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.
