Geoguide 2
Guide to
Site Investigation
Geotechnical Engineering Office
Civil Engineering and Development Department The Government of the Hong Kong
Special Administrative Region
Geoguide 2
Guide to
Site Investigation
Geotechnical Engineering Office
Civil Engineering and Development Department The Government of the Hong Kong
Special Administrative Region
© 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
Continuously updated e-version, 18 December 2017
Prepared by:
Geotechnical Engineering Office,
Civil Engineering and Development Department, Civil Engineering Building,
101 Princess Margaret Road, Homantin, Kowloon,
Hong Kong.
This Geoguide is a continuously updated version incorporating amendments issued since the Geoguide was last published. The continuously updated version is released in e-format only on the CEDD website. This Geoguide is to be cited as “GEO (2017). Guide to Site Investigation (Geoguide 2) (Continuously Updated E-Version released on 18 December 2017).
Geotechnical Engineering Office, Civil Engineering and Development Department, HKSAR Government, 349 p.”
Foreword
This Geoguide presents a recommended standard of good practice for site investigation in Hong Kong, the need for which was formally recognized as early as July 1983 by the Subcommittee of the Building Authority Working Party on Geotechnical Regulations. In its format and content, the Geoguide follows closely the British Standard BS 5930 : 1981, Code of Practice for Site Investigations, but the recommendations in the British Standard have been adapted to suit local conditions and practices. It should be used in conjunction with the companion document, Guide to Rock and Soil Descriptions (Geoguide 3). These Geoguides expand upon, and largely replace, Chapter 2 of the Geotechnical Manual for Slopes.
This Geoguide covers Sections l to 7 of BS 5930, while Section 8 is dealt with in Geoguide 3. It has been prepared in such a way that the organization and format of the British Standard have generally been preserved. Where portions of BS 5930 have been adopted in the text without significant amendment, this is clearly denoted by the use of an italic typeface.
It should be noted that this Geoguide gives guidance on good site investigation practice and, as such, its recommendations are not mandatory. It is recognized that the practitioner will often need to use alternative methods. There will also be improvements in site investigation practice during the life of the document which will supersede some of its recommendations.
The Geoguide was prepared in the Geotechnical Control Office (GCO) under the general direction of Mr J.B. Massey. The main contributors to the document were Dr A. Cipullo, Mr K.S. Smith and Mr D.R. Greenway, with significant contributions during the final stages of preparation from Dr P.L.R. Pang and Dr R.P. Martin. Many other members of the GCO made valuable suggestions and contributions.
To ensure that the Geoguide would be considered a consensus document of the civil engineering profession in Hong Kong, a draft version was circulated widely for comment in early 1987 to contractors, consulting engineers and Government Departments. Many organizations and individuals made useful and constructive comments, which have been taken into account in finalizing the Geoguide, and their contributions are gratefully acknowledged.
Practitioners are encouraged to comment at any time to the Geotechnical Control Office on the contents of this Geoguide, so that improvements can be made to future editions.
E.W. Brand
Principal Government Geotechnical Engineer September 1987
Contents
Page No.
Title Page 1
Foreword 3
Contents 4
Part I : Introduction 17
1. Scope 18
2. Terminology 20
Part II : General Considerations 21
3. Primary Objectives of Site Investigation 22
4. General Procedures 23
4.1 Extent and Sequence of Investigation 23
4.1.1 General 23
4.1.2 Adjacent Property 23
4.2 Desk Study 24
4.3 Site Reconnaissance 25
4.4 Detailed Examination and Special Studies 25
4.5 Construction and Performance Appraisal 25
4.6 Site Investigation for Tunnel Works 26
5. Earlier Uses of the Site 28
5.1 General 28
5.2 Tunnels 28
5.3 Mines and Quarries 28
5.4 Waste Tips 28
5.5 Other Earlier Uses 28
5.6 Ancient Monuments 29
6. Aerial Photographs 30
6.1 General 30
6.2 Topographic Maps and Aerial Photographic Imagery 30
Page No.
6.2.1 Map and Plan Scales 30
6.2.2 Aerial Photographic Imagery 30
6.2.3 Orthophoto Maps and Plans 31
6.3 Aerial Photograph Interpretation 31
6.3.1 Identification and Interpretation of Ground 31 Features
6.3.2 Examples of API in Hong Kong 32
Part III : Planning the Ground Investigation 33
7. Introduction to Ground Investigation 34
7.1 Objectives 34
7.2 Planning and Control 35
8. Types of Ground Investigation 37
8.1 Sites for New Works 37
8.2 Defects or Failures of Existing Features or Works 37
8.3 Safety of Existing Features and Works 38
8.3.1 Effect of New Works upon Existing Features and Works 38
8.3.2 Types of Effects 38
8.3.3 Procedure 39
8.4 Materials for Construction Purposes 39
9. Geological Mapping for Ground Investigation 41
10. Extent of the Ground Investigation 42
10.1 General 42
10.2 Character and Variability of the Ground 42
10.3 Nature of the Project 43
10.3.1 General 43
10.3.2 Slope and Retaining Wall Construction 43
10.3.3 Foundations for Structures 43
10.4 Preliminary Investigation 43
10.5 Location 44
Page No.
10.6 Spacing 44
10.7 Depth of Exploration 45
10.7.1 General 45
10.7.2 Foundations for Structures 45
10.7.3 Embankments 47
10.7.4 Cut Slopes 47
10.7.5 Pavements 47
10.7.6 Pipelines 47
10.7.7 Marine Works 48
10.8 Specific Guidance for Silt- and Clay-rich Layers of Rock Cut 48 Slopes
10.9 Specific Guidance for Tunnel Works 50
10.9.1 Preliminary Design Stage 50
10.9.2 Detailed Design Stage 51
11. Selection of Ground Investigation Methods 53
11.1 General 53
11.2 Site Considerations 53
12. Effect of Ground Conditions on Investigation Methods 55
12.1 General 55
12.2 Granular Soils Containing Boulders, Cobbles or Gravel 55
12.3 Granular Soils 56
12.4 Intermediate Soils 57
12.5 Very Soft to Soft Cohesive Soils 57
12.6 Firm to Stiff Cohesive Soils 57
12.7 Cohesive Soils Containing Boulders, Cobbles or Gravel 58
12.8 Fill 58
12.9 Rock 59
12.10 Soils Derived from Insitu Rock Weathering 59
12.11 Discontinuities 60
12.12 Cavities 61
Page No.
