A coupled stress–strain and hydraulic hysteresis model for unsaturated soils: Thermodynamic analysis and model evaluation

The thermodynamics of coupled mechanical deformation and hydraulic hysteresis in unsaturated soils is discussed, and the energy dissipation associated with the elastoplastic process and the main wetting and drying processes is derived. Based on a newly developed elastoplastic model (Hu et al., 2014) and a hysteretic water retention curve for deformable soils (Hu et al., 2013), a coupled hydromechanical model is formulated. In the coupled model, the newly defined inter-particle bonding factor from our recent work (Hu et al., 2014) plays a key role both in representing the influence of water retention curve on the mechanical behavior and in governing the evolution of the hardening force in the stress–strain response. The effect of deformation on the water retention behavior is determined according to the variation in the void ratio. It is proved that the total energy dissipation is strictly positive once plastic deformation and/or an irreversible change in the degree of saturation occurs, and therefore, our model satisfies the second law of thermodynamics. The model is validated by comparison with experimental data, and it is shown that the coupled model is able to capture the main features of coupled hydro-mechanical behavior in unsaturated soils, including hydraulic hysteresis and mechanical hysteretic behaviors during drying–wetting cycles.