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

To address the face stability problem of deep-buried tunnels under nonlinear conditions, using Power-Law (P-L) criterion the approximate piecewise and by multi-tangent technique, a novel multi-cone failure mechanism is developed for tunnel faces based on the kinematic limit analysis theorem. A theoretical framework considering the tension cut-off (T-C) is proposed to evaluate the stability of tunnel faces. The reliability of the proposed method is validated through comparison with that in the existing literatures. Based on the back-calculated stress distribution and geometric feature of the failure mechanism, the influences of T-C under different parameters on the stability of tunnel faces are investigated. The results indicate that: (1) under the linear condition, the effects of T-C on the critical support pressure are more significant with high cohesion and small internal friction angle. Under 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 effects of T-C on the stability of tunnel faces are 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.