Transport tunnels under Hooghly set a record

On June 20, the Kolkata Metro Rail Corporation (KMRC) completed the construction of India’s first set of underwater transportation tunnels. The two tunnels, 16.1 metres apart, with a diameter of 5.5 metres each, link the two banks of the Hooghly river — Howrah Maidan and Sealdah on one side with Kolkata on the other. The tunnels are at a depth of 13 metres below the riverbed, and 30 metres from the land surface.

On April 14, a day considered auspicious in many parts of the country, a German-made Earth Pressure Balancing Tunnel Boring Machine named Rachna touched the riverbed — and it reached the other end on May 23. The same day, the second tunnel boring machine named Prerna started tunnelling the riverbed and came out on the other side on June 20.

“This is the first underwater transportation tunnel passing through a mighty river. The tunnels are an engineering marvel, which we have been able to complete in just 66 days,” Satish Kumar, Managing Director, KMRC, told The Hindu .

With the completion of the tunnels, India has joined a select group of nations that have underwater transport, he said, adding that the feat comes almost 33 years after the first metro train in the country opened its doors for the public in Kolkata in 1984.

Mr. Kumar explained that the two tunnel-boring machines could not work parallel to each other because of the soft clay soil under the riverbed. After one machine completed the tunnelling, the second TBM followed.

Complex engineering

The tunnels are part of the 16.6 km East West Metro route, a major transport project in the city with an estimated cost of ₹8,996 crore, of which about 5.8 km traverses an elevated corridor and 10.8 km will run underground. The 520 metre-long tunnels under the river are part of the 10.8 km underground stretch.

Mr. Kumar pointed out that tunnelling under the river was a high risk exercise involving a lot of protective measures. Along with the tunnelling, a dense concrete layer was being used to seal the tunnel and prevent seepage of water, which was the primary concern for engineers working in any water tunnel.

“Each concrete liner segment is fitted with an imported neoprene main gasket and hydrophilic auxiliary gasket. The hydrophilic gasket expands when in contact with water and prevents any water seepage,” a senior KMRC official said.

Engineers involved in the construction said the maximum face pressure below the river shall be approximately three times the atmospheric pressure.

Interestingly, the two bridges over the Hooghly — connecting Kolkata with Howrah — took a longer time. While it took six years for the iconic Howrah Bridge to be completed since its commissioning in 1943, for Vidyasagar Setu the time taken was much longer — about 13 years. KMRC authorities said that about 250 workers worked round the clock in two shifts.

ASI’s nod

Two days before the completion of the tunnel, the last hurdle for the mega transport project was resolved. The Archaeological Survey of India (ASI) had been holding back permission for underground construction near the city’s Dalhousie area on the ground that three protected monuments — the Beth El Synagogue, the David Maghen Synagogue and the Currency Building — came within 100 metres of the metro tunnelling.

Laying down the conditions for such construction, the National Monument Authority (NMA) and competent authority Nandini Bhattacharya Sahu said all construction above the ground, including the Mahakaran Metro Station, shall be beyond the 100-metre prohibited area of Centrally Protected Monuments. The Mahakran Metro Station will be underground.


What next

While the city will have to wait till 2020 for train services between Howrah Maidan and Salt Lake, the first phase of the East-West Metro between Salt Lake and Phoolbagan in the city will start next year, KMRC authorities said.

The Corporation awaits delivery of 14 coaches from Bharat Earth Movers. The first rake will come in December. Each train will have six coaches with a maximum capacity of 2,068 passengers. Esplanade, also underground, will be a mega station connecting North South and the East West.

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Printable version | Aug 20, 2022 2:11:40 am | https://www.thehindu.com/news/cities/kolkata/transport-tunnels-under-hooghly-set-a-record/article19142194.ece