U I T I E S

12.12 C A V I T I E S

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13, AGGRESSIVE GROUND AND GROUNDWATER 65 13.1 GENERAL 65

13.2 INVESTIGATION O F POTENT IAL DETERIORATION O F CONCRETE 65

13.3 INVESTIGATION O F POTENT IAL CORROSION O F S T E E L 65

13.4 INVESTIGATION O F F I L L CONTAINING INDUSTR IAL WASTES 66

14, GROUND INVESTIGATIONS OVER WATER 14.1 GENERAL

14.2 S TAGES AND PLATFORMS

14.3 FLOATING CRAFT

14.4 WORKING BETWEEN T I D E L E V E L S

14.5 LOCATING BOREHOLE POS I T IONS

14.6 DETERMINATION O F REDUCED L EVEL S

14.7 DRILL ING , SAMPL ING AND TEST ING

15. PERSONNEL FOR GROUND INVESTIGATION 15.1 GENERAL

15.2 PLANNING AND D IRECT ION

15.3 SU PERV I S I ON I N THE F I E LD

15.4 LOGGING AND DESCR I PT ION O F GROUND CONDIT IONS

15.5 LABORATORY T E S T I NG

15.6 S P E C I A L I S T ADVICE

15.7 INTERPRETAT ION

15.8 OPERAT IVES

16. REVIEW DURING CONSTRUCTION 16.1 GENERAL

16.2 P UR PO S E

16.3 INFORMATION REQUIRED

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16.3.1 Soil and R o c k

16.3.2 W a t e r

16.4 I N S T R U M E N T A T I O N

PART I V : GROUND INVESTIGATION METHODS

17, INTRODUCTION TO GROUND INVESTIGATION METHODS

18, EXCAVATIONS AND BOREHOLES 18.1 SHALLOW T R I A L P I T S AND S L O P E S U R F A C E S T R I P P I N G

18.2 D E E P T R I A L P I T S AND C A I S S O N S

18.3 HEADINGS OR A D I T S

18.4 HAND A U G E R BORING

18.5 L I G H T C A B L E P E R C U S S I O N BORING

18.6 MECHANICAL A U G E R S

18.7 ROTARY O P E N H O L E D R I L L I N G AND ROTARY CORE D R I L L I N G

18.7.1 G e n e r a l

18.7.2 F l u s h i n g M e d i u m

18.7.3 Inclined D r i l l i n g

18.8 WASH BORING AND OTHER METHODS

18.8.1 W a s h B o r i n g

18.8.2 O t h e r M e t h o d s of B o r i n g

18.9 B A C K F I L L I N G EXCAVATIONS AND B O R E H O L E S

19, SAMPLING THE GROUND 19.1 G E N E R A L

19.2 S A M P L E Q U A L I T Y

19.3 D I S T U R B E D S A M P L E S FROM BORING T O O L S OR

EXCAVATING E Q U I P M E N T

19.4 O P E N - T U B E S A M P L E R S

19.4.1 P r i n c i p l e s o f D e s i g n

19.4.2 Sampling Procedure

19.4.3 T h i n - W a l l e d S a m p l e r s

19.4.4 G e n e r a l P u r p o s e 100 mm D i a m e t e r

O p e n - T u b e Sampler

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19.4.5 Split Barrel S tandard Penetration Test

Sampler

19.5 THIN-WALLED STATIONARY PISTON SAMPLER

19.6 CONTINUOUS SOIL SAMPLING

19.6.1 General

19.6.2 The Delft Continuous Sampler

19.7 SAND SAMPLERS

19.8 R O T A R Y CORE SAMPLES

19.9 BLOCK SAMPLES

19.10 HANDLING AND LABELLING OF SAMPLES

19.10.1 General

19.10.2 Labelling

19.10.3 Disturbed Samples of Soil and Hand

Specimens of Rock

19.10.4 Samples Taken with a Tube Sampler

19.1.0.5 Rotary Core Extrusion and Preservat ion

19.10.6 Block Samples

20, GROUNDWATER 20.1 GENERAL

20.2 METHODS OF DETERMINING GROUNDWATER PRESSURES

20.2.1 Response Time

20.2.2 Observations in Boreholes and Excavations

20.2.3 Standpipe Piezometers

20.2.4 Hydraulic Piezometers

20.2.5 Electrical Piezometers

20.2.6 Pneumatic Piezometers

20.2.7 Installation of Piezometers

20.2.8 Varying Groundwater P ressures

20.2.9 Soil Suction

20.3 GROUNDWATER SAMPLES

21, TESTS I N BOREHOLES 21.1 GENERAL

21.2 STANDARD PENETRATION TESTS

21.2.1 General Principles

21.2.2 Preparat ion for Testing

21.2.3 Advantages and Limitations

21.2.4 Results and Interpretat ion

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21.3 VANE TESTS

21.3.1 General P r inc ip les

21.3.2 A d v a n t a g e s a n d Limitations

21.4 PERMEABILITY TESTS

21.4.1 General P r inc ip les

21.4.2 P r e p a r a t i o n s f o r t h e T e s t

21.4.3 Variable-head T e s t

21.4.4 Cons tan t -head T e s t

21.4.5 Analys i s of Resu l t s

21.4.6 Formulae f o r Borehole Permeabi l i ty T e s t s

21.4.7 A d v a n t a g e s a n d Limitations

21.5 PACKER (WATER ABSORPTION) TESTS

21.5.1 General P r i n c i p l e s

21.5.2 P a c k e r s

21.5.3 Application a n d Measurement of P r e s s u r e

21.5.4 Measurement of Flow

21.5.5 Execut ion of T e s t

21.5.6 Resu l t s a n d I n t e r p r e t a t i o n

21.6 PLATE TESTS

21.6.1 General

21.6.2 Limitations

21.6.3 P r e p a r a t i o n

21.6.4 Bedding of t h e P la te

21.6.5 Application a n d Measurement of Load

21.6.6 Measurement of Deflection

21.6.7 Execut ion of T e s t

21.6.8 U s e s of t h e T e s t

21.6.9 S u p p l e m e n t a r y T e s t

21.6.10 Horizontal P l a t e T e s t s

21.7 PRESSUREMETER TESTS

21.7.1 T e s t Descr ipt ion

21.7.2 Equipment Cal ibra t ion

21.7.3 Forming t h e T e s t P o c k e t

21.7.4 Resu l t s a n d I n t e r p r e t a t i o n

21.7.5 T e s t s i n Rock

21.8 BOREHOLE DISCONTINUITY SURVEYS

21.8.1 Impress ion P a c k e r S u r v e y

21.8.2 Core O r i e n t a t o r s

22, FREQUENCY OF SAMPLING AND TESTING IN BOREHOLES

22.1 GENERAL PRINCIPLES

22.2 DETERMINATION OF THE GROUND PROFILE

22.3 ROUTINE DETERMINATION OF SOIL AND R O C K PROPERTIES

129

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22.4 DOUBLE-HOLE SAMPLING 130

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22.5 SPECIAL TECHNIQUES

23 , PROBING AND PENETRATION TESTING 23.1 GENERAL

23.2 DYNAMIC PROBING

23.3 STATIC PROBING O R CONE PENETRATION TESTING

23.3.1 General Descr ipt ion

23.3.2 Mechanical Cone P e n e t r o m e t e r s

23.3.3 Electrical Cone Pene t romete rs

23.3.4 General Recommendations

23.3.5 Uses a n d Limitations of t h e T e s t

23.3.6 P r e s e n t a t i o n of Resul ts

23.4 STATIC-DYNAMIC PROBING

PART V : FIELD AND LABORATORY TESTS

24 , FIELD TESTS 24.1 GENERAL

24.2 R O C K STRENGTH INDEX TESTS

24.2.1 Point Load S t r e n g t h

24.2.2 Schmidt Hammer Rebound Value

24.3 INFILTRATION TESTS

25 , PUMPING TESTS 25.1 GENERAL PRINCIPLES

25.2 GROUNDWATER CONDITIONS

25.3 TEST SITE

25.4 PUMPED WELLS

25.5 OBSERVATION WELLS

25.6 TEST PROCEDURES

25.7 ANALYSIS OF RESULTS

26, DISCONTINUITY SURVEYS 26.1 GENERAL

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26.2 D I S C O N T I N U I T Y R O U G H N E S S S U R V E Y S

