India's First Undersea Tunnel Nears Completion On Mumbai-Ahmedabad Bullet Train Route

The construction of India's first undersea tunnel, part of the Mumbai-Ahmedabad bullet train corridor, has reached a significant milestone. This seven kilometre stretch under the Thane Creek is set to become a pivotal part of the 21-kilometre tunnel, which will be fully operational when the bullet train service begins between Ahmedabad and Mumbai in 2028.

The engineering feat under Thane Creek presents a complex challenge due to the diverse geological layers beneath the sea. The tunnel will be a single tube structure, designed to accommodate two tracks, one for each direction. To accomplish this, Tunnel Boring Machines (TBMs) with cutter heads measuring 13.6 metres in diameter will be employed, a significant increase compared to the usual 6-8 metre diameter cutter heads typically used for metro tunnels, which accommodate only a single track.

According to the National High-Speed Rail Corporation Limited (NHSRCL), the first TBM for this project is expected to be launched by the end of this year. Excavation work is currently underway at three key locations: Ghansoli, Shilphata, and Vikhroli. The first TBM is set to be lowered at the Ghansoli site, at a depth of 39 metres, where it will begin excavating towards the creek.

The NHSRCL has reported that 218 controlled blasts using gelatin were conducted within a month to facilitate the lowering of the TBMs. Progress has been swift, with 120 metres excavated towards the Bandra-Kurla Complex (BKC) and 110 metres towards Shilphata.

However, the project has been advancing more rapidly in Gujarat compared to Maharashtra. The overall 502-kilometre stretch of the bullet train corridor, of which 352 kilometres pass through Gujarat, is expected to be operational by 2027. A section between Surat and Bilimora, covering 50 kilometres, is anticipated to open as early as August 2026.

The entire bullet train project is expected to be completed by the end of 2028, six years later than originally planned. In Maharashtra, work is also progressing on the underground section of the corridor that runs through Mumbai and Thane.

To ensure the safety of civil structures and utilities near construction sites, highly sensitive geotechnical monitoring instruments are being deployed across all locations in Maharashtra. The tunnel beneath Thane Creek will be excavated at a depth ranging from 25 to 57 metres below ground level. For this purpose, three TBMs will be employed to dig 16 kilometres of the 21-kilometre underground section, including the 7-kilometre undersea stretch.

The remaining five kilometres will be constructed using the New Austrian Tunnelling Method (NATM), a technique that involves continuous monitoring to optimise wall reinforcement strategies depending on the type of rock encountered as the excavation progresses.

This project represents a significant achievement in India's infrastructure development, promising not only to enhance connectivity between Ahmedabad and Mumbai but also to showcase India's growing expertise in complex engineering solutions.