Mumbai: Work on undersea tunnel for bullet train to start soon

The alignment of the entire 21-km-long tunnel

Two weeks after work on the underground Mumbai bullet train station was finalised, the construction of India's first undersea tunnel, at 21 km, between Shilphata and Vikhroli, for the Mumbai-Ahmedabad bullet train project, is set to commence.

"This is the first time that a tunnel for a high-speed railway will be opened anywhere in India. It will be located between the underground station at Bandra Kurla Complex (BKC) and Shilphata near Kalyan," an official said. The approximately seven-km-long single-tube undersea tunnel at Thane creek (inter-tidal zone) will accommodate twin tracks. It will also have 39 equipment rooms at 37 locations to be constructed adjoining the tunnel location.

An artist's depiction of the tunnel

To construct this tunnel, tunnel boring machines (TBMs) with a cutter head of 13.1-m in diameter will be used, which is double the size of those used for the Metro. Three TBMs will be used to make about 16 km of the tunnel portion and the remaining 5-m stretch will be through a method called the new Austrian tunnelling method (NATM). This tunnel will be about 25 to 65 metres deep from ground level and the deepest construction point will be 114 metres below Parsik hill near Shilphata.

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Three shafts - at BKC, Vikhroli and Sawli near Ghansoli - with approximate depths of 36, 56 and 39 metres respectively will facilitate the construction, and a 42-m inclined shaft at Ghansoli and tunnel portal at Shilphata will facilitate the tunnelling via NATM. The financial bidding for the work was opened earlier this week and Afcons Infrastructure Limited quoted the lowest bid.

The BKC High-Speed Rail (HSR) station is the only underground station on the corridor. The platform is planned at a depth of about 24 metres from ground level. There will be three floors - the platform, concourse and service floors. Two entry/exit points have been planned, one to facilitate access to the nearby Metro station on the 2B line and the other towards the MTNL building.

Tree loss and gain

Asked about the loss of trees due to construction, officials said tree-cutting permissions have been sought at Vikhroli, where 1,828 trees will be affected in a 3.9252-hectare area. The second location is at the traction substation and distribution substation where 1,243 trees will be affected on a two-hectare plot. About 585 trees will be affected at the construction zone of the tunnel shaft and ventilation building on a 1.9-hectare plot; 86.4 per cent of trees (1,579) are of less than 30-cm girth while 13 per cent are in the category of 30 cm to 60 cm. Also, more than 67 per cent of the trees (1,234) are invasive non-native (exotic type) generally used for livestock feed. About 141 trees will be transplanted and compensatory plantation of more than 5,300 trees will be undertaken by the NHSRCL for this.