India’s first undersea rail tunnel is one step closer to reality as a massive Tunnel Boring Machine (TBM) for the Mumbai-Ahmedabad Bullet Train project began its journey beneath Thane Creek on Saturday. The launch of the second machine marks the beginning of excavation over a subsea distance of 7 kilometres, which is the first of its kind for any rail corridor in the country. The second TBM has started tunneling from Savli (Ghansoli) towards Vikhroli in Maharashtra. The undersea portion is part of a 10-km tunnel drive, of which 7 km is under Thane Creek.The Mumbai-Ahmedabad Bullet Train project consists of 21 km of underground tunnel, out of which 16 km between Savli (Ghansoli) and Bandra Kurla Complex (BKC) will be excavated using tunnel boring machines. Work on the second section is already underway after the first TBM commenced 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 Tunneling Method (NATM).
One of the largest tunnel boring machines in India
Designed for challenging ground conditions, the newly deployed machine is one of the largest tunnel boring machines used for rail tunnel construction in India. Its cutterhead is 13.6 meters wide – approximately the height of a four-storey building, while the machine weighs 3,200 tonnes, the equivalent of about 500 Asian elephants. Its length is 96 meters and 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 special gantries that support tunneling operations. Configured as a Mixshield-type, semi-automatic, slurry-based machine, it uses a pressurized bentonite slurry circuit to stabilize the tunnel face during excavation. The technology has been selected for the Mumbai suburban section as it provides better control over ground settlement while minimizing surface disturbance.It also includes a Semi-Continuous Advance (SCA) system, which allows tunnel excavation and segment ring installation to be performed simultaneously. This allows faster progress while maintaining safety during construction.
39 meter deep launch shaft
Launching the machine required the construction of a 39 meter deep shaft at Savli, approximately the depth of a 12-storey building from ground level. Since there was limited space inside the shaft, the TBM was lowered in separate sections. The gantry was placed first and pushed into the already completed NATM tunnel, followed by the main shield and cutterhead.Behind the machine, four two-storey backup gantries, each measuring approximately 18 by 20 metres, house systems such as roller crushers, slurry pumps, hydraulic equipment, operator cabins, grouting systems, power units, emergency refuge rooms, workshop areas, cable trays and hose reels.
Security systems and waterproof design
The TBM is equipped 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 on designated escape routes.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 includes a real-time surveillance system to monitor ground activity and protect nearby structures. Instruments such as surface settlement points, optical displacement sensors, inclination meters, bi-reflector targets, strain gauges and seismographs are being used during excavation.The tunnel being dug using TBM has been designed as a completely waterproof structure. The continuous monitoring system will track structural performance, groundwater behavior and overall construction safety. To prevent water ingress, the tunnel lining will use a double-layer ethylene propylene diene monomer (EPDM) gasket with a hydrophilic seal to ensure long-term durability and safety.
