Experimental study on face instability of shield tunneling in saturated sands at various cover depths

Abstract

Strict control of the support pressure is critical for shield tunneling in urban metro projects. However, due to an insufficient understanding of the three-dimensional failure mechanism in saturated sands, the limit support pressure is difficult to determine, and an improper selection can easily lead to face instability. In this study, a series of model tests on face instability were conducted in saturated sands at four different cover depths. The test results indicate that: (1) The support pressure and displacement curve exhibits a four-stage characteristic. As the cover-to-diameter ratio increases, the soil arching effect becomes more significant. For deep tunnels, the maximum ground settlement is only 1/80 of that for shallow tunnels. (2) Face instability induces stress redistribution in the surrounding soil. For shallow tunnels, the failure zone develops more rapidly and propagates to the ground surface, whereas for deep tunnels, the failure zone has a wider horizontal extent, and its height stabilizes at 3 to 4 times the tunnel diameter. The inclination angle of the wedge in the failure zone decreases as the cover-to-diameter ratio increases. (3) Compared to dry sands, the ratio of the limit support pressure to the initial support pressure is larger in saturated sands. The overburden load sustained by the soil arching effect is reduced, and the normalized displacement required to reach the critical settlement S_cr increases significantly, indicating a delay in the formation and failure of the soil arch. These findings are primarily applicable to medium-dense, well-graded, saturated medium-to-coarse sands.