Stop excessive withdrawal of water to avoid subsidence in Bowbazar, says expert

A senior ground water expert from Kolkata has opined that Kolkata Municipal Corporation should immediately stop withdrawing underground water in Bowbazar and adjacent zone within central Kolkata to halt further subsidence and collapse of houses in the area being triggered by an accident catered by East – West metro construction first in 2019; and again, repeated recently.

“Kolkata Municipal Corporation should make an effort to Immediately stop withdrawal of groundwater, especially pumping through deep tube well, as far as possible from the adjoining area of 1 km radius and , if possible, artificially recharge the sand layer containing water so that water level comes up leading to increase of hydrostatic pressure in the area that is likely to halt the land subsidence process” said PradipSikdar, a groundwater expert and a professor of environment in Indian Institute of Social Welfare and Business Management, Kolkata.

Several studies earlier pointed out that groundwater has been getting lowered at an alarming rate in both south and central Kolkata due to rapid withdrawal of groundwater.

Sikdar was the lead scientist in a study that showed how a rapid withdrawal of underground water leading to the lowering of subsurface water level had played a key role in destabilization of landmass and collapse of several buildings in Bowbazar area within central Kolkata being triggered by an accident caused by the construction of East – West metro in 2019.

The expert feels that similar dynamics apparently got repeated a few days back when a fresh spell of collapse started to set in within several buildings of the area.

“On 2nd September 2019, TBM (tunnel boring machine) named ‘Chundee’ hit an unidentified sand bed at a depth of 14 m bgl (below ground level) resulting in flooding of the metro tunnel and … buildings at DurgaPithuri Lane, Sankra Para Lane, Gaur De Lane and Hidaram Banerjee Lane in Bowbazar of Central Kolkata developed cracks. … Land subsidence (occurred) due to decline of piezometric (ground water level) surface which results in vertical compression of the sub-surface materials” reads a paper published in Journal Geological Society of India in November 2020 on the Bowbazar disaster that caused collapse of around 25 buildings in the neighourhood.

The paper was co-authored by Sumanta Banerjee with Sikdar.

“There is a layer of clay about 14 meters below land in the Bowbazar area and then comes a sand layer. When the accident during east- west metro construction happened, excessive withdrawal of groundwater had already lowered the water level into the sand layer. As the TBM punctured the sand layer, its water gushed out in the tunnel leading to the corresponding loss of water from the clay layer and, hence, unequal settling of the clay mass and triggered collapse” explained Sikdar to this correspondent.

“Recently the dynamics got apparently repeated when, while trying to pull out the two TBM from the underground, a fresh incision might have been made leading to fresh release of water from the sand layer, and another spell of collapse”, pointed out the scientist.

“Apart from stopping withdrawal and recharging as far as possible, the opening of the tunnel created due to extrication of two tunnel boring machines should be plugged at the earliest” added Sikdar.