Implementation and validation of a hydro – mechanical model based on the Clay and Sand Model extended to unsaturated conditions

Wetting -induced collapse settlements are a brittle and irreversible phenomenon that can cause significant structural damage and economic losses. These deformations primarily occur in loose and unsaturated soils triggered by water infiltration due to precipitation, pipe leakage, or a rising water table. Modelling this phenomenon requires a comprehensive hydro-mechanical (H-M) coupling approach to capture the interaction between hydraulic and mechanical processes within the soil. This approach involves the simultaneous solution of the water mass balance and mechanical equilibrium equations. In this study, the commercial software COMSOL Multiphysics® was used as an implementation platform to develop a numerical solver to simulate different laboratory tests, including triaxial tests under constant and variable suction. The Clay and Sand Model (CASM), extended to unsaturated conditions by incorporating Bishop’s effective stress and suction as stress variables, called U-CASM, was employed as the constitutive model. The CASM framework provides a unified elastic-plastic model capable of capturing the behaviour of both clayey and sandy soils. The use of symbolic algebra facilitated an efficient implementation, with an integration strategy based on the Current Increment Corrects Error (CICE) method, which reduces potential integration drift. The model was verified against predictions obtained by other numerical tools with the same constitutive model and subsequently calibrated and validated considering experimental results available in literature. The comparison with experimental data demonstrated that the model accurately reproduces the soil behaviour under different suction and loading conditions.