The Mumbai–Ahmedabad High Speed Rail (MAHSR) project has reached another significant milestone as the second Tunnel Boring Machine (TBM) has begun excavating India's first undersea rail tunnel. According to the latest reports from PIB Mumbai, this development marks the start of tunnelling operations on the 7-km undersea stretch that forms part of the 10-km tunnel drive for the Mumbai–Ahmedabad Bullet Train Project. The undersea section represents the first of its kind for any rail corridor in the country, with the second machine starting tunnelling from Sawli (Ghansoli) towards Vikhroli in Maharashtra. The 21-km underground section between BKC and Sawli is one of the most complex parts of the project, with 16 km built using two giant TBMs while the remaining 5 km has already been completed using the New Austrian Tunnelling Method (NATM).
The newly deployed machine is among the largest Tunnel Boring Machines ever used for railway tunnel construction in India, featuring a cutterhead measuring 13.6 metres across—roughly the height of a four-storey building. As reported by PIB Mumbai, the machine weighs 3,200 tonnes, equivalent to around 500 Asian elephants, and stretches 96 metres in length, almost as long as a football field. The TBM is configured as a Mixshield-type, semi-automatic, slurry-based machine using pressurised bentonite slurry circuit to stabilise the tunnel face during excavation. According to PIB Mumbai, the machine comprises several key components including the cutter wheel/head, main bearing, jaw crusher, erector, main shield, tail shield, and four specialized gantries that support tunnelling operations. The machine is designed to safely excavate through Mumbai's challenging geology while minimising ground settlement and disruption on the surface, with the semi-continuous advance system allowing tunnel excavation and lining work to take place simultaneously, speeding up construction. The technology was specifically chosen to minimise surface disruption in the urban Mumbai suburban sector.
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. According to PIB Mumbai, 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). The Mixshield method utilises a pressurized liquid bentonite slurry circuit to actively stabilize the tunnel face during excavation, providing highly reliable technology for driving through complex geological conditions.
Launching the machine required construction of a 39-metre-deep shaft at Sawli, nearly the depth of a 12-storey building below ground level. As reported by PIB Mumbai, the TBM was lowered in separate sections due to limited space inside the shaft, with gantries placed first and pushed into the completed NATM tunnel. The machine is fitted with a real-time multi-gas monitoring system to detect methane, oxygen, carbon monoxide and carbon dioxide, along with fire safety measures including automatic detection and extinguishing systems and safety water curtain. The surface shaft area has been equipped with a comprehensive support ecosystem including water and slurry treatment plants, bentonite storage tanks, a dedicated power substation with backup generators, and a ready-mix concrete plant for grouting. The TBM is equipped with advanced safety features including real-time gas monitoring, automatic fire protection systems and emergency refuge chambers to ensure worker safety during underground operations.
The tunnel being excavated using TBMs is designed as a fully waterproof structure with continuous monitoring systems tracking structural performance, groundwater behaviour and construction safety. According to PIB Mumbai, the tunnel lining will use double-layer Ethylene Propylene Diene Monomer (EPDM) gaskets along with hydrophilic seals to ensure long-term durability and safety. The project incorporates real-time monitoring systems using instruments such as Surface Settlement Points, Optical Displacement Sensors, Tilt Meters, Bi-Reflective Targets, strain gauges and seismographs throughout the excavation process. To prevent water ingress, the tunnel lining is protected using double-layer EPDM gaskets combined with hydrophilic seals, ensuring long-term structural durability and safety. The project is also being monitored through a network of sensors tracking ground movement, vibration and structural behaviour to ensure safe tunnelling beneath one of Mumbai's busiest urban regions.