27, FIELD DENSITY TESTS 27 .1 GENERAL P R I N C I P L E S

27.2 SAND R E P L A C E M E N T METHOD

27.3 CORE C U T T E R METHOD

27 .4 WEIGHT I N WATER METHOD

27.5 WATER D I S P L A C E M E N T METHOD

27 . 6 R U B B E R BALLOON METHOD

27 . 7 N U C L E A R M E T H O D S

27.8 WATER R E P L A C E M E N T METHOD FOR ROCK F I L L

28, INSITU STRESS MEASUREMENTS 28.1 G E N E R A L

28.2 S T R E S S M E A S U R E M E N T S I N ROCK

28.3 S T R E S S M E A S U R E M E N T S I N S O I L S

29, BEARING TESTS 29.1 V E R T I C A L LOADING T E S T S

29.1.1 G e n e r a l P r i n c i p l e s

29.1.2 L i m i t a t i o n s o f t h e T e s t

29.1.3 S i t e Preparation

29.1.4 T e s t A r r a n g e m e n t

29.1.5 M e a s u r e m e n t s

29.1.6 T e s t M e t h o d s

29.1.7 A n a l y s i s o f R e s u l t s

29.1.8 Interpretation o f R e s u l t s

29.2 HORIZONTAL AND I N C L I N E D LOADING T E S T S

29.3 P R E S S U R I Z E D CHAMBER T E S T S

29.4 I N S I T U C A L I F O R N I A B E A R I N G RATIO ( C B R ) T E S T S

29.4.1 G e n e r a l

29.4.2 T e s t M e t h o d

29.4.3 L i m i t a t i o n s and U s e o f T e s t

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30. INS I TU DIRECT SHEAR TESTS 30.1 GENERAL PRINCIPLES

30 .2 SAMPLE PREPARATION

30 .3 TEST ARRANGEMENT

30.4 MEASUREMENTS

30.5 TEST METHODS

30.6 ANALYSIS OF RESULTS

31, LARGE-SCALE F I E LD TR IALS 31.1 GENERAL

31 .2 METHODS OF INSTRUMENTATION

31 .3 TRIAL EMBANKMENTS AND EXCAVATIONS

31.4 CONSTRUCTION TRIALS

32. BACK ANALYSIS 32.1 GENERAL

32 .2 FAILURES

32 .3 OTHER CASES

33. GEOPHYSICAL SURVEYING 33.1 GENERAL

33 .2 LAND GEOPHYSICS

33.2.1 Res i s t iv i ty

33.2.2 Gravimetric

33.2.3 Magnet ic

33 .2 .4 Se i smic

33.3 MARINE GEOPHYSICS

33.3.1 General

33 .3 .2 Echo-Sounding

33.3.3 Cont inuous Se i smic Reflection Prof i l ing

33.3.4 S i d e S c a n S o n a r

33.4 BOREHOLE LOGGING

33 .5 CORROSION TESTING

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34 , PRINCIPLES OF LABORATORY TESTING

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35. SAMPLE STORAGE AND INSPECTION FACIL IT IES 35.1 HANDLING AND LABELLING

35 .2 STORAGE OF SAMPLES

35 .3 INSPECTION FACILITIES

36, VISUAL EXAMINATION 36.1 GENERAL

36 .2 SOIL

36 .3 ROCK

36.4 PHOTOGRAPHIC RECORDS

37, TESTS ON SOIL 37.1 GENERAL

37 . 2

37 .3

SAMPLE QUALITY

SAMPLE SIZE

37.4 TEST CONDITIONS

37.5 RELEVANCE OF TEST RESULTS

38 , TESTS ON ROCK

PART V I : REPORTS AND INTERPRETATION

39, FIELD REPORTS

40, S ITE INVESTIGATION REPORT 40.1 GENERAL

40.2 DESCRIPTIVE REPORT 40.2.1 Report as Record 40 .2 .2 In t roduc t i on 40 .2 .3 Descr ip t ion o f S i t e 40.2.4 Geology

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40.2.5 Field Work 40.2.6 Borehole Logs 40.2.7 Incidence and Behaviour of Groundwater 40.2.8 Location of Boreholes 40.2.9 Laboratory Test Results and Sample Descriptions

40.3 ENGINEERING INTERPRETATION 40.3.1 Matters t o be Covered 40.3.2 Data on which Interpretat ion i s Based 40.3.3 Presentation of Borehole Data 40.3.4 Design 40.3.5 Construction Expedients 40.3.6 Sources of Materials 40.3.7 Failures 40.3.8 Calculations 40.3.9 References

REFERENCES

TABLES LIST OF TABLES

TABLES

FIGURES LIST OF FIGURES

FIGURES

PLATES LIST OF PLATES

PLATES

APPEND ICES APPENDIX A : INFORMATION REQUIRED FOR DESK STUDY APPENDIX B : SOURCES OF INFORMATION

APPENDIX C : NOTES ON SITE RECONNAISSANCE

APPENDIX D : INFORMATION REQUIRED FOR DESIGN AND CONSTRUCTION

APPENDIX E : SAFETY PRECAUTIONS

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PART I

INTRODUCTION

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1, SCOPE

This Geoguide deals with t h e investigation of s i tes in Hong Kong for t h e purposes of assessing the i r suitability for civil engineering and building works. and of acquir ing knowledge of s i t e characteris t ics t h a t affect t h e design and construct ion of such works a nd t h e secur i ty of adjacent propert ies . I t is essentially BS 5930 : 1981, Code of Practice for Site Investigations (BSI, 1981a), modified a s considered desirable for use in Hong Kong.

While t h e basic structure and philosophy of BSI (1981a) has been maintained in th is Geoguide, topics of part icular importance in Hong Kong have been supplemented o r rewrit ten in t h e l ight of local conditions and experience. Other sections of BSI (1981a) have been repeated herein without significant amendment, and th i s has been denoted by a n 12al1'cscript . Less re levant o r rare ly-used portions of BSI (1981a) have been incorporated only by reference, or have been specifically deleted.

In th i s Geoguide, a s in BSI (1981a). t h e expression "site investigation" has been used in i t s wider sense. I t i s often used elsewhere in a narrow sense t o describe what has been termed herein "ground investigation". The use of soil and rock a s construct ion materials is t rea ted only briefly: f u r t h e r information on th is is given in BSI (1981b).

From Part I1 onwards. this Ceoguide is divided as follows .-

Part I/. Part /I deals with those matters of a technical, legal or environmental character that should be taken into account in selecting the site for in determining whether a proposed site is suitable) and in preparing the design of the works. Part I/I. Part 111discusses general aspects and planning of ground in vestl'gahon, including the influence of general condihons and ground condihbns on the selection of methods of in vestigation.

Par ts IV and V. Pa r t s IV and V discuss methods of ground investigation, sub-divided a s follows : Par t IV deals with excavation, boring. sampling, probing and tests in boreholes; Pa r t V deals with field tests and laboratory tests on samples.

Part V/. Part V/ deals with the preparation of field reports and borebole logs, the interpretation of the data obtahed from the hvescigafhn and the preparahon of the final site investigation report.

The las t section of BSI (1981a), which deals with t h e description of soils and rocks , is not covered in th is Geoguide. A companion document. Geoguide 3 : Guide t o Rock and Soil Descriptions (GCO . 1988), has been devoted entirely to th i s topic, and t h e r eader should re fe r t o i t for guidance on t h e description and classification of Hong Kong rocks and soils.

I t may be noted t h a t t h e r e are some imbalances in t reatment of t h e various topics, with, in some cases. more comprehensive coverage given t o methods t h a t a r e less f requent ly used. Because i t would not be possible t o include full coverage of all available s i t e investigation techniques, methods t h a t a r e well documented elsewhere in t h e l i te ra ture receive abbrevia ted coverage in

this Geoguide.

This Geoguide represents a standard of good practice and therefore takes the form of recommendations. Compliance with it does not confer immunity from relevant statutory and legal requirements. The recommendations given are intended only a s guidance and should not be taken as mandatory. In this respect, it should be realized that improvements to many of the methods will continue to evolve.

2, TERMINOLOGY

A few commonly-used descript ive terms for geological materials and t ype s of ground investigation a r e often in te rp re ted in different ways and therefore r equ i re definition. In th i s Geoguide, t h e terminology given in t h e following paragraphs has been adopted.

"Rock" re fe r s t o all solid material of natural geological origin t h a t cannot be broken down by hand. "Soil" r e fe r s t o any naturally -formed ea r t h material o r fill t h a t can be broken down by hand and includes rock which has weathered insi tu t o t h e condition of an engineering soil. Fur the r guidance on t he use of t h e s e terms is given in Geoguide 3 (GCO, 1988).

