What Happened
- The National High Speed Rail Corporation Limited (NHSRCL) released a detailed plan for constructing the 16-kilometre Tunnel Boring Machine (TBM) section of the 21-km underground stretch of the Mumbai-Ahmedabad High Speed Rail (MAHSR) corridor.
- The 21-km tunnel connects Bandra Kurla Complex (BKC) in Mumbai to Shilphata in Thane district — the longest tunnel on the entire 508-km bullet train corridor.
- Of the 21 km, 16 km will be bored using TBMs and the remaining 5 km will use the New Austrian Tunnelling Method (NATM); as of early April 2026, the 5-km NATM section has been fully excavated.
- NHSRCL also announced the assembly of the first TBM at the casting yard in Mahape, with excavation expected to begin within 97 days.
Static Topic Bridges
Mumbai-Ahmedabad High Speed Rail (MAHSR) Corridor
The Mumbai-Ahmedabad High Speed Rail corridor is India's first dedicated high-speed rail line, developed as a joint project between India and Japan under Official Development Assistance (ODA). It is modelled on Japan's Shinkansen (bullet train) technology. The total corridor length is 508 km, running through Maharashtra (155 km) and Gujarat (352 km), with one station in Dadra & Nagar Haveli.
- Implementing agency: NHSRCL — a joint venture of the Ministry of Railways (50%), Government of Maharashtra (25%), and Government of Gujarat (25%).
- Financing: Japan International Cooperation Agency (JICA) loan covering 81% of project cost at 0.1% interest, 50-year repayment with 15-year moratorium.
- Design speed: 320 km/h (operating); max 350 km/h.
- Stations: 12 stations including Mumbai, Thane, Virar, Boisar, Vapi, Bilimora, Surat, Bharuch, Vadodara, Anand/Nadiad, Ahmedabad, Sabarmati.
- Project cost: Approximately ₹1.08 lakh crore (revised estimates higher).
Connection to this news: The 21-km underground tunnel through Mumbai's dense urban geology is the most technically complex section of the corridor, requiring TBM boring under Thane creek — India's first undersea rail tunnel.
Tunnel Boring Machines (TBMs) and Undersea Tunnelling
A Tunnel Boring Machine is a large rotating disc with cutting tools that excavates cylindrical tunnels while simultaneously installing precast concrete lining segments. Modern TBMs can excavate through hard rock, mixed ground, and waterlogged strata.
- The MAHSR TBM has a diameter of 13.2 metres — among the largest used in India — to accommodate the shinkansen gauge and clearances.
- Tunnel lining: precast segments of M70 grade (ultra-high-strength) concrete; each ring consists of 9 curved segments + 1 key segment; 77,000 segments required for the 16-km TBM section (7,700 rings).
- Undersea section: 7 km will pass under Thane Creek at a depth of approximately 25–65 metres below the seabed — India's first rail undersea tunnel.
- A 394-metre ADIT (Addit Tunnel) for TBM access was completed in May 2024.
- Casting yard: Mahape, Navi Mumbai — dedicated facility for producing tunnel lining segments.
Connection to this news: NHSRCL's release of the TBM construction plan formalises the excavation schedule; assembly of the first TBM marks the start of the technically critical undersea boring phase.
New Austrian Tunnelling Method (NATM)
NATM is a tunnelling approach that relies on the inherent rock strength of the surrounding geology to form the primary support. Unlike TBM which uses precast segments, NATM uses shotcrete (sprayed concrete) and rock bolts to stabilise freshly excavated rock.
- NATM is suited for varying geological conditions and irregular cross-sections.
- The 5-km NATM section of the MAHSR tunnel lies between Shilphata and the ADIT access point.
- NATM requires careful monitoring of ground deformation using instruments (extensometers, inclinometers).
- Widely used in metro tunnels in India (Mumbai, Delhi, Bangalore).
Connection to this news: The 5-km NATM section has already been excavated, confirming construction is ahead on the non-TBM portion; the 16-km TBM section now becomes the critical path for the Maharashtra segment's completion.
Key Facts & Data
- Total MAHSR corridor length: 508 km (Maharashtra: 155 km, Gujarat: 352 km, DNH: 1 km)
- Mumbai underground tunnel: 21 km (BKC to Shilphata) — longest on the corridor
- TBM section: 16 km; NATM section: 5 km (already excavated)
- TBM diameter: 13.2 metres; tunnel lining: M70 grade concrete
- Undersea section: 7 km under Thane Creek — India's first rail undersea tunnel
- Casting yard: Mahape, Navi Mumbai — 77,000 lining segments to be produced
- TBM assembly start: April 2026; excavation expected to begin ~97 days later
- NHSRCL shareholding: Ministry of Railways 50%, Maharashtra 25%, Gujarat 25%
- JICA loan: 81% of project cost, 0.1% interest, 50-year tenure