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Hyperbaric Tunneling


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The Nanjing Yangtze River Crossing Tunnel

The Nanjing Yangtze River Crossing Tunnel is a 2.990 m long twin tube crosses the river Yangtze in Nanjing, China. Two Slurry-TBMs (Ø 14.96 m) are in use to excavate the tunnel in soft alluvium strata. The strata are mainly silt and fine sand.

The tunnel invert’s deepest point is 65 m below sea level.Due to tidal influence the water level of the river is typically between +/-1.5 m above/below sea level.

On this project, welding in compressed air was the major task to carry out. From our experience and research of welding in compressed air and under water we knew that it is not a real problem. But this time it was very extreme. The buckets of 6 arms of the TBM had to be renewed. Therefore we welded new supports on the side arms of the cutterhead. The total time of this work took more than 12 weeks, day and night. The pressure was up to 5.4 bar overpressure in air. To keep the support pressure stable we used bentonite with a special mixture of high density and viscosity.

Maintenance and repair under compressed air was performed at up to 5.4 bar air pressure for works at the cutterhead and up to 6.5 bar for works at the stone crusher. Regular compressed air (no mixed gases) and oxygen decompression is successfully in use. In total more than 4.000 h of compressed air works and more than 945 total man interventions are performed. Only 3 minor cases of decompression illness are reported.

For the welding operation we used the first time a new special designed compressed air helmet with triple air supply, two regulators and one free flow, communication and an integrated welding shield with sensors.

The Nanjing Yangtze River Crossing Tunnel