Excluding any boulders o r cobbles, a "fine-grained soil" or a "fine soil" i s one t h a t contains about 35% or more of f ine material (silt and clay size particles). A "coarse-grained soil" o r a "coarse soil" contains less than 35% of f ine material and more than 65% of coarse material (gravel and sand size particles). Fur the r guidance is given in Geoguide 3.

A "cohesive soil" is one which, usually by v i r tue of its fines content. will form a coherent mass. Conversely a "granular soil" o r a "cohesionless soil" will not form a coherent mass. These simple terms are useful in t h e classification of materials dur ing ground investigation fo r t h e purpose of choosing a suitable method fo r sampling t h e ground. A fine soil i s generally cohesive.

The "matrix" of a composite soil r e fe r s t o t h e fine-grained material enclosing, o r filling t h e spaces between, t h e l a rge r gra ins o r particles in t h e soil.

"Boring" is a method of advancing a cased o r uncased hole (viz a "borehole") in t h e ground and includes auge r boring, percussion boring and r o t a ry drilling , in which a dri l l bi t i s rotated into t h e ground for t h e purpose of forming t h e hole. Although t h e term "drillhole" is commonly used in Hong Kong because of t h e popular u s e of t h e ro tary core drilling method in ground investigations, t h e general term "borehole" i s used throughout th i s Geoguide for simplicity, whether t h e hole is bored, augered o r drilled.

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PART I 1

GENERAL CONSIDERATIONS

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3, PRIMARY OBJECTIVES OF SITE INVESTIGATION

Investigatrbn of the site fk an essentrtral prelimnary to the constructlbn of a// u'vil engineering and building works. and the objectrives h making such investrgatrons are as follows :

/a1 Suitability. To assess the general suitabfXty of the site and environs for the proposed works.

/b1 Des~gn. To enable an adequate and economic design to be prepared incfud~ng the design of temporary works.

fc1 Construction. To plan the best method of construction; to foresee and provide aganst d~Yficulties and delays that may arise during construction due to ground and other l o d conditrbns; in appropriate cases, to explore sources of indigenous mater%ds for use in constructrbn /seeSectron 8.4); and to se/ect sites for the dfkpod of waste or surplus materials.

/dl Effect of Changes. To determine the changes that may arise in the ground and environmental conditrons, eeither naturdy or as a result of the works, and the effect of such changes on the works, on aaacent works, and on the environment in general.

/e1 Choice of Site. Where alternatives emkt, to advise on the relative suitabh!ity of dXferent sites, or different parts of the same site.

In additon, site hvestigatrons may be necessary in reportrng upon the safety of existrng features and works /see Sectrbn 8.31, for the design of exte~wons, vertrical or hor~zontal, to emstrng works, and for invesligating cases where failure has occurred /see Sectron 8.21.

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4, GENERAL PROCEDURES

4.1 EXTENT AND SEQUENCE OF INVESTIGATION 4.1.I General

The extent o f the hvestigation depends primarily upon the magnitude and nature of the proposed works and the nature of the site.

A site ~nvestigation wifl norma//y proceed in stages, as fo/laws :desk study; site reconnaissance; detailed examination for design, including ground invesogation, topographic and hydrographic survey and spec12 studies; follow- up investigatJons during constructJon (Figure 1). This may be fohwed by appraisal of performance. Some of the stages may overlap, or be taken out of sequence; for example, the site reconnaissance may well take place before completion o f the desk study.

The costs of a site investigation are low in refatJon to the overall cost of a project and may be further reduced b y intelligent forward planning. Discussion at an e&y stage with a speciaht contractor wifl help to formdate an efficient and economic plan. The technicaf requirements of the investigation shoufa' be the overriding factor in the selection of investigatory methods, rather than the]> cost.

A s far as possibfe, assembly of the desk study hformation shoufd be complete, at least h respect of t h e aspects refated to ground condizbns, before ground in vestJgatJon begins. A preliminary ground investigathn may be desirable to determine the extent and nature of the main ground investigation. The extent of the ground investigation is discussed in Chapter 10.

For regional s tudies o r s i t e investigation of projects covering large areas, e.g. road, tunnel o r transmission line routes, techniques such as engineering geological and geomorphological mapping, t e r ra in classification and hazard analysis may be useful t o delineate critical a reas so t h a t detailed investigations can be concentrated in areas where they are most requi red (Brand e t al. 1982; Griffiths & Marsh. 1984; Hansen. 1982).

4.1.2 Adjacent Proper ty Because of t h e dense u rban development in Hong Kong, construct ion

activities can often affect adjacent property. I t is therefore essential t h a t investigations should cover all fac tors t h a t may affect adjacent proper ty . including fea tures such a s slopes and retaining walls (see Chapter 7 and Section 8.3). Where possible, records of ground levels, groundwater levels and re levant part iculars of adjacent propert ies should be made before, dur ing and a f t e r construct ion. Where damage t o existing s t r uc tu r e s i s a possibility, adequate photographic r ecords should be obtained.

Adjacent buildings, s t r uc tu r e s and buried services, including pipes conveying water, gas o r sewage, should be specifically considered, a s they may be affected by vibrat ions, ground settlement o r movement, o r changes in groundwater levels dur ing and a f t e r construct ion activities on t h e site. Hospitals and o the r buildings containing sensi t ive ins t ruments o r appara tus should be given special consideration.

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Special permission o r approval must be obtained when t h e s i t e i s above o r near t h e Mass Transi t Railway Corporation's tunnels o r s t ruc tu res . o r is within t h e Mid-levels Scheduled Area (see Appendices A and B; s ee also Chapter 7). The approximate locations of these two fea tu res a r e shown in Figure 2.

4.2 DESK STUDY

A s a f i r s t s t age in a s i te investigation, a desk s t udy is necessary and AppendixA indicates t h e t ype s of information t h a t may be requi red . Much information about a s i te may already be available in existing records . A summary of t h e important sources of information is given in Appendix B.

A new geological su rvey is cu r ren t ly underway in Hong Kong t o replace t h e existing 1:50 000 scale geological maps and memoir (Allen & Stephens , 1971): new 1:20 000 scale geological maps will become available between 1986 and 1991 (Figure 3). The new geological su rvey u se s different nomenclature fo r certain major rock divisions and rock types (Addison. 1986; GCO. 1988; S t range & Shaw. 1986); th i s should be used wherever possible.

An important source of basic geotechnical information is t h e Geotechnical Area Study Programme (GASP) publications available from t h e Government Publications Centre. Systematic t e r ra in evaluation has been under taken at a scale of 1:20 000 covering t h e en t i r e Terri tory (Brand et al. 1982). These publications generally contain Engineering Geology. Terrain Classification, Erosion. Landform and Physical Constraint Maps. Selected a reas of t h e Terri tory have also been evaluated a t t h e 'district ' scale of 1:2 500. b u t these have not been published. The GASP programme and the areas covered by t he GASP publications a r e shown i n Figure 4, and examples of some of t h e 1:20 000 maps a r e given in Figure 5.

The Geotechnical Information Unit also contains numerous records of boreholes from throughout t h e Terri tory, a s well a s useful r ecords of landslides. rainfall and piezometric data. and laboratory t e s t resul t s on soil and rock samples. Relevant da ta can be easily accessed by geographical location of t h e site. Fur the r details of t h e Geotechnical Information Unit a r e given in Appendix B.

A useful bibliography on t he geology and geotechnical engineering of Hong Kong is also available (Brand, 1992). Local maps and plans a r e easily obtained (Table 1). and as-buil t records of pr iva te developments a r e retained by t he Buildings Ordinance Office o r t h e Public Records Office (see AppendixB). Valuable information may often be obtained from aerial photographs. a s discussed in Chapter 6.

4.3 SITE RECONNAISSANCE

A t a n early stage. a thorough visual examination should be made of t h e site. The extent t o which ground adjacent t o t h e s i te should also be examined is. in general , a matter of judgement ( see Section 4.1.2). In t h e intensely-developed u rban a reas of Hong Kong, i t will usually be necessary t o inspect existing slopes and retaining walls within and su r round ing t he s i t e and adjacent propert ies dur ing t h e s i t e reconnaissance s tage . Appendix C gives a summary of t h e procedure fo r s i t e reconnaissance and t h e main points t o be considered bu t should not be regarded as necessarily covering all requirements.

