3D mechanical response and evolution characteristics of load bearing zone for surrounding rock mass in large section tunnels

Abstract In deep-buried tunnels, surrounding rock acts as both a load source and load-bearing structure via the internal load-bearing zone, whose three-dimensional (3D) evolution governs support design and tunnel long-term stability. This study investigates the 3D spatial effects of the load-bearing zone, which are governed by the stress release coefficient. Numerical calculation methods are employed to analyze the 3D characteristics of the surrounding rock load-bearing zone, considering the influence of geological conditions and tunnel engineering parameters. The findings reveal that the 3D shape of the tunnel’s load-bearing zone resembles a thick “bullet” shape. The intermediate principal stress influences the longitudinal dimension of the load-bearing zone, while the horizontal principal stress affects its cross-sectional geometry. Weak bedding interfaces induce significant anisotropy of the load-bearing zone, where oblique principal-stress-bedding intersections block stress transfer, causing local disconnection and delayed stress convergence in layered surrounding rock.As the tunnel span increases, the longitudinal dimension of the 3D “bullet-shaped” load-bearing zone elongates, and the proportion of non-load-bearing areas relative to the tunnel cross-sectional area increases. This results in greater instability of the surrounding rock and less favorable stress conditions in the vertical direction. The research provides insights into optimizing support design and enhancing tunnel stability under weak rock mass.