Abstract：Through virtue of numerical simulation and theoretical analysis the problem of passive failure mechanism of shield tunnel is studied. Firstly, the numerical simulations with finite element software are performed to reveal the partial instability modes of passive failure of a pressurized tunnel face. The partial instability ratio and limit support pressure for passive failure of shield tunnel face are obtained. Moreover, the parameter sensitivity analysis of friction angle and the relative cover depth on partial instability ratio and limit support pressure are also discussed. Secondly, a new passive failure mechanism of tunnel face considering the partial instability modes is proposed based on limit analysis methods. The new passive failure mechanism is composed of five truncated cones and a distributed force acting on the truncated cones. The distributed force is determined using a modification of the silo theory for passive modes. The upperbound solution for passive failure mechanism are calculated by numerically optimizing with respect to the defined angles and partial instability diameter on tunnel face. Then the effects of friction angle and the relative cover depth on the partial instability ratio are conducted. He results of partial instability ratio obtained from numerical simulation and theoretical analysis in this paper are compared. Finally, the obtained passive limit support pressures and the existing approaches are compared, which indicates that new passive failure mechanism provides relatively satisfactory results for limit support pressures.