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Nearby cut sfopes can reveal soil and rock types and their stablzty characteristics, as can old excavations and quarries. Shilarly, in the vicinity there may be embankments or bu~idings and other structures having a settfement history because of the presence of compress~Ble or unstable soih. Other important evidence that might be obtained from an inspection is the presence of underground excavations, such a s basements and tunnefs. The beha viour of structures simiar to those intended shoufa' also pro vide useful hformatJon, and the absence of such structures may be significant, a s may be also the presence of a vacant site zn the midst of otherwise intensive de vehpment.

Exampfes of earlier uses of the site that may affect new construct~on works are given in Chapter 5.

4.4 DETAILED EXAMINA TION AND SPECIAL STUDIES

For most projects, the design and planning of construction will require a detaded examination of the site and its surround~ngs /see afso Append~kD). Such requirements may necessitate a detailed land survey /see Append~xD.ZI, or an investigation of liabifity to flooding. The hvestigation of ground conditions is dealt with in Parts IZI and IK Other requirements may entail studies of special subjects such a s hydrography /see Appendix D. 3); micrometeorofogy /see Append~kD. 4); sources of mater~als (see Appendix D. 5); disposal of waste materials /see Appendix 0.6); or other en vironmental considerations.

Tbe poss~B~Xty of disused tunnels affecting the site should also be considered (see Section 5.21.

In areas where underground cavities are suspected (Culshaw & Waltham, 1987). it may be necessary to carry out a special study to assess the suitability of the site for development (see Section 7.1 ).

4.5 CONSTRUCTION AND PERFORMANCE APPRAISAL

Construction and performance appraisal are discussed in Chapter 16.

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31

5, EARLIER USES OF THE SITE

5.1 GENERAL

If a site has been used for other purposes in the past, this can have a significant effect on the present intended use. A careful visual inspechon of a site and the vegetation i t sustains may reveal clues suggesting interference with the natural subsoil conditons at some t h e in the past. Examples are given in Sectlbns 5.2 to 5.6.

Due t o t h e relatively s ho r t history of development in Hong Kong, many instances of previous use of a s i te can be discovered by an inspection of early maps, aerial photographs and o the r historical records (see Appendices A and B).

5.2 TUNNELS

The presence of nearby tunnels may have a profound effect on t h e intended use of t h e si te , and should be fully considered. In addition t o tunnels in act ive u s e for water supply, sewage conveyance, roads and railways, underground shel ters and disused tunnels (of ave rage dimensions 2 m high and 3 m wide) exist in places throughout t h e Terri tory as a r e su l t of previous wartime activities.

5.3 MINES AND QUARRIES A relatively minor amount of e i the r opencast o r underground mining has

been under taken in Hong Kong, bu t quarry ing fo r rock products has been extensive at some locations, as have borrow a r ea operations. Where th i s has occurred , detailed consideration must be given t o i t s influence on affected sites.

5.4 WASTE TIPS

Waste t ips, used for t h e disposal of domestic refuse , indus t r ia l waste and o the r refuse , may be found in places throughout t h e Territory. The location of pas t o r p resen t 'controlled tips' opera ted by Government a r e documented, b u t o t he r t ips may also exist. Harmful indust r ia l wastes may also be encountered. The u se of waste t ip s i tes for o the r purposes must consider fully t h e effects of combustible gas, toxic leachate and ground settlement. Furthermore, sites in t h e proximity of waste t i p s may also be sub jec t t o t h e effects of laterally migrating combustible gas and leachate.

5.5 OTHER EARLIER USES

Much of t h e low-lying land of Hong Kong has been ' extended by successive s tages of reclamation in t h e pas t 80 t o 90 years . Former seawalls and o the r obs t ruct ions may therefore be encountered beneath these areas . The fill materials used have been variable, often containing l a rge boulders and building debris . The fill is often underlain by soft compressible marine sediments.

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Natural slopes and boulders, and older c u t and fill slopes and retaining walls, a r e often prone to landslides and o the r forms of instability. I t is of paramount importance t h a t all slope fea tu res on o r adjacent t o t h e s i t e should be examined for areas of past , c u r r e n t o r potential instability at a n early s t age in t h e s i t e investigation.

5.6 ANCIENT MONUMENTS

A l i s t of gazet ted historical s i tes is maintained by t h e Antiquities and Monuments Office of t h e Government Secretariat , and a permit is r equ i red before commencement of any work within a gazetted historical site. I t is advisable t o consult t h e Antiquities and Monuments Office before enter ing any historical s i te , even ungazetted sites. During s i te investigation, any discovery of ant iquit ies or supposed antiquit ies should be repor ted t o t h e Antiquities and Monuments Office (see Appendices A and B).

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6, AERIAL PHOTOGRAPHS

6.1 GENERAL

Aerial photographs can be used in t h e preparat ion and revision of maps and plans, and they can as s i s t in t h e identification and general assessment of na tura l and man-made fea tu res , including geology, geomorphology, hydrology and vegetation. on o r in relation to a site. They a r e part icular ly useful in t h e assessment of s i t e his tory (i.e. changes in form. materials and land use ) and can provide valuable information f o r t h e assessment of slope stability (Geological Society, 1982).

Black and white aerial photograph coverage of Hong Kong is extensive. Although partial coverage of t h e Terr i tory is available from 1924, t h e f i r s t complete coverage was obtained in 1963, a s summarised in Table 2. For almost any s i te i n t h e Terr i tory, repeated aerial photograph coverage r ecords t h e land use and development changes t h a t have occurred , as well a s any history of r ecen t instability. The small scale black and white photographs obtained a t flying heights of ove r 6 000 m a r e more sui table fo r obtaining an overall view of t h e Terr i tory. A small number of t r u e colour. (false) colour inf rared and black and white inf rared photographs a r e also available. Advice on how t o obtain t h e aerial photographs i s given in Appendix 8.1.3.

6.2 TOPOGRAPHIC MAPS AND AERIAL PHOTOGRAPHIC IMAGERY

6.2.1 Map and Plan Scales

Accurate topographic maps and plans can b e produced from aerial photo- g raphs . A part ial catalogue of maps and plans available from t h e Lands Department i s given in Table 1 (see also Appendix B.l.1). Large scale plans (scales 1:500 t o 1:l 000) a r e usually most appropr ia te fo r s i te invest igat ions of small a reas , whereas plans with scales of 1:5 000 t o 1:20 000 a r e more appropr ia te fo r d is t r ic t o r regional s tudies .

6.2.2 Aerial Photographic Imagery

The scale of an image on an aerial photograph i s proportional t o t h e distance between t h e camera and t h e subjec t . For an aerial photograph taken vertically, tall objec ts ( tops of hills and buildings) , and objec ts nea r t h e c en t r e of t h e photograph. c rea te images a t slightly l a rge r scales than low t e r ra in o r similar objec ts nea r t h e e dg e of t h e photograph. Radial distortion about t h e optical axis of t h e camera displaces t h e t r u e vert ical away from cen t r e of t h e photograph, an effect which becomes more pronounced nea r t h e edges of a photograph. This may c rea te dramatic effects on large-scale photographs with considerable changes in elevation from one portion t o another .

Despite these sources of distortion. fo r s i tes which can b e identified within t h e cent ra l t h i r d of a vert ical aerial photograph and which contain t e r r a in of broadly similar elevation, reasonably accura te scaled images can b e obtained by proportioning t h e distances between objec ts identifiable on a map ( o r plan) and a contact p r i n t of an aerial photograph, and by using th i s proportion rat io a s an enlargement factor . Most aerial photography has been obtained using cameras with la rge format negatives. Prior t o 1963, t h e sizes of t h e contact p r in t s v a ry , b u t a r e usually 162 mm by 175 mm. With few

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exceptions, t h e negatives obtained from 1963 t o t h e p resen t a r e 228 mm by 228 mm. The image produced on contact pr in ts is extremely sha rp , and clear images can be obtained e i the r by viewing t he contact p r in t s with a magnifying lens o r stereoscope, o r by enlarging all o r p a r t of t h e negative. Enlarged p r in t s can be used even fo r s tudies of small a reas of t h e size of a n individual building site.

6.2.3 Orthophoto Maps and Plans

Orthophoto maps and plans, which consist of rectified ( t r ue t o scale) photographs overpr in ted with contours o r g r ids can be made (overseas only) fo r both vert ical and oblique aerial photography. Rectification of t h e image can be performed optically o r digitally; t h e accuracy is determined by t h e number of control points supplied, t h e degree of rectification desired and t h e scale of t h e original photography.

