Three-Dimensional Passive Partial Failure Analysis of the Excavation Face of a Shield Tunnel

Previous analytical models for the passive stability analysis of the shield tunnel face always assumed the global failure at the excavation face. However, recent experimental and numerical results have refuted this hypothesis and found that partial failure is more likely to occur at the excavation face when the passive failure happens. To better explore the passive stability of excavation faces, two improved three-dimensional (3D) failure models were developed considering the possibility of the partial failure at the excavation face. To check the passive stability, the limit passive pressure and the corresponding failure surface were derived using a combination of the upper-bound limit analysis method (UBLAM) and a hybrid optimization method. Validations illustrate that the 3D transition circular partial failure model is preferable for the passive stability analysis. The parametric and sensitive analysis shows that the importance sequence of the analytical variables on the stability issue is friction angle > relative cover depth > ground surcharge > cohesion > soil gravity. The extension of the developed method in nonhomogeneous soils shows that, for these selected cases, the passive failure is more likely to occur from the layered interface or above the layered interface, and larger limit passive pressure triggers smaller upper partial failure as the layered interface rises. The application to a practical tunnel case further validates the effectiveness of the developed method.