A hypoplastic model for unsaturated sand accounting for drying and wetting cycles

Existing constitutive models for unsaturated sand cannot capture the irreversible volumetric strain in the drying and wetting (DW) cycles. This study develops a novel hypoplastic model that aims to capture the behaviour of unsaturated soils, with a specific emphasis on the DW cycles experienced by unsaturated sand. The formulation integrates a hypoplastic model proposed by von Wolffersdorff for saturated sand with Bishop's effective stress. The simulation of soil water characteristic curve hysteresis is achieved through the implementation of the scanning rule for the main drying/wetting curve and scanning curve. The incorporation of a new tensorial term, Hs, into the hypoplastic equations, captures the irreversible volumetric strain in the DW cycle. A novel pyknotropy factor, referred to as fus, is proposed to replicate the rate at which irreversible volumetric strain accumulates during the DW cycle. Furthermore, the influence of suction effects on the critical state line is taken into account to represent the shear behaviour of unsaturated sand. The evaluation of the model performance is conducted through existing element tests on unsaturated sand. The comparison illustrates that the proposed model has the capability to capture certain essential characteristics, such as irreversible volumetric strain in the DW cycles and critical state behaviour of unsaturated sand. Due to its simplicity, the proposed model lends itself to straightforward implementation and potential utilization in future engineering applications, particularly in the analysis of slope stability in the presence of rainfall-induced conditions.