Report No. : GEO Report No. 60

Report Title : Assessment of Geological Features Related to Recent Landslides in Volcanic Rocks of Hong Kong Phase 2A - Chai Wan Study Area (1998), 78 p. plus 6 drgs.

Author : S.D.G. Campbell & N.P. Koor

Abstract

This report cover the phase 2A assessment of geological features related to recent landsliding in the Chai Wan area. Phase 2B covering the Aberdeen area will be reported separately. The study involved aerial photograph interpretation (API), a geological survey and an engineering geology assessment, including a desk study with limited field checking. A sensitivity analysis of the stability with respect to planar failure of slopes recommended for further investigations was also carried out. The reports includes eight 1:5 000-scale thematic maps.

This study concentrated on identifying slopes that are geologically similar to the one that failed at Fei Tsui road, and that have had similar histories of persistent seepage and multiple minor failure. The key geological features that influenced the Fei Tsui Road landslide were thick, laterally-extensive concentrations of kaolin dipping out of the slope face subparallel to the volcanic fabric and persistent steep joint sets.

The study concluded that mime slopes (out of 193 registerable cut slopes and retaining walls) require more detailed inspection, (e.g. chunam/shotcrete strips, vegetation clearance) and further engineering geological assessment. Seven are in volcanic rocks and two in fine- to medium-grained granite. The slopes were selected because of the presence of, or potential for, adversely-orientated geological features, including kaolin seams. A sensitivity analysis concluded that plane failure was likely if the conditions at the Fei Tsui Road landslide site exist in the nine slopes. Further recommendations are made for GI of two photolineaments identified by API, and of large-scale landform features suspected to be associated with landsliding, and for a study of the engineering properties of kaolinitic clay-bearing, completely to highly decomposed tuff based on block samples from up to two slopes recommended for further investigation.

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