Study on the Stability and Influencing Factors of the Tunnel Surrounding Rock in a Ductile Shear Zone: Insights from Numerical Simulations

To elucidate the evolutionary patterns and influencing factors affecting the stability of the surrounding rock during tunnel excavation in ductile shear zones, this study takes the tunnel section within the ductile shear zone of the mountain area as a case study. Numerical simulation methods were employed to analyze vertical deformation, plastic zone development, and stress redistribution characteristics of the surrounding rock under a three-step excavation approach. Furthermore, this research investigates how burial depth and surrounding rock grade impact stability. The findings indicate that during the tunnel excavation process, the vertical deformation, plastic zone, and extrusion deformation at the face exhibit significant phased characteristics. Notably, the weak zone of the fractured surrounding rock serves as a concentrated area of deformation. Upon completion of the excavation, the maximum settlement observed at the arch top reached 246.07 mm. The plastic zone primarily experienced shear failure and showed a tendency to stabilize after reaching section Y = 35 m during excavation. The burial depth exerts a significant influence on the stability of the surrounding rock. As the burial depth increases from 400 m to 550 m, there is an observable upward trend in the settlement at the top of the arch, uplift at the bottom of the arch, and maximum principal stress; notably, the rate of increase for maximum principal stress remains stable. The instability mechanism of the surrounding rock, primarily characterized by shear failure, has not altered. The classification grade of the surrounding rock serves as a critical factor influencing stability. The vertical deformation scale, extent of plastic zones, and values for maximum principal stress in Grade IV surrounding rock are considerably smaller than those observed in Grade V. Enhanced mechanical properties and integrity within the rock mass can significantly improve stability conditions for surrounding rocks. These research findings provide a theoretical foundation and engineering reference for optimizing support systems in tunnels traversing ductile shear zones.