ENGINEERING GEOLOGY OF NORTH LANTAU, HONG-KONG

Results of an engineering geology study of the northern part of Lantau Island in the west of Hong Kong are described. Over the next few years this area will be subject to major changes as a result of the planned new international airport at Chek Lap Kok. As a consequence of the airport, high-speed road and rail links are also planned along the coast of North Lantau, together with two new towns, industrial developments and a major port development at the eastern end. The study identified specific engineering geological problems and hazards with major safety or economic implications for the planned developments. The western end of the island is mostly rugged mountainous terrain with elevations rising to more than 900 m above sea level. Towards the east, the topography is more subdued, although hillslopes are generally still steep. The solid geology of the area is dominated by volcanic and intrusive igneous rocks of Mesozoic age, including volcanic tuffs and lava, granites and quartzphyric and feldsparphyric rhyolite dykes. These have weathered and a mantle of saprolite and residual soil covers much of the terrain. The onshore superficial soils include colluvium and alluvium whilst offshore, thick deposits of Holocene marine muds and Pleistocene alluvium overlie the weathered bedrock. The main geological and geotechnical constraints to development on land in the North Lantau area are: the possible adverse effect of relict joints on the stability of cut slopes, potential stability problems on natural slopes due to steep, colluvium covered slopes, boulder fields and the possible remobilization of colluvium within drainage courses during rainstorms. Difficulties may also be encountered with piled foundations due to the presence of large boulders in deposits of alluvium and colluvium. Offshore, the presence of thick deposits of compressible marine mud represents the main difficulty for the planned reclamations along the coastline.