Stability Control for Surrounding Rock of Undercrossing Subway Tunnels with Large Section and Small Clear Spacing

With the acceleration of urbanization in China, there are lots of subway tunnels with large section and small clear spacing (LSSCS). Due to the complexity and uncertainty of mechanical characteristics of surrounding rock, subway tunnels with LSSCS are prone to instability and failure. The tunnel of Zhengzhou Subway Line 5 crossing under the existing tunnel of South Jingguang Expressway is selected to analyze the stress characteristics of surrounding rock and the law of ground settlement. Based on the results of numerical simulation and field monitoring data, stability control measures for surrounding rock and modified theoretical formulas of surrounding rock pressure (SRP) are proposed. The main contributions are as follows: (1) The maximum horizontal stress is found at the outer arch foot of the tunnel, and the maximum vertical stress is found at the inner arch foot. (2) With the increase of the tunnel clear spacing, the ground settlement decreases, and the ground settlement above the first tunnel is a little lager than that above the latter tunnel. (3) The excavation sequence of K2 in CRD method and the reasonable clear spacing of 3.4 m can be used as the optimal construction scheme to effectively improve the surrounding rock stability (SRS). (4) The modified theoretical formulas of SRP based on numerical simulation results and field monitoring data can provide theoretical guidance for controlling the SRS of this undercrossing tunnel and other similar tunnels.