13. Aggressive Ground and Groundwater 62
13.1 General 62
13.2 Investigation of Potential Deterioration of Concrete 62
13.3 Investigation of Potential Corrosion of Steel 62
13.4 Investigation of Fill Containing Industrial Wastes 63
14. Ground Investigations over Water 64
14.1 General 64
14.2 Stages and Platforms 65
14.3 Floating Craft 65
14.4 Working between Tide Levels 66
14.5 Locating Borehole Positions 66
14.6 Determination of Reduced Levels 66
14.7 Drilling, Sampling and Testing 67
15. Personnel for Ground Investigation 68
15.1 General 68
15.2 Planning and Direction 68
15.3 Supervision in the Field 68
15.4 Logging and Description of Ground Conditions 69
15.5 Laboratory Testing 70
15.6 Specialist Advice 70
15.7 Interpretation 70
15.8 Operatives 70
16. Review during Construction 72
16.1 General 72
16.2 Purpose 72
16.3 Information Required 73
16.3.1 Soil and Rock 73
16.3.2 Water 73
16.4 Instrumentation 73
16.5 Tunnel Works 73
Page No.
Part IV : Ground Investigation Methods 75
17. Introduction to Ground Investigation Methods 76
18. Excavations and Boreholes 77
18.1 Shallow Trial Pits and Slope Surface Stripping 77
18.2 Deep Trial Pits and Caissons 78
18.3 Headings or Adits 78
18.4 Hand Auger Boring 78
18.5 Light Cable Percussion Boring 79
18.6 Mechanical Augers 79
18.7 Rotary Open Hole Drilling and Rotary Core Drilling 79
18.7.1 General 79
18.7.2 Flushing Medium 80
18.7.3 Inclined Drilling 81
18.8 Wash Boring and Other Methods 82
18.8.1 Wash Boring 82
18.8.2 Other Methods of Boring 82
18.9 Backfilling Excavations and Boreholes 83
19. Sampling the Ground 84
19.1 General 84
19.2 Sample Quality 85
19.3 Disturbed Samples from Boring Tools or Excavating 85 Equipment
19.4 Open-tube Samplers 86
19.4.1 Principles of Design 86
19.4.2 Sampling Procedure 87
19.4.3 Thin-walled Samplers 88
19.4.4 General Purpose 100 mm Diameter Open-tube Sampler 88 19.4.5 Split Barrel Standard Penetration Test Sampler 89
19.5 Thin-walled Stationary Piston Sampler 89
19.6 Continuous Soil Sampling 89
Page No.
19.6.1 General 89
19.6.2 The Delft Continuous Sampler 90
19.7 Sand Samplers 90
19.8 Rotary Core Samples 90
19.9 Block Samples 92
19.10 Handling and Labelling of Samples 92
19.10.1 General 92
19.10.2 Labelling 93
19.10.3 Disturbed Samples of Soil and Hand Specimens of Rock 93
19.10.4 Samples Taken with a Tube Sampler 94
19.10.5 Rotary Core Extrusion and Preservation 95
19.10.6 Block Samples 96
20. Groundwater 97
20.1 General 97
20.2 Methods of Determining Groundwater Pressures 98
20.2.1 Response Time 98
20.2.2 Observations in Boreholes and Excavations 98
20.2.3 Standpipe Piezometers 99
20.2.4 Hydraulic Piezometers 99
20.2.5 Electrical Piezometers 100
20.2.6 Pneumatic Piezometers 100
20.2.7 Installation of Piezometers 101
20.2.8 Varying Groundwater Pressures 102
20.2.9 Soil Suction 103
20.3 Groundwater Samples 103
21. Tests in Boreholes 105
21.1 General 105
21.2 Standard Penetration Tests 105
21.2.1 General Principles 105
21.2.2 Preparation for Testing 105
Page No.
21.2.3 Advantages and Limitations 106
21.2.4 Results and Interpretation 106
21.3 Vane Tests 107
21.3.1 General Principles 107
21.3.2 Advantages and Limitations 107
21.4 Permeability Tests 108
21.4.1 General Principles 108
21.4.2 Preparations for the Test 108
21.4.3 Variable-head Test 109
21.4.4 Constant-head Test 109
21.4.5 Analysis of Results 110
21.4.6 Formulae for Borehole Permeability Tests 110
21.4.7 Advantages and Limitations 111
21.5 Packer (Water Absorption) Tests 113
21.5.1 General Principles 113
21.5.2 Packers 114
21.5.3 Application and Measurement of Pressure 114
21.5.4 Measurement of Flow 115
21.5.5 Execution of Test 115
21.5.6 Results and Interpretation 116
21.6 Plate Tests 116
21.6.1 General 116
21.6.2 Limitations 117
21.6.3 Preparation 117
21.6.4 Bedding of the Plate 117
21.6.5 Application and Measurement of Load 118
21.6.6 Measurement of Deflection 118
21.6.7 Execution of Test 118
21.6.8 Uses of the Test 118
21.6.9 Supplementary Test 118
21.6.10 Horizontal Plate Tests 119
Page No.
21.7 Pressuremeter Tests 119
21.7.1 Test Description 119
21.7.2 Equipment Calibration 119
21.7.3 Forming the Test Pocket 120
21.7.4 Results and Interpretation 120
21.7.5 Tests in Rock 120
21.8 Borehole Discontinuity Surveys 121
21.8.1 Impression Packer Survey 121
21.8.2 Core Orientators 122
22. Frequency of Sampling and Testing in Boreholes 123
22.1 General Principles 123
22.2 Determination of the Ground Profile 123
22.3 Routine Determination of Soil and Rock Properties 124
22.4 Double-hole Sampling 124
22.5 Special Techniques 124
23. Probing and Penetration Testing 126
23.1 General 126
23.2 Dynamic Probing 126
23.3 Static Probing or Cone Penetration Testing 127
23.3.1 General Description 127
23.3.2 Mechanical Cone Penetrometers 127
23.3.3 Electrical Cone Penetrometers 128
23.3.4 General Recommendations 128
23.3.5 Uses and Limitations of the Test 129
23.3.6 Presentation of Results 129
23.4 Static-dynamic Probing 129
Part V : Field and Laboratory Tests 130
24. Field Tests 131
24.1 General 131
Page No.
