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.

Mechanical response of soils under simple shear conditions has long been a subject of significant interest in the field of geomechanics. When subjected to simple shear loading, soils experience rotations of the principal stress directions. To provide a unified description for the simple shear behavior of clay and sand, this paper proposes a novel critical state model that accounts for the influence of principal stress rotation (PSR), based on the unified critical state model for overconsolidated clay and sand with subloading surface (CASM-S). The novelty of the newly proposed model, which is named as CASM-SP, lies in its ability to consider the influence of direction of the stress increment when establishing the plastic flow rule that is suitable for both clay and sand. Therefore, the model can capture the mechanical response of soils resulting from the PSR loading mechanism, such as noncoaxial behaviors. Then, the newly proposed model is validated through the comparisons with a series of experimental data of clay and sand under both drained and undrained simple shear conditions. Results predicted by the CASM-SP model agree well with those from the experiments, demonstrating that CASM-SP can reasonably describe the simple shear behavior of both clay and sand.