Kolkata metro mishap: Bulkhead ready, water may be pumped in from today

KOLKATA: With the completion of the bulkhead, KMRC is now preparing to pump in water into the tunnel. This is likely to begin on Tuesday or Wednesday as the 3m thick concrete wall needs at least 24 hours to firm up. But engineers were not sure if they would manage to stop the water seepage by Wednesday.

Around 100m of the tunnel, in which the 75m long tunnel boring machine (TBM) currently stands, needs to be filled with water. “The concrete must be given 24 hours to firm up. Only after that can the water injection start to fill up the void around the TBM,” said an engineer, adding, “This may take 6-12 hours or even two days. We don’t know how much water will be needed to arrest the seepage of groundwater.”
Building a bulkhead or concrete wall across the tunnel to stop the flow of water into the tunnel was recommended by the international experts. The experts were flown in by Kolkata Metro Rail Corporation (KMRC) to supervise the restoration and recovery work after the tunnelling disaster that led to extreme soil subsidence and caused several buildings to fall.
“The problem is that the tunnel is below the groundwater table and this water is continuously flowing from the top to bottom. The bulkhead will block the front end of the tunnel, which will become full after injection of water from the other end. Then no more water can seep in,” an engineer said.
He was confident that the bulkhead is strong enough to withstand water pressure and will not give way like the previous one built for the same purpose. The new bulkhead was built with 54 cubic metre of concrete material. It’s 3.3m thick. After the bulkhead strengthens, water will be pumped in through pipes inserted into the bulkhead. The water will fill up the space inside the tunnel faster. This is how the water flow will be arrested as an equilibrium will be struck. The water will remain still and stop flowing out.
While the bulkhead was given the time to dry, KMRC workers on Monday started dewatering the tunnel. “Dewatering is required because the tunnel must be absolutely dry to prevent water seepage,” an engineer said. The tunnel has remained flooded with water since the TBM, digging the westbound tunnel for East-West Metro, hit an aquifer and groundwater started gushing out. This caused subsidence and, consequently, the house collapses at Durga Pituri Lane off Bowbazar.