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.