6.3 AERIAL PHOTOGRAPH INTERPRETATION

6.3.1 Identification and Interpretat ion of Ground Features

Aerial photographs can be in terpre ted a t a r ange of scales and levels of detail t o provide information valuable t o both t h e design of s i te investigations and t o t h e interpretat ion of t h e results . The design of s i te investigations for la rge projects such as rou te corr idors (e.g. roads, railways, pipelines o r transmission lines) can benefit enormously from a preliminary aerial photograph in terpre ta t ion (API) su rvey . This can highlight t h e na tura l and man-induced characteris t ics of t h e te r ra in , noting in part icular hazards and resources t h a t may have a significant effect on t h e feasibility o r design of t h e project. Even when performed fo r smaller s i tes , a n API s t udy can often provide useful information on t h e distribution and th ickness of natural and fill materials, and may reveal potential problems originating from adjacent land. Sequences of aerial photographs taken a t different da tes can be compared to determine t h e location, extent and approximate time of filling and reclamation, and t h e sequence of development of a n area.

Aerial photographs, particularly when examined stereoscopically, can often be used t o identify and delineate specific ground fea tures such a s t h e distr ibution of soil types (e.g. colluvial and alluvial deposi ts) , soil th ickness , bedrock type , dep th t o bedrock, f r ac tu re pa t t e rns and spacings, as well a s local relief. API is of part icular value in t h e mapping of "photolineaments". This term re fe r s t o s t r a igh t o r gently-curving fea tures on aerial photographs which a r e usually t h e su r face expression of variations in t h e s t r u c t u r e o r materials of t h e underlying bedrock. Photolineaments a r e usually marked by topographic highs o r lows in t h e t e r ra in b u t sometimes they may be more sub t l e fea tures , which can only be identified by different vegetation growth. reflecting underlying changes in soil type , soi! th ickness or moisture content . Well-defined l inear depressions usually indicate t h e location of less res is tant bedrock o r of discontinuities in t h e bedrock s t r u c t u r e such as faults , f r ac tu re zones o r major joints. Local linear topographic highs o r lines of boulders o r rock outcrops may indicate t h e presence of a rock uni t t h a t is more re s i s t an t t o weathering.

All t h e f ea tu res mentioned above may be important for t h e interpretat ion of s i te conditions. Early identification by API of major changes in soil and rock t ype s and fea tu res t h a t a r e likely t o have a significant influence on t he

local groundwater regime can be of g r e a t assistance in t h e design of t h e ground investigation and in establishing a geological model for t h e site.

Reviews of API and related mapping techniques a r e contained in Geological Society (1982). Some good examples of t h e use of API techniques a r e provided by Lueder (1959). Van Zuidam & Van Zuidam-Cancelado (1979), Verstappen & Van Zuidam (1968) and Way (1978).

6.3.2 Examples of API in Hong Kong

In Hong Kong, API techniques have been successfully applied t o both specific problems and regional appraisals . Examples of t h e former a r e given in Brimicombe (1982). Bryant (1982) and Koirala e t a1 (1986). Systematic regional API s tudies have been under taken within t h e Geotechnical Area Studies Programme (GASP) t o provide information for planning, resource appraisal and engineering feasibility s tudies (see Section 4.2). The f i r s t of t h e regional GASP repor t s was available in 1987 ( G CO , 1987) and a f u r t he r eleven repor t s in t h e ser ies were published between 1987 and 1989. All GASP maps a r e available for inspection in t h e Geotechnical Information Unit ( see Appendix B) .

F igure 6 shows an example of a vert ical black and white aerial photo- g raph of pa r t of Hong Kong Island and includes t h e corresponding portion of t h e 1:20 000 scale geological map ( G C O , 19861. Some fea tures of t h e bedrock s t r u c t u r e can be in terpre ted from t h e aerial photograph. For example, t h e location of t h e faul t line shown on t h e geological map can be clearly seen a s a s t ra ight , deep valley in t h e cent re-eas t p a r t of t h e photograph. Near t h e north eas te rn co rne r of t h e photograph, t h e photolineaments indicated on t he map can be seen t o correspond t o less clearly-defined valleys.

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PART I I 1

PLANNING THE GROUND INVESTIGATION

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7, INTRODUCTION TO GROUND INVESTIGATION

7.1 OBJECTIVES

For new works, t h e objectives of ground investigation are t o obtain reliable information t o produce an economic and safe design and t o meet t ende r and construction requirements. The investigation should be designed t o verify and expand information previously collected. In Hong Kong, because of in tense u rban development, i t is often necessary t o invest igate t h e effects of new works on t he safe ty of existing fea tures and works: in part icular , t h e effects on t h e stability of existing slopes and retaining s t r uc tu r e s (see Sections 4.1.2 and 8.3).

The objective of ground investigation related to defects or faihres of existhg features or works (see Section 8.21, or to safety of existing features and works (see Section 8.31, wiiY be directly related to the particular problems ~n vol ved The requirements for i n vestljrathn of materials for constructhn purposes are discussed i n Sectlbn 8.4.

An understanding of the geology of the site i s a fundamentaf require- ment in the planning and interpretation of the ground i n vestigatlbn. I n some cases where the geology is relatively straightforward and the engineering problems are not complex. suffic~ent geofogical hformation may have been provided by the desk study, subject to confirmatibn by t r i d p i t s or boreholes or both. I n other cases, it may be necessary to undertake geolog~cal mapping, which is discussed i n Chapter 9.

O fp rha r y hportance w i l be the establishment of the so17profile or sod and rock prof i / , and the groundwater condit~ons. The profile shouM be obtained by close visual inspection and systematic description of the ground using the methods and term~irology given fir Geoguide 3 ( G CO , 1988), or a suitable alternative system. I n many cases, this. supplemented by limited insitu or laboratory testing, will su f f ie . I n others, it w~Yl be necessary to determine i n detail the engineering properties of the soils and rocks. The extent of the ground hvestigatfon i s discussed i n Chapter 10. Mere appropriate, the geometry and nature of discontinuities shoufd be estabhbed (see Section 12.11).

In many cases. especially in slope design, i t will b e ve ry important t o determine t h e variations in t h e groundwater regime in response t o rainfall.

The investigation should embrace al/ ground i n which temporary or permanent changes may occur as a result of the works. These changes include :changes i n stress and associated strain, changes ~n moisture content and associated volume changes, changes li, groundwater level and flow pattern. and changes in soil properties such as strength and compress1;3ility. Materials placed in the ground may deteriorate. It is therefore necessary to provide informaaon from which an estimate of the corros~~vity of the ground can be made (see Chapter 131.

Special measures may be requi red to locate disused tunnels o r under-ground cavities, which may collapse, result ing in damage t o s t r uc tu r e s (see Sections 8.3.2. 10.3.3 and 10.7.2). Other hazards may ar ise from earl ier uses of t h e s i te (see Chapter 5 ) .

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7.2 PLANNING AND CONTROL

Before commenuhg ground in vestgaton, dl relevant informaton collected frwn the sources discussed in Part I1 should be considered together to obtiui, a prehinary concepton of the ground conditions and the engineering problems that may be involved T h k will assist in plannhg the amount and types of ground hvestigatrbn required

Plannhg of the ground investigation should be flexible so that the work can be varied as necessary h the light of fnsh information. On occasions, especiafly on large or extended sites, a prefimhwy hvestigation may be necessary in order that the main investgatian may be planned to best advantage fsee Sectbns 4.1.1 and 10.4).

The ground investigation should be largely completed before t h e works are finally designed. I t i s therefore important t h a t sufficient time fo r ground investigation (including dealing with all legal. environmental, contractual and administrative matters , repor t ing and in terpre ta t ion) is allowed in t h e overall programme fo r any scheme. For example, in slope design. piezometers should be installed well in advance t o obtain sufficient groundwater da ta fo r t h e design. Should changes in t h e projec t occur a f t e r completion of t h e main investigation, additional ground investigation may be required. If so, t h e programme should be ad jus ted to allow fo r t h e additional time requi red .

Somethes, conditons necessitate additondl investigation after the works commence. In tunnelfihg, for example, probing ahead of the face may be required to give warning of hazards or changes in ground conditlbns. The propertes of the ground and also the groundwater levels may vary with the seasons. In plannihg the ~hvestigation, consideraton should be given to predicting the ground conditions at other thes of the year.

The ~hpositon of limitatons on the amount of ground investgation to be undertaken, on the grounds of cost and the, may result h insufficient infomaton beihg obtained to enable the works to be designed, tendered for and constructed adequately, economicdly and on the. Add1'0ond investi- gations carried out at a later stage may prove more costly and result in delays.

