Dynamic failure characteristics of surrounding rocks under different lateral pressure coefficients in deep tunnel transient excavation

A novel transient unloading testing system was adopted to simulate the transient excavation of tunnels under different lateral pressure coefficients (k(0)). The results show that the transient excavation of a tunnel induces significant stress redistributions and concentrations, particle displacements and vibrations to the surrounding rocks. The decrease of k(0) enhances the dynamic disturbance of transient tunnel excavation, and especially when k(0) = 0.4 and 0.2, the tensile stress can be observed on the top of the tunnel. The peak particle velocity (PPV) of the measuring points on the top of the tunnel decreases with the increasing distance between the tunnel boundary and measuring point. The transient unloading wave is generally concentrated on lower frequencies in the amplitude-frequency spectrum under the same unloading conditions, especially for lower k(0) values. In addition, the dynamic Mohr–Coulomb criterion was used to reveal the failure mechanism of a transient excavated tunnel by involving the loading rate effect. It is found that the excavation damaged zone (EDZ) of the tunnel is dominated by the shear failure, and the number of the shear failure zones increases with the decrease of k(0). The EDZ of tunnels after transient excavations varies from ring-shape to egg-shape and X-type shear with the decrease of k(0). The evolution of the EDZ induced by the transient unloading is associated with k(0), i.e., the shear failure of surrounding rocks mainly occurs in the stress redistribution stage under high k(0) (1.0–0.7), while the dramatic destruction of surrounding rocks is more prone to occur after the transient unloading process when k(0) ≤ 0.6.