Numerical modelling and field observations on the failure mechanisms of deep tunnels in layered surrounding rock

Tunnelling through layered rock mass with high geo-stress often leads to large deformation disasters, including rock collapses and structural failures. This study investigates the asymmetrical deformation and failure behaviors of deep tunnels in layered phyllite strata in Chengdu-Lanzhou railway (China). It has been observed that the mechanical behavior of these tunnels exhibits evident correlations with the orientations between geo-stress conditions and bedding plane. For in-depth analysis, a modeling method was first proposed considering the difference between interlayer bedding planes and intralayer joints. The excavation disturbance behaviors of unsupported tunnels with various bedding angles, such as deformation characteristics, crack generation and propagation, and stress distribution, were simulated and compared with on-site phenomena. Afterward, the crack distributions were classified into four typical types based on simulation results, and their causes, differences, characteristics, and evolutionary process mechanisms were also systematically analyzed. Additionally, the asymmetric failure caused by the weak plane effect was investigated and discussed. The results revealed that the bedding angle, the direction of principal stress and the effect of layer thickness primarily influenced the failure patterns of layered surrounding rock, which resulted in variations in both the direction and depth of failure. Engineering practice demonstrated that the classification method, based on the proposed numerical modeling approach, can serve as a valuable reference for tunnel support design.