Failure mechanism and control technology of large deformation for Muzhailing Tunnel in stratified rock masses

Deep tunnels in soft rock with high in situ stresses always exhibit large deformations. A case study in the Muzhailing Tunnel was carried out to explore the failure mechanism and investigate the control technology of the tunnel. The numerical simulation software Universal Discrete Element Code (UDEC) was used to establish a model, the results of which were analyzed. Combined with the rock mass strength criteria of the Geological Strength Index (GSI), the parameters of the rock mass around the tunnel were evaluated, and the microparameters of the rock mass from the UDEC were calibrated. Considering the results of the original support scheme to those calculated without support, failure first occurred in the roof and floor surfaces and then developed deeper into the surrounding rock. In the end, the tunnel suffered from a large asymmetric deformation with roof subsidence, rib shrinkage, and steel arch frame bending and twisting, which could be clearly observed from the stress, displacement, and crack evolution. Considering the failure of the original support with insufficient strength and stiffness, a new support scheme with high constant resistance anchor cables was proposed to control the large deformation of the tunnel. The anchor cables have been proved to be effective with high constant resistance and large deformation by the laboratory test, numerical simulation and field test. The results showed that the new support could effectively control the deformation of the tunnel. The new support scheme transforms the large deformation and nonuniform stress distribution of the tunnel into a small deformation and uniform stress distribution, which could provide useful references for support design in deep underground engineering.