Collapse mechanism of shield tunnel face and evaluation method for the minimum required support pressure considering soil strain softening effect

During shield tunneling, accurately revealing the collapse mechanism and evaluating the minimum required support pressure are key issues to avoid tunnel face failure. The strain softening effect of sandy soil is one of the vital factors for tunnel face stability analysis. However, it has not been explicitly considered in previous research. In this study, a practical strategy was proposed to consider the strain softening effect in finite element simulations. A novel method was also proposed for the evaluation of the minimum required support pressure. The results show that the stratum failure zone can be divided into three types during tunnel face collapse, which correlates with the cover to diameter ratio of the tunnel and can be uniformly delineated by the wedge-prim model. The process of tunnel face collapse was accompanied by the development of local stratum strain softening. The soil's friction angle at the collapse zone boundary was found in a range between the peak value and the residual value. A coefficient representing the strain softening degree was defined to improve the traditional wedge-prim model. Moreover, by comparing with existing tests and theoretical models, the effectiveness of this method was well validated. The proposed method could accurately assess the minimum required support pressure for construction safety concerns.