India’s ambitious Mumbai-Ahmedabad high-speed rail (MAHSR) project is charting new engineering frontiers with the construction of the country’s first undersea rail tunnel – a formidable 7 km tunnel beneath Thane Creek. This undersea tunnel is a critical component of the 21 km underground stretch of the bullet train corridor. This monumental project, especially the tunnel, spearheaded by National High Speed Rail Corporation Limited (NHSRCL), signals a new era of tunnel engineering, where innovation has become a necessity.
Breaking ground below the waters
The MAHSR project is set to revolutionise intercity travel. Spanning approximately 508 km, this corridor is designed to connect Mumbai with Ahmedabad, with the primary objective to dramatically cut the travel time between these two major cities from over seven hours to just under three hours. There will be 12 stations in total. In Maharashtra, the stations will be located in Mumbai, Thane, Virar and Boisar, while in Gujarat, they will be in Vapi, Bilimora, Surat, Bharuch, Vadodara, Anand, Ahmedabad and Sabarmati.
Within this extensive corridor, the 21 km underground tunnel, which includes a critical 7 km undersea section beneath Thane Creek, holds particular strategic importance and is currently under construction. Engineered for high speed bidirectional travel, the tunnel is designed to accommodate bullet trains operating at speeds of up to 250-320 km per hour. It connects Mumbai’s Bandra-Kurla Complex (BKC) station, which is planned underground with multiple levels, to Shilphata in Maharashtra. This is presented as the direct catalyst for a wide array of economic benefits, including enhanced business interactions (considering Mumbai and Ahmedabad involved), increased productivity, and urban growth.
Financing front
The MAHSR project has an estimated total cost of approximately Rs 1.08 trillion (around $15 billion). The financing structure for this monumental infrastructure asset is primarily supported by an international partnership.
The Japan International Cooperation Agency (JICA) is the principal funding partner, providing a substantial loan of Rs 880 billion (approximately $12 billion). This loan has highly favourable terms, including a low interest rate of 0.1 per cent and a generous moratorium on repayments for up to 15 years after the line becomes operational. This financial arrangement significantly de-risks the project for India in its initial operational years. The remaining costs will be borne by the Government of India, which will contribute approximately Rs 170 billion, and the state governments of Maharashtra and Gujarat, which will collectively contribute Rs 50 billion.
Package C2, the tunnel segment of the project initially estimated at Rs 1 billion, has experienced substantial cost increases to Rs 100 billion, demonstrating the complex engineering demands and sophisticated technologies needed for the country’s first underwater rail tunnel.
Sanctioned in December 2015, the project has so far incurred an expenditure of Rs 674.86 billion.
However, the overall collaborative financing structure has enabled the undertaking of such a capital-intensive and technologically demanding project.
Tunnel design and techniques
The tunnel will be constructed at varying depths, ranging from 25 to 65 metres below ground level, and will reach its deepest point at approximately 114 metres near Parsik Hill. The design features a single-tube configuration, yet it is engineered to accommodate two tracks. The construction of this complex tunnel employs two advanced methodologies: tunnel boring machines (TBMs) and the New Austrian Tunnelling Method (NATM). TBMs are being utilised for the excavation of 16 km of the tunnel. These are not ordinary machines; they are massive TBMs equipped with cutter heads measuring approximately 13.1-13.6 metres in diameter. This size is notably larger than the 5-6 metres diameter cutter heads typically used in urban rail projects. A total of three mega TBMs are planned for deployment across the tunnel section. The first of these TBMs is anticipated to be lowered at the Ghansoli shaft, from where it will commence excavation towards Thane Creek.
The remaining 5 km of the tunnel is being constructed using NATM. This method is better for precise excavation and is adaptable to variable geological conditions. The strategic use of TBMs and NATM is a clear indication of an adaptive engineering approach. This choice is explicitly linked to the varying and complex geological conditions encountered (silt, weathered rock, basalt, high water pressure), where TBMs are more efficient for homogeneous ground and NATM is preferred for variable or challenging geology due to its adaptability and real-time monitoring capabilities.
Construction progress
On the execution side, around 3.3 km of the overall tunnel length has been completed. The excavation at Shaft 1, located at the Mumbai HSR station, is currently under way at a depth of 36 metres. Shaft 2, situated in Vikhroli, has completed excavation to a depth of 56 metres and is now ready for TBM deployment, indicating readiness for the next phase of mechanical boring. Shaft 3, near Savli, in Ghansoli, has been successfully excavated to a depth of 39 m. At the Shilphata portal, over 602 metres of the 1,628 metres NATM stretch has been excavated, demonstrating steady advancement in this section. Furthermore, engineers have meticulously carried out 218 blasts using gelatin explosives to dig 120 metres towards BKC and 110 metres on the Shilphata end in less than a month, showcasing the rapid pace and precision of the excavation work.
The construction of seven mountain tunnels using the advanced NATM is progressing in Palghar district of Maharashtra. Meanwhile, the only mountain tunnel in Gujarat has been successfully completed.
The additionally driven intermediate tunnel (ADIT) spanning around 394 metres was completed back in May 2024 by Afcons Infrastructure. This tunnel facilitated additional NATM excavation, enabling the completion of 1,111 metres of length. Furthermore, it provides vehicular access for construction and emergency evacuation.
Safety and sustainability strides
The decision to construct this undersea tunnel was a deliberate and strategic choice, driven by the necessity to navigate highly challenging terrain, densely populated urban areas, and ecologically sensitive zones. Specifically, the undersea alignment was chosen to avoid disturbing the Thane Creek Flamingo Sanctuary and the adjacent mangrove forests, unlike previous subaqueous tunnels such as the Kolkata metro’s passage beneath the Hooghly river or Mumbai Metro Line 3’s alignment under the Mithi river.
On the safety front, adequate ventilation is maintained inside the tunnels, excavated material is disposed of as per state guidelines, and continuous monitoring of surrounding structures is conducted using geotechnical instruments, including inclinometers, vibration monitors and tilt meters, to prevent risks related to tilt, settlement or deformation.
Overall, all high quality safety norms have been adopted in the construction of the country’s first undersea tunnel project.
Project potential
The bullet train project is steadily transforming from blueprint to reality, with viaducts, tunnels and stations progressing across states. Operations are likely to commence by 2026-27. It will also include modern amenities such as adjustable reclining seats, facilities for passengers with mobility challenges and advanced entertainment systems, making the journey both efficient and comfortable.
As India’s flagship high speed rail venture, it symbolises a leap towards next-generation transport, promising faster intercity connectivity, cutting-edge engineering and a powerful boost to national infrastructure ambitions. Interestingly, the project was implemented using advanced Japanese technology Shinkansen. This technology will enable trains to operate at a speed of 320 km per hour. One train set of two Shinkansen variants – E5 and E3 – will be delivered to India in early 2026, after they are fitted with inspection equipment. While the benefits are galore, these train sets are yet to undergo rigorous testing to be deemed fit for India’s climate conditions and terrains. It is, however, estimated that with targeted modifications, these trains will further enhance the country’s constant efforts to leverage new technologies and
innovations.
Central to the MAHSR corridor, the undersea tunnel is not simply a physical passage but a critical enabler that underpins the entire economic rationale and transformative impact. With this, the tunnel has now set new construction benchmarks. Moreover, the completion of this tunnel is expected to deliver major benefits, including reduced travel time and improved accessibility for commuters.