24.2 Rock Strength Index Tests 132
24.2.1 Point Load Strength 132
24.2.2 Schmidt Hammer Rebound Value 133
24.3 Infiltration Tests 133
25. Pumping Tests 135
25.1 General Principles 135
25.2 Groundwater Conditions 136
25.3 Test Site 136
25.4 Pumped Wells 136
25.5 Observation Wells 138
25.6 Test Procedures 138
25.7 Analysis of Results 139
26. Discontinuity Surveys 140
26.1 General 140
26.2 Discontinuity Surveys for Rock Cut Slopes 140
26.3 Discontinuity Roughness Surveys 140
27. Field Density Tests 142
27.1 General Principles 142
27.2 Sand Replacement Method 142
27.3 Core Cutter Method 143
27.4 Immersion in Water Method 143
27.5 Water Displacement Method 143
27.6 Rubber Balloon Method 143
27.7 Nuclear Methods 144
27.8 Water Replacement Method for Rock Fill 144
28. Insitu Stress Measurements 146
28.1 General 146
28.2 Stress Measurements in Rock 146
28.3 Stress Measurements in Soils 147
Page No.
29. Bearing Tests 149
29.1 Vertical Loading Tests 149
29.1.1 General Principles 149
29.1.2 Limitations of the Test 149
29.1.3 Site Preparation 150
29.1.4 Test Arrangement 150
29.1.5 Measurements 151
29.1.6 Test Methods 151
29.1.7 Analysis of Results 152
29.1.8 Interpretation of Results 153
29.2 Horizontal and Inclined Loading Tests 154
29.3 Pressurized Chamber Tests 154
29.4 Insitu California Bearing Ratio (CBR) Tests 155
29.4.1 General 155
29.4.2 Test Method 155
29.4.3 Limitations and Use of Test 155
30. Insitu Direct Shear Tests 157
30.1 General Principles 157
30.2 Sample Preparation 157
30.3 Test Arrangement 158
30.4 Measurements 159
30.5 Test Methods 159
30.6 Analysis of Results 159
31. Large-scale Field Trials 160
31.1 General 160
31.2 Methods of Instrumentation 160
31.3 Trial Embankments and Excavations 161
31.4 Construction Trials 162
32. Back Analysis 163
32.1 General 163
Page No.
32.2 Failures and Design of Slope Preventive or Remendial Works 163
32.3 Other Cases 164
33. Geophysical Surveying 166
33.1 General 166
33.2 Land Geophysics 167
33.2.1 Resistivity 167
33.2.2 Gravimetric 167
33.2.3 Magnetic 167
33.2.4 Seismic 168
33.2.5 The Gamma Density Method 168
33.2.6 The Spectral Gamma Ray Method 169
33.2.7 The Neutron Porosity Method 170
33.3 Marine Geophysics 170
33.3.1 General 170
33.3.2 Echo-sounding 170
33.3.3 Continuous Seismic Reflection Profiling 170
33.3.4 Side Scan Sonar 170
33.4 Borehole Logging 170
33.5 Corrosion Testing 172
33.6 Identification of Weak Layers in the Ground 172
34. Principles of Laboratory Testing 173
35. Sample Storage and Inspection Facilities 174
35.1 Handling and Labelling 174
35.2 Storage of Samples 174
35.3 Inspection Facilities 174
36. Visual Examination 176
36.1 General 176
36.2 Soil 176
36.3 Rock 176
Page No.
36.4 Photographic Records 176
37. Tests on Soil 178
37.1 General 178
37.2 Sample Quality 178
37.3 Sample Size 178
37.4 Test Conditions 178
37.5 Relevance of Test Results 179
38. Tests on Rock 180
Part VI : Reports and Interpretation 181
39. Field Reports 182
40. Site Investigation Report 183
40.1 General 183
40.2 Descriptive Report 183
40.2.1 Report as Record 183
40.2.2 Introduction 183
40.2.3 Description of Site 183
40.2.4 Geology 184
40.2.5 Field Work 184
40.2.6 Borehole Logs 184
40.2.7 Incidence and Behaviour of Groundwater 187
40.2.8 Location of Boreholes 187
40.2.9 Laboratory Test Results and Sample Descriptions 188
40.3 Engineering Interpretation 188
40.3.1 Matters to be Covered 188
40.3.2 Data on which Interpretation is Based 188
40.3.3 Presentation of Borehole Data 189
40.3.4 Design 190
40.3.5 Construction Expedients 191
40.3.6 Sources of Materials 192
Page No.
40.3.7 Failures 192
40.3.8 Calculations 193
40.3.9 References 193
40.4 Documentation for Site Investigation of Tunnel Works 193
References 194
Tables 215
List of Tables 216
Tables 217
Figures 233
List of Figures 234
Figures 237
Plates 284
List of Plates 285
Plates 286
Appendices 298
Appendix A: Information Required for Desk Study 399
Appendix B: Sources of Information 307
Appendix C: Notes on Site Reconnaissance 331
Appendix D: Information Required for Design and Construction 336
Appendix E: Safety Precautions 341
Part I
Introduction
1. Scope
This Geoguide deals with the investigation of sites in Hong Kong for the purposes of assessing their suitability for civil engineering and building works, and of acquiring knowledge of site characteristics that affect the design and construction of such works and the security of adjacent properties. It is essentially BS 5930 : 1981, Code of Practice for Site Investigations (BSI, 1981a), modified as considered desirable for use in Hong Kong.
While the basic structure and philosophy of BSI (1981a) has been maintained in this Geoguide, topics of particular importance in Hong Kong have been supplemented or rewritten in the light of local conditions and experience. Other sections of BSI (1981a) have been repeated herein without significant amendment, and this has been denoted by an italic script.
Less relevant or rarely-used portions of BSI (1981a) have been incorporated only by reference, or have been specifically deleted.
In this Geoguide the expression "site investigation" has been used in its wider sense. It is often used elsewhere in a narrow sense to describe what has been termed herein "ground investigation". The use of soil and rock as construction materials is treated only briefly;
further information on this is given in BSI (2009). [Amd GG2/01/2017]
From Part II onwards, this Geoguide is divided as follows :
Part II. Part II deals with those matters of a technical, legal or environmental character that should be taken into account in selecting the site (or in determining whether a proposed site is suitable) and in preparing the design of the works.
Part III. Part III discusses general aspects and planning of ground investigation, including the influence of general conditions and ground conditions on the selection of methods of investigation.
Parts IV and V. Parts IV and V discuss methods of ground investigation, sub-divided as follows : Part IV deals with excavation, boring, sampling, probing and tests in boreholes;
Part V deals with field tests and laboratory tests on samples.
Part VI. Part VI deals with the preparation of field reports and borehole logs, the interpretation of the data obtained from the investigation and the preparation of the final site investigation report.
