Natural terrain hazard assessemnt : a case stury
M.Sc. in Applied Geosciences GEOS8004 Project
Natural Terrain Hazard Assessment - Case Study
NATURAL TERRAIN HAZARD ASSESSMENT
A CASE STUDY - THE PROPOSED NORTH-SOUTH LANTAU LINK
The results of a natural terrain hazard assessment are presented for a proposed new road linking the existing road networks on the northern and southern sides of Lantau Island. Hazard maps (based on aerial photograph interpretation, engineering geological mapping and the results of ground investigation) and landslip susceptibility maps (assembled using Natural Terrain Landslide Study data) have been constructed and used as a basis for a quantitative risk assessment (QRA) for the proposed road. Following identification of the areas requiring further analysis using QRA, a detailed deterministic approach based on calculated factors of safety (FOS) has been applied for a catchment.
The basis of this study is a project which comprises the construction of a new road link between North and South Lantau connecting with the North Lantau Highway in Tai Ho Wan in the north and South Lantau Road in the vicinity of Mui Wo in the south. During the investigation stage for the road a comprehensive geotechnical study was undertaken across a broad study corridor involving desk study, aerial photograph interpretation (API), engineering geological mapping, bore holes, trial pits, geophysical surveys (microgravity, magnetic and seismic), laboratory testing followed by detailed analyses and preliminary design (Refs. 1 to 13). This study expands on the natural terrain study carried out by the author as part of the Investigation Assignment for the project performed by Mouchel Asia Limited for Highways Department of the Government of the Hong Kong Special Administrative Region.
1.1 Location and topography
A ridge of high ground, roughly trending NNE-SSW along the spine of the island, dominates Lantau. The route is planned to cross over a low saddle between the section of ridge dominated by Lin Fa Shan (elevation +766mPD) and high ground to the east associated with Lo Fu Tau (+465mPD). The proposed scheme traverses a variety of terrain as illustrated in Figure 1, typified by steep boulder strewn slopes in the northern section; low rounded ridges through the central saddle area; and wooded foothills above a poorly drained alluvial plain. Gradients are typically between 25and 40 The steepest gradients are found on the upper slopes of the catchments below Lin Fa Shan, where grades of over 40and 60are common.
The geology through the study area is illustrated on Figure 2 (based on Sheet 10, Solid and Superficial Geology, Edition 1, 1991). The geology along the majority of the route is dominated by a swarm of ENE-WSW trending dykes of feldsparphyric rhyolite. In the north, the feldsparphyric rhyolite dykes encompass a larger dyke of porphyritic microgranite (the results of field mapping and ground investigation carried out for the project indicate that most of the feldsparphyric rhyolite dykes may have microgranite cores). To the south, and across most of the length of the project, dykes of feldsparphyric rhyolite cut through country rock consisting of medium grained granite. At the extreme southern end of the route, near South Lantau Road, the bedrock geology consists for the main of fine grained quartz syenite with small outcrops of metamorphosed siltstone and tuff. The entire study area is bounded to the west by volcanic rocks of the Lantau Formation (Refs. 14 to 18).
The oldest rock suite through the study area is the medium grained granite country rock through the central
Natural Terrain Hazard Assessment
May 2000 Page 3
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:landslides china hong kong slopes soil mechanics
Date of Publication:01/01/2000