As ground investigations in Hong Kong must often be under taken in u rban areas (Plate lA ) , i t is often necessary t o obtain road excavation permits. temporary licences or way leaves before commencing t h e ground investigation. For some s i tes i t will be necessary to coordinate t h e works with t h e requirements of t h e t raf f ic police and o the r authori t ies (Plate 1B). Proper identification and maintenance of utilities encountered by t h e works is essential; high voltage power cables. gas distribution lines and o the r utilities often p r e s en t significant safe ty hazards.

Since backfilled pits and boreholes might in te r fe re with subsequen t construct ion, they should be si ted and backfilled with care. I t is essential t h a t t h e precise location of eve ry excavation, borehole or probing is properly referenced to t h e 1980 Hong Kong Metric Grid and recorded dur ing t h e execution of t h e fieldwork. I t is also essential t o establish and record t h e ground levels of t h e s e locations. The records should be such t h a t t h e locations and levels can be readily incorporated in to t h e r epo r t on t h e invest igat ions (see Sections 10.5 and 40.2.8).

Invest igat ions fo r new works and all o the r building works within t h e

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Mid-levels Scheduled Area (Figure 2) must comply with the provisions of the Buildings Ordinance (Government of Hong Kong. 19851, including the submission of the ground investigation plan to the Buildings Ordinance Office for approval and consent to commence the work.

Where the proposed investigation is in the vicinity of the Mass Transit Railway, o r within the limits of the railway 'protection boundary', details and locations of the proposed works, including the depths of any proposed boreholes, should be forwarded to the Mass Transit Railway Corporation for agreement prior to commencement of the work.

Should i t appear during the course of the investigation tha t items of archaeological o r historical significance have been encountered, the Antiquities and Monuments Office should be notified (see Section 5.6).

To obtain the greatest benefit fm a ground investigation, it is

essential that there is adequate directrbn and supervisbn of the work by

competent personnel who have approprhte knowledge and exper~ence and the

authoriy to decide on var~atrons to the ground investigation when required

(see Chapter 15).

In planning ground investigations, particular attention should be paid to the safety of personnel. Certain methods present special safety problems, and recommendations a re given in the relevant sections. Other methods involve normal safety precautions appropriate to site o r laboratory work. A list of statutory regulations which may apply to ground investigations is given in Appendix E; this list is not necessarily complete, and if there is doubt over safety precautions, fur ther advice should be obtained.

Appendix A summarises the types of information tha t may be required in planning a ground investigation.

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8, TYPES OF GROUND INVESTIGATION

8.1 SITES FOR NEW WORKS

In vestigat~ons for new works d~Wer from the other types of hvestigatlbn mentioned ILJ Chapter Z in that they are u M y wider li, scope because they are required to y~eld information to assist h selecting the most suitable location for the works, and the des~gn and construction of the works. For example, when slope excavathn has to be carr~ed out, a knowledge of the subsurface materi*als and groundwater cond~*tibns should indkate :

whether removal of t h e material will be difficult.

whether t h e s ides of t h e excavation will b e stable if unsuppor ted o r will r equ i re suppor t ,

whether groundwater conditions will necessitate special measures such a s groundwater dra inage o r o t he r geotechnical processes.

whether t h e na tu re of t h e ground will change a s a r e su l t of excavation, e.g. opening of rel ict joints in t h e soil mass,

what form of su r face protection is required.

On the desjyn side, it is necessary to assess such considerations as bearing capacity and setuement of foundations, stability of slopes ~ i , embankments and cuttings, earth pressures on supporthg structures. and the effect of any chemically aggressive ground conditom. For the design of new works, it is important that the range of cond~'tions, including least favourable conditions, should be known. This entays not only a study of the degree of var~kbifityin the soil and rock profiles over the area of the site, but also an appreciation of the possibe hjur~ous effects of groundwater var~ations and weather c0nd~'tibns on the propertis of the various subsurface materials. Where works requ~k excavations into or within rock. iduding weathered rock, the orientatrbn and nature of discontinuities in the rock may be the most hportant factors.

Often, a preliminary design of t h e proposed works is of g rea t ass is tance in t h e identification of parameters t h a t are required t o be obtained from t h e ground investigation.

Investigations should assess whether t h e proposed works may induce ground movements which could affect adjacent land, services and s t ruc tu res . and whether t h e hydrogeological regime may be adverse ly affected (see Sections 4.1.2 and 8.3) .

8.2 DEFECTS OR FAILURES OF EXISTING FEATURES OR WORKS

The investigation of a site where a falure has occurred is often necessary to estabfish the cause of the faiure and to obtarh the hfonnation required for the design of remedid measures.

Observations and measurements of t h e f ea tu re o r s t r u c t u r e to determine

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t h e mode o r mechanism of failure a r e f i r s t needed, and t he se will often sugges t t h e origin of t h e trouble, o r at least indicate whether t h e ground conditions were par t ly o r wholly responsible. If th i s is t h e case, a n investigation will b e requi red t o ascertain t h e ground and groundwater conditions relevant t o t h r e e phases of t h e s i te history, i.e. before t h e works were cons t ructed , at t h e time of failure and as they exist a t p resen t (see also Chapter 32). Each problem will need t o be considered on its merits. Indications of t h e probable cause of a failure will often resu l t in detailed at tent ion being directed t o a part icular aspect o r t o a part icular geological feature.

In the case of slope failure, or where such faihre is considered imminent, 12 is common practice to monitor movements both of the surface and underground The former is conducted b y conventionalsurvey methods and the latter by means of slip indicators or incfinometer measurements. These techniques are fu/y described in BSI 11981b) and in the Geotechnical Manual fo r Slopes (GCO, 1984). It is also usually necessary to monitor groundwater pressures within the various underlying zones (see Chapter 201.

fierefore, an investigation to determine the causes of a failure may be much more detailed in a partzkuhr respect than would n o r d y be the case in an investiga&bn of new works.

8.3 SAFETY OF EXISTING FEATURES AND WORKS

8.3.1 Effect of New Works upon Existing Features and Works

Because of t h e dense u rban development in pa r t s of Hong Kong, i t is often necessary t o invest igate existing fea tures and works in t h e immediate vicinity o r even remote from t h e s i te of t h e proposed new works, t o decide whether t h e existing works a r e likely t o be affected by changes in t h e ground and groundwater conditions b rough t about by t h e new works.

8.3.2 Types of Effects

Existing slopes and s t r uc tu r e s may be affected by changed conditions such as t h e following :

Impeded drainage, which may resu l t in a r i se in t h e groundwater level. This can cause softening of cohesive materials and reduction of shea r s t r eng th of permeable materials, and give r i se t o increased pore p r e s su r e s affecting t h e stability of slopes and retaining walls; swelling may resu l t in ground heave.

Excavations o r demolitions in t h e immediate vicinity. which may cause a reduction in suppo r t t o t h e slope o r s t r uc tu r e , e i the r by general ground deformation o r b y slope instability.

S t res ses t h a t t h e new s t r u c t u r e may impose on existing slopes o r s t r uc tu r e s , o r on t h e foundation materials below adjacent s t ruc tu res . which can cause slope instability o r d i s t r e s s t o existing s t ruc tu res .

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( d ) Vibrations and ground movement result ing from traff ic , v ibra tory compaction, piling o r blasting in t h e immediate vicinity, o r from o the r construct ion activities.

/el Lowering the groundwater level by pumping from wels or dewatering of excavations or tunnels w17l cause an increase in the effective stress in the subsoil affected which can lead to excessive settlement of adjkcent structures. Also, ifpumps do not have an adequate filer, the leaching of tihes from the subs017 can easily result in excessive settlement of structures at considerable distance from the pump. In areas where natura l underground cavities can occur. e.g. k a r s t fea tures in t h e Yuen Long basin, increase in effective s t r e s s o r downward ravelling of soil due t o heavy pumping may lead t o subsidence o r t h e formation of sinkholes (Siu & Wong. 1984).

f f ) Tunnelling operations in the ne~ghbourhood which may cause deformations and subsidence; the effect of tension and compress~un on drainage should not be overlooked.

fg) Alterahon in stream flow of a waterway, wh~ch may cause ~nderc~t t ing of banks or scouring of foundations of wafls, bridges and p~ers, and may be due to works carr~ed out some distance away.

fh) Silahon of the approaches of harbour works changing of navigation channel afignments.

or the

8.3.3 Procedure

In the ~hvest~gat~on of the safety of existing features or works, the /list requirement is an apprec~htion of the changes to the ground that are likely to occur. The ground investigahon wifl need to provide knowledge of the subsurface materials, together with the examination and testing of samples to assess the effect that the changed cond12ions are likely to have on these mater~kls. In some cases, it may be necessary to carry out a detailed analysis to estimate the effect of the changed conditins on the safety of the existing features and works.