The last section of BSI (1981a), which deals with the description of soils and rocks, is not covered in this Geoguide. A companion document, Geoguide 3 : Guide to Rock and Soil Descriptions (GCO, 1988), has been devoted entirely to this topic, and the reader should refer to it for guidance on the description and classification of Hong Kong rocks and soils.
It may be noted that there are some imbalances in treatment of the various topics, with, in some cases, more comprehensive coverage given to methods that are less frequently used.
Because it would not be possible to include full coverage of all available site investigation techniques, methods that are well documented elsewhere in the literature receive abbreviated 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 as 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 descriptive terms for geological materials and types of ground investigation are often interpreted in different ways and therefore require definition. In this Geoguide, the terminology given in the following paragraphs has been adopted.
"Rock" refers to all solid material of natural geological origin that cannot be broken down by hand. "Soil" refers to any naturally-formed earth material or fill that can be broken down by hand and includes rock which has weathered insitu to the condition of an engineering soil. Further guidance on the use of these terms is given in Geoguide 3 (GCO, 1988).
Excluding any boulders or cobbles, a "fine-grained soil" or a "fine soil" is one that contains about 35% or more of fine material (silt and clay size particles). A "coarse-grained soil" or a "coarse soil" contains less than 35% of fine material and more than 65% of coarse material (gravel and sand size particles). Further guidance is given in Geoguide 3.
A "cohesive soil" is one which, usually by virtue of its fines content, will form a coherent mass. Conversely a "granular soil" or a "cohesionless soil" will not form a coherent mass.
These simple terms are useful in the classification of materials during ground investigation for the purpose of choosing a suitable method for sampling the ground. A fine soil is generally cohesive.
The "matrix" of a composite soil refers to the fine-grained material enclosing, or filling the spaces between, the larger grains or particles in the soil.
"Boring" is a method of advancing a cased or uncased hole (viz a "borehole") in the ground and includes auger boring, percussion boring and rotary drilling, in which a drill bit is rotated into the ground for the purpose of forming the hole. Although the term "drillhole" is commonly used in Hong Kong because of the popular use of the rotary core drilling method in ground investigations, the general term "borehole" is used throughout this Geoguide for simplicity, whether the hole is bored, augured or drilled.
Part II
General Considerations
3. Primary Objectives of Site Investigation
Investigation of the site is an essential preliminary to the construction of all civil engineering and building works, and the objectives in making such investigations are as follows :
(a) Suitability. To assess the general suitability of the site and environs for the proposed works.
(b) Design. To enable an adequate and economic design to be prepared, including the design of temporary works.
(c) Construction. To plan the best method of construction; to foresee and provide against difficulties and delays that may arise during construction due to ground and other local conditions; in appropriate cases, to explore sources of indigenous materials for use in construction (see Section 8.4);
and to select sites for the disposal of waste or surplus materials.
(d) Effect of Changes. To determine the changes that may arise in the ground and environmental conditions, either naturally or as a result of the works, and the effect of such changes on the works, on adjacent works, and on the environment in general.
(e) Choice of Site. Where alternatives exist, to advise on the relative suitability of different sites, or different parts of the same site.
In addition, site investigations may be necessary in reporting upon the safety of existing features and works (see Section 8.3), for the design of extensions, vertical or horizontal, to existing works, and for investigating cases where failure has occurred (see Section 8.2).
4. General Procedures
4.1 Extent and Sequence of Investigation 4.1.1 General
The extent of the investigation depends primarily upon the magnitude and nature of the proposed works and the nature of the site.
A site investigation wil1 normally proceed in stages, as follows : desk study; site reconnaissance; detailed examination for design, including ground investigation, topographic and hydrographic survey and special studies; follow-up investigations during construction (Figure 1). This may be followed 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 of the desk study.
The costs of a site investigation are low in relation to the overall cost of a project and may be further reduced by intelligent forward planning. Discussion at an early stage with a specialist contractor will help to formulate an efficient and economic plan. The technical requirements of the investigation should be the overriding factor in the selection of investigatory methods, rather than their cost.
As far as possible, assembly of the desk study information should be complete, at least in respect of those aspects related to ground conditions, before ground investigation begins. A preliminary ground investigation 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 studies or site investigation of projects covering large areas, e.g. road, tunnel or transmission line routes, techniques such as engineering geological and geomorphological mapping, terrain classification and hazard analysis may be useful to delineate critical areas so that detailed investigations can be concentrated in areas where they are most required (Brand et al, 1982; Griffiths & Marsh, 1984; Hansen, 1982).
4.1.2 Adjacent Property
Because of the dense urban development in Hong Kong, construction activities can often affect adjacent property. It is therefore essential that investigations should cover all factors that may affect adjacent property, including features such as slopes and retaining walls (see Chapter 7 and Section 8.3). Where possible, records of ground levels, groundwater levels and relevant particulars of adjacent properties should be made before, during and after construction.
Where damage to existing structures is a possibility, adequate photographic records should be obtained.
Adjacent buildings, structures and buried services, including pipes conveying water, gas or sewage, should be specifically considered, as they may be affected by vibrations, ground settlement or movement, or changes in groundwater levels during and after construction activities on the site. Hospitals and other buildings containing sensitive instruments or apparatus should be given special consideration.
Special permission or approval must be obtained when the site is above or near the Mass Transit Railway Corporation's tunnels or structures, or is within the Mid-levels Scheduled Area (see Appendices A and B; see also Chapter 7). The approximate locations of these two features are shown in Figure 2.
4.2 Desk Study
As a first stage in a site investigation, a desk study is necessary and Appendix A indicates the types of information that may be required. Much information about a site may already be available in existing records. A summary of the important sources of information is given in Appendix B. Readers are advised to take note of any warning messages on the data, check with the relevant data owners on the reliability, accuracy and completeness of the data they require where necessary, taking into account the needs of their project. Readers are also invited to provide feedback to the GEO should the need to update this Geoguide 2 be identified.
[Amd GG2/01/2017]
A new geological survey is currently underway in Hong Kong to replace the 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 survey uses different nomenclature for certain major rock divisions and rock types (Addison, 1986; GCO, 1988; Strange & Shaw, 1986); this should be used wherever possible.
An important source of basic geotechnical information is the Geotechnical Area Study Programme (GASP) publications available from the Government Publications Centre.
