Consistent Modelling of Expansive and Collapsive Response of Unsaturated Soils

In unsaturated soil modelling, collapse is viewed as an ordinary response, whereas swelling upon an increase of moisture is considered as a special class of problems. However, an investigation based on fundamentals of plasticity indicates that the two seemingly opposite responses are unified within the same ordinary elastoplasticity framework. In fact, similar to the well-known contractive and dilative responses in the critical state framework, the collapsive and expansive responses of unsaturated soils differ only in the sign of the dissipative hydrostatic stress. For a soil with a low applied net stress, the dissipative mean normal stress during wetting is likely to be negative, and the plastic volumetric strain is kept negative (swelling) as well, such that the dissipation is always positive, complying with the second law of thermodynamics. When the applied net stress is high, the dissipative mean normal stress may turn to be positive, resulting in a positive plastic volumetric strain (collapse). This paper explains the concept of this ‘ordinary’ modelling framework, and uses a bounding surface triaxial model to demonstrate the concept. The numerical results are compared with experimental observations reported in the literature.