Numerical Analysis of Gas-Bearing Ground Deformation and Failure Mode around the Parallel Tunnel Cross-Passage

The failure of the tunnel and the ground collapse due to the gas migration and release in the gas-bearing confined stratum during the tunnel project construction are the geotechnical problems that may be encountered in the shallow gas geological area. In this study, a parallel tunnel and the cross-passage failure caused by gas-bearing formation near the Nak-Dong River in South Korea were evaluated through numerical simulation employing multiphase fluid and mechanical coupling equations via COMSOL Multiphysics. Based on the modeling results, it is found that the high-pressure gas in the gas-bearing formation migrates into the deep cement mixing (DCM) to potentially expand fractures. Accordingly, after the tunnel is excavated, the water-gas mixture in the gas-bearing formation overflowing along the DCM fracture can cause stress disturbance around the tunnel, which leads to damage and formation subsidence. In addition, this study analyzes the effect of different gas pressures on the extent of the failure, and the effect of the location and number of potential cracks on tunnel failure was analyzed. This study shed light on the failure modes of tunnels caused by gas migration in gas-bearing soils and the DCM fracture, emphasizing the importance of quality control for DCM when reinforcing in situ soils in gas-bearing areas.