A constitutive model considering the interaction between evolution of microstructure and hydro-mechanical behaviour of unsaturated soils

The evolution of microstructure induced by loading and unloading has a significant impact on the hydro-mechanical behaviour of soils, including volume change, shear strength, water retention and permeability. In this paper, a constitutive model based on the evolution of microstructure is established building on the approach of an existing mechanistic model. In this model, the evolution of microstructure is represented via changes in the pore size distribution (PSD) and assumed to be related solely to the change of void ratio induced by loading and unloading. A PSD-dependent Bishop’s effective stress coefficient χ*, which represents the coupled impact of PSD evolution on hydro-mechanical behaviour of soils, is used to replace the Bishop’s effective stress coefficient χ. The model can reproduce and predict the hydro-mechanical behaviour and evolution of microstructure and their interaction within a unified framework. It also has potential in studying the soil-water characteristic curve and multi-field-coupling of soils. Model response and sensitivity analysis are reported based on idealized parameters to give a primary evaluation on the model’s performance and feasibility of using PSDs from mercury intrusion porosimetry. It is found that whilst the model is sensitive to parameters representing inter-aggregate pore size distributions it can be satisfactorily applied to represent the hydro-mechanical behaviour and microstructural evolution of unsaturated soils.