A first in India, the two 520-metre structures are part of a 10.8 km underground stretch and are crucial to the East-West Metro project

On April 14, a day considered auspicious in many parts of the country as it marks the beginning of New Year, a gigantic Tunnel Boring Machine (TBM) began cutting deep inside the bed of the Hooghly river, commencing the construction of the first underwater metro tunnel in the country.

By the end of July, two tunnels running parallel will connect the twin cities of Howrah and Kolkata located on either side of the Hooghly. The tunnels are being dug at a depth of 30 metres below the earth’s surface and 13 metres below the riverbed.

The twin tunnels, about 520 metres long, are crucial to the much-awaited Rs. 8,900 core East West Metro project that will connect Salt Lake Sector V in the eastern part of Kolkata to the Howrah Maidan across the river.

Of the 16.6 km East West Metro route, 5.8 km is on an elevated corridor and 10.8 km will run underground. The 520-metre tunnels under the river are part of the 10.8 km underground stretch.

Optimistic estimate

“As per our optimistic estimate by the end of May or the first week of June this year, the first under-river tunnel of India will be complete. One tunnel-boring machine is already under the water, and the second machine will also start working by the end of May. Both the tunnels will be complete by July,” Satish Chandra, Managing Director of the Kolkata Metro Railway Corporation Limited (KMRCL) told The Hindu .

TBM 2, which has gone deep into the bed of the river, has already progressed a distance of 100 metres.

Explaining the nature of the soft clay soil under the riverbed, Mr. Chandra, who is also Assistant General Manager, Eastern Railways, said the two TBMs could not work simultaneously. “One machine goes ahead and the second follows. They cannot work together for various technical reasons. For instance, the ground movement will be very high if they work simultaneously,” Mr. Chandra said.

The diameter of the two tunnels running across the river is 5.5 metres and the distance between the two tunnels will vary at different places — 16.1 metres under the river, 18 metres at the Howrah station, and 13 metres at the Howrah Maidan (from where tunnelling started in May 2016).

About 250 people are working round the clock on the tunnelling. Alongside, a dense concrete layer is being used to seal the tunnel, prevent the subsidence of earth, and stop seepage of water — a primary concern for the engineers working on any water tunnel.

While tunnelling and construction of East West Metro Project are in full swing, it has had to overcome a number of challenges.

When one of the German-made TBM machines went under the ground at the Howrah Maidan, cracks appeared in the Howrah District Library. The KMRCL authorities had to take a lot of precaution when the tunnel reached near Colvin Court, a 94-year-old railway building made of red bricks, which now serves as residential quarters for railway staff.

The protective measures subsequently taken by the engineers included widening the foundation of the building and grouting of the soil, the authorities said.

Last hurdle

However, the last hurdle being faced by the East West Metro Project is the threat the tunnelling work may pose to three heritage buildings in Kolkata. The Currency Building, an Italian structure that served as one of first banks of the country, and two 19th century Jewish monuments — the Maghen David Synagogue and the Beth-El Synagogue — stand close to the Metro tunnels.

As per the existing rules, construction and mining operations are prohibited within 100 meters of protected monuments — but the Currency Building is 24 metres away from the Metro’s alignment, and the Beth-El and Maghen David synagogues are at a distance of about 17 metres and 9.8 metres respectively.

After the issue was raised by the Archaeological Survey of India, an expert committee from the Indian Institute of Technology, Kharagpur, was constituted to look into the matter. The committee said there would be no “ adverse affect” on the protected monuments.

Union Minister for State for Heavy Industries Babul Supriyo, who inspected the project, expressed hope that that the last hurdle would be cleared soon.