8.4 MATERIALS FOR CONSTRUCTION PURPOSES

Invest~gations of sites are sometimes requked : fa) to assess the suitability, and quantities, for construchon

work of materials that become available from excavations or dredging, e.g. whether spoil from cuts in road and rayway works win be suitable for li//s in other places,

( b ) t o f ind suitable materials for specific purposes, e.g. t o locate borrow pits o r a reas for ear thworks ( a common problem in Hong Kong where in tense u rban development demands a cons tant search for sui table fill materials); t o

assess the suitability of materials in waste tips that may

need to be removed for environmental reasons,

fcl to Jocate suitabJe dXsposaJ s12e.s for waste and dredged rnaterids.

9, GEOLOGICAL MAPPING FOR GROUND I N VEST I GA T I ON

The object of geological mapping is t o a s sess t h e charac ter , distr ibution and s t r u c t u r e of t h e soils and rocks underlying t he area. Interpretat ion of t h e geological conditions a t t h e s i te may not be possible without mapping a l a rge r area. An unders tanding of geological f ea tu res is a pre-requisi te t o in terpre t ing t he geological conditions a t t h e site, and a sui tably t ra ined specialist should normally under take th is task.

A s a base map o r reference, t h e new 1 2 0 000 scale geological maps a r e most useful. These maps will become available between 1986 and 1991 (Figure 3). Two existing 1:50 000 geological maps (which cover t h e en t i r e Terr i tory) a r e also available (Allen & Stephens, 1971). Methods used fo r geological mapping a t t h e regional scale a r e equally sui table fo r s i te specific mapping for ground investigations (Geological Society, 1982; Strange. 1986) and may be supplemented by interpretat ion of aerial photographs (see Chapter 6) and geophysical investigations (see Chapter 33).

Natural exposures and artificial exposures, such as cut slopes and quarries, beyond the site may provide data on the material and mass characterisbcs of soils and rocks. ~hcludhg, for example, the orientabon, frequency and character of bedding and jbinbhg discontinu~'bes, weathering profiies, and the nature of the junction between superficial and soM formabbns. Such information should be used as a guide only to conditoons likely to be present at the site. Caution is needed in extrapohling data; geologcal deposits may vary laterdy, and very important geo/ogialstructures, such as faults and other majar disconbhuities, may have only a restricted extent.

It may be expedient to hvestigate local conditions at an early stage of the mapphg, ushg mechanically excavated s M o w pits and trenches. The walls of excavations and, where appropr~ate, the f/oorshould be mapped at a suitably large scale and sampled before backijlfing takes place.

Slope surface s t r ipping is also commonly used in Hong Kong fo r t h e purpose of geological mapping (see Section 18.1).

Recording of geo/ogcal information should be undertaken at a// stages of the works.

Fur the r information and examples of engineering geological mapping a r e given elsewhere (Burnet t & Styles, 1982; Geological Society, 1972; IAEG, 1981; ICE, 1976; UNESCO, 1976).

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10, EXTENT OF THE GROUND INVESTIGATION

10.1 GENERAL

The extent of the ground investigafion is determined by the character and variability of the ground and groundwater, the type of project, and the amount o f existing information. I t is important that the general character and variabifity of the ground should be estabhhed before deciding on the basic principles of the design of the works.

In Hong Kong. soils derived from insitu rock weathering generally exhibit great variability even within relatively short distances. Granitic and volcanic rocks, which together form the major portion of the solid geology of the Territory, may be weathered to soils typically to depths of 30 m and 10 m respectively. Under certain geological conditions, granitic rocks may be weathered to over 100 m deep. Examples can be found in the Mid-levels area. Ma On Shan and Yuen Long. It i s important to recognize that ground conditions may not always improve with depth; on occasions, hard rock at the ground surface may be underlain by thick zones of weaker material. Similarly. fill materials within reclamations may vary considerably. Hong Kong soils and rocks are further discussed in Geoguide 3 (GCO . 1988).

Investigations include a range of "methods '; e.g. excavations, borehok, probing, see Chapters I7 to 23. The factors determining the selection of a particular method are diicussed in Chapters If, I2 and I7 to 23. In genera4 the recommendations in Sections 10.2 to 10.7appfy irrespective of the method adopted, and the term "exploration point" is used to describe a position where the ground is to be exdored b y any partkular method

Each combination of project and site is Me& to be unique, and the foflowing generalpohts should therefore be considered as guidance inplanning the ground investigation and not as a set of rules to be applied rigidly in every case.

10.2 CHARACTER AND VARIABILITY OF THE GROUND

The greater the natural variabifity of the ground, the greater wi'l be the extent of the ground jhvestigation requii-ed to obtain an ihdicatlbn of the character of the ground The depth of exp/orahon is generafly determined by the nature of the works projected, but it may be necessary to exp/ore to greater depths at a lihited number of poihts to establish the overall geological conditions. The technical development of the project should be kept under continuous review, since decisions on the design will influence the extent of the jir vestigation.

It ii jhportant to realize that the detailed geology of a site can be no more than inferred from aerial photography, surface outcrops and subsurface information at the positions of the explorathn points. The possibility remains that significant undetected variations or discontinuities can exist, includihg lateral or vertical varjahons within a given layer. The uncertainties can be reduced but, except by complete excavation. can never be whofly eliminated b y a more ihtensive inveshgation. The use of angled boreholes can in certain cases greatly assist interpreting vari~hons between vertical boreholes (seealso Section 10.l.81. In some circumstances, additonal information between investigation points can be obtained b y geophysical methods (see Chapter 331.

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10.3 NATURE OF THE PROJECT

3 . General

The investigation should yield sufficient data on which to base an adequate and econom~'cal design of the project. It should in aadit~on be sufficient to cover possible methods of construction and, where appropriate, to indicate sources of constructhn materials. The lateral and vertical extent of the invesbgathn should cover aLf ground that may be s~gnificantfy affected by the new works or their constructzbn. Two typical examples are the zone of stressedgruund beneath the bottom of a group of pii'es; and an adJ;lcent slope, the stabZty of which may be reduced by the works.

10.3.2 Slope and Retaining Wall Construction

Due t o t h e extensive construct ion of slopes and retaining walls in Hong Kong, detailed guidance on t h e na tu re and content of s i t e investigation for these f ea tu res is given in Tables 3 and 4. Fur the r discussion of t h e design and construct ion of slopes and retaining walls is given in t h e Geotechnical Manual for Slopes (GCO. 1984) and in Geoguide 1 : Guide t o Retaining Wall Design (GEO, 1993).

10.3.3 Foundations fo r S t ruc tu res

Most s t r uc tu r e s in Hong Kong a r e founded on piles. Hand-dug caissons. driven piles, machine-bored piles and ba r re t t e s a r e commonly used. A general approach t o planning a ground investigation suitable for pile design purposes is given in ICE (1978). The investigation should make a full appraisal of t h e s i te and t h e ground conditions should be invest igated a t dep ths well below t he proposed pile toe level t o allow for variations in t h e pile design. Knowledge of t h e groundwater conditions i s also required. Fur the r advice on ground investigation fo r foundations is given in Section 10.7.2. BSI (1986) and Weltman & Head (1983).

In areas where major s t ruc tu ra l defects in rock may occur (e.g. k a r s t fea tures in t h e Yuen Long basin, o r major shea r o r faul t zones) , more intensive investigation and g rea te r exploration dep ths than normally recommended may be requi red . Consideration may need t o be given t o locating underground cavities within t h e zone of influence of t h e loaded area, a nd t o identifying o the r possible significant fea tures such as steeply-dipping rockhead, f r ac tu res and al ternat ing soil and rock layers.

Recommendations on t h e dep th of exploration fo r foundations for s t r uc tu r e s , including shallow foundations, a r e given in Section 10.7.2.

10.4 PRELIMINARY INVESTIGATION

Before deciding on a full investigation programme, i t may be advisable t o excavate t r ial pits o r t o s t r i p t h e su r face cover from slopes fo r a preliminary assessment of ground conditions. These should be carefully examined, logged and sampled (see Section 18.1).

For la rge projec ts requi r ing s taged ground investigations, i t will often be useful dur ing t h e f i r s t s t age t o c a r r y o u t a geophysical s u rvey in addition t o

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some widely-spaced boreholes t o identify those a reas which requ i re more detailed investigation.

