Repairing a shield tunnel damaged by secondary grouting.

This paper reports on a repair work which has recently been conducted for a metro tunnel in Hefei city, China. The tunnel has been originally constructed using shield method where synchronous grouting was used to fill the gaps between the tunnel segments and soil. Following a regular maintenance inspection of the tunnel, several leakage issues were identified between three stations. Secondary grouting was adopted as a solution to block the tunnel leakage, however, shortly after the start of grouting work, the track and track bed were found to be unevenly uplifted with significant cracks in the tunnel’s segments. The paper describes and discusses key aspects of this case study including ground conditions, leakages patterns of the tunnel, recorded volumes and injection pressure of the secondary grouting, as well as survey data of track displacement and segment cracks. The investigation confirmed that the situation was caused by an inappropriate implementation of the secondary grouting, particularly by high grouting pressure (significantly higher than the geostatic pressure), large volumes of injected grout, and poor selection of grouting locations. Ground Penetrating Radar (GPR) was conducted to inspect the tunnel conditions before commencing the structural repair work, which revealed that there were no voids under the track bed of the affected zone. The study presents simplified strategies used to repair the damage while maintaining minimum disturbance to the affected segments.