Monitoring and field tests for controlling large tunnel deformation in squeezing ground: a case study

The large squeezing deformation is a characteristic disaster in tunnel construction under complex geological conditions. This study presents a case study on the squeezing characteristics and support schemes used in the Zongsi tunnel, which is located in a carbonaceous shale stratum. The tunnel is situated in a well-known seismic zone in China, where intense geological activities result in broken surrounding rock, thereby making tunnel support difficult. First, the tunnel excavation method and potential problems related to squeezing were introduced. Second, the in situ monitoring was carried out to analyze the characteristics of tunnel deformation and the stress state of the supporting structures. Finally, the numerical inversion method was used to obtain the lateral pressure coefficient at the tunnel site, which was calculated based on the proportional relationship between horizontal and vertical displacements. Optimization measures and field tests for controlling the large squeezing deformation of the Zongsi tunnel were discussed. Various measures were adopted during field tests resulting in a decrease in the over-deformation ratio from 77.9 to 0.8%. The results reveal that effective measures to control large deformation consisted of optimizing the tunnel cross section based on the lateral pressure coefficient, providing adequate reserved deformation, enhancing the tunnel excavation method, and strengthening the supporting structure. The experiences in controlling large squeezing deformation and optimizing the support scheme provide critical insights for similar projects.