Face stability of the slurry shield–driven tunnel with an impermeable filter cake in saturated medium sand

The most crucial role in the safe construction of shield tunneling is how to keep up the tunnel face stability, especially for those in saturated sand. An issue of collapse associated with insufficient support pressure can be observed during shield tunneling when the face support pressure is not well managed. In this regard, the face stability of slurry shield tunnels with an impermeable filter cake in saturated sand was analyzed in this study. A series of finite element models were developed and verified against published experimental data. The failure mechanism of tunnel face and limit support pressure at collapse were investigated with consideration of the effects of cover depth ( d 1 ), water level ( d 2 ), and internal friction angle ( φ ). The results indicated that the wedge-prism model could represent the tunnel face’s failure zone. The internal friction angle had a significant impact on the failure zone of the tunnel face. The increasing friction angle caused the reduced height of the failure zone and the increased slope of the failure surface. As the cover depth increased, the size of the wedge-prism model increased until the soil arch formed (i.e., h ≈ 0.5 D in this paper). The collapse mode of the tunnel face was less affected by changes in water level. When the cover depth reached the height of the soil arch, the hydraulic head pressure above the soil arch determined the limit support pressure ( p hh = γ w ( d 1 + d 2 − h )). The limit support pressure increased linearly with d 1 and d 2 , while the impact of internal friction angle on the limit support pressure of a tunnel in saturated sand was slight. About − 0.16 kPa increments are added to the limit support pressure as the internal friction angle reduces 1°. Moreover, an empirical approach to estimate the limit support pressure on the slurry shield tunnel face in saturated sand was proposed. The calculated limit support pressure by the prediction approach was consistent with the numerical results. Overall, the conducted investigations could offer helpful references for the slurry shield operation in saturated sand.