Deformation and failure mechanisms of surrounding rock and group anchorage in tunnels traversing fault fracture zones with different fault geometries

Tunnels traversing fault fracture zones frequently encounter severe engineering difficulties, primarily because the rock mass in these regions is highly fractured and mechanically weakened, rendering the surrounding rock particularly susceptible to excessive deformation and instability during excavation. However, existing studies have not sufficiently clarified the influence of fault geometries on the deformation behavior and failure mechanisms of the surrounding rock and support system. Therefore, three-dimensional numerical simulations were conducted herein to investigate the mechanical behavior of tunnels traversing fault fracture zones. Two representative excavation approaches, namely the center diaphragm and bench cut methods, were first compared to assess the influence of excavation scheme selection on the surrounding rock response. Subsequently, numerical analyses were performed to examine stress distribution, deformation characteristics, and plastic-zone evolution under varying fault geometries. Building on this framework, an orthogonal experimental design was employed to optimize rock-bolt support parameters. Particular attention was paid to load distribution within bolt groups and to the nonuniform anchorage performance observed in fault fracture zones. The results revealed marked spatial variation in bolt axial force, with local stress concentrations developing in specific bolts. Accordingly, a targeted support optimization strategy is proposed to improve load-bearing coordination within bolt groups and enhance surrounding rock stability. The main novelty of this study lies in establishing an integrated numerical framework to systematically reveal the coupled effects of excavation method, fault geometry, and support design. The findings provide a useful basis for selecting excavation methods, optimizing support design, and controlling deformation and instability in tunnels crossing fault fracture zones.