India’s largest rail TBM launched in Mumbai

The Ahmedabad-Mumbai Bullet Train project achieved a significant engineering breakthrough on Sunday as India’s largest rail tunnel boring machine (TBM) was launched from the Vikroli shaft, beginning excavation toward the under-construction Bandra-Kurla Complex (BKC) station.

This marks the first TBM deployment for the project’s ambitious 21-kilometre underground section in Mumbai. Of this, 16 km between Saki (Ghansoli) and BKC will be constructed using TBM technology, while the remaining 5 km has already been completed using the New Austrian Tunnelling Method (NATM).

The newly launched TBM will bore a single-tube, 6-km-long tunnel that will accommodate both up and down tracks for the high-speed train. The alignment passes beneath dense urban infrastructure, including multi-storey buildings, major roads, the Mithi River, and other critical structures, making precision and safety paramount. Weighing approximately 3,100 tonnes — equivalent to about 500 Asian elephants — and stretching 96 metres in length (roughly the size of a football field), this TBM is among the largest ever used for rail tunnels in India. Its massive 13.6-metre-diameter cutterhead, comparable in height to a four-storey building, rotates at 4 revolutions per minute. The machine is equipped with state-of-the-art components, including a cutter wheel, main bearing, jaw crusher, erector, main shield, tail shield, and four special gantries.

The project is employing advanced Mixshield TBM technology, widely regarded as one of the most sophisticated methods for tunnelling in mixed geological conditions and high groundwater pressure. By using pressurised bentonite slurry to stabilise the tunnel face, the technology significantly reduces the risk of ground settlement, ensuring minimal surface disruption in Mumbai’s crowded urban environment.

A key feature of the TBM is its ability to excavate and install concrete segment linings simultaneously, enhancing both speed and structural safety. To facilitate operations, a 56-metre-deep shaft — equivalent to the height of a 20-storey building — was constructed at Vikroli. The site includes a water treatment plant, slurry treatment facility, bentonite storage, dedicated power substation, backup generators, ready-mix concrete plant, and sewage treatment systems. Real-time monitoring using surface settlement points, optical displacement sensors, tilt meters, 3D targets, strain gauges, and seismographs will continuously track nearby structures to ensure safety.

Meanwhile, at a specialised casting yard in Mahape, Thane, spanning 11.17 hectares, workers are producing 77,000 concrete segments to form 7,700 tunnel rings.

Each 100-tonne ring consists of nine curved segments and one key segment. The entire tunnel will feature double-layer EPDM gaskets and hydrophilic seals, making it fully waterproof for long-term durability.

Officials described the TBM launch as the most critical phase of underground construction. The development is expected to accelerate progress on India’s first high-speed rail corridor, bringing the dream of bullet trains closer to reality.