Making waves underground: India’s first undersea rail tunnel takes shape
India’s first undersea rail tunnel has moved a step closer to reality, with a giant Tunnel Boring Machine (TBM) beginning its journey beneath Thane Creek on Saturday for the Mumbai–Ahmedabad Bullet Train Project. The launch of the second machine marks the start of excavation on the 7-km undersea stretch, the first of its kind for any rail corridor in the country. The second TBM has started tunnelling from Sawli (Ghansoli) towards Vikhroli in Maharashtra. The undersea section forms part of a 10-km tunnel drive, with 7 km running below Thane Creek.The Mumbai–Ahmedabad Bullet Train Project includes a 21-km underground tunnel, of which 16 km between Sawli (Ghansoli) and Bandra Kurla Complex (BKC) will be excavated using Tunnel Boring Machines. Work is already underway on another stretch after the first TBM began its 6-km drive from Vikhroli towards BKC on July 5, 2026. The remaining 5 km of the underground section has already been completed using the New Austrian Tunnelling Method (NATM).
One of India’s biggest tunnel boring machines
Designed for challenging ground conditions, the newly deployed machine is among the largest Tunnel Boring Machines ever used for rail tunnel construction in India. Its cutterhead measures 13.6 metres across—roughly the height of a four-storey building, while the machine weighs 3,200 tonnes, equivalent to around 500 Asian elephants. Stretching 96 metres in length, it is almost as long as a football pitch.The TBM is equipped with a cutterhead, main bearing, jaw crusher, erector, main shield, tail shield and four specialised gantries that support tunnelling operations. Configured as a Mixshield-type, semi-automatic, slurry-based machine, it uses a pressurised bentonite slurry circuit to stabilise the tunnel face during excavation. The technology has been selected for the Mumbai suburban section because it offers better control over ground settlement while minimising disruption on the surface.It also features a Semi-Continuous Advance (SCA) system, allowing tunnel excavation and segment ring installation to take place simultaneously. This enables faster progress while maintaining safety during construction.
A 39-metre-deep launch shaft
Launching the machine required the construction of a 39-metre-deep shaft at Sawli, nearly the depth of a 12-storey building below ground level. Since there was limited space inside the shaft, the TBM was lowered in separate sections. The gantries were placed first and pushed into the already completed NATM tunnel, followed by the main shield and cutterhead.Behind the machine, four double-storey backup gantries, each measuring about 18 to 20 metres, carry systems such as the roller crusher, slurry pumps, hydraulic equipment, operator cabin, grouting systems, power units, emergency refuge chambers, workshop area, cable trays and hose reels.
Safety systems and waterproof design
The TBM is fitted with a real-time multi-gas monitoring system to detect methane, oxygen, carbon monoxide and carbon dioxide. Fire safety measures include automatic fire detection and extinguishing systems, a safety water curtain and an active sprinkler network along the designated escape route.The launch shaft is supported by infrastructure including a water treatment plant, slurry treatment plant, bentonite storage tanks, a dedicated power substation, backup generators, a ready-mix concrete plant for grouting, a slurry transport system, a sewage treatment plant and other logistics facilities.The project also incorporates a real-time monitoring system to track ground movement and protect nearby structures. Instruments such as Surface Settlement Points, Optical Displacement Sensors, Tilt Meters, Bi-Reflective Targets, strain gauges and seismographs are being used throughout the excavation.The tunnel being excavated using TBMs is designed as a fully waterproof structure. Continuous monitoring systems will track structural performance, groundwater behaviour and overall construction safety. To prevent water ingress, the tunnel lining will use double-layer Ethylene Propylene Diene Monomer (EPDM) gaskets along with hydrophilic seals to ensure long-term durability and safety.
