Mumbai, July 5 (HS): India's largest Tunnel Boring Machine (TBM) commenced excavation on Saturday from the Vikhroli shaft for the underground section of the Mumbai-Ahmedabad High-Speed Rail (Bullet Train) project, marking a major milestone in the country's first high-speed rail corridor.

The Tunnel Boring Machine will excavate a six-km-long single-tube tunnel from Vikhroli to the under-construction Mumbai Bullet Train station at Bandra Kurla Complex (BKC). The tunnel will accommodate both up and down tracks.

The 21-km underground section of the project comprises 16 km to be constructed using Tunnel Boring Machines between Sawli (Ghansoli) and BKC, while the remaining five-km stretch has already been completed using the New Austrian Tunnelling Method (NATM).

According to the National High Speed Rail Corporation Limited (NHSRCL), the TBM is among the largest ever deployed for rail tunnel construction in India. It has a cutterhead diameter of 13.6 metres, weighs around 3,100 tonnes and measures 96 metres in length.

The machine will tunnel beneath densely populated urban areas, including multi-storeyed buildings, roads, the Mithi River and other critical infrastructure. It employs advanced Mixshield technology, a slurry-based tunnelling system designed for excavation in mixed geological conditions and areas with high groundwater pressure.

The technology uses pressurised bentonite slurry to stabilise the tunnel face, helping minimise ground settlement and surface disruption in congested urban areas. It also enables simultaneous tunnel excavation and installation of concrete lining segments, improving both safety and construction speed.

To facilitate the launch of the TBM, a 56-metre-deep shaft has been constructed at Vikhroli. The site has been equipped with supporting infrastructure, including slurry and water treatment plants, bentonite storage facilities, a dedicated power substation, backup generators, a ready-mix concrete plant, sewage treatment facilities and other logistics systems.

The project also incorporates a comprehensive real-time monitoring system comprising surface settlement points, optical displacement sensors, tilt meters, strain gauges, bi-reflective targets and seismographs to ensure safe tunnelling and protect nearby structures.

For the 16-km TBM section, an 11.17-hectare casting yard at Mahape in Thane district is manufacturing 77,000 concrete segments to build 7,700 tunnel rings. Each tunnel ring consists of nine curved segments and one key segment, with every completed ring weighing approximately 100 tonnes.

The underground tunnel has been designed as a fully waterproof structure using double-layer Ethylene Propylene Diene Monomer (EPDM) gaskets and hydrophilic seals to prevent water ingress and ensure long-term durability and operational safety.

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Hindusthan Samachar / Indrani Sarkar