A state-dependent bimodal soil water retention model considering evolutions of pore-scale soil–water interaction

Existing soil water retention models can capture the effects of void ratio and/or net mean stress on the changes in soil water retention curve (SWRC). However, most of them, especially those for bimodal soil that is composed of macro- and micro-pore systems, do not consider the simultaneous evolutions of pore structure and particle contacts, and neglected the drainage processes of bulk water and meniscus water when subjected to any external hydro-mechanical loading condition. This paper proposes a state-dependent bimodal SWRC model considering pore structure evolution and pore-scale water drainage. In this model, an elastoplastic relationship is introduced to describe the variations of void ratio with net mean stress and suction, whilst the corresponding changes in the pore structure are modelled by the shift and scaling of the pore-size distribution and the volumetric proportion between the macro- and micro-pore systems. By relating stress-induced changes in void ratio and coordination number to the mean pore radius and total meniscus water volume, respectively, the soil water content at different stages of water drainage can be determined. The model is validated against existing and new data, and can capture the effects of void ratio, isotropic stress, and K0 stress for unimodal and bimodal soils.