The significance of primary volcanic fabrics and clay distribution in landslides in Hong Kong

Most rain-induced landslides on Hong Kong’s volcanic rocks are shallow and small-scale. However, two major landslides that occurred on Hong Kong Island in 1995 highlight relationships between primary volcanic foliations and kaolin accumulations that may have general significance in determining loci of larger failures. In the first case, a bedding-parallel shear zone was the locus for kaolin concentration in a shallowly dipping tuff. This formed a weak, planar, low-permeability layer which acted as the surface of rupture of a translational failure. In the second case, steeply inclined fabrics (eutaxitic foliations) and subparallel, closely spaced joints occurred within a zone of syn-volcanic folding and shearing. Deep weathering developed along the zone, with kaolin concentrations present mainly above the soil-rock interface. The irregular and variably concave to planar rupture surface was closely associated with kaolin-infilled relict joints towards the base of the soil profile. Kaolin distribution is largely associated with the weathering profile, and may have been both physically and chemically redistributed within it. Halloysite is more abundant than kaolinite in most clay samples at and near both rupture zones, and may have developed from kaolinite in response to strain. Detailed mapping of volcanic fabrics around the major landslides suggests that kaolinized bedding-parallel shear zones are uncommon, but that deep weathering zones associated with shearing and faulting are common, mappable features.