ABSTRACT: A study was undertaken by GEO on the applicability of modern downhole geophysical methods for site investigation of slopes in Hong Kong.  The study evaluated the capabilities of a number of downhole geophysical techniques to locate and define discontinuities, and in particular weak clay-rich layers in saprolite. The techniques reviewed included acoustic borehole televiewer, spectral gamma ray, gamma density, neutron porosity, electrical cylinder, four-arm dipmeter, three-arm caliper, and self potential.   This paper describes the results obtained using these techniques at a calibration test site with controlled simulated geological conditions, and subsequent field trials at the Tseung Kwan O field trial site.  The results show that certain downhole geophysical methods can be used to detect weak clay-rich layers greater than 30mm in thickness.

 ABSTRACT: Two unusually large landslides occurred in Hong Kong in August 1995. Following these landslides, the distribution, mineralogy and shear strength of kaolin-rich zones in Hong Kong have been thoroughly assessed. The findings indicate that kaolin migrates physically and chemically during weathering. This can result in both broad dissemination of kaolin throughout weathered material and preferential infill of relict discontinuities. The infills also show evidence of shearing, brecciation and multiple generations of kaolin. These may represent the dissipation of weathering-induced stresses by movement along the kaolin-infilled relict discontinuities and this would explain the generation of many slickensides observed in the field. This movement would also result in the dilation of the discontinuities allowing yet further kaolin accumulation to occur. The occurrence of kaolin concentrations has significant implications for slope design in Hong Kong. Recent observations have already led to some selective re-engineering of slopes and guidelines for ground investigation have been revised accordingly.

 ABSTRACT: Obtaining representative discontinuity strength data is of critical importance for cut slopes in a jointed rock mass. However, the strength of infilled discontinuities within saprolites is difficult to determine using standard laboratory testing equipment. This paper reviews the use and results of large-shear-box testing to obtain data for weak infilled discontinuities within a saprolite mass for two sites in Hong Kong and highlights the importance of clay mineralogy on the strength values obtained.

ABSTRACT: In situ stress measurements using the hydraulic fracturing and over-coring methods were carried out as part of the site investigation for the proposed Nam Wan Tunnel on Tsing Yi Island in Hong Kong. The tunnel is part of the Highways Department proposed Route 9 between Tsing Yi and Cheung Sha Wan.  Measurements were conducted in site investigation drillholes on both the western and eastern sides of Nam Wan Hill and at a range of depths to provide a design profile of in situ stress with depth. The hydraulic fracturing measurements yield a WNW ¡V ESE orientation for the maximum horizontal stress (S_H = 105¢X ¡Ó 9¢X to 111¢X ¡Ó 13¢X). The measurements indicate that the vertical principal stress is the minimum principal stress at depths less than 150 m and that the minimum horizontal stress becomes the minimum principal stress at depths greater than 150 m.  Both the magnitude and the orientation of the principal stresses determined using the hydraulic fracturing method were consistent with published measurements for Hong Kong. The magnitude and orientation of the principal stresses determined using the over-coring method were strongly influenced by the presence of rock mass discontinuities and by small-scale defects in the rock mass and as a result the measurements show a significant scatter.

ABSTRACT: Clay-rich zones in weathered profiles developed over igneous rocks have played a significant role in the occurrences of several recent, relatively large-scale landslides in Hong Kong. This paper demonstrates applications of various techniques to study the nature and origin of clay minerals. The techniques utilized for a comprehensive analysis involve X-ray diffraction (XRD), optical microscopy, scanning-electron microscopy (SEM) and transmission electron microscopy (TEM). This is the first integrated study using facilities in Hong Kong at the University of Hong Kong.

ABSTRACT: In this paper, results of chemical analyses of 148 weathered granite specimens from three different sites of the Kowloon Pluton will be reported and discussed. Some quantitative chemical weathering indices are derived and are correlated with the six-fold material decomposition grades. In addition, physical properties including void ratio and dry density of 34 specimens have been studied and interpreted in relation to the proposed quantitative chemical weathering indices and the existing six-fold material decomposition grades. It is found that chemical weathering potential index (WPI), mobiles index (I_mob) and alumina to calcium-sodium oxide ratio (ACN) exhibit a good correlation with the decomposition grade. Weathering potential index (WPI) shows a distinct variation and decreasing trend with decomposition grade. On the contrary, mobiles index (I_mob) and alumina to calcium-sodium oxide ratio (ACN) show an increasing trend but smaller variations with decomposition grades. The chemical weathering indices, WPI and I_mob, exhibit a reasonable trend with dry density and void ratio, although considerable scatter is observed. These quantitative chemical weathering indices may have a potential to assist in identifying the degree of weathering and to deduce some physical properties of weathered granite in Hong Kong.