Systematic terrain evaluation has been undertaken at a scale of 1:20 000 covering the entire Territory (Brand et al, 1982). These publications generally contain Engineering Geology, Terrain Classification, Erosion, Landform and Physical Constraint Maps. Selected areas of the Territory have also been evaluated at the 'district' scale of 1:2 500, but these have not been published. The GASP programme and the areas covered by the GASP publications are shown in Figure 4, and examples of some of the 1:20 000 maps are given in Figure 5.
The Geotechnical Information Unit also contains numerous records of boreholes from throughout the Territory, as well as useful records of landslides, rainfall and piezometric data, and laboratory test results on soil and rock samples. Relevant data can be easily accessed by geographical location of the site. Further details of the Geotechnical Information Unit are given in Appendix B.
A useful bibliography on the geology and geotechnical engineering of Hong Kong is also available (Brand, 1992). Local maps and plans are easily obtained (Table 1), and as-built records of private developments are retained by the Buildings Ordinance Office or the Public Records Office (see Appendix B). Valuable information may often be obtained from aerial photographs, as discussed in Chapter 6.
4.3 Site Reconnaissance
At an early stage, a thorough visual examination should be made of the site. The extent to which ground adjacent to the site should also be examined is, in general, a matter of judgement (see Section 4.1.2). In the intensely-developed urban areas of Hong Kong, it will usually be necessary to inspect existing slopes and retaining walls within and surrounding the site and adjacent properties during the site reconnaissance stage. Appendix C gives a summary of the procedure for site reconnaissance and the main points to be considered but should not be regarded as necessarily covering all requirements.
Nearby cut slopes can reveal soil and rock types and their stability characteristics, as can old excavations and quarries. Similarly, in the vicinity there may be embankments or buildings and other structures having a settlement history because of the presence of compressible or unstable soils. Other important evidence that might be obtained from an inspection is the presence of underground excavations, such as basements and tunnels. The behaviour of structures similar to those intended should also provide useful information, and the absence of such structures may be significant, as may be also the presence of a vacant site in the midst of otherwise intensive development.
Examples of earlier uses of the site that may affect the new construction works are given in Chapter 5.
4.4 Detailed Examination and Special Studies
For most projects, the design and planning of construction will require a detailed examination of the site and its surroundings (see also Appendix D). Such requirements may necessitate a detailed land survey (see Appendix D.2), or an investigation of liability to flooding.
The investigation of ground conditions is dealt with in Parts III and IV. Other requirements may entail studies of special subjects such as hydrography (see Appendix D.3);
micrometeorology (see Appendix D.4); sources of materials (see Appendix D.5); disposal of waste materials (see Appendix D.6); or other environmental considerations.
The possibility of disused tunnels affecting the site should also be considered (see Section 5.2).
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.
4.6 Site Investigation for Tunnel Works [Amd GG2/01/2017]
Pre-tender site investigation should be as comprehensive as possible to provide adequate information for the design of tunnel works and contract preparation. In addition to the geological and hydrogeological conditions, the site investigation should identify utilities and buried installations to ascertain whether they will interfere with or be affected by the tunnel works (see ETWB TC(W) No. 17/2004 for government projects). [Amd GG2/01/2017]
There are inherent uncertainties in the subsurface geology and hydrogeology, regardless of the extent of site investigation. Also, physical constraints, e.g. existing buildings and subsurface installations could limit the pre-tender site investigation for particular sections of tunnel works. Therefore, it is essential to make provision for additional ground investigation in the works contract to check and monitor continuously the actual conditions against those assumed, and to take measures to deal with conditions not anticipated but having significant impact on the design, construction, or on life and property.
[Amd GG2/01/2017]
The US Army Corps of Engineers Manual (USACE, 1997) includes a practical guide to the relative cost of site investigation as a proportion of the estimated construction cost. Based on this guide, the typical cost of site investigation for a deep tunnel located in difficult ground conditions and in a dense urban area is about 3-4% of the estimated construction cost.
Notwithstanding, the cost of site investigation for a particular project depends greatly on the quality, suitability and adequacy of available information, and the data needed for the design and risk management of the types of tunnel works involved. The client should include adequate funding for site investigation in the project cost estimate. [Amd GG2/01/2017]
Site investigation for projects involving tunnel works should be phased. This approach is necessary as different phases of the project have different requirements. Also, the tunnel alignment and design requirements can change during route planning or design.
[Amd GG2/01/2017]
Using the data obtained at each phase, the impact of the proposed excavation method on the sensitive receivers identified and the geotechnical risks at each tunnel section should be assessed. The risk assessment should be reviewed when the tunnel alignment is fixed and as
more information becomes available. [Amd GG2/01/2017]
Some simple guidelines on site investigation for tunnelling are given in ITA (2009).
An outline of the engineering considerations and site investigation techniques for rock tunnels, based on IMMM (2003), is given in Table 13. Supplementary information on ground investigation techniques is given in Appendix F. [Amd GG2/01/2017]
For sources of information and expertise, reference should be made to Appendix B of this Geoguide and GEO (2016) for general guidance on, and sources of information for SI and tunnels in Hong Kong. Reference should also be made to Geoguide 4 (GEO, 1992), which contains guidance on site investigation for cavern schemes, much of which is also applicable to
tunnels and shaft construction. [Amd GG2/01/2017]
Information on the pre-Quaternary geology of Hong Kong is given in Sewell et al (2000).
The Hong Kong Geological Survey (HKGS) section of GEO/CEDD has the most detailed information on the geology of Hong Kong and offers an advisory service. HKGS should be consulted, especially at the planning stage of new projects involving tunnel works, in the formulation of geological models, anticipation of difficult areas, and the verification of significant geological features (faults, dykes, contact zones between geological units, etc.).
This consultation process in actual projects also allows feedback of important geological information from the project to existing geological archives maintained by HKGS.
[Amd GG2/01/2017]
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 inspection of a site and the vegetation it sustains may reveal clues suggesting interference with the natural subsoil conditions at some time in the past. Examples are given in Sections 5.2 to 5.6.
Due to the relatively short history of development in Hong Kong, many instances of previous use of a site can be discovered by an inspection of early maps, aerial photographs and other historical records (see Appendices A and B).
5.2 Tunnels
The presence of nearby tunnels may have a profound effect on the intended use of the site, and should be fully considered. In addition to tunnels in active use for water supply, sewage conveyance, roads and railways, underground shelters and disused tunnels (of average dimensions 2 m high and 3 m wide) exist in places throughout the Territory as a result of previous wartime activities.
