Construction of Multiple Intersections of Tunnel Structures in Limestone and Mudstone Sections: A Case Study

Limestone and mudstone have the low bearing capacity and poor physical and mechanical properties. In this weak surrounding rock, a tunnel project consisting of inclined shafts, connecting air ducts, and main tunnels are proposed. The structure is complicated and the construction method changes frequently. There are great challenges in design, construction and safety control. Due to the large cross-section of the tunnel and multiple intersections, the excavation procedures are numerous, which will cause multiple disturbances to the surrounding rock, therefore, the construction is extremely risky. Tongzi Tunnel located in Guizhou is introduced as a case for the above-mentioned engineering problems. In this study, a solution to ensure the stability of the weak surrounding rock and a smooth construction process is proposed. The characteristic of this innovative plan is to first determine the excavation sequence of each intersection in combination with the construction location and surrounding rock conditions, and then flexibly convert the two-step method, the three-step method, and the ring-shaped reserved core soil method for construction. The settlement of the dome of the cross structure tended to be stable after 7 days of monitoring, and the settlement value was only 9.1 mm. The level of convergence decreased after 6 days, and the convergence value was only 8.6 mm. Field monitoring data show that this method causes little disturbance to the surrounding rock in the application of a tunnel excavation. It ensures safe and efficient construction of intersections and connecting air ducts, which verified the safety of the construction method.