Two unusually large landslides in Hong Kong in August 1995, both of which caused loss of life and substantial damage to property, were influenced by localised concentrations of low strength, kaolin-rich zones within the weathered rock mass (Kirk et aI., 1997). Plate 1 shows one of the failures, at Shum Wan Road on the south of Hong Kong Island. This landslide involved 26,000m3 of material, killed two people and resulted in considerable economic loss. The presence of kaolin in Hong Kong has often been ascribed to hydrothermal events (e.g. Strange & Shaw, 1986; Irfan, 1994). However, recent field observations suggest a close spatial relationship between the presence of kaolin, groundwater movement and weathering (Campbell et aI., 1998). The development of an appropriate geological model for occurrences and origins of kaolin-rich zones is of considerable importance in engineering practice in Hong Kong, with its concentrated infrastructure and development close to steep terrain, and the abundance of slopes already formed (>30,000 cut slopes in excess of 3m high). As a result, considerable research into this· problem has recently been carried out by the Geotechnical Engineering Office of the Hong Kong SAR Government.
During the course of this study, some twenty five sites, the majority related to slope formation projects, have been selected for field examination and assessment, largely based on the observations that kaolin-infilled discontinuities were present at the sites. The kaolin-rich infill varies in thickness from <1 mm to > 50 mm. It most commonly occurs as an infilling within relict discontinuities in the saprolite derived from both rhyolitic volcanic and granitic rocks. The infill is typically white (7.5YR 8/4) but can vary significantly from pink (7.5YR 7/3) to reddish yellow (5YR 6/8), and varies in strength from very soft to very stiff, depending on its moisture content.