5.3 Mines and Quarries
A relatively minor amount of either opencast or underground mining has been undertaken in Hong Kong, but quarrying for rock products has been extensive at some locations, as have borrow area operations. Where this has occurred, detailed consideration must be given to its influence on affected sites.
5.4 Waste Tips
Waste tips, used for the disposal of domestic refuse, industrial waste and other refuse, may be found in places throughout the Territory. The location of past or present 'controlled tips' operated by Government are documented, but other tips may also exist. Harmful industrial wastes may also be encountered. The use of waste tip sites for other purposes must consider fully the effects of combustible gas, toxic leachate and ground settlement.
Furthermore, sites in the proximity of waste tips may also be subject to the effects of laterally migrating combustible gas and leachate.
5.5 Other Earlier Uses
Much of the low-lying land of Hong Kong has been extended by successive stages of reclamation in the past 80 to 90 years. Former seawalls and other obstructions may therefore be encountered beneath these areas. The fill materials used have been variable, often containing large boulders and building debris. The fill is often underlain by soft compressible
marine sediments.
Natural slopes and boulders, and older cut and fill slopes and retaining walls, are often prone to landslides and other forms of instability. It is of paramount importance that all slope features on or adjacent to the site should be examined for areas of past, current or potential instability at an early stage in the site investigation.
5.6 Ancient Monuments
A list of gazetted historical sites is maintained by the Antiquities and Monuments Office of the Government Secretariat, and a permit is required before commencement of any work within a gazetted historical site. It is advisable to consult the Antiquities and Monuments Office before entering any historical site, even ungazetted sites. During site investigation, any discovery of antiquities or supposed antiquities should be reported to the Antiquities and Monuments Office (see Appendices A and B).
6. Aerial Photographs
6.1 General
Aerial photographs can be used in the preparation and revision of maps and plans, and they can assist in the identification and general assessment of natural and man-made features, including geology, geomorphology, hydrology and vegetation, on or in relation to a site. They are particularly useful in the assessment of site history (i.e. changes in form, materials and land use) and can provide valuable information for the assessment of slope stability (Geological Society, 1982).
Black and white aerial photograph coverage of Hong Kong is extensive. Although partial coverage of the Territory is available from 1924, the first complete coverage was obtained in 1963, as summarised in Table 2. For almost any site in the Territory, repeated aerial photograph coverage records the land use and development changes that have occurred, as well as any history of recent instability. The small scale black and white photographs obtained at flying heights of over 6 000 m are more suitable for obtaining an overall view of the Territory. A small number of true colour, (false) colour infrared and black and white infrared photographs are also available. Advice on how to obtain the aerial photographs is
given in Appendix B.3.3. [Amd GG2/01/2017]
6.2 Topographic Maps and Aerial Photographic Imagery 6.2.1 Map and Plan Scales
Accurate topographic maps and plans can be produced from aerial photographs. A partial catalogue of maps and plans available from the Lands Department is given in Table 1 (see also Appendix B.3.l). Large scale plans (scales 1:500 to 1:1 000) are usually most appropriate for site investigations of small areas, whereas plans with scales of 1:5 000 to l:20 000 are more appropriate for district or regional studies. [Amd GG2/01/2017]
6.2.2 Aerial Photographic Imagery
The scale of an image on an aerial photograph is proportional to the distance between the camera and the subject. For an aerial photograph taken vertically, tall objects (tops of hills and buildings), and objects near the centre of the photograph, create images at slightly larger scales than low terrain or similar objects near the edge of the photograph. Radial distortion about the optical axis of the camera displaces the true vertical away from centre of the photograph, an effect which becomes more pronounced near the edges of a photograph. This may create dramatic effects on large-scale photographs with considerable changes in elevation from one portion to another.
Despite these sources of distortion, for sites which can be identified within the central third of a vertical aerial photograph and which contain terrain of broadly similar elevation, reasonably accurate scaled images can be obtained by proportioning the distances between objects identifiable on a map (or plan) and a contact print of an aerial photograph, and by using this proportion ratio as an enlargement factor. Most aerial photography has been obtained
using cameras with large format negatives. Prior to 1963, the sizes of the contact prints vary, but are usually 162 mm by 175 mm. With few exceptions, the negatives obtained from 1963 to the present are 228 mm by 228 mm. The image produced on contact prints is extremely sharp, and clear images can be obtained either by viewing the contact prints with a magnifying lens or stereoscope, or by enlarging all or part of the negative. Enlarged prints can be used even for studies of small areas of the size of an individual building site.
6.2.3 Orthophoto Maps and Plans
Orthophoto maps and plans, which consist of rectified (true to scale) photographs overprinted with contours or grids can be made (overseas only) for both vertical and oblique aerial photography. Rectification of the image can be performed optically or digitally; the accuracy is determined by the number of control points supplied, the degree of rectification desired and the scale of the original photography.
6.3 Aerial Photograph Interpretation
6.3.1 Identification and Interpretation of Ground Features
Aerial photographs can be interpreted at a range of scales and levels of detail to provide information valuable to both the design of site investigations and to the interpretation of the results. The design of site investigations for large projects such as route corridors (e.g. roads, railways, pipelines or transmission lines) can benefit enormously from a preliminary aerial photograph interpretation (API) survey. This can highlight the natural and man-induced characteristics of the terrain, noting in particular hazards and resources that may have a significant effect on the feasibility or design of the project. Even when performed for smaller sites, an API study can often provide useful information on the distribution and thickness of natural and fill materials, and may reveal potential problems originating from adjacent land.
Sequences of aerial photographs taken at different dates can be compared to determine the location, extent and approximate time of filling and reclamation, and the sequence of development of an area.
Aerial photographs, particularly when examined stereoscopically, can often be used to identify and delineate specific ground features such as the distribution of soil types (e.g.
colluvial and alluvial deposits), soil thickness, bedrock type, depth to bedrock, fracture patterns and spacings, as well as local relief. API is of particular value in the mapping of
"photolineaments". This term refers to straight or gently-curving features on aerial photographs which are usually the surface expression of variations in the structure or materials of the underlying bedrock. Photolineaments are usually marked by topographic highs or lows in the terrain but sometimes they may be more subtle features, which can only be identified by different vegetation growth, reflecting underlying changes in soil type, soil thickness or moisture content. Well-defined linear depressions usually indicate the location of less resistant bedrock or of discontinuities in the bedrock structure such as faults, fracture zones or major joints. Local linear topographic highs or lines of boulders or rock outcrops may indicate the presence of a rock unit that is more resistant to weathering.
