In previous chapters, it has been shown that the principle of effective stresses can be applied to the shear strength, the tensile strength and the volumetric behavior of unsaturated soils. This chapter shows that the critical state line for unsaturated soils shifts with respect to the saturated critical sate line in a quantity that depends on the suction stress. Taking into account this phenomenon and the influence of hydro-mechanical coupling on the behavior of unsaturated soils, a fully coupled general constitutive models for soils is developed. This model is based on the modified Cam-Clay model and includes a yield surface with anisotropic hardening that takes into account the shift of the critical state line with suction. The result is a very simple model with a symmetric stiffness matrix that can be used for saturated, unsaturated and compacted materials.
Keywords: Constitutive models, Critical state concept, Critical void ratio, Effective stress, Elastic zone, Elastoplastic framework, Failure surface, Plastic deformations, Preconsolidation stress, Suction hardening, Tensile strength, Virgin consolidation line, Volumetric behavior, Yield surface.