Although universal in practical engineering, the soil arching effect induced by tunnel face unloading (TFU) in the unsaturated sandy ground (USG) hardly receives academic concerns for its complicacy. In this study, a physical model and a discrete element method (DEM) incorporating the interparticle capillary water force (ICWF) were established and verified. With the combination of experimental and numerical TFU, the intrinsic mechanism of soil arching effect in the USG was innovatively investigated from macroscale to mesoscale. The results indicate that the tunnel face limit support pressure in sandy ground decreases firstly, and then increases with the increase of saturation degree and its minimum value can be less than 22% of that in the dry sandy ground (DSG). Meanwhile, distinct from the global collapse in DSG, a self-stabilized soil arch emerges above the tunnel crown in USG and prevents the loosening zone from further development. With more effective stress transfer under the stronger soil arching effect, the cover-ratios of transition zone and weak deflection zone for the major principal stress in USG can decrease to 24% and increase to 47% respectively as compared to those in the DSG. Additionally, the coordinate number, weak contact proportion, porosity, and contact anisotropy can effectively reflect the meso-mechanical characteristics of soil arching effect in the USG. This work provides precious evidence for evaluating the tunnel face stability in the USG.

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.