Deformation Mechanism and Control Measures of Large-Span Tunnels in Steep Layered Surrounding Rock

Large-span tunnels often encounter large deformation and failure when crossing layered sedimentary structures, particularly sedimentary structures that have been steeply inclined by geologic processes. This paper takes the Yongle Tunnel on the G544 line in Jiuzhaigou County, Sichuan Province, China, as an example and analyzes the deformation and failure characteristics of the tunnel through a field investigation and field test. Then, through theoretical analysis and numerical simulation, the influence of the section size, rock dip angle, and rock thickness on the deformation of the tunnel is analyzed to reveal the tunnel's deformation mechanism. Finally, effective control measures are put forward based on the examination. The results show that the tunnel suffers from unobvious asymmetric deformation. The convergence deformation of the tunnel side wall and the sensitivity to the section size are much larger than the vault settlement and haunch convergence. The large deformation of the tunnel is caused by improper construction methods and weak structural planes. The deformation of the tunnel was significantly reduced by adapting the center diaphragm method, with anchors placed as vertically as possible to the rock or at large intersection angles and by strengthening the radial grouting of the surrounding rock. The research results can provide a reference for tunnels in similar strata.