Safety factor analysis of a tunnel face with an unsupported span in cohesive-frictional soils

Abstract Ensuring the stability of the tunnel face and the unsupported span is a critical issue in conventional tunnelling. An improved prism-wedge model is established to reasonably evaluate the safety factor of a tunnel face and an unsupported span in cohesive-frictional soils. In this model, two failure modes are considered: the failure of an unsupported span alone and the simultaneous failure of a tunnel face and an unsupported span. The contribution of soil arching to the stability of a tunnel face and an unsupported span is considered by the silo theory, and the failure surface in front of a tunnel face is assumed to be log-spiral shaped. To validate the accuracy of this model, the limit support pressure and the safety factor of a tunnel face are respectively contrasted with those obtained by other typical theoretical models and numerical simulations, which shows great agreement and reliability of the proposed model. The critical failure mode of a tunnel face in the critical limit equilibrium state with different parameters is obtained by the strength reduction technique. Further, to popularise this model in preliminary tunnel design, a series of nomograms are presented for the safety factor analysis of a tunnel face with different parameters.