CASM-U: a unified critical state model for unsaturated clays and sands

This paper proposes a coupled hydro-mechanical constitutive model for unsaturated clay and sand (CASM-U) in a critical state framework. The mechanical behaviour of unsaturated soils is modelled by modifying the unified clay and sand model (CASM) with Bishop's effective stress, bounding surface concept and loading collapse (LC) yield surface. The hydraulic behaviour is described by a soil-water characteristic curve (SWCC) with nonlinear scanning law, considering the coupled effects of soil deformation and hysteresis. CASM-U is implemented into a commercial finite element software through the user-defined material subroutine (UMAT), and the implementation is benchmarked by a new semi-analytical cavity expansion solution adopting CASM-U. Finally, the performance of CASM-U in predicting hydro-mechanical behaviour of unsaturated clays and sands is examined by comparing with experimental data from tests along various loading paths, including isotropic compression, cyclic drying-wetting, triaxial shearing, and their combinations. It is shown that CASM-U can provide reasonable predictions for hydro-mechanical behaviour of unsaturated soils with a total of 15 material parameters.