Evolution characteristics of the surrounding rock pressure and construction techniques: A case study from Taoshuping tunnel

The surrounding rock of the Taoshuping tunnel is fine silty sand in a plastic flow state after encountering water, which leads to inferior stability of the surrounding rock and extremely high construction risk. Therefore, the Taoshuping tunnel project is considered to be a rare worldwide problem in China. This article aims to review the engineering problems, construction techniques, and technical points faced in Taoshuping tunnel's construction, as well as reveal the evolution characteristics of surrounding rock pressure and evaluate structural safety. First, through the analysis of the characteristics and difficulties of the project, the combined dewatering scheme of vacuum wellpoint dewatering and collecting wells was formulated. Three kinds of tunnel excavation methods were developed: the double-heading at bottom method, both-side heading method with 9 steps, and ADECO-RS (Analysis of COntrolled DEformation in Rocks and Soils). Second, the variation law of the water content for the surrounding rock during excavation was studied through experiments. The water content that can ensure the stability of the surrounding rock is approximately 6.5%. Then, the evolution characteristics of the surrounding rock pressure of the first lining, the liner contact pressure, and the axial force of the reinforcement during construction were monitored and analyzed. The safety of the liner was evaluated by the pressure ratio of the liner and the strength-based safety factor. Finally, the deformation characteristics of the surrounding rock in the context of different excavation methods were investigated by numerical simulation. The lessons learned from the Taoshuping tunnel can provide insights for the increasing number of similar projects.