A laboratory testing program was conducted to investigate the effects of microstructure on the water retention curve (WRC) and wetting–drying induced volume change in loess. The axis translation and vapor equilibrium techniques were adopted to control suction in the range of 0–400 kPa and 4–140 MPa, respectively. Hysteresis in the WRC of loess was observed for the entire range of suction studied. Compared to re-compacted loess, intact loess exhibits a more pronounced hysteresis in the suction range below 20 kPa, which can be explained by the ink-bottle pore neck effect or constricted pores. The hypothesis is supported by microstructural evidence of mercury intrusion porosimetry and scanning electron microscopy tests. However, re-compacted loess exhibits larger hysteresis than intact loess for suctions above 30 kPa. A conceptual model was introduced, which links WRC to the corresponding pore-size density (PSD) function. Regarding volume change, more noticeable drying-induced shrinkage, but yielding at a lower suction, was observed for re-compacted loess. This is consistent with the compression test results. Stress has a significant effect on change of PSD and constricted macropores leading to a shift in the main wetting curve and a less pronounced hysteresis. Intact loess exhibits a stress-dependent wetting-induced collapse and drying-induced shrinkage.

A state-dependent elasto-plastic constitutive model for both saturated and unsaturated soils is presented in this paper. The model, which is developed under an extended critical-state framework, uses two independent stress state variables: net normal stress and matric suction. In addition, the influence of a simplified non-linear soil-water characteristic curve on mechanical behaviour is incorporated in the model by using a bi-linear function. Most importantly, a state-dependent dilatancy formulation is introduced to account for the effects of stress level or stress ratio, internal state (density) and soil suction. In this paper, details of the model formulation and the determination of model parameters are described and reported. By using the new model, numerical simulations of triaxial tests have been carried out on both saturated and unsaturated fine-grained (sandy silt) and coarse-grained (gravelly sand) weathered soils. The simulated triaxial tests include undrained shear tests on saturated specimens, constant water content tests and wetting tests under constant deviator stress on unsaturated specimens. The computed results show that the stress–strain and volumetric-shear–strain relationships at both loose and dense and saturated and unsaturated states can be modelled properly with a single set of parameters. Moreover, the model is capable of capturing some key observed features, such as a sudden increase in shear strain and volumetric strain as suction is reduced under a constant deviator stress.

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