Analysis of the Influence of Tunnel Span on the Stability of Unlined Circular Tunnels Subjected to Intense Dynamic Load

This study investigates the influence of tunnel span on the dynamic response of rock masses with high integrity under intense dynamic load, analyzing an unlined circular tunnel excavated in intact surrounding rock with a uniaxial compressive strength of fr=57 MPa. Using a combined approach of physical model testing and numerical simulation, the influence mechanism of span on tunnel stability under different intense dynamic loads is systematically analyzed. The research results indicate the following: (1) When the peak intense Dynamic load is below 0.51fr, the surrounding rock mass remains in an elastic state. (2) When the peak load ranges between 0.51fr and 0.54fr, plastic zones emerge at the tunnel wall. (3) Once the peak load exceeds 0.70fr, the influence of the tunnel span on stability becomes significantly more pronounced with increasing load intensity. In small-span tunnels, plastic zones primarily distribute along the wall sides, whereas in large-span tunnels, they extend further upward and downward. (4) At a peak load of 0.70fr, the ratio of the maximum extent of the plastic zone in a 20 m span tunnel to that in a 5 m span tunnel is 10.70, and the ratio of the maximum relative displacement between the vault and invert is 4.67. When the peak load increases to 1.40fr, the plastic zone extent ratio rises to 13.94, and the vault–invert displacement ratio increases to 6.17. The conclusions of this study provide theoretical foundations for the design of tunnels with varying spans under intense dynamic load.