Bullet train corridor: Largest of the 20 giant tunnel boring machines to be deployed for project

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A whopping 20 Tunnel Boring Machines (TBMs) will be deployed by Afcons for various underground rail tunnel projects this financial year.

The company is building India's first undersea rail tunnel in Mumbai for the ambitious Mumbai Ahmedabad High Speed Rail (HSR) project, aka bullet train corridor.

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The HSR C2 Tunnelling package is daunting because it not only throws the gauntlet of constructing the country's first undersea tunnel but also the alignment between BKC and Vikhroli passes through a highly urbanised area making underground tunnelling a tricky affair. The shaft excavations could prove challenging, too, since the shaft depths could go beyond 50 metres.

"A total of 17 TBMs are being mobilised this year and three more will be deployed early next year," said V Manivannan, Afcons' Director, Construction Plant and Equipment (CPE). "We own all these TBMs, and we are probably the only organisation in the country, at present, to own and deploy so many TBMs at a time," he adds.

The National High-Speed Rail Corporation Limited (NHSRCL) recently signed a contract with Afcons for constructing India's first undersea rail tunnel, which is part of a 21-KM long tunnel for the Mumbai-Ahmedabad bullet train project. The undersea tunnel, at Thane Creek, will be 7 km long and around 25 to 65m below the ground level. Sixteen kilometres of the tunnel will be executed using TBMs and five kilometres will be constructed with the New Austrian Tunnelling Methodology (NATM).

"Our equipment fleet is strategic, not generic in nature," says Manivannan. "Most of our equipment is custom built and that's why we are capable of taking on technically challenging projects, and completing them on time, or, ahead of schedule," he adds. One of the largest TBMs will be deployed for the construction of the High-Speed Rail's C2 package. "A slurry TBM will be deployed with a diameter of 13.1m. It will be one of the largest in the country," Manivannan says.

Considering that the undersea portion of the tunnel will fall under the inter-tidal zone, it is possible that the water pressure could be high leading to extremely high overburden. However, Afcons has previously executed underwater tunnelling projects and has constructed the country's first underwater Metro tunnels below a river in Kolkata.

Afcons constructed three underground Metro stations which are joined by two underground tunnels of 3.8 km each. A portion of the tunnels - nearly 520M - is under the mighty river Hooghly.

"The underwater tunnels are one of the biggest achievements in the East-West Metro project in Kolkata. We executed the project under poor geology and congested topography. Before and after the river area, the tunnels passed below several dilapidated buildings, railway offices and yards, busy roads, bridges, flyovers and heritage structures. Our highly experienced tunnel crew made sure the TBMs were provisioned to shut down like a submarine in case of the slightest water ingress. The underwater tunnels were completed in a record time of 66 days," he added.