Deformation and protection of tunnels influenced by excavation dewatering in soft soil strata with leaky aquifers

Abstract

Deep excavation engineering often causes deformation and destruction of adjacent existing shield tunnels. In previous studies, the influence of deep excavation on tunnel was mainly concentrated on tunnel deformation caused by retaining structure deformation, and the maximum range of the influence zone was approximately 4 times the excavation depth (4H_e). However, there has been little research on tunnel deformation caused by groundwater drawdown when tunnels are located outside the traditional influence range (4H_e) of the excavation. In this study, the deformation and damage characteristics of tunnels caused by dewatering in a deep excavation project were analysed using field data, and control methods of tunnel deformation caused by excavation dewatering in leaky aquifers were proposed and discussed. In this project, the maximum settlement reached 8.23 mm for tunnel at the location far than 4H_e from the excavation, and the influence range of the dewatering on tunnel was nearly 8H_e. Furthermore, the higher stiffness of the station reduced the settlement and convergence but aggravated the dislocation of the tunnels within approximately 40 m from the station, causing many leakage points. To protect the tunnels, groundwater recharge and deep-shallow-well dewatering scheme (dewatering wells in phreatic aquifer and confined aquifer were set independently) were proposed and applied during subsequent construction, which effectively avoided further tunnel settlement. Groundwater recharge also induced slight uplift and horizontal deformation of the tunnels to the opposite side of the excavation. In addition, recharge should be started in advance and remain in operation until the groundwater level was fully restored. For deep excavations near important infrastructures in soft soil strata with leaky aquifers, the same dewatering and recharge system in this case study is suggested to adopted.