Failure mechanism of the deep-buried metro tunnel in mixed strata: Physical model test and numerical investigation

Lining cracking is a typical damage phenomenon during the construction of metro tunnels, especially in the mixed strata with soft upper and lower hard rock, which severely affects the safety of the tunnel. In this paper, the physical model test was conducted to investigate the failure mechanism of metro tunnels. The full-face excavation apparatus was used to simulate tunnel excavation. The pressure cells, displacement transducers, strain gauges, and quartz sand marking method were adopted to monitor the mechanical response of the tunnel. Moreover, the failure evolution process of tunnel lining was recorded. The effectiveness of the model test was verified by numerical simulation. Meanwhile, the influences of the soil mechanical parameters, tunnel geometry, and lining stiffness on the failure behaviors of the tunnel were systematically discussed. The test results indicate that the tunnel excavation induced a slight deep soil disturbance. At buried depths ranging from 50 to 500 m, the soil settlement near the tunnel entrance was significant due to the uneven longitudinal compression, and the settlement decreased with increasing depth. The attenuation magnitude of the ground pressure in the vertical direction was greater than that in the horizontal direction. For the spring line, it was most affected by the coupling of axial compression and bending moment. The bearing capacity of the lining reached the limit when the buried depth was 150 m. The damage of the tunnel invert was the most severe, followed by the crown and the spring lines. In addition, the damage of the left part of the lining was more severe than that on the right part due to asymmetrical pressure. The entrance of the tunnel had little convergence, while the convergence at the end of the tunnel was the most significant.