On 13 August 1995, two major landslides in Hong Kong, both resulting in fatalities, were influenced by localised concentrations of low strength, kaolin-rich zones within the weathered rock mass (Kirk et al, 1997). The first at Fei Tsui Road in Chai Wan released about 14 000m3 of debris and the second, at Shum Wan Road near Aberdeen, had a volume of 26 000m3. In his independent reviews of the landslide investigations, Knill (1996a) noted that at Fei Tsui Road "the presence of kaolinite is not unusual in altered or weathered rocks in Hong Kong. In this case what is unusual is that the kaolinite was present as a relatively thick continuous layer". Knill (1996a) went on to say "at this stage the factors which resulted in the development of the kaolinitic clay layer associated with the altered tuffs at Fei Tsui Road cannot be stated with sufficient precision to enable such materials to be predicted". At Shum Wan Road, Knill (1996b) noted that "the presence of kaolinitic clay seams and clay filled joints at shallow depth within the rock mass was a major contributory factor both to the relatively low shear strength as well as to controls on shallow groundwater flow". Following the landslides, the Geotechnical Engineering Office (GEO) of the Civil Engineering Department, Hong Kong SAR Government carried out a series of studies focusing on kaolin and its engineering significance (Campbell & Parry, 2002). The influence of mineralogy on shear strength has been well-documented (Wesley, 1977; Anon, 1997), as has the influence of previous shearing on shear strength (Skempton, 1985; Rossatto et al., 1992). In addition, a particular objective, in view of Knill's conclusions (1996a), was to improve the understanding of the distribution of kaolin-rich zones and so develop a geological model to explain the occurrences, origins and characteristics of kaolin-rich zones. Hence the study comprised sampling, supported by detailed field observations, a range of mineralogical tests and shear strength testing.
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