India’s 1st underwater tunnel: Blink-and-you-miss-it experience for West Bengal passengers

The East-West Metro Corridor's first underwater tunnel, being built beneath the Hooghly River in West Bengal at a cost of around Rs 120 crore, will be a blink-and-you-miss-it experience for passengers, with trains crossing the 520-meter section in only 45 seconds.

The tunnel, which is the Indian equivalent of Eurostar's London-Paris route, is 13 metres below the riverbed and 33 metres below ground level.

The 520-meter tunnel is part of Kolkata's East-West Metro Corridor, which runs from Salt Lake Sector V in the east to Howrah Maidan in the west across the river.

The tunnel's construction is complete, and it is expected to be completed in December 2023, after the completion of a 2.5-kilometer length of the corridor between Esplanade and Sealdah.

Sailesh Kumar, General Manager (Civil) of the Kolkata Metro Rail Corporation on December 30 said that the tunnel is necessary for the East-West Corridor and was critical. 

''With residential neighbourhoods and other technical challenges involved, the under-the-river alignment was the only option," he added.

He also asserted that this metro line between Howrah and Sealdah reduces commuting time from 1.5 hours to 40 minutes. It will also help to relieve congestion at both ends.

He estimated that crossing the tunnel would take 45 seconds and the metro rail's East-West Corridor has been plagued by delays and expense increases. 

''It was sanctioned in 2009, at a cost of Rs 4,875 crore, with an estimated completion date of August 2015,'' he said.

According to authorities, the total cost has now risen to Rs 8,475 crore, with Rs 8,383 crore already spent and the tunnel's interior diameter will be 5.55 metres and its exterior diameter will be 6.1 metres.

The up-and-down tunnels will be 16.1 metres apart from one another. 

The tunnel's inner walls are built of high-quality M50 grade reinforced concrete segments with a thickness of 275mm apiece.

Six of these pieces will form a circular lining around the tunnel's circumference and the segments are cast in specialized molds supplied from Korea. 

Besides this, several safeguards have been put in place to avoid water infiltration and leaks in the tunnel.

To reduce water permeability, concrete mixes including fly ash and micro silica were employed for the segments. The segments are being sealed with a complicated grouting procedure that fills the area between the segments and the tunnel boring machine's shield (TBM).

To fill the gaps, a two-component grout mix including a slurry of water, cement, bentonite, and sodium silicate is utilized.

The liner segments are connected with neoprene and hydrophilic auxiliary gaskets developed in Germany, which expand when in contact with water to prevent ingress via segmental joints.

Prerna and Rachna, two German-made tunnel boring machines (TBMs), were used in the procedure. Inside the tunnel, emergency evacuation shafts up to 760 metres in length are provided in compliance with National Fire Protection Association (NFPA) rules.

For passenger safety, evacuation shafts are being installed at Howrah Station in the west and Strand Road in the east.