A multiscale analysis of adjacent fault dislocation mechanism induced by tunnel excavation based on continuous-discrete coupling method

Abstract

Tunnelling usually causes adjacent fault dislocation that further induces serious disasters and threats, such as earthquakes, while its physical mechanism is still not clear. In this paper, we establish a multiscale numerical model of tunnel excavation adjacent to a fault by a coupled continuous-discrete method. The discrete element method (DEM) is employed to simulate the meso-mechanical behaviours of the fault fracture zone, and the finite difference method (FDM) is used to describe the macrodynamic characteristics of the hanging wall and footwall. A synthetic rock mass (SRM) model is developed to build the fractured rock to present the discontinuity and heterogeneity of the fault zone. The macro and meso mechanism of fault dislocation induced by tunnel excavation are analyzed, respectively. Results show that the fault experiences a different local dislocation form changing from upward movement to downward movement along the fault surface at the macro-scale. The weak transfer ability of the fault for deformation between the hanging wall and footwall, the reduced macro shear strength of the fault during the unloading process, and the meso joint slip and intact rock failure in the fault fracture zone are three main reasons for fault dislocation. The random distribution characteristics of the joints have remarkable influence on fault deformation. The fault slips more easily when the joint dip is close to the fault orientation. A higher joint density and a larger joint length provide a favourable environment to accumulate more connected joint surface, resulting in a larger slip displacement. Sensitivity analyses are conducted on several critical parameters including tunnel location, fault thickness, and stress condition, with the purpose of evaluating the safety of a potential tunnel site and predicting the potential seismic risks when tunnelling adjacent to a fault.