Creating a working space for modifying and maintaining the cutterhead of a large-diameter slurry shield: A case study of Beijing railway tunnel construction

During the railway tunnel excavation of the Beijing Underground Zhijingxian Project, which used a large-diameter slurry shield, excessive wear of the cutterhead and cutting tools was found. Cutterhead modifications as well as frequent maintenance on the cutterhead and cutting tools were necessary for sustained shield tunneling, and creating a reliable working space near the cutterhead remained critical to performing the maintenance operations. The tunnel crossing beneath densely populated areas and the tunnel face stability should be maintained during the intervention. A free air working space was created for the first cutterhead modification by excavating a shaft before the cutterhead. Inspection stops with ground reinforcement by the sleeve-pipe injection were well planned and arranged in advance. At the location for the second cutterhead modification, reinforcement by cast-in-place bored piles combined with the sleeve-pipe compensation grouting was employed. Repairs to the cutterhead were performed in an airtight space created in a two-meter diameter pile. In uttermost cases, ground improvement was not allowed. The compressed air working space was chosen. The hyperbaric interventions were safely performed with well-regulated air pressures and field-tested solutions against ground air leakage. In the different working spaces created in the complex geologies and under strict environmental constraints, the intervention works were safely completed. The shield advance was generally maintained at an accepted level, and the tunnel excavation was safely completed. Fortunately, the interventions were performed with less injury to the working personnel, and no decompression illness was reported. Undoubtedly, safety-oriented principles that characterize reinforcing, airproofing and stabilizing the ground can be readily adopted to create a workspace for intervention activities in similar conditions.