Optimization of excavation method for large-section metro station tunnel: a case study

The Baijusi Station Tunnel of Chongqing Rail Transit Line 18, excavated in moderately weathered sandstone, was used as a case study to optimize the excavation method for a large-section metro station tunnel. The double side drift (DSD) and optimized three-bench (OTB) methods were first compared using Hoek-Brown-based numerical simulations. The results showed comparable tunnel stability for the two methods, while the OTB method offered advantages in construction efficiency and working space. A trial section was then excavated to verify the OTB method, with the central pillar dismantled as the upper bench advanced. Field monitoring showed that deformation and support responses remained within acceptable ranges, and the spatial distribution of microseismic events reflected micro-crack evolution broadly consistent with the numerically predicted rupture surfaces. Under the investigated geological, loading, and construction conditions, the central pillar did not play a decisive role in controlling overall tunnel stability. The OTB method was subsequently applied to the entire tunnel under the same monitoring scheme, and the monitored deformation, support stress, and microseismic responses remained within acceptable ranges. These results indicate that, with appropriate numerical assessment, trial verification, and field monitoring, eliminating the central pillar can be feasible under comparable conditions, achieving a balance between tunnel stability and construction efficiency.