Kolkata: The underground route of East-West Metro project in Kolkata met "nasty surprises" in the form of water-pockets and one of which caused damaged to 75 buildings, many of which are heritage structures, in the colonial Bowbazar area following subsidence because of water seepage into the tunnel after the boring machine hit a mammoth aquifer.
The tunnel work left three prominent heritage structures, old Currency Building, Writers’ Buildings and St Andrew’s Church, in BBD Bag area unhurt.
But when the tunnel reached congested Bowbazar area dotted with hundreds of old buildings on the surface, the problem began. Earlier, the heritage house of Rani Rashmoni, the founder of Dakshineshwar temple, in Janbazar area, a pocket adjoining to Bowbazar, developed cracks.
According to sources in the Kolkata Metro Railway Corporation, the agency implementing the project, the tunnel boring machine had narrowly missed several aquifers before reaching Bowbazar area.
“It was an accident. We were aware of the water-bearing strata but there was no way to know about its size. It starts 5 m under the surface and extends till 24 m. The tunnel is 20 m under the surface. After travelling 200 m through the aquifer, the water pressure rose so much that the tunnel boring machine gave in,’’ said an engineer involved in the project.
Before starting tunnel works in BBD Bag area in 2016, the KMRC had approached the National Monument Authority (NMA) to conduct a survey of heritage structures including the old Currency Building built in 1833 and 201-year-old St Andre’s Church, the city’s only Scottish church which was opened to public in 2018.
The NMA is responsible for granting permissionof construction-related activities in prohibited and regulated area.
“We started construction work in the tunnel after getting a nod from the NMA. In Bowbazar, a detail survey was conducted with the help of expert agencies,’’ said Biswanath Diwanjee, chief engineer (civil), KMRC.
Metro officials conducted a survey in Bowbazar and its adjoining areas before work started underground. “We examined the buildings to see whether there were already any cracks on it.
The Kolkata Municipal Corporation’s engineers too examined the health condition of the buildings,’’ said a Metro official.
The descendants of Rani Rashmoni had written to the Metro authorities after the 215-year-old heritge building developed cracks. Earlier, they had requested the authorities for hydrographic survey of the building.
The Metro authorities carried out repairing work but the descendants are not happy.
‘’Water is seeping and the old structure is under threat. Metro authorities should have conducted a thorough survey before starting work of the underground tunnel,’’ said Prasun Hazra, one of the descendants.
Work stalled
The underground work of the East-West project was suspended by Kolkata High Court after one of the owners of the damaged buildings moved a petition. The KMRC brought a number of international experts after the underground mishap took place.
Cambridge-educated John Endicott, based in Hong Kong who has been involved in boring tunnels since 1975, advised the KMRC about how to deal with the present situation in Bowbazar.
“The condition will be safe again after the flow of water and soil into the tunnel is stopped. Unless the water seepage is stopped, subsidence will continue. Another tunnel boring machine can be taken down through the other end. It will take some time for the machine to reach the affected area,’’ Endicott advised the Metro.
It was an accident. We were aware of the water-bearing strata but there was no way to know about its size. It starts 5 m under the surface and extends till 24 m. The tunnel is 20 m under the surface.
After travelling 200 m through the aquifer, the water pressure rose so much that the tunnel boring machine gave in KMRC engineer
We started construction work in the tunnel after getting a nod from the NMA. In Bowbazar, a detail survey was conducted with the help of expert agenciesBiswanath Diwanjee