ABSTRACT: Hong Kong first began educating local university graduates with the academic background to become engineering geologists in 1995 and some of these are now working in the site investigation industry logging boreholes. One of the key training aspects of site investigation work is the geologist being able to produce accurate and comprehensive preliminary and final borehole logs in the shortest possible time. Initial descriptions from graduate geologists can be as simple as ¡§red clay¡¨ or contain so much detail that there is not enough space for printing on the borehole log. Such descriptions are of little use to civil engineers and take too long to get right for the contractors and hence significant training investment is required for the graduate geologist.

ABSTRACT: Site investigations comprise two phases.  The first phase involves involve the use of maps, aerial photographs, literature surveys, and field reconnaissance.  The objective is to develop a geological model that anticipates potential difficulties and provides a basis for planning the subsequent ground investigation phase.  These studies often lack an appreciation of the site geomorphology, which is particularly important for the identification of zones of deep weathering, for planning borehole locations, and for optimising the orientation and spacing of seismic survey lines.  Several case histories are presented that illustrate the contribution of geomorphological interpretations to ground investigation planning, and also to show how they can assist with resolving borehole logging problems at sites with complex geology, particularly in weathered rock profiles, superficial deposits, and areas of cut and fill.  It is argued that a knowledge of the pre-development geomorphology, bedrock geology, and structural grain is particularly important when investigating sites in the extensively modified urban environment of Hong Kong.

 ABSTRACT: The presence in rock of horizontal stresses equal to or greater than the vertical stress is well known.  What happens to the horizontal stress when rock weathers to a saprolite is, however, very poorly understood, especially in areas where differential surface erosion is shaping the landscape during the weathering process.  Geotechnical practitioners designing excavations in saprolites commonly rely on a theoretical analysis of the subject (Vaughan & Kwan, 1984) based largely on the behaviour of a homogenous linear elastic solid and they assume that K_o approximates to that of a deposited soil.  However, the assumptions in the analytical treatment presuppose no retention of strain energy during weathering, thereby predetermining the conclusion that higher horizontal stresses are not maintained.  The actual level of horizontal stress in a saprolite is likely to be dependent on a variety of factors but elastic theory and this theoretical analysis do not provide the answer.

 ABSTRACT: An offshore geological model based on sedimentological characteristics and eustatic sea-level changes dating back almost 0.5 million years is proposed for the superficial deposits on the seabed of Hong Kong. The model showing five interglacial-glacial cycles is in agreement with the high-resolution record identified in the Vostok ice core of Antarctica. Selected engineering applications of the model are briefly described. They include refinement in the interpretation of offshore seismics, improvement in understanding of the engineering properties of superficial seabed deposits, the provision of a conceptual model for the exploration and exploitation of offshore sand and gravel deposits, and, the dating of terrestrial landslide deposits of Quaternary age for assisting their risk assessment.

ABSTRACT: The reclamation at Tung Chung New Town, Lantau Island is partially underlain by complex ground conditions characterized by the presence of metasedimentary rocks, cavity-fill sediments and other karst-related deposits, and depressions in rockhead to over 180m below Principle Datum. This paper describes in detail the geology at one development site where, during a ground investigation, such conditions were encountered. Laminated clay, silt and sand, loose sand and gravel, and diamict were deposited in a cavity that had formed within a marble block enclosed in granite. The cavity was at least 30m across and 50m high, and its floor is about -150mPD. Large, completely decomposed, granite blocks set in a variable diamict, clay and sand matrix overlie the cavity-fill sediments. These deposits were formed during the progressive collapse of the granite roof to the former cavity. In places, the completely decomposed granite blocks are penetrated by vertical, de-watering, pipes filled with fine sand and silt. The implications for the construction of deep pile foundations within these, or similar, ground conditions are discussed.