The commissioning of East-West Metro’s Sealdah station, which officials think will make the route viable as more passengers will use the system rather than the fragmented run from Salt Lake Sector V to Phoolbagan, is now deferred indefinitely because of the collapse of tunnel boring machine in Bowbazar, said officials.
After the August 31 accident, which damaged several buildings above ground in Bowbazar and hundreds of residents had to be evacuated, the machine burrowing the west-bound tunnel has become defunct.
The other machine will now have to go till Sealdah station boring the east-bound tunnel and then make an about-turn. It will then complete burrowing the portion of west-bound tunnel left incomplete by the damaged machine, said officials of Kolkata Metro Rail Corporation Limited (KMRCL), the agency implementing the project that will connect Salt Lake Sector V with Howrah Maidan.
“It will take two months to make the machine turn around at Sealdah station. It is a complicated process,” said a railway official. “Till the machine makes a U-turn and completes burrowing remaining of the other tunnel, Sealdah station’s underground platforms cannot be commissioned,” he said.
Metro gives a lowdown of the U-turn plan.
How to make U-turn
The tunnel boring machine will burrow for about 750 metres before reaching Sealdah. It is now under Nirmal Chunder Street, getting overhauled, said engineers.
On reaching Sealdah, the machine will be dismantled into small pieces, said engineers. At Sealdah station, there are several columns supporting the construction, like any other underground Metro station, and these would provide impediments for the process to make the turn around, they said.
On the space between two pairs of columns, steel rails would be laid temporarily on which the segments of the tunnel boring machine will be made to slide from one track to the other, said the engineers.
There will also be a turning pad made of steel on which portions of the machines will be placed. The turning pad will help the machine make the U-turn and place it up against the edge of the wall from where burrowing of the parallel tunnel will start.
“First, the front portion of the machine with the cutter would be turned with the help of the pad and placed near the wall. Rest of the portions will follow the same process,” said the official. Once all the portions are placed in sequence and joined, the tunnel boring work will start.
The shaft
A vent shaft installed at Sealdah station will have been used to take out both the tunnel boring machines once they had reached Sealdah from Esplanade.
But with the Bowbazar accident, the plan has changed. Now a shaft has to be built at Bowbazar to take out both the machines there.
“However, the shaft at Sealdah needs to be kept because the concrete segments that will form the walls of the tunnel, will be brought down through the shaft,” said a KMRC official. Also, the muck produced during the excavation will be taken out through the shaft.
“To bring in the segments through the shaft and take out muck through it we need an area inside the Sealdah station. So, until tunnel boring is complete till Bowbazar, completion work of Sealdah station cannot be undertaken,” he said.
When will work end
The tunnel boring from Nirmal Chunder Street cannot start without the permission of Calcutta High Court. The court had stalled the work after the accident. Recently, it had given permission to move the second machine for five metres for maintenance.
Engineers said the machine, although in working condition, required to be overhauled with some key equipment replaced before the technical committee could approach the court seeking permission for the final phase of tunnel boring. It would take about a year for the tunnel boring work to be complete, said officials. After that they would take up finishing work. “We have to be sure that the machine can burrow safely through the congested area,” said an engineer.
The first machine had hit an aquifer and collapsed.