Progressive arching effect and damage evolution process of shallow tunnels with jointed hard rock under overlying loads

The influence of the arching effect on the loosening pressure in shallow tunnels with jointed hard rock is not clear. In order to solve this problem, the analytical model of loosening pressure in shallow tunnels is modified by considering the role of stress deflection and shear resistance under the arching effect of the stratum based on the traditional analytical model of loosening pressure. Relying on the Qingdao Metro Line 6 project, this paper carries out similar model test and numerical simulations, verifies the applicability of the analytical solution, and clarifies the influence of different joint surfaces on the tunnel stability. The results show that: (1) The shallow tunnel stratum arch of jointed hard rock under overlying loads is formed by progressive stepwise evolution. In the limit state, the final arch reduces the surrounding rock pressure acting on the supporting structure, so that the surrounding rock pressure is only 3.8 times of the self-weight of the rock mass in the loosening zone, and the test and numerical results are more in agreement with the analytical solution. (2) In the process of overlying loads, the macro-mechanical properties of the surrounding rock are greatly influenced by the joint surfaces. Preferential cracking of the rock at the arch waist where the joint surfaces are located, with the cracks extending in a figure of eight pattern along the joints. (3) When the joint surfaces tend to be parallel to the direction of the maximum principal stress, the mechanical properties of the rock body are better; with the increase in the number of joint surfaces, the rock body anisotropy is more obvious, and the rock body between the joint surfaces is more fragile and then damage to the detachment. The research results provide a certain reference and guidance for the stability analysis of tunnels with joint surfaces under the timely active support system considering the arching effect.