Strata Response Analysis of Subway Tunnel Construction Under Hidden Karst Caves in the Tunnel Arch

［Objective］In urban subway tunnel projects, it is common to encounter shallow-buried hidden karst caves in the tunnel arch. Therefore, it is necessary to study the strata response of subway tunnel construction under such condition. ［Method］The indoor model test is used to simulate the tunnel excavation process. By analyzing and monitoring the deformation of surrounding rock, release of soil pressure, internal force of primary support arch and ground settlement laws during tunnel construction, the disturbance effect of tunnel construction on karst strata is studied. ［Result & Conclusion］Under Grade IV surrounding rock, when the excavation tunnel span is 6.52 m, the tunnel arch cave diameter is 4 m, and the net distance from cave bottom to metro tunnel top is 2 m, the surrounding rock of the hidden cave in the tunnel arch could maintain stability under mechanical excavation of the tunnel, with no surrounding rock instability and damage. Under the condition with vault cavity, the change amount of surrounding rock soil pressure at the tunnel vault is significantly smaller than that under the condition without cavity, and the final value of surrounding rock soil pressure in the former condition is 53.8% of that in the latter condition. The existence of hidden cavity in the arch will increase the range of ground settlement, and the peak ground settlement value directly above the top of the excavation tunnel is 1.73 times that under the condition without cavity. When the tunnel vault develops cavity, the axial force value at the arch vault is 120.3% of that under the condition without cavity, and the bending moment value at the vault is 53.3% of that under the condition without cavity.