Kolkata Metro starts building shaft to pull out TBMs

The Kolkata Metro Rail Corporation (KMRC) has started construction of shaft for retrieving the two giant tunnel borers for the East-West Metro corridor at Durga Pithuri Lane, off Bowbazar. It will take six months to complete the shaft.

This is where the first tunnel boring machine (TBM), Chundi, which was digging the west-bound tunnel, hit an aquifer on August 31, last year, and is now lying defunct 16m below the ground. A 300 m radius around the spot came to be known as the cave-in zone ever since.

Several houses had collapsed when Chundi hit the sand aquifer. Water gushed into the tunnel, triggering largescale subsidence. The subsidence zone, strengthened with elaborate grouting, has now become an open space since 27 damaged houses in the area either fell or were razed for safety reasons.

The retrieval shaft is being built in this open space to pull out both the machines, which were moving from Esplanade to Sealdah for the last leg of 2.5 km East-West Metro tunnelling. Rig machines from Austria are being used to build piles for the 22m deep retrieval shaft, 40m in length and 15m wide. The piles will be built for 130 piers of the retrieval shaft being dug directly on the line of the west-bound tunnel.

Cutting the damaged TBM (Chundi) into pieces will be a challenge because it its 9m-wide front shield is covered with concrete that was poured-in to seal the water leakage. Along with the front shield, the first few metres of the concrete rings will also be pulled out of the shaft. And the wall created (in the tunnel) to prevent seepage of water will also be dismantled.

A huge crane will be used to pull out the machine parts, each machine of which weighs around 300 tonnes. The second TBM, Urvi, building the east-bound tunnel, is now burrowing towards its destination, Sealdah station, from where it will make a U-turn and dig the rest of the affected tunnel and proceed back towards Durga Pituri Lane.

Urvi should be able to reach Sealdah station by mid-September. It will be disintegrated inside a box at Sealdah station, reassembled and veered in the opposite direction, to Durga Pituri Lane and thus complete the underground link. By this time, the retrieval shaft will be ready. It would take a month to complete the retrieval of the two machines.

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