Case study on repair work for excessively deformed shield tunnel under accidental surface surcharge in soft clay

As the metro system is being developed rapidly these years in China, the metro tunnels are frequently influenced by nearby construction activities, of which the extreme surface surcharge probably is the most serious one. It will seriously threaten the safety of the lining structure and its serviceability. A field case of metro tunnel with an unexpected surcharge 7 m in height in Shanghai is introduced in details at first. The inspected defects and monitored deformation of the tunnel caused by such an accidental surcharge are introduced. The largest convergence has reached 34.5‰ times the tunnel outer diameter D. The improvement measures, including the unloading, FRP reinforcement and steel plate reinforcement, for the lining structures are then proposed. Based on the monitoring and inspection data, the effect of improvements on the deformation is finally discussed. The unloading has significantly reduced the longitudinal settlement of lining and even made the ring heave in some typical rings. But it has little effect on the tunnel transverse convergence. The reinforcement by using FRP and steel plate has enhanced the bearing capacity for the tunnel lining ring. It should be helpful from this detailed case to study the influence of the repair work on the metro tunnels under the extreme surface surcharge.