Influence of fault on the surrounding rock stability of a tunnel: Location and thickness

Abstract For an underground tunnel or cavern, the stability of the surrounding rockmass and the safety of the supporting system are predominantly controlled by the fault located near the underground opening, especially during the construction period. In this paper, a series of finite element method (FEM)-based experiments were performed to study the displacements, stresses and plastic zones in the surrounding rockmass and the stresses in the sprayed concrete lining (SCL) in different scenarios, including fault location, fault thickness and the surrounding rockmass quality. To simulate the real behaviour of a fault, an interface model that can represent the behaviour of a fault in rockmass was introduced, which can reflect such features as the fault’s closing, slipping, and opening at different stress states based on the theory of the contact-friction interface (COJO) element. To simulate faults with different thicknesses without remeshing in one FEM mesh, a practical method was established, and a series of formulas for the equivalent parameters of the fault zone is derived based on the equivalent theory on deformation and strength. In the analysis, the location of the fault around the tunnel was defined by arch, shoulder and sidewall. The discrepancy in displacements, stresses and plastic zones in the surrounding rockmass was analyzed on the cases with different fault thickness and locations, as well as the inner forces and stresses in the SCL system.