Clay and Sand Model Extended to Unsaturated Conditions: Implementation and Validation Within a Symbolic Algebra–Based Framework

This study presents a fully coupled hydro‐mechanical implementation of the Clay and Sand Model (CASM) extended to partially saturated conditions (U‐CASM) within the COMSOL Multiphysics environment. The formulation adopts Bishop's effective stress and suction as stress variables and incorporates alternative loading–collapse surfaces, dilatancy laws, and a porosity‐dependent water retention curve. The main contribution lies in a fully customised implementation strategy based on symbolic algebra, which enables user‐defined constitutive modelling, simplifies equation integration, and reduces programming effort within a multiphysics framework. The model is verified against benchmark simulations for clays and sands under drained and undrained conditions in both saturated and unsaturated states. Validation against triaxial and oedometer data confirms the ability of the model to reproduce soil behaviour over a wide range of stress paths and suction levels, while highlighting the influence of alternative constitutive assumptions. A field‐scale application further demonstrates the capability of the framework to capture key hydro‐mechanical features of partially saturated soils, including collapse upon wetting.