All the features mentioned above may be important for the interpretation of site
conditions. Early identification by API of major changes in soil and rock types and features that are likely to have a significant influence on the local groundwater regime can be of great assistance in the design of the ground investigation and in establishing a geological model for the site.
Reviews of API and related mapping techniques are contained in Geological Society (1982). Some good examples of the use of API techniques are provided by Ho et al. (2006), Lueder (1959), Van Zuidam & Van Zuidam-Cancelado (1979), Verstappen & Van Zuidam (1968)
and Way (1978). [Amd GG2/01/2017]
6.3.2 Examples of API in Hong Kong
In Hong Kong, API techniques have been successfully applied to both specific problems and regional appraisals. Examples of the former are given in Brimicombe (1982), Bryant (1982) and Koirala et al (1986). Systematic regional API studies have been undertaken within the Geotechnical Area Studies Programme (GASP) to provide information for planning, resource appraisal and engineering feasibility studies (see Section 4.2). The first of the regional GASP reports was available in 1987 (GCO, 1987) and a further eleven reports in the series were published between 1987 and 1989. All GASP maps are available for inspection in the Geotechnical Information Unit (see Appendix B).
Figure 6 shows an example of a vertical black and white aerial photograph of part of Hong Kong Island and includes the corresponding portion of the 1:20 000 scale geological map (GCO, 1986). Some features of the bedrock structure can be interpreted from the aerial photograph. For example, the location of the fault line shown on the geological map can be clearly seen as a straight, deep valley in the centre-east part of the photograph. Near the north eastern corner of the photograph, the photolineaments indicated on the map can be seen to correspond to less clearly-defined valleys.
Part III
Planning the Ground Investigation
7. Introduction to Ground Investigation
7.1 Objectives
For new works, the objectives of ground investigation are to obtain reliable information to produce an economic and safe design and to meet tender and construction requirements.
The investigation should be designed to verify and expand information previously collected.
In Hong Kong, because of intense urban development, it is often necessary to investigate the effects of new works on the safety of existing features and works; in particular, the effects on the stability of existing slopes and retaining structures (see Sections 4.1.2 and 8.3).
The objective of ground investigation related to defects or failures of existing features or works (see Section 8.2), or to safety of existing features and works (see Section 8.3), will be directly related to the particular problems involved. The requirements for investigation of materials for construction purposes are discussed in Section 8.4.
An understanding of the geology of the site is a fundamental requirement in the planning and interpretation of the ground investigation. In some cases where the geology is relatively straightforward and the engineering problems are not complex, sufficient geological information may have been provided by the desk study, subject to confirmation by trial pits or boreholes or both. In other cases, it may be necessary to undertake geological mapping, which is discussed in Chapter 9.
Of primary importance will be the establishment of the soil profile or soil and rock profile, and the groundwater conditions. The profile should be obtained by close visual inspection and systematic description of the ground using the methods and terminology given in Geoguide 3 (GCO, 1988), or a suitable alternative system. In many cases, this, supplemented by limited insitu or laboratory testing, will suffice. In others, it will be necessary to determine in detail the engineering properties of the soils and rocks. The extent of the ground investigation is discussed in Chapter 10. Where appropriate, the geometry and nature of discontinuities should be established (see Section 12.11).
In many cases, especially in slope design, it will be very important to determine the variations in the groundwater regime in response to rainfall.
The investigation should embrace all ground in which temporary or permanent changes may occur as a result of the works. These changes include : changes in stress and associated strain, changes in moisture content and associated volume changes, changes in groundwater level and flow pattern, and changes in soil properties such as strength and compressibility.
Materials placed in the ground may deteriorate. It is therefore necessary to provide information from which an estimate of the corrosivity of the ground can be made (see Chapter 13).
Special measures may be required to locate disused tunnels or underground cavities, which may collapse, resulting in damage to structures (see Sections 8.3.2, 10.3.3 and 10.7.2).
Other hazards may arise from earlier uses of the site (see Chapter 5).
7.2 Planning and Control
Before commencing ground investigation, all relevant information collected from the sources discussed in Part II should be considered together to obtain a preliminary conception of the ground conditions and the engineering problems that may be involved. This will assist in planning the amount and types of ground investigation required.
Planning of the ground investigation should be flexible so that the work can be varied as necessary in the light of fresh information. On occasions, especially on large or extended sites, a preliminary investigation may be necessary in order that the main investigation may be planned to best advantage (see Sections 4.1.1 and 10.4).
The ground investigation should be largely completed before the works are finally designed. It is therefore important that sufficient time for ground investigation (including dealing with all legal, environmental, contractual and administrative matters, reporting and interpretation) is allowed in the overall programme for any scheme. For example, in slope design, piezometers should be installed well in advance to obtain sufficient groundwater data for the design. Should changes in the project occur after completion of the main investigation, additional ground investigation may be required. If so, the programme should be adjusted to allow for the additional time required.
Sometimes, conditions necessitate additional investigation after the works commence.
In tunnelling, for example, probing ahead of the face may be required to give warning of hazards or changes in ground conditions. The properties of the ground and also the groundwater levels may vary with the seasons. In planning the investigation, consideration should be given to predicting the ground conditions at other times of the year.
The imposition of limitations on the amount of ground investigation to be undertaken, on the grounds of cost and time, may result in insufficient information being obtained to enable the works to be designed, tendered for and constructed adequately, economically and on time.
Additional investigations carried out at a later stage may prove more costly and result in delays.
As ground investigations in Hong Kong must often be undertaken in urban areas (Plate lA), it is often necessary to obtain road excavation permits, temporary licences or way leaves before commencing the ground investigation. For some sites it will be necessary to coordinate the works with the requirements of the traffic police and other authorities (Plate 1B).
Proper identification and maintenance of utilities encountered by the works is essential; high voltage power cables, gas distribution lines and other utilities often present significant safety hazards.
Since backfilled pits and boreholes might interfere with subsequent construction, they should be sited and backfilled with care. It is essential that the precise location of every excavation, borehole or probing is properly referenced to the 1980 Hong Kong Metric Grid and recorded during the execution of the fieldwork. It is also essential to establish and record the ground levels of these locations. The records should be such that the locations and levels can be readily incorporated into the report on the investigations (see Sections 10.5 and 40.2.8).
Investigations for new works and all other building works within the Mid-levels
Scheduled Area (Figure 2) must comply with the provisions of the Buildings Ordinance (Government of Hong Kong, 1985), 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, or 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 it appear during the course of the investigation that items of archaeological or historical significance have been encountered, the Antiquities and Monuments Office should be notified (see Section 5.6).
