The ground report: Going deep inside Mumbai's under-river metro tunnel

There is a steady clanking of metal chains. Labourers are busy at work, fixing disc cutters onto Tansa, under Mumbai's Mithi river. About 24 kilometres away, near Mumbai's Taj Vivanta, is Surya, tasked with another tunneling work.

Surya and Tansa are part of a fleet of tunnel boring machines (TBM), requisitioned for the Rs 30,000-crore underground transit network — the third line of the Mumbai Metro.

At the southern-most end of the city, Cuffe Parade metro station is under construction opposite Taj President. Temporary scaffolding is used to reach 23 metres beneath the base level, where metro rail tracks will be laid. This station will be similar to the Chhatrapati Shivaji Maharaj Terminus (CST), as metro trains will terminate and start here. To facilitate this, two tunnels of 200 metres each have been constructed.

Engineers at the site say tunnels are being made using the new Austrian tunneling method, different from TBMs, and cheaper. TBM Surya is at work on the opposite end, boring a tunnel that will connect Cuffe Parade metro station to 25 other stations on metro line 3, including the Bandra Kurla Complex (BKC) station.

The target now is to complete the Cuffe Parade station by June 2022, a 10-month delay from the initially expected timeline. “There are various challenges, one of them is that we need to wait for lean traffic hours to move heavy material and equipment," said an engineer.

Moving materials in Mumbai's traffic, keeping Mithi's water out of the metro tunnels and meeting a deadline, initially set for 2021, are few of the many challenges for city planners. Kolkata, too, in February, got a second phase of its metro system, which includes tunnels under the Hooghly river.

The BKC station is part of the work package that includes construction of two tunnels passing under the Mithi. Around 1.1 km stretch of this tunnel will run under the river bed, with about 250 metres under the water channel and 70 metres under an active water channel.

The tunnel can be accessed through an 18-metre descend using temporary scaffolding and a ride in a makeshift rake, operated over temporary tracks. Usage of special gaskets to line segments of this tunnel, ensure water from the river does not enter it.

At the other end of this tunnel, where Tansa TBM is deployed. The Tansa is stationed there to bore its way out towards the Dharavi-end. The breakthrough is expected by middle of March. “The 42 disc cutters on TBMs need to be regularly serviced. The labourers are servicing it today,” said one of the engineers present there.

The BKC station is planned to be a three-line station, two lines for regular operations and the third to allow reversing of trains. For every two trains that come from Cuffe Parade to BKC, one would be reversed to Cuffe Parade and one will continue ahead northwards. This planned station is expected to be an intersection for two different metro lines and will allow scope for direct connection to the proposed bullet train network.

It takes more than an hour to reach the under-construction BKC station from Cuffe Parade, a 24 km journey. Once Surya, Tansa and other TBMs complete their work, city authorities say the travel time will be shorter. For now the wait extends to 2021 for the north stretch of metro line 3, while the second stretch towards Cuffe Parade may take longer.