Some aspects of numerical modelling of hydraulic hysteresis of unsaturated soils

The Soil Water Characteristic Curve (SWCC) plays an essential role in the response of unsaturated soils as it quantifies the ability of soils to maintain or lose their available moisture. It is generally understood that the SWCCs exhibit hysteresis when subjected to cycles of wetting and drying, and more generally the relationship between suction, volume changes, and the saturation degree for a given soil is not unique and can be path-dependent. In particular, the SWCC may show strong sensitivity to volume changes in cohesive soils. Due to the complexities involved in considering the hysteretic response of the SWCC and its dependency on volume changes, these two features are often ignored in numerical studies of unsaturated soils. To facilitate their use in numerical modeling, a model for the SWCC equation based on the bounding surface concept is proposed. The resulting non-linear model is presented in an incremental form relating the rate of saturation degree to the rate of suction and volume changes. It is shown that the proposed model has the ability to replicate the shape and the evolution of the scanning curves observed in the experiments compared to currently available alternatives. The model has benefits for numerical application as it improves the convergence properties of the overall numerical analysis and includes a robust integration scheme with automatic error control for updating the saturation degree during the analyses. Results are presented to show the ability and the robustness of the proposed scheme in modeling the unsaturated soil response under various loading conditions including static and dynamic analyses.