The characterisation and assessment of geotechnical properties of residual soils is a complex subject given the fact that these soil formations are a product of the physical, chemical and biological weathering processes of the rock. This in-situ decomposition of the parent rock and rock minerals produces characteristic features of mechanical behaviour that cannot necessarily be approached by conventional geotechnical design methods due to one or more of the following reasons (Schnaid et al., 2004a, 2004b):

1 The soil state is variable due to complex geological conditions. 2 Classical constitutive models do not offer a close approximation of its true nature. 3 These soil formations are difficult to sample and the soil structure cannot be

reproduced in the laboratory. As a consequence, the mechanical behaviour and geotechnical properties are assessed directly from in-situ testing data in most geotechnical design problems.