Optimization Study on Key Parameters for Mechanical Excavation of Deep-Buried Large-Section Metro Station

When mechanically excavating deep-buried large-section metro stations, stringent deformation control requirements for the surrounding rock must be adhered to. Calculations indicate that horizontal convergence in certain areas of the station exceeds acceptable limits, necessitating adjustments to construction parameters to comply with these requirements. This study, based on a project for the Chongqing Metro Line 18, establishes a three-dimensional numerical analysis model for an underground excavation station by utilizing the characteristics of the stratum-structure model. A comprehensive 3D numerical simulation was conducted to evaluate the deformation characteristics of the stratum and surrounding rock resulting from excavation, and to determine optimal excavation parameters based on deformation control. The key findings are as follows: (1) Under the original excavation design parameters, the horizontal convergence displacement at the arch foot met specification requirements and was smaller than that at the sidewall. However, the horizontal convergence displacement at the sidewall exceeded the 20 mm limit specified by the relevant codes, failing to satisfy deformation control standards. (2) The deformation of the surrounding rock increased with factors such as the distance between the excavation face and the initial support, as well as the length of the excavation step. While the spacing between adjacent pilot tunnels had a relatively minor impact on overall station deformation, the number of pilot tunnels, in conjunction with other parameters, proved beneficial for controlling surrounding rock deformation. (3) Among the parameters examined, the distance between the excavation face and the initial support, along with the excavation step length, exerted the greatest influence on deformation. Based on deformation control criteria, the optimal excavation parameters were determined as follows: the distance between the excavation face and the initial support should not exceed 6 m; the excavation step length is set to 1.5 m; the number of pilot tunnels is established at 11; and the spacing between adjacent pilot tunnels is set at 10.5 m. (4) Field monitoring data closely corresponded with the effects observed from implementing the optimized parameters, thus validating the reliability of the optimization scheme. The results of this study provide a valuable reference for the excavation of metro stations under similar conditions in the future.