Differential settlement remediation for new shield metro tunnel in soft soils using corrective grouting method: case study

In the Yangtze River Delta of China, a large number of metro tunnels have been constructed in soft soils. The excessive and differential tunnel settlement may impair the serviceability of the metro system. The treatment of such excessive and differential settlement in rheologic and sensitive soft soils is a challenge because the tunnel may incur a larger settlement due to construction disturbances. In this paper, a case study of the differential settlement treatment of the new shield tunnel of Ningbo Metro line 2 is presented. A maximum tunnel settlement of 214 mm was observed several months after construction of the tunnel was completed. To uplift the deviated tunnel axis, a grouting and lifting measure named "bottom grouting, inner support, real-time monitoring and immediate adjusting" is proposed. The settlement treatment section is successfully uplifted with an average value of 30 mm, and the maximum final uplift amount of the tunnel is 41 mm, which reached the target value of uplift. The maximum convergence deformation caused by the grouting is 10 mm, which is less than the maximum acceptable deviation, i.e., 15.5 mm. The corrective grouting method and the valuable monitoring data presented in this study can provide a reference for projects with similar problems in the future.