Determination of the Rock Mass Bearing Mechanism Following Excavation of Circular Tunnels

The self-supporting capacity of the surrounding rock mass is a critical factor in maintaining tunnel stability after excavation and is a key determinant of tunnel support structures safety. Despite being widely applied, the mechanism behind the self-supporting effect remains a challenging issue to clarify. In this paper, we have used the finite difference method (FDM) to investigate the formation mechanism and characteristics of the surrounding rock mass's self-supporting zones. By analyzing the redistribution of a stress and an energy using stress concentration factor, energy concentration factor, we propose a self-supporting mechanism of the surrounding rock mass under different horizontal to vertical stress ratios. The further analysis results were subsequently validated through non-contact strain acquisition experiments. Our findings suggest that in cases of unequal horizontal and vertical stress, a “primary load-bearing zone” is formed in the direction of the vertical maximum principal stress within the rock mass. The rock mass areas aligned parallel to the direction of the maximum principal stress transfer the maximum principal stress to the primary load-bearing zone, causing compression towards the “primary load-bearing zone” and resulting in the formation of a “passive load-bearing zone”. The primary load-bearing zone resembles the footings of an arch, while the passive load-bearing zone resembles the arch's intrados. When the horizontal and vertical stresses are equal, the formation of the self-supporting zone is caused by the radial deformation of the surrounding rock into the tunnel cavity, followed by the mutual compression of the surrounding rock in the circumferential direction. The surrounding rock mass quality and depth of burial influence the size of the self-supporting zone, with the surrounding rock mass quality having a greater impact than the depth of burial.