Thermodynamics-based constitutive framework for unsaturated soils. 2: A basic triaxial model

Following the theoretical framework established in Part 1, a basic elastoplastic unsaturated soil model is formulated. The model is a combination of a soil skeleton model and an SWCC (soil-water characteristic curve) model. For simplicity, the model deals only with isotropic soils in triaxial compression space under a constant pore air pressure. The paper shows that the central element in the model that differentiates itself from a saturated model is its dual plastic hardening, which not only reflects the evolution of the internal structure of the soil skeleton but is also associated with the evolution of the solid–fluid interactive structure. For more flexible shear stress–strain responses, the non-associated flow rule (the alpha model) by Collins & Kelly, based on a thermodynamic analysis, is adopted. The present model introduces six new material constants, which can be calibrated following a systematic procedure. The model successfully simulates the multiple series laboratory tests reported by Sivakumar & Wheeler, including wetting, consolidation and shearing, on a compacted clay.