Morphology and distribution characteristics of tunnel bearing arch in horizontal layered surrounding rock

Tunnel excavation induces stress redistribution in the surrounding rock, forming a bearing arch to sustain ground stress. As a typical and special type of rock formation, the bearing arch morphology in (approximately) horizontally layered surrounding rock remains unclear due to the influence of structural planes. Additionally, the effects of factors such as ground stress, layer thickness, and interlayer contact conditions on the distribution of the bearing arch are not well understood. To address this issue, non-contact strain acquisition experiments combined with numerical calculations were used to determine the morphology of the bearing arch, followed by sensitivity analysis of the parameters. The results indicate that non-uniform rock masses under unfavorable stress conditions may fail at stress levels far below the ultimate strength of the rock blocks. For tunnels excavated in horizontally layered surrounding rock, vertical loads easily cause bending-induced tensile stress in the roof and floor, while increasing initial horizontal stress improves the bearing condition. The bearing arch in horizontally layered surrounding rock exhibits two extreme states after excavation: "butterfly-shaped" and "rounded rectangular." Under extreme conditions, "bearing zone discontinuity" may occur in the vertical direction. Overall, the lateral pressure coefficient and interlayer bonding strength significantly influence the bearing arch, while the layer thickness has a relatively smaller effect. The lateral pressure coefficient mainly affects the arch's morphology, while interlayer bonding strength and layer thickness primarily influence the stress redistribution during the post-excavation stress equilibrium process. In practical tunnel engineering, engineering measures should aim to enhance the overall integrity of the surrounding rock to fully utilize the bearing arch effect.