By Manjiri Joshi,News18

Copyright news18

For the country’s financial capital, Mumbai, where every inch of space is precious, a maze of Metro lines is being laid to ease commute. While some move along the overground or elevated route, for some areas, packed with commercial and residential complexes and sky high realty prices, the path is being laid underground through tunnelling.
The process to the outcome, News18 takes you through the journey of making an underground Metro.
How many underground Metro lines does Mumbai have?
The city’s first underground Metro line is Metro-3, a 33-5-km corridor that links Colaba to Bandra to Seepz. Tunnels pass beneath Marine Drive, CSMT Heritage Zone, Mithi River, existing rail corridors (WR, CR).
Other lines such Metro 2B (DN Nagar-Mandale) has underground stretches near CSMT (still being planned); Mumbai Metro-4 (Wadala-Thane) has twin underground tunnels being planned in dense sections; Metro-11 (Wadala to Gateway extension of Line 4) will be entirely underground and Metro-9 (Dahisar to Mira-Bhayander) extension have been planned with underground sections.

Surya-1, Tunnel Boring Machine (TBM) for pkg-1, Cuffe Parade to Hutatma Chowk manufactured by @Robbins_Co completes Factory Acceptance Test(FAT). TBM will soon be shipped to Mumbai. #MumbaiLifeLine3 #ComingSoon @CMOMaharashtra @TheMetroRailGuy @rajtoday pic.twitter.com/rEN7zOc6OM
— MumbaiMetro3 (@MumbaiMetro3) April 20, 2018

Making of a Metro tunnel: What are the machines used?
1. Tunnel Boring Machines (TBMs) are primarily used in the process. They are massive cylindrical machines (80-100m long) that dig through soil/rock, while simultaneously laying tunnel segments.
They are used to achieve:

Minimal surface disruption in dense city zones
Ensuring precision under old buildings, roads, and utility lines.

A case in point: 17 TBMs had to be used while making Line 3, making it one of the largest TBM operations in India.
Work of the tunnel for Colaba-Bandra-Seepz Metro-3 begins from Naya Nagar at Mahim in Mumbai. (PTI)
2. New Austrian Tunnelling Method (NATM): This is another method used for the process, although not as frequently as a TBM. It relies on controlled blasting and spray concrete support and is suitable for hard rock conditions. It used only in cases where TBMs are less feasible or in shorter stretches.
For Metro-3, some short sections used NATM to build station access tunnels. In case of Metro-4, TBMs will be used for tunnelling under Thane Creek and dense urban pockets and NATM for tricky transitions into elevated segments. In case of Metro-11, TBM will be used. For Metro-9, which has short underground portions near dense city nodes, a mix of cut-and-cover for shallow tunnels will be used.

Metro 3 network a project of national importance is shaping up rapidly with 17 TBMs tunneling 52 kms of tunnel with total 14 TBM shafts and 32 drives. 12 kms of total tunneling completed now. 80% tunneling to be completed by Dec 2019. #WhatsHappeningBeneath #MetroMission2021 pic.twitter.com/YO9EuOQGD1
— MumbaiMetro3 (@MumbaiMetro3) November 14, 2018

Metro tunnel: Why is it an engineering feat?
Heritage sites: The tunnels for Mumbai Metro lines pass under heritage sites. For instance, in case of Mumbai Metro-3, the line will pass under CSMT, Flora Fountain and Marine Drive. As scenic as it may sound, it requires detailed geotechnical surveys and vibration monitoring to avoid structural damage.
Crowded subsurface: The tunnels have to avoid existing rail lines (WR & CR), sewerage lines, gas mains, telecom ducts, and storm drains. So they are often bored at 25-40m depth to stay below these utilities.
Water ingress: South Mumbai has high water tables and reclaimed land, which, in turn, raise flooding risks. Waterproof tunnel segments, sealing rings, and real-time groundwater pressure monitoring is used to reduce this risk.
Digital monitoring systems: Tunnelling involves the use of real-time deformation sensors, ground movement detectors, AI-based TBM control software to enable adjustments to TBM pressure and speed in response to soil changes.

CM @Dev_Fadnavis was at the First breakthrough of TBM (Tunnel Boring Machine) for 33.5 km long fully underground @MumbaiMetro line 3 (Colaba-Bandra-SEEPZ) at T2, Chhatrapati Shivaji Maharaj International Airport, Mumbai. Minister @Ranjitpatil_Mos too present. pic.twitter.com/5QWrajfBGc
— CMO Maharashtra (@CMOMaharashtra) September 24, 2018

Mumbai Metro tunnels: What are the risks involved?
News18 asked ChatGPt and here’s the table it threw up:

Ground subsidence
Real-time monitoring, controlled boring, compensation grouting

Damage to old buildings
Structural audits, vibration dampeners, legal notifications

Utility strikes
Pre-surveys with GPR (ground-penetrating radar), coordination with BMC

Flooding/ water ingress
Pre-injection grouting, sealed segments, 24/7 sump systems

Human safety
Strict SOPs, emergency exits every 250m, fire suppression systems

Cost overruns & delays
Modular construction, global consultants (e.g., AECOM, MMRC)

How did Mumbai still manage the underground work?
One of the key elements in the process was taking assistance through global consortia from Japan, China, Italy, Turkey, and India wherever required. Another was involvement of world-class engineering firms for tunnel alignment, seismic resilience, and ventilation systems.
Do you know innovative ways in which Mumbai-specific tunnelling challenges were handled?
Here are some examples from media reports:

Mumbai’s underground has hard basalt, so TBMs had to be customised and even get cutter-head design to handle varying strata from clay to basalt.
Cross-passages had to be built for emergency evacuation and air circulation between twin tunnels.
In some cases, media reports highlighted how robotic scanning was used to detect cracks inside finished tunnels.
A station like BKC is designed to be LEED-certified, with natural ventilation shafts and solar-equipped roofs (above ground entry points).
To tunnel below Mithi River, reports highlighted that special TBM pressure calibration and flood-proof tunnel rings had to be used.
For precise alignment while crossing the already existing Metro-1, which is elevated, 3D digital twin models were used.
For cramped areas like Girgaon, top-down construction method was used to minimise footprint and impact on the residents.

With Agency Inputs