EXTENT OF GROUND INVESTIGATION .1 General Sites

The extent of a ground investigation is dependent on the complexity of the ground and, to a certain degree, the form of the proposed development and type of structures and the intended foundation types. Adequate investigation should be carried out to ensure no particular foundation options will be precluded due to a lack of information on ground conditions. Sufficient information should be obtained to allow engineers to have a good understanding of the ground conditions and material properties within the zone of influence of the foundations. Although no hard and fast rules can be laid down, a relatively close borehole spacing of say 10 m to 30 m will often be appropriate for general building structures.

In reclamation areas, closely-spaced boreholes may be needed to delineate buried obstructions such as remnants of an old seawall where this is suspected from a desk study of the site history.

In general, boreholes should be extended through unsuitable founding materials into competent ground beyond the zone of influence of the proposed foundations. The zone of influence can be estimated using elasticity theory.

Where pile foundations are considered to be a possibility, the length of pile required usually cannot be determined until an advanced stage of the project. Some general guidance in this instance is given in Geoguide 2 : Guide to Site Investigation (GCO, 1987). The traditional ground investigation practice in Hong Kong is to sink boreholes to at least 5 m into grade III or better rock to prove that a boulder has not been encountered. This practice

should be backed by a geological model prepared by a suitably experienced professional.

It is good practice to sink sufficient boreholes to confirm the general geology of the site. Consideration should also be given to sinking boreholes immediately outside the loaded area of a development in order to improve the geological model. It is also important to continually review the borehole findings throughout the investigation stage to ensure adequate information has been obtained.

For piles founded on rock, it is common practice to carry out pre-drilling, prior to pile construction, to confirm the design assumption and predetermine the founding level of the piles. For large-diameter bored piles founded on rock, one borehole should be sunk at each pile position to a depth of 5 m into the types of rock specified for the piles or the bases of the rock sockets, whichever is deeper. In the case of diaphragm wall panels carrying vertical load by end-bearing resistance, the boreholes should be sunk at about 10 m spacings. For small-diameter piles, such as H-piles driven to bedrock, socketed H-piles and mini-piles, the density of the pre-drilling boreholes should be planned such that every pile tip is within a 5 m distance from a pre-drilling borehole. The above approaches should always be adopted in Hong Kong in view of the inherent variability of ground conditions and the possible presence of corestones in the weathering profile.

Where appropriate, geophysical methods may be used to augment boreholes. A range of surface, cross-hole and down-hole geophysical techniques (Braithwaite & Cole, 1986;

GCO, 1987) are available. The undertaking and interpretation of geophysical surveys require a sound knowledge of the applicability and limitations of the different techniques, proper understanding of geological processes and the use of properly calibrated equipment. The data should be processed in the field as far as possible in order that apparent anomalies may be resolved or confirmed. Geophysical techniques are generally useful in helping to screen the site area for planning of the subsequent phases of investigation by drilling.

The design of foundations on or near rock slopes relies on a comprehensive study of the geology and a detailed mapping of exposed joint conditions. In some cases, the rock face cannot be accessed for detailed mapping for different reasons, e.g. the rock face is outside the development boundary. Adequate drillholes or inclined drillholes may be necessary to determine the continuity and orientation of discontinuities. The ground investigation should include measurement of discontinuities from drillholes, using impression packer tests or acoustic televiewer method. The presence of low strength materials, such as kaolin, should be carefully assessed. The strength of the such low strength materials could well dictate the stability of the rock slope under the foundation loads. Good quality rock core samples should be obtained and it may sometimes require the use of better sampling equipment, such as triple tube core barrels and air foam.

2.4.2 Sites Underlain by Marble

Given the possible extreme variability in karst morphology of the marble rock mass, the programme of ground investigation should be flexible. It is important that the borehole logs and cores are continuously reviewed as the works progress so that the investigation works can be suitably modified to elucidate any new karst features intercepted.

For high-rise developments on sites underlain by marble, the investigation should be staged and should be carried out under the full-time supervision of technical personnel. For preliminary investigation, it is recommended that there should be a minimum of one borehole per 250 m², drilled at least 20 m into sound marble rock, i.e. rock which has not been or is only slightly affected by dissolution (e.g. Marble Class I or II (Chan, 1994a)). The depth of boreholes should correspond with the magnitude of the load to be applied by the structure.

The position of subsequent boreholes for determining the extent of dissolution features, such as overhanging pinnacles and deep cavities, should be based on the findings of the preliminary boreholes. It is anticipated that boreholes on a grid of about 7 m to 10 m centres will be required to intercept specific karst features. Boreholes in other parts of the site should be sunk on a grid pattern or at points of concentration of piles, to a depth of 20 m into sound marble. Attention should be given to logging the location and size of cavities, the nature of the cavity walls, infilling materials and discontinuities. If the infill is cohesive in nature, good quality tube samples of cavity infill may be obtained using a triple-tube sampler with preferably air foam as the flushing medium.

A lower density of borehole may be sufficient for low-rise developments. Where the loading is small or where the superficial deposits above the marble rock are very thick, drilling may be limited to a depth where there is a minimum of 20 m of competent founding material. Nevertheless, it is strongly recommended that at least one deep borehole is sunk at each site underlain by marble, say to 100 m below ground level, to obtain a geological profile.

Surface geophysical methods can produce useful results to identify the potential problematic areas. The cost of ground investigation can be reduced by targeting drilling over the problematic areas. The micro-gravity method works best in relatively flat ground and without any influence from high density objects in the surroundings. Leung & Chiu (2000) used this method to detect the presence of karst features in a site in Yuen Long. The ground investigation field works were carried out in phases using both conventional rotary drilling and micro-gravity geophysics to supplement each other in refining the geological model.

Kirk et al (2000) described the investigation of complex ground conditions in the northshore of Lantau Island using gravity survey to identify areas of deeply weathered zones and supplement conventional ground investigation works. The accuracy of the gravity methods depends on careful calibration and interpretation of the field data.

Borehole geophysical techniques, including cross-hole seismic shooting and electro-magnetic wave logging, have been found to give meaningful results. Lee et al (2000) described the use of tomography technique to analyse the images of cross-hole ground penetration radar and predict the karst location. This technique is suitable when there is a good contrast in the dielectric permittivity between sound marble and water (in cavities). It is not suitable in highly fractured marble or marble interbeds with other rocks, such as meta-siltstone and meta-sandstone (Lee & Ng, 2004).

While recent experiences in geophysics have demonstrated their capabilities in identifying karst features, geophysics should be regarded as supplementary ground investigation tools in view of their inherent limitations and the simplifications involved in the interpretation. The value of geophysical testing is that it gives a greater level of confidence in the adequacy of the ground investigation, particularly in relation to the ground conditions between adjacent boreholes. In addition, the results may be used to help positioning the boreholes of the subsequent phase of ground investigation.

All boreholes must be properly grouted upon completion of drilling. This is especially important in the case of drilling into cavernous marble in order to minimise the risk of ground loss and sinkhole formation arising from any significant water flow that may otherwise be promoted.