Inject liquid nitrogen into the ground to freeze the underground water. Increase air pressure inside the underground tunnels to prevent seepage of water.
Experts have suggested some modern techniques of tunnel boring for East-West Metro’s tunneling work in Bowbazar, where water seepage has led to displacement of hundreds of residents and traders since the first subsidence in August 2019.
The area has since seen subsidence twice — the latest on Friday last week. In all three instances, the subsidence, caused by the tunneling work of East-West Metro, triggered cracks in buildings.
All three disasters led to suspension of the tunneling work.
“Artificial ground freezing” and “compressed air tunnelling” are two methods engineers experienced in building tunnels using modern technology suggested as way ahead for East-West Metro.
“Both methods have been used successfully to bore tunnels for many years. These can be effective to prevent further seepage of groundwater when East-West tunneling work resumes,” said a senior engineer, who has been part of several tunnel-boring projects across the world.
“Both methods have been used in several parts of Asia.”
In the artificial ground freezing method, according to the engineer, a series of closely spaced pipes are installed by drilling holes into the ground.
“A coolant, usually liquid nitrogen, is circulated through these pipes. The liquid nitrogen extracts heat from the ground and freezes the water in soil pores, which is then converted into a strong material. That minimises chances of seepage,” said the engineer, who was also once involved in East-West Metro.
In the compressed air tunneling method, according to another engineer who has built tunnels in several parts of the world, two diaphragms are built to block both ends of a tunnel.
“Artificial air is then pumped in to increase the pressure inside the tunnel. In Kolkata, one-and-a-half atmospheric pressure of air would be sufficient,” the engineer pointed out. “This method is suited for excavation in unstable soil, such as in Bowbazar, where high volume of water is present. The artificial air pressure prevents water from seeping into the tunnel.”
Both engineers, however, said the Kolkata Metro Rail Corporation (KMRC), implementing agency of East-West Metro, has to hire proper international agencies to employ these two methods.
“Both methods require setting up of plants,” said one of the engineers.
KMRC sources said they had asked the agency working at the Bowbazar site to provide plans for tunnelling work once it resumes.
