Failure control of large-scale exposed tunnels under the combined effects of excavation damage and dynamic disturbance at a depth of 1240 m.

The stability of deeply buried tunnels is significantly influenced by the combined effects of primary joint fissures, blasting-induced damage, high-stress environments, and dynamic disturbances, all of which are key contributors to rock instability. The instability characteristics of rock masses under varying disturbance frequencies and amplitudes remain unclear, making it difficult to establish a reliable basis for tunnel management. This study measured the distribution of joint fissures on the tunnel surface at a burial depth of 1240 m, investigated rock failure characteristics through low-frequency perturbation true triaxial experiments, and analyzed support designs incorporating various combinations of metal mesh, bolts, anchor cables, and shotcrete. The results indicate that as the amplitude and frequency of disturbances increase, the number of cracks in the rock rises significantly and irregularly, while the fractal dimension of the rock's fracture direction decreases. When the disturbance reaches 10 MPa and 10 Hz, the fractal dimension decreases to a minimum value of 0.62. Additionally, the frequency of pore orientation at angles between 80° and 120° peaks at 52% of its maximum value, approximately 1.68 times that of the original rock. This suggests that the stress experienced by the particles within the rock becomes uneven after disturbance, leading to stress concentration and a pronounced fracture direction. Furthermore, as the amplitude and frequency of disturbances increase, the micropore area observed in scanning electron microscope (SEM) images initially increases rapidly, then continues to grow at a slower rate, with the rate of increase progressively diminishing. Simulations reveal that standard bolts in tunnels subject to dynamic disturbances can effectively resist disturbances with strengths below 40 MPa. However, when the disturbance intensity exceeds 70 MPa, the anchor's bearing capacity reaches its limit. In the case of bolt-supported tunnels subjected to dynamic disturbances, characteristics such as shallow anchoring depth, low preload force, significant separation of deep surrounding rock, and poor anti-damage ability of the bolts are observed. The use of highly prestressed anchor cable support can resist dynamic disturbances up to 100 MPa and enhance the tunnel's damage resistance. By combining stress and peak ground acceleration (PGA), the tunnel is classified into five potential risk levels (I to V). Based on this classification, a tunnel support strategy under high-stress disturbances is proposed. Practical applications demonstrate that implementing this strategy reduces the deformation of the surrounding rock by 42.47% to 51.07%, significantly improving the tunnel's stability.