Two engineers from two different parts of the world, who are experts in tunnel boring and have been associated with the East-West project in the past, told The Telegraph on Saturday what they suspected went wrong on Friday morning.
At least 10 houses developed cracks in the Bowbazar area following water seepage during construction of a passage to connect two tunnels on Friday.
Railway officials said the water flow stopped around 11pm on Friday but work at the vulnerable point was stalled.
Both the engineers, who had been involved in tunnelling work in several countries, said accidents can happen as it did on Friday, but when two almost similar disasters had taken place in the near past on the same stretch, more precautions should be taken.
The engineers request not to be named.
They said they did not have the knowledge of whether their prescribed precautions were taken or not at Bowbazar because they were not involved in the project work there.
The Telegraph tried to find out what should have been the ideal way to work with safety as priority based on the conversation with the two engineers and several Metro Railway officials.
Risk assessment
According to the two engineers, it was obvious that there could be aquifers or sand containing large pools of water, underground because two similar incidents had happened recently in the same stretch.
“The advantage of constructing a cross passage compared to boring a tunnel, is it needs manual construction. One can progress with caution,” said one of the engineers.
“Usually, when the terrain has such soft soil, we move with caution,” he said.
According to him, several holes are made for two purposes, one is for grouting and the other to drain out the water.
“As long as the water is coming out, it is not safe to start construction,” said the engineer.
“Till there is even little seepage, one should continue grouting. Even after that accidents can happen but the risk is much less.”
Ground treatment
According to the engineers, while doing construction on such terrain like Bowbazar, grouting with cement mixed with chemicals is the only way.
“We do grouting continuously to replace the soft soil with a mixture of cement and chemicals that forms a hard surface to prevent soil subsidence. That is the proven method,” said one of the two engineers.
Monitoring
“Risk assessment can best be done by experienced senior engineers. In such terrains like Bowbazar with previous histories of disaster, such monitoring by senior engineers should happen 24X7,” said one of the engineers.
In the case of Bowbazar, all three mishaps happened either late in the night or early morning. “We are looking into whether senior engineers were monitoring the work at such hours,” said an official of Kolkata Metro Rail Corporation, the implementing agency for the East West project.
Preventive measures
Both the engineers said at such vulnerable stretches, the residents of buildings that are within the affected zone should have been evacuated before starting the work and preventive measures like setting up iron proppings should have been done.
Several displaced residents have alleged no such measures were taken before the disaster set in.
Official speak
“We have sought explanations from the agency, that was given the contract for the work about the disaster. We had hired a general consultant to monitor the work 24X7. We have sought an explanation from the consultant also,” said CN Jha, managing director of the KMRC.
