Constitutive modelling of partially saturated soils: Hydro-mechanical coupling in a generic thermodynamics-based formulation

Hydro-mechanical coupling is a crucial element in constitutive modelling of partially saturated soils, given the dependence of the macro behaviour on the interaction between frictional sliding, grain rearrangement and ruptures of liquid bridges and their redistributions at the grain contacts. The inseparable nature of this interaction requires the interdependence of all internal variables describing the inelastic behaviour of a continuum model. We propose a new generic thermodynamics-based approach to coupling the effects of deformation and saturation in modelling partially saturated soils taking into account the interdependence of all internal variables. This approach allows the derivation of models from only two explicitly defined energy and dissipation potentials, leading to coupled hydro-mechanical behaviour governed by a single yield surface in stress-suction space and two evolution rules for plastic strains and irrecoverable saturation. This coupling provides a path-dependent hydraulic response, reflecting the nature of the hydro-mechanical interactions at the grain scale, while removing the use of a separate Soil Water Characteristic Curve (SWCC). The benefits are the reduction in number of parameters in conjunction with the identification and calibration of all model parameters from standard tests. An extensive experimental validation shows the capabilities of the model and the advantages of the proposed thermodynamics-based approach.