India’s First Underwater Twin Tunnels a Crucial Link for East-West Corridor of Kolkata Metro

India’s first underwater tunnel, built 30 metre below the riverbed underneath Hooghly River, a crucial link for East-West Corridor of Kolkatta Metro to connect the twin cities of Howrah and Kolkata located on either side of the Hooghly, has been completed as tunnel-boring machine punched through the diaphragm of the proposed Metro station at Esplanade in Kolkata and came out on the surface of the Earth on March 22, 2018. This tunnelling is an  engineering marvel as the tunnels pass less than 100 metres away from three Heritage buildings that include two Jewish synagogues, Raj Bhavan and a few dilapidated Railway buildings; besides boring the tunnel below the riverbed. IIT Kharagpur had done an independent study and certified that the tunnelling methods being used will ensure no damage to the heritage buildings and permission had also been sought from Archeological Survey of India. Kolkata had the honour of getting India’s first Metro. Now, it will get yet another honour of getting India’s first metro under the river.

East-West Corridor also known as Line 2 of the Kolkatta Metro is a rapid transit system under construction to connect Salt Lake in Kolkatta with Howrah by going underneath the Hooghly River or the Bhagirathi-Hooghly traditionally called Ganga, in West Bengal.
Corridor would consist of 6 elevated and 6 underground Stations from Salt Lake Sector V in the east to Howrah Maidan in the west covering a total distance of 16.6 km. of which 5.77 km is elevated 10.83 km is underground tunnel and. With total estimated cost of the project at around Rs 9000 crore the East-West Metro is scheduled to be operational by August 2019.

The underwater twin tunnels, one east-bound and the other west-bound, tunnelling work commenced on April 21, 2016 with lowering into the ground on the other side of the river German-made 90-metres-long tunnel-boring machine christened, Rachna and Prerna, named after the daughters of a project officer who died in a road accident. Prerna has completed the boring and Rachna which is running parallel is catching up and is just a few metres behind:

• Having traversed a distance of 520 metres below the riverbed, the two tunnels, about 3.8 km long, are the first underwater transport tunnels in India.
• Depth of the crown of the tunnel below the riverbed is 13 metres.
• Diameter of each tunnel is 5.55 metre and the thickness of the wall is 275 milimetre.
• Distance between the two tunnels is 16.1 metre.
• Special concrete sections are being used to line the tunnel.
• These sections have special gaskets that expand if contacted with water to rule out leakages.
• There will be walkways in the tunnels for evacuation of passengers in case of emergency.
• Commuters between Howrah and Mahakaran metro stations will be under the river for only about a minute when the metro train will pass through the tunnel at a speed of 80 km per hour