To obtain the greatest benefit from a ground investigation, it is essential that there is adequate direction and supervision of the work by competent personnel who have appropriate knowledge and experience and the authority to decide on variations 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 are given in the relevant sections. Other methods involve normal safety precautions appropriate to site or 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, further advice should be obtained.
In the project planning stage for tunnel works, alternative tunnel routes and potential shaft locations are typically considered. It is sufficient to have only a general picture of the subsurface geology and hydrogeology, to define the preferred route corridor and to estimate the order of project cost. The site investigation should largely comprise desk studies and site reconnaissance, and include only limited ground investigation, if any is needed. Reference should be made to information available from nearby tunnel projects (see, for example, the
references given in GEO (2016)). [Amd GG2/01/2017]
Appendix A summarises the types of information that may be required in planning a ground investigation.
8. Types of Ground Investigation
8.1 Sites for New Works
Investigations for new works differ from the other types of investigation mentioned in Chapter 7, in that they are usually wider in scope because they are required to yield information to assist in selecting the most suitable location for the works, and the design and construction of the works. For example, when slope excavation has to be carried out, a knowledge of the subsurface materials and groundwater conditions should indicate :
(a) whether removal of the material will be difficult,
(b) whether the sides of the excavation will be stable if unsupported or will require support,
(c) whether groundwater conditions will necessitate special measures such as groundwater drainage or other geotechnical processes,
(d) whether the nature of the ground will change as a result of excavation, e.g. opening of relict joints in the soil mass, (e) what form of surface protection is required.
On the design side, it is necessary to assess such considerations as bearing capacity and settlement of foundations, stability of slopes in embankments and cuttings, earth pressures on supporting structures, and the effect of any chemically aggressive ground conditions. For the design of new works, it is important that the range of conditions, including least favourable conditions, should be known. This entails not only a study of the degree of variability in the soil and rock profiles over the area of the site, but also an appreciation of the possible injurious effects of groundwater variations and weather conditions on the properties of the various subsurface materials. Where works require excavations into or within rock, including weathered rock, the orientation and nature of discontinuities in the rock may be the most important factors.
Often, a preliminary design of the proposed works is of great assistance in the identification of parameters that are required to be obtained from the ground investigation.
Investigations should assess whether the proposed works may induce ground movements which could affect adjacent land, services and structures, and whether the hydrogeological regime may be adversely 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 failure has occurred is often necessary to establish the cause of the failure and to obtain the information required for the design of remedial measures.
Observations and measurements of the feature or structure to determine the mode or mechanism of failure are first needed, and these will often suggest the origin of the trouble, or at least indicate whether the ground conditions were partly or wholly responsible. If this is the case, an investigation will be required to ascertain the ground and groundwater conditions relevant to three phases of the site history, i.e. before the works were constructed, at the time of failure and as they exist at present (see also Chapter 32). Each problem will need to be considered on its merits. Indications of the probable cause of a failure will often result in detailed attention being directed to a particular aspect or to a particular geological feature.
In the case of slope failure, or where such failure is considered imminent, it is common practice to monitor movements both of the surface and underground. The former is conducted by conventional survey methods and the latter by means of slip indicators or inclinometer measurements. These techniques are fully described in the Geotechnical Manual for Slopes (GCO, 1984). It is also usually necessary to monitor groundwater pressures within the various underlying zones (see Chapter 20). [Amd GG2/01/2017]
Therefore, an investigation to determine the causes of a failure may be much more detailed in a particular respect than would normally be the case in an investigation 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 the dense urban development in parts of Hong Kong, it is often necessary to investigate existing features and works in the immediate vicinity or even remote from the site of the proposed new works, to decide whether the existing works are likely to be affected by changes in the ground and groundwater conditions brought about by the new works.
8.3.2 Types of Effects
Existing slopes and structures may be affected by changed conditions such as the following :
(a) Impeded drainage, which may result in a rise in the groundwater level. This can cause softening of cohesive materials and reduction of shear strength of permeable materials, and give rise to increased pore pressures affecting the stability of slopes and retaining walls; swelling may result in ground heave.
(b) Excavations or demolitions in the immediate vicinity, which may cause a reduction in support to the slope or structure, either by general ground deformation or by slope instability.
(c) Stresses that the new structure may impose on existing slopes or structures, or on the foundation materials below adjacent
structures, which can cause slope instability or distress to existing structures.
(d) Vibrations and ground movement resulting from traffic, vibratory compaction, piling or blasting in the immediate vicinity, or from other construction activities.
(e) Lowering the groundwater level by pumping from wells or dewatering of excavations or tunnels will cause an increase in the effective stress in the subsoil affected, which can lead to excessive settlement of adjacent structures. Also, if pumps do not have an adequate filter, the leaching of fines from the subsoil can easily result in excessive settlement of structures at considerable distance from the pump.
In areas where natural underground cavities can occur, e.g.
karst features in the Yuen Long basin, increase in effective stress or downward ravelling of soil due to heavy pumping may lead to subsidence or the formation of sinkholes (Siu &
Wong, 1984).
(f) Tunnelling operations in the neighbourhood, which may cause deformations and subsidence; the effect of tension and compression on drainage should not be overlooked.
(g) Alteration in stream flow of a waterway, which may cause undercutting of banks or scouring of foundations of walls, bridges and piers, and may be due to works carried out some distance away.
(h) Siltation of the approaches of harbour works or the changing of navigation channel alignments.
8.3.3 Procedure
In the investigation of the safety of existing features or works, the first requirement is an appreciation of the changes to the ground that are likely to occur. The ground investigation will need to provide knowledge of the subsurface materials, together with the examination and testing of samples to assess the effect that the changed conditions are likely to have on these materials. In some cases, it may be necessary to carry out a detailed analysis to estimate the effect of the changed conditions on the safety of the existing features and works.
8.4 Materials for Construction Purposes
Investigations of sites are sometimes required :
(a) to assess the suitability, and quantities, for construction work of materials that become available from excavations or dredging, e.g. whether spoil from cuts in road and railway works will be suitable for fills in other places,
(b) to find suitable materials for specific purposes, e.g. to locate borrow pits or areas for earthworks (a common problem in Hong Kong where intense urban development demands a constant search for suitable fill materials); to assess the suitability of materials in waste tips that may need to be removed for environmental reasons,
(c) to locate suitable disposal sites for waste and dredged materials.
