Face stability analysis of shield-driven tunnels using multi-tangent technique

To address the face stability problem of deep-buried tunnels under nonlinear conditions, this study employs a multi-tangent technique to piecewise approximate the Power-Law (P-L) criterion and develops a novel multi-cone failure mechanism for tunnel faces based on kinematic limit analysis theorem. A theoretical framework considering the tension cut-off (T-C) is proposed to evaluate tunnel face stability. The reliability of the proposed method is validated through comparison with existing literature. Based on the back-calculated stress distribution and geometric feature of the failure mechanism, this study investigates the influence of T-C under different parameters on the tunnel face stability. Results indicate that: (1) In the linear condition, the effect of T-C on the critical support pressure is more significant with high cohesion and small internal friction angle; In the nonlinear condition, an increase in the nonlinear coefficient will not exacerbate the impact of T-C on the face stability; (2) For higher tensile strength, the effect of T-C on the face stability is significant, as the tensile strength decreases, the failure mechanism governed by T-C gradually shrinks, and the top of the failure mechanism exhibits noticeable blunting and looks like a dome.