An elasto-plastic critical state framework for unsaturated soil

Data from a series of controlled suction triaxial tests on samples of compacted speswhite kaolin were used in the development of an elasto-plastic critical state framework for unsaturated soil. The framework is defined in terms of four state variables: mean net stress, deviator stress, suction and specific volume. Included within the proposed framework are an isotropic normal compression hyperline, a critical state hyperline and a state boundary hypersurface. For states that lie inside the state boundary hypersurface the soil behaviour is assumed to be elastic, with movement over the state boundary hypersurface corresponding to expansion of a yield surface in stress space. The pattern of swelling and collapse observed during wetting, the elastic-plastic compression behaviour during isotropic loading and the increase of shear strength with suction were all related to the shape of the yield surface and the hardening law defined by the form of the state boundary. By assuming that constant-suction cross-sections of the yield surface were elliptical it was possible to predict test paths for different types of triaxial shear test that showed good agreement with observed behaviour. The development of shear strain was also predicted with reasonable success, by assuming an associated flow rule.