Thermodynamics-based constitutive framework for unsaturated soils. 1: Theory

Unlike the elastoplastic modelling of saturated soil behaviour, the constitutive modelling of unsaturated soils has not been firmly based on engineering science, and in particular on the thermodynamic laws. To establish a thermodynamics-based modelling framework for unsaturated soils, one needs to examine and deal with the work–energy–dissipation relations for an open multiphase thermodynamic system, as well as the coupling effects among the phases. This paper presents the detailed derivation of such a modelling framework, with all major assumptions clearly stated. This framework will pave a way for developing specific unsaturated soil models with an examinable basis.