The extent of Thursday’s subsidence on the underground stretch of East-West Metro in Bowbazar was “unusual”, said a tunnelling expert.

John Endicott, a specialist in design and construction of underground structures, urged extreme caution while working at such a vulnerable location.

“It is unusual and surprising that subsidence of such an extent took place again in Bowbazar,” Endicott, who was in Bowbazar after the first subsidence in 2019 to find a way to deal with the water that leaked, told Metro on Friday.

Endicott was then in Calcutta to advise Kolkata Metro Rail Corporation, the implementing agency of East-West Metro, or the Green Line.

A 200-metre stretch in Bowbazar that has seen four incidents of subsidence since August 31, 2019, stands in the way of making the Esplanade-Sealdah stretch of East-West Metro operational.

“I know the soil condition in Bowbazar is extremely difficult to deal with. Good engineers know the ground condition is difficult and they take mitigating measures accordingly,” Endicott said.

“There can be cracks but people should be geared up to stop it at an initial level. Usually, waterproof materials like cement are injected into the ground to prevent water
seepage.”

Till now nearly 100 buildings have been affected by subsidence since August 2019 and among the displaced people, more than 400 are yet to
return home.

“Once a small crack appears in such a vulnerable ground condition, water can flow in and it becomes very difficult to stop the water. So, one has to be very careful beforehand,” Endicott said.

In Bangkok, he said, a similar problem of inflow of water had happened during tunnel-boring work but the problem was tackled.

According to Endicott, the earlier three incidents in Bowbazar could be the reason for Thursday night’s subsidence.

“Cumulative settlement from the original one in 2019 could be causing more damage. If this (Thursday’s) incident was the first subsidence, then probably the scale would not have been so big that people had to be evacuated. But already, there is settlement of soil from earlier occasions, making it worse,” he pointed out.

“After I left Calcutta in 2021, there were leaks during the underground construction in Bowbazar, which were stopped by my colleague and a Swedish consultant by grouting,” said Endicott.

He said the fragile conditions of the buildings made things difficult.

“During the 2019 subsidence, many of the buildings had to be completely brought down because of the extent of damage. Many of the buildings are fragile and so are affected by the subsidence,” said Endicott.

Evacuation, he said, is a protective measure and was the right thing to do.