The tunneling work under the Hooghly river for the East West Metro project, the first such under-river project in the country, will take a month-and-a-half to wrap up, officials said here on Wednesday.
The under water tunnelling is technologically comparable to the Eurostar - connecting Paris and London.
The Metro corridor bridges two of the most populous areas, Howrah and Kolkata, and has been planned to drastically cut down on travel cost and time, almost by an hour and a half.
Once it is commissioned in 2019, the track would entail travelling at a depth of over a 10-storied building below the water surface for almost half a kilometer across the 460 metre-wide Hooghly.
The 16.6 km project aims to connect Salt Lake's Sector V, Kolkata's IT hub with Howrah Maidan. Out of the total 16.55 km line, 5.8 km is on an elevated corridor while 10.8 km will run underground.
The 520-metre twin tunnels under the river are part of the underground stretch.
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Deployed in 2016, a pair of Rs 100-crore tunnel boring machines (TBM), almost two-storey high, were used to bore the twin tunnels, 30 metres beneath the water level, in April this year.
The German-make machines, christened Prerna and Rachna, can bore upto 15 metres a day, excavating up to 500 metre cube of earth a day. One tunnel is east-bound and the other west-bound.
Rachna, the TBM that entered the river bed from Howrah side in April, reached the bank along Kolkata earlier this month.
"We are progressing with a steady regular pace of 11 metres per day and by doing this we can ensure that all the parameters are followed and mining is done in a steady controlled manner.
"We hope the twin tunneling work will be completed in a month and a half," an official of Kolkata Metro Rail Corporation Ltd., the implementing agency of East-West Metro, told IANS.
The reinforced concrete segmental tunnel linings have a thickness of more than a quarter of a metre and are caulked with a composite gasket of neoprene and hydrophilic rubber to seal the tunnel from ingress of water. Afcons-Transtonnelstroy JV is in-charge of the tunnel building.
"This marvel of modern technology is a resounding first in India as well as a rare venture worldwide. Though the startling scale of this project is self-evident, the Eurostar (connecting Paris and London) offers an apt analogy for the level of technology harnessed by KMRCL," according to the organisation's website.
Around 250 people are working round the clock on the project.
The East West Metro project is now estimated to cost nearly Rs 9,000 crore, with its first phase -- connecting the IT hub of Salt Lake Sector V with Phoolbagan -- expected to be completed by June 2018.
The target date for completion of the second phase, connecting Phoolbagan with Howrah Maidan, is August 2019.
The Metro project -- connecting two of the world's busiest railway stations Sealdah and Howrah -- was originally slated to be operational by 2012, but the deadline has got pushed back repeatedly due to land acquisition and other issues.
In line with the Make In India initiative of the central government, the tender for procurement of 84 coaches -- 14 cars with six coaches in each -- was awarded to the Bharat Earth Movers Ltd in February last year. It would cost Rs 900 crore.
The projected daily passenger load is seven lakh in 2020, 8.10 lakh in 2025 and nine lakh in 2035. The total number of stations is 12 -- six elevated and six underground.
--IANS
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