India's first undersea rail tunnel has taken a major step forward with a giant Tunnel Boring Machine (TBM) beginning its journey beneath Thane Creek on Saturday, advancing the Mumbai–Ahmedabad Bullet Train Project. The launch of the second TBM marks the start of excavation on the 7-km undersea stretch, the first-of-its-kind for any rail corridor in the country.
TBM Specifications and Capabilities
Designed for challenging ground conditions, the newly deployed TBM is among the largest 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 configured as a Mixshield-type, semi-automatic, slurry-based machine. It uses a pressurised bentonite slurry circuit to stabilise the tunnel face during excavation, offering 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 for faster progress.
| Specification | Value |
|---|---|
| Cutterhead diameter | 13.6 m |
| Weight | 3,200 tonnes |
| Length | 96 m |
| Type | Mixshield slurry TBM |
| Key feature | SCA system for simultaneous excavation and lining |
Project Progress and Tunnel Details
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 TBMs. 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 second TBM has started tunnelling from Sawli (Ghansoli) towards Vikhroli in Maharashtra. Launching the machine required construction of a 39-metre-deep shaft at Sawli—nearly the depth of a 12-storey building. Due to 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 about 18 to 20 metres long, support operations including slurry pumps, hydraulic equipment, operator cabin, grouting systems, power units, emergency refuge chambers, workshop area, and cable trays.
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 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 with continuous monitoring systems to track structural performance, groundwater behaviour, and overall construction safety.
Implications for Logistics and Connectivity
While primarily a passenger rail project, the completion of this undersea tunnel will significantly enhance connectivity in the Mumbai metropolitan region, potentially freeing up existing rail corridors for freight movement. The use of advanced TBM technology minimises surface disruption—critical for a densely populated area—and sets a precedent for future underground logistics infrastructure in India. For freight forwarders and logistics managers, faster intercity connectivity could improve supply chain reliability for time-sensitive goods moving between Mumbai, Ahmedabad, and beyond. The project demonstrates India's growing capability in complex tunnelling, which could support future rail or road freight tunnels in urban areas.
Sources:
