Shear strength behaviour and the measurement of shear strength in residual soils

The selection of appropriate strength parameters and the prediction of deformation and failure are important steps in the design of foundations and cut slopes in residual soils. In countries with widespread residual soils, strength testing practice, both in the laboratory and in the field mostly follows standard procedures, employing triaxial and shear box tests in the laboratory and some form of penetration test, vane or plate loading test in the field (Brand & Phillipson, 1985). The need for a clear understanding of the difference between transported and residual soils arises mainly with the preparation and handling of the specimens, and with the interpretation of the test results. A knowledge of the genesis of residual soils and of the factors affecting their shear strength will enable both engineers engaged in design and in material testing to appreciate the peculiarities of these materials in their response to deformation and shear, and will thus facilitate the selection of the most suitable design values for their work. Residual soils develop a particular fabric, grain structure and particle size distribution in situ, which may make them fundamentally different from transported soils. The latter develop their fabric as a result of their mode of deposition and their stress history after deposition. In the following section, the special features which affect the stress-strain behaviour and the strength of residual soils will be summarized and discussed.