Model test and numerical verification of surrounding rock stability of super-large-span and variable-section tunnels

There are great risks and difficulties during the construction of super-large-span and variable-section tunnels, as it is very easy to cause tunnel rock instability, landslide, roofing and other accidents. Through the case of Qizishan Tunnel Project, the second phase of Suzhou International Rapid Logistics Corridor, a model test system has been established for the mechanical effect study of the construction of super-large-span and variable-section tunnels. The construction has been simulated of the tunnel's multi-arch section turning to large-span and variable section in this paper. In this way, the mechanical effect of super-large-span and variable-section road tunnel construction has been explored. Based on the monitoring data, the overall mechanical response and the stress change of the surrounding rock in the tunnel space have been studied during the change of construction section. By monitoring the deformation and stress change of the surrounding rock, the deformation and stress release mechanism of the surrounding rock has been revealed during the construction of super-large-span and variablesection tunnels. The results of the study show that the bias pressure caused by the geometric asymmetry at the variable section of the tunnel makes the lining stress at the right sidewall relatively large. The excavation of the upper step has a great impact on the vertical displacement of the surrounding rock at the tunnel vault, and the settlement change of the vault caused by the excavation of the left half of the upper step is smaller than that of the right half. The vault settlement reaches the maximum when the excavation passes through the monitoring section of the variable section. The upper step supports the surrounding rock at the vault, and the remaining excavation steps have little effect on the settlement of the surrounding rock at the vault. The numerical simulation results of super-large-span and variable-section tunnels are basically consistent with the deformation law of surrounding rock in the model test results, which shows good accuracy of the model test.