Report No. : GEO Report No. 67

Report Title : Assessment of Geological Features Related to Recent Landslides in Volcanic Rocks of Hong Kong Phase 2B - Aberdeen Study Area (1999), 106 p. plus 8 drgs.

Author : C.A.M. Franks, S.D.G. Campbell & W.W.L. Shum

Abstract

This report covers the Phase 2B assessment of geological features related to recent landsliding in the Aberdeen area. Phase 2A covering the Chai Wan area has already been reported separately. The study involved aerial photograph interpretation (API), a geological survey and an engineering geology assessment, comprising a desk-study with limited field check. The report includes eight 1:5 000-scale thematic maps.

The study concentrated on identifying slopes that are geologically similar to those that failed at Shum Wan Road and Fei Tsui Road in August 1995, and that have had histories of persistent seepage and multiple minor failure. The key geological feature of the Shum Wan Road landslide was the presence of weak clay seams and clay-infilled joints. These were associated with a zone of anomalously deep weathering associated with locally subvertical volcanic fabric striking subparallel to very closely-spaced joints and to the direction of the slope that failed. The key geological features of 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 geological and topographic factors pre-disposing steep slopes to instability include: weathering profiles with abrupt changes in decomposition grade (V/IV to II/I), with either very thin or absent grade III and containing local zones of deeply weathered rock >10 m thick, especially along faults and shear zones; high or transiently high ground water tables; kaolin seams within relict joints in weathered rock masses; closely-jointed rock masses; proximity to coastal cliff sites and to dykes or other intrusions. Intersections of low-angle stress release joints which daylight in the slope also influence landslide development.

Eight natural slope areas with clusters of previous failures may indicate geological conditions adverse to slope stability. All are associated with old coastal cliffs subjected to long periods of in situ weathering and coastal erosion. Some adversely-orientated, weathered stress release joints within cliffs, which often contain kaolin infills, will have provided preferential failure surfaces.

The study recommends that twelve slopes (three of which are unregistered) require more detailed inspection, including chunam and shotcrete strips and vegetation clearance, and further engineering geological assessment. These slopes were selected because of the presence of, or potential for, adversely-orientated geological features, including kaolin seams, localised deeply weathered zones, high groundwater conditions and with a history of previous landsliding.

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