Coupling of hydraulic hysteresis and stress–strain behaviour in unsaturated soils

Consideration of the different roles of pore air pressure, pore water pressure within bulk water and pore water pressure within meniscus water suggests that the degree of saturation will have a significant influence on the stress–strain behaviour of an unsaturated soil, in addition to any influence of suction. This suggestion is supported by experimental evidence. In the light of this, a new elasto-plastic framework for unsaturated soils is proposed, involving coupling of hydraulic hysteresis and mechanical behaviour. Within the proposed framework, plastic changes of degree of saturation influence the stress–strain behaviour, and plastic volumetric strains influence the water retention behaviour. A specific constitutive model for isotropic stress states is proposed, and model predictions are compared with experimental results, in order to demonstrate some of the capabilities of the new framework. Forms of behaviour that can be represented include proper transitions between saturated and unsaturated types of response, the occurrence of irreversible compression during the drying stages of wetting–drying cycles, and the influence of a wetting–drying cycle on subsequent behaviour during isotropic loading.