10 .5 LOCATION

The points of exploration, (e.9. boreholes, soundings, pits) should be located so that a general geological view of the whole site can be obtained, together with adequate detays of the engineering properhes of the soils and rocks and of groundwater conditlbns. More de tded informahon shouM be obtained at positbns of important structures and earthworks, at points of special engineering difficulty or importance, and where ground conditions are compficated, e.9. suspected buried va/lys, old s/ipped areas and underground cavities. Rigid, preconceived patterns of pits, boreholes or soundkgs shouM be avoided. In some cases, it wig not be possible to locate subsurface features until much of the ground investigation data has been obtained In such cases, the programme of investigations shou/d be modfled accordingly.

The locations of boreholes and o the r exploration points should only be planned a f t e r t h e desk s t udy , s i te reconnaissance and geological mapping a r e completed. I t i s often useful t o locate boreholes at t h e intended positions of la rge deep foundations. For slopes, boreholes should generally be located along anticipated critical slope sections, a s well a s uphill and downhill for area and regional stability s tudies.

For tunnels and incfined shafts, boreholes should be offset so as not to interfere with subsequent construction. For other structures, the need to offset boreboles and trial excavahbns from criticapoints should be considered In most cases, boreholes should be carefufly backfilled, concreted or grouted up. Tr~klexcavations shouM be located outside proposed foundation areas.

I t is essentid that accurate locations and ground levels for all exploration points should be estabfished, I necessary b y survey (see Section l.2).

10.6 SPACING

Although no hard and fast rules can be laid down, a relatively close spacing between points of &orahon, e.g. I0 to 30 m, wi/ often be appropriate for structures. For structures small in plan area, explorahon should be made at a minimum of t h e points ifpossible. Where a structure consists of a number of adjacent units, one exp/oration poht per unit may suffice. Certain engineerhg works, such as dams, tunnels and major excavations, are particularly sensih've to geological conditons, and the spacing and locahon of exploration points shouM be more closely related to the detailed geology of the area than I> usual for other works.

In t h e case of a proposed c u t slope extending from soil into rock, t h e level of bedrock along t h e face of t h e cutt ing is important. Consideration should be given t o obtaining t h e subsurface profile by additional drilling and geophysical means. In t h e case of reclamation, v e r y closely spaced boreholes may be requi red t o locate and delineate buried obstruct ions such a s remnants of an old seawall.

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10.7 DEPTH OF EXPLORATION

10.7.1 General

The dep th of exploration is governed by t h e dep th t o which t h e new project will affect t h e ground and groundwater o r b e affected by them. Normally, exploration should be taken below all deposits t h a t may be unsuitable for foundation purposes, e.g. fill and weak compressible soils, including any weak materials overlain by a s t r o n g e r layer. The exploration should be taken th rough compressible soils likely t o cont r ibute significantly t o t h e settlement of t h e proposed works, generally t o a depth where s t r e s s increases cease t o b e significant, o r deeper.

In Hong Kong, i t is common practice to drill into rock fo r a dep th of a t least 5 m t o establish whether corestone, boulder o r bedrock has been encountered. However, t h e final depth of drilling will depend on t h e need t o prove bedrock. In some cases, i t will be necessary t o drill deeper than 5 m t o establish conclusively t h e presence of bedrock, for example, i n investigations fo r end bearing piles (see Section 10.7.2). In o the r instances, i t may not be necessary t o terminate drilling in rock (see Section 10.7.4).

More specifica//y. the recommendations given ~nSections I'D. Z 2 to I'D. Z 8 may be considered for certain types of work. I t is not always necessary that every exploration should be taken to depths recommended in SectJons IO.l.2 to 10.Z8. In many instances, it wi// be adequate IF one or more boreholes are taken to those depths in the early stages of the field work to estabfish the general subsurface profile, and then the remainder sunk to some lesser depths to exp/ore more thoroughly the zone near the surface which the initial exphrahon had shown to be most relevant to the problem in hand

10.7.2 Foundations for S t ruc tu res

In the case of foundations for structures, the depth of exphratron should be at least one and a half times the width of the loaded area. Commonsense will indicate exceptions to this guidefine; for example, 12 would not usually be necessary to continue drilfing for long distances in strong rock. For foundations near the surface, the loaded area is considered as either :

(a/ the area of an individual footing, or (bl the plan area of the structure, where the spacing

foundahbn footings is less than about three tJhes tbreadth, or where the floor loading is significant, or

of he

(cl the area of a foundation raft. In each case, the depth should be measured below the base of the

footing or raft. Where piled foundations are considered to be a possibi/ity, the length of

pile usually cannot be decided unt17 an advanced stage of the project. No exp/icit rules can be given for the depth of exploration, but the fohwing offer some guidance :

(a) Fill and weak compressible soils seldom contr ibute t o t h e sha f t res is tance of a pile and may add down d r a g t o t h e

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load on it. The whole pile load. possibly with t h e addition of down drag . will have t o be borne by t h e s t ronger materials lying below t he weak materials, e i the r in end bearing o r th rough sha f t resis tance.

The length of driven piles is often determined initially by t he driving resistance, and subsequent ly checked by load t e s t s . Hence, in such cases, t h e length of t h e pile is not accurately known until t h e piling cont rac t begins, b u t i t may be possible t o gain a n early indication from s tandard penetration t e s t (SPT) blow counts. In t h e case of end bearing piles in s t rong rock, boreholes should be of sufficient depth t o establish conclusively t h e presence of bedrock. The rock should then be f u r t h e r explored, usually by means of ro ta ry drilling, t o such a dep th t h a t t h e engineer directing t he investigation (see Section 15.2) is satisfied t h a t t h e r e is no possibility of weaker materials occurring lower down t h a t could affect t h e performance of t h e piles. This will usually r equ i re penetrat ing a t leas t 5 m, o r two and a half times t h e diameter of t h e pile, whichever is l a rge r , below t he proposed founding level of t h e pile. For boreholes car r ied ou t dur ing construction t o prove sat isfactory pile founding levels, t h e dep th of penetration may have t o be increased where la rge corestones o r boulders a r e suspected o r have been identified nearby.

In weathered rocks, i t may not always be feasible t o locate underlying f resh rock. Foundations in th is case must often be founded in t h e weathered rock, and proving t he s t r eng th and continuity of t h e material below t h e intended founding level and t h e location, na tu re and orientation of discontinuities may then suffice.

Pile-supported ra f t s on clay may be used solely t o reduce settlement. In these cases, t h e depth of exploration is governed by t h e need t o examine all subsurface materials t h a t could contr ibute significantly t o t h e settlement. Similarly, for pile groups on clay, i t will be necessary t o e n s u r e t h a t t h e dep th of exploration i s sufficient t o prove t he adequacy of t h e founding material below t h e toe of t h e piles.

Based on the informat~bn of the probable subsurface profile obtained from the desk study, the genera/ guidance given in fal to (el above,and an assessment of the types of pile likely to be considered, the engineer directing the hvestigathn shouh' determine the depth of exploraation and be ready during the course of the field work to modify this depth as appears to be necessary. In any event, exp/oration should at some pofnts be taken below the depth to which it is cons~'deredMely that the longest piles wil penetrate.

I t should be noted t h a t if any s t r u c t u r e i s likely t o be affected by subs idence due t o collapse of underground cavities (e.g. k a r s t fea tures in t h e Yuen Long basin) o r any o the r causes, g rea te r exploration dep ths than those recommended in th i s Section may be requi red .

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10.Z3 Embankments

For embankments on alluvial and marhe soils, the depth of the exploraation should be sufficient to check possible shear failure through the foundation materials and to assess t%re likely amount of any settlement due to compress~Ble materids. In the case of water-retaining embankments, investigation should explore all materials through which piping could be initiated or significant seepage occur.

10.7.4 C u t Slopes

The dep th of exploration for c u t slopes should be sufficient t o permit full assessment of t h e stability of t h e slope. This may necessitate proving t he full dep th of any relatively weak o r impermeable materials such as decomposed dykes (Au. 1986). In general, exploration for slopes should extend a minimum of 5 m below t h e toe of t h e slope o r 5 m in to bedrock, whichever i s shallower. However, one o r more exploration points should in all cases extend below t h e toe of t h e slope o r excavation, i rrespective of bedrock level. Groundwater conditions, including t h e possibility of perched o r multiple water tables, should also be determined.

10.7.5 Pavements

For pavements, t h e dep th of exploration should be sufficient t o determine t h e s t r eng th and drainage conditions of possible sub-grades . Exploration t o a dep th of 2 t o 3 m below t he proposed formation level will probably be sufficient in most cases.

For shallow s d p~pefines,it wil frequently b