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 chi*, which represents the coupled impact of PSD evolution on hydro-mechanical behaviour of soils, is used to replace the Bishop's effective stress coefficient chi. 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.

The structure of unsaturated loess has a significant impact on its hydraulic and mechanical properties. An elastoplastic model considering the structured evolution of unsaturated loess is presented in this paper using double stress variables consisting of average skeleton stress and suction. The model divides the structure of unsaturated loess into inherent structure and suction-induced structure from the structured composition of loess and gives isotropic compression equation for unsaturated loess that considers the structure evolution. At the same time, a soil-water characteristic curve considering the influence of the void ratio is introduced to reflect the coupling between the hydro-mechanical behaviours of unsaturated loess. The proposed isotropic compression equation is extended to axisymmetric stress space with the aid of the yield surface and flow law in the modified Cam-clay model. The proposed model can not only reflect the structured evolution of loess, but also predict reasonably the mechanical and hydraulic behaviour of loess under different stress paths. The reasonableness and availability of the proposed model are initially verified by comparison with the results of the unsaturated loess isotropic compression tests at constant suctions, the wetting test at constant net stresses, the complex stress path tests and the triaxial shear tests as well.