Second giant TBM begins work on India’s first undersea rail tunnel

A second large-diameter tunnel boring machine has started excavation on the Mumbai-Ahmedabad High-Speed Rail project, advancing one of the most technically demanding parts of India’s first bullet train corridor.

The machine was launched from the Sawli launch shaft at Ghansoli and will drive towards Vikhroli as part of Package C2. This underground package includes the 21 km tunnel section between Sawli and Bandra Kurla Complex, with 7 km passing beneath Thane Creek. That section will form India’s first undersea tunnel for a railway corridor.

The first TBM began tunnelling from Vikhroli towards BKC on 5 July 2026. Together, the two machines will excavate 16 km of the underground alignment, while the remaining 5 km has already been completed using the New Austrian Tunnelling Method.

The newly launched TBM is one of the largest machines deployed for railway tunnelling in India. It has a cutterhead diameter of 13.6 m, is 96 m long and weighs around 3,200 tonnes.

It is a slurry-based Herrenknecht Mixshield TBM, a type of machine used where face stability and groundwater control are critical. The pressurised bentonite slurry circuit supports the excavation face, allowing the machine to work through complex ground while reducing the risk of instability and water ingress.

Launching the TBM required a 39 m deep shaft at Sawli. Because space was limited, the machine had to be lowered in parts and assembled underground. Its backup system includes four double-deck gantries carrying slurry pumps, hydraulic systems, grouting equipment, electrical systems, workshops and emergency refuge chambers.

The machine also includes gas monitoring for methane, oxygen, carbon monoxide and carbon dioxide, together with automatic fire detection and suppression. These details are important because long underground drives require the TBM itself to function as a controlled industrial environment.

The completed tunnels are being designed as fully waterproof structures. The segmental lining uses double EPDM gaskets and hydrophilic seals to prevent water ingress and improve long-term durability.

Real-time monitoring will also be central to the works. Settlement markers, tilt meters, strain gauges and vibration sensors will track the behaviour of the ground, nearby structures and critical infrastructure during excavation. In a dense city such as Mumbai, this monitoring is not optional. It is a core part of risk management.

Once completed, the Mumbai-Ahmedabad High-Speed Rail corridor will extend approximately 508 km and significantly reduce travel times between the two cities. For now, the start of the second TBM marks a major underground milestone, especially for the undersea section beneath Thane Creek.