Based on the Wheeler model

a hydro-mechanical coupling model of unsaturated porous media is presented. The formulation of capillary hysteresis with internal state variables is adopted as the soil-water characteristic curve in this model. The effect of capillary hysteresis in the hydraulic history on the deformation of unsaturated porous media and the effect of deformation of unsaturated porous media on the flow processes can be simulated by this new model. The strength of unsaturated soil is related to suction and the saturation degree. Although the soil experienced drying and wetting have the same suction value

the saturation degrees are different. So the strengths of unsaturated soils are different too. The irrecoverable change in volume fraction of water and the plastic volume deformation are used to be hardening parameters. The new model can be used to describe the effect of the saturation degree on strength and deformation

in addition to the hardening effect of matric suction on the unsaturated soils. The comparisons between the model predictions and test results confirm that the model can simulate the main features of unsaturated soil behavior. The theoretical framework is developed for the isotropic loading condition; so its generalization to a more general stress space is needed in the future