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.

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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.

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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.

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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.

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