Multi-scale investigation on tunnel face failure and soil arching in unsaturated sandy ground

Abstract

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.