REHABILITATION OF DAMAGED SOFT GROUND TUNNELS

The paper presents the rehabilitation of soft ground tunnels that were damaged in an accident at the ventilation shaft A of the Pan-chiao Line of Taipei Rapid Transit Systems. Subsoil conditions and the preceding operations at the tunnel-shaft interface are introduced. The accident at shaft A is described and ground settlement data induced by the accident are presented. Procedures taken by the contractor during emergency rescue, damage assessment, and rehabilitation of shaft and tunnels are reported. Possible reasons that caused the accident included drift wood in the ground, cleavage between soilcrete and shaft, and non-verticality of grouting rods. It is concluded that the ground improvement outside the shaft before the shield’s arrival played an important role in this accident. The geotechnical engineer should be extremely cautious when breaking the mirror-face on the diaphragm wall under the threat of high groundwater pressure.