How Kolkata Built India’s First Underwater Metro


On April 12, India’s first Metro network again made history when trains ran through the Hooghly. The launch of Kolkata Metro’s North-South corridor in 1984 was no mean achievement, but the latest feat on the East-West route makes it India’s answer to London’s ‘Underground’ that dives below the Thames, and the Paris Metro going under the Seine.

Wednesday’s event was a precursor to the line’s eventual commissioning early next year. Two six-coach rakes were transported from the under-construction Esplanade Metro station in Kolkata’s central business district (CBD) to East-West Metro’s Howrah Maidan terminal station via the Hooghly, a distributary of the Ganga.
Trials with the empty rakes have already started, and once all the systems like signals, ventilation, platform screen doors and fire alarms are synchronised, the section, including the 520m underwater stretch, should get the safety regulator’s nod for commercial launch of India’s first underwater Metro.

For those involved in the Rs 10,000-crore East-West project, the first crossing was a momentous occasion because the 520m twin tunnels had been ready and waiting for almost six years. When the line is commissioned, you will shoot across the river at a depth of 12 storeys below the water’s surface in less than a minute, an engineer from Kolkata Metro Rail Corporation (KMRC), the implementing agency, told TOI. Like the London and Paris river tunnels, Kolkata’s river tunnels have been built in such a way that you won’t know the underwater part from the land, he said.

An Engineering Marvel
The East-West route, also known as Green Line or Line 2, will be 16.6km long but currently runs a truncated 9.4km between Sector V in east Kolkata and Sealdah, one of Asia’s busiest railway stations. It sees about 50,000 users daily, but the ridership will shoot up once the full line opens.

The contract to build the river tunnels was awarded in 2010. For the project, construction major Afcons formed a strategic joint venture with the Russian company Transtonnelstroy that had experience of building a road under the sea in Iran. Afcons started digging the Hooghly tunnels in April 2017and completed them in July 2017 – taking all of 67 days.

Thousands of tonnes of mud was dug underneath a flowing river to build foolproof transportation tracks. The bottom of the tunnel is 26m from the water’s surface and trains will ply 16m below the riverbed. Tunneling below the river was a challenge, say engineers. Waterproofing and designing the gaskets were major issues because the tunnels have been builtfor a service of 120 years.

“Not a drop of water can enter the river tunnels. There are hydrophilic gaskets in between the concrete of the tunnels. If water enters the tunnels, the gaskets will open up,” explained an engineer. The material used can withstand quakes in seismic zone 3, in which Kolkata lies.

Afcons deployed a highly experienced tunnel crew, in case it became necessary to enter the cutting chamber of the tunnel boring machine (TBM). If water entered the TBMs they would have shut down like a submarine for safe evacuation.

Unlike conventional tunneling, river tunneling, once started, can’t stop because “stoppages would attract ground loss, leakages and various other problems,” said an official. So, a robust plan with 24×7 crew deployment was in place. “The TBM’s cutter-head interventions were carried out just before plunging into the river so that no intervention was needed once inside the river.”

The TBMs were equipped withemergency inflatable seals, shield joints, etc, to stop leaks and could dig through poor soil conditions. The tunnels were aligned to pass through “comparatively competent geology” below the river. So they had to go deeper below the riverbed. “The TBMs were constantly supervised so that there were no functional issues,” an engineer said.

To match river depth and the tunnel’s gradient, Afcons also had to build India’s deepest Metro station-30m below Howrah railway station – and the country’s deepest ventilation shaft at 43. 5m under the ground. The river tunnels and the ventilation shaft near the bank are a stone’s throw from Howrah bridge, another engineering marvel built last century to connect Kolkata and its twin.
In 2020, at the peak of the pandemic, Afcons completed the 15-storey deep ventilation shaft next to the bridge, on the Kolkataside riverbank. It is a crucial feature because it will pump air in and out of the river tunnels and be used for evacuating passengers during emergencies.

Some Hurdles Remain
After crossing the river below Howrah Bridge, the Metro tunnels snake through Kolkata’s congested streets to reach Mahakaran station that’s coming up below Laldighi at Dalhousie Square, and burrow beyond to Esplanade.

The daunting task of tunneling through Kolkata’s mostly soft soilseemed to have ended, but exactly a year ago the project faced two setbacks. Land subsidence occurred in May 2022 and again in October in the Bowbazar (central Kolkata) zone, jeopardising the East-West corridor’s final run to its Howrah terminal side.
The 2.5km stretch from Esplanade to Sealdah has proved tricky before also. In August 2019, subsidence resulted in houses falling like a pack of cards. So, the authorities have decided to skirt the “troubledsection” for now and do a truncated 4.8km run between Esplanade and Howrah Maidan sometime next year.

“Once East-West Metro joins Kolkata and Howrah, it should cater to a million commuters by 2035,” a KMRC official said. That will take at least two more years, depending on the hurdles at the Bowbazar cave-in zone. And then, the East-West Metro will run its full route from Sector V to Howrah Maidan, via the Hooghly, in 27 minutes flat.

Source: Times of India