Numerical Assessment of Safe Rock Pillar Thickness for Tunneling in High-Pressure CO2 Strata: A Case Study from a Deep Tunnel in Western China

The expansion of deep-buried tunnels into complex geological settings has heightened the risk of encountering high-pressure gas strata. This study addresses a critical knowledge gap regarding safe rock-pillar thickness when tunneling through high-pressure CO2-bearing formations, motivated by a 2.3 MPa CO2 blowout event encountered during the geological investigation of a deep railway tunnel in western China. Numerical simulations were conducted using Phase2/RS2 (2D plane-strain models for the tunnel floor) and FLAC3D (3D models for the tunnel face) to evaluate plastic zone evolution and displacement responses under prescribed equivalent static CO2 pressure conditions and rock-mass degradation scenarios. The simulations represent a mechanical assessment under a prescribed pressure condition rather than a fully coupled gas-flow–mechanical analysis. Under the equivalent static CO2 pressure assumption, the calculated plastic zone depth increased from 10.2 m in the no-pressure case to 17.4 m under the 2.3 MPa pressure condition, while the maximum floor displacement increased from 0.4 cm to 11.0 cm. These results represent a conservative mechanical response under the adopted pore-pressure efficiency assumption and should not be interpreted as a direct simulation of gas compressibility, capillary effects, pressure diffusion, or gas–water two-phase behavior. Under the adopted parametric degradation scenarios, rock-mass strength reduction further increases the calculated plastic zone depth and displacement. In the strong degradation case, the plastic zone depth reaches 32.6 m and the maximum displacement reaches 19.0 cm. These values should be interpreted as sensitivity-analysis results for the assumed degraded rock-mass conditions, rather than as general predictions for all fractured or weathered rock masses. For face stability, the critical coalescence distance between excavation-disturbed and high-pressure-affected zones was identified as 15 m for intact rock, advancing to 20 m and 30 m under 10% and 20% strength reductions, respectively. Based on these findings, preliminary conservative reference values are proposed for risk identification when tunneling toward high-pressure CO2-bearing fractured zones. The calculated floor plastic zone depth of 17.4–32.6 m and the face coalescence distance of 15–30 m should be interpreted as mechanical warning indicators under the adopted equivalent static pressure assumption. These values have not yet been validated by construction-stage monitoring data and should therefore be updated using gas-pressure measurements, deformation monitoring, support response, drainage performance, and field back-analysis during tunnel construction.