Review the selected volcanic CHASE slopes in the context of the area studies initiated after the 1995 fatal landslides

Abstract

The CHASE study in the early 1980s’ was to develop an empirical approach for stability assessment of soil cut slopes. Given the state of knowledge at the time, the data collection was focused on the geology, morphology and material descriptions, etc. This information is suitable for determining weathering grades and material strengths, but it is not that useful for assessing slope stability. This dissertation study is to establish the relationship between material attributes and the stability-critical geological features, thereby the attributes could be used as proxies to assess the potential of instability of the volcanic CHASE slopes. In August 1995, a cut slope at Fei Tsui Road in Chai Wan and a natural slope at Shum Wan Road in Aberdeen collapsed following the intense and prolonged rainfall and had resulted in loss of life and damage to property. Detailed investigations concluded that both landslides were influenced by kaolin-rich zones. The GEO then carried out engineering geological studies in four areas, namely Chai Wan, Aberdeen, Tseung Kwan and Mount Davis, having the same volcanic terrain setting as the two landslide sites. Apart from reinforcing the common occurrences of kaolin, the studies have recognized the geological features of extensive presence of volcanic (eutaxitic) fabrics and rock head depressions which may adversely affect slope stability. A review of the area studies suggests that fine grained tuff could be a potential proxy for the presence of eutaxitic fabric because this is preferential developed in fine grained tuffs. Therefore, those CHASE slopes consisting of fine-grained materials would have greater instability concern in respect of volcanic fabrics. Previous studies suggest that kaolin accumulations in Hong Kong are of a weathering origin, hence material attributes responding well to different degrees of weathering could be potential proxies for indicating the likely presence of kaolin. Material attributes from 16 CHASE slopes that fall within the four study areas have been analyzed under this study to relate them with the extent of weathering. The analyses of material attributes against decomposition grades have shown that Percent Fines (i.e. silt and clay), ‘N’ and ‘L’ Types Schmidt Hammer rebound values, Hand Penetrometer strength, field test strength estimate, feldspar strength and slake test are useful for differentiating the extent of weathering, especially between grade IV materials and grade V and VI materials. Some index tests appear to provide more consistent results (e.g. Schmidt Hammer rebound values and slaking in water) than others (e.g. % Fines and field test strength estimate). Although the basic rock type of the four areas is fine ash tuff, the lithologies and compositions vary, influencing the determination or estimation of the various attributes. Therefore, when the indexes are used collectively, they work better for differentiating decomposition grades and hence for assessing the potential instability of the CHASE slopes.