Tunnel Boring Machine (TBM) Technology

A tunnel boring machine, or TBM, is a giant machine that digs a round tunnel and lines it with concrete at the same time. It works like a huge mechanical earthworm: a rotating wheel at the front cuts the rock or soil, and the machine slowly crawls forward, building the tunnel wall behind it. TBMs are used to build metro lines, railway tunnels, road tunnels, water and sewer tunnels, and tunnels for hydropower projects.

Why does it matter?

Cities are crowded, and land is costly. Digging underground lets a metro or railway pass below buildings, roads and even rivers without disturbing life above. Compared with older methods like blasting, a TBM:

• digs faster and more steadily in long tunnels;
• causes far less vibration and less sinking of the ground above (important under old buildings);
• produces a smooth, ready-lined tunnel;
• is safer for workers, who stay inside a protective steel shell.

The main drawback is cost and set-up time. A large TBM can cost hundreds of crores of rupees, and it must be custom-made for each project's soil and tunnel size.

Where did it come from?

• 1825: Engineer Marc Isambard Brunel began the Thames Tunnel in London using a tunnelling shield, a frame that protected workers while they dug by hand. The tunnel opened in 1843.
• Late 1800s: Peter Barlow and James Henry Greathead improved the shield into a round form, used for early London underground railways.
• 1952: American engineer James S. Robbins built what is widely called the first modern TBM, for the Oahe Dam project in South Dakota, USA. A few years later, his machines began using disc cutters, which became the standard for cutting hard rock.
• 1970s onwards: Japanese and German engineers developed slurry and earth pressure balance machines that can dig safely through soft, wet ground below cities and rivers.

How does a TBM work?

Step by step:

1. Cutting: The cutterhead, a large rotating wheel fitted with disc cutters or scrapers, grinds the rock or soil in front.
2. Pushing: Powerful hydraulic jacks (thrust cylinders) push the machine forward by pressing against the concrete rings already fitted behind it.
3. Removing the muck: The cut material (called muck) is carried back by a screw conveyor and belt, or pumped out mixed with liquid.
4. Lining: A robotic arm called the segment erector places curved precast concrete pieces (segments) to form a complete ring. Ring after ring becomes the tunnel wall.
5. Grouting: The small gap between the ring and the ground is filled with grout (a cement mix) to lock it in place and stop the ground above from sinking.

Behind the cutting shield, a long train of trailers (the "backup") carries electrical systems, pumps and supplies. A big TBM can be over 90 metres long.

Main types of TBM

The type depends on the ground:

• Open (gripper) TBM: For hard, stable rock. It braces itself against the tunnel walls using side grippers. Used in mountain and hydropower tunnels.
• Single and double shield TBM: For rock that may be broken. A steel shield protects the machine and workers.
• Earth Pressure Balance (EPB) TBM: For soft soil, common in city metros. It uses the dug-out soil, turned into a thick paste, to push back against the tunnel face and stop it from collapsing.
• Slurry TBM (including "Mixshield"): For soft, very wet ground and tunnels below rivers or the sea. It pumps a bentonite clay slurry (a thick liquid) into the cutting chamber to hold up the face, and the muck is pumped out with the slurry.

India's position and Indian examples

• Metros: TBMs have dug underground sections of the Delhi, Mumbai, Kolkata, Chennai, Bengaluru, Lucknow and other metros.
• Kolkata underwater metro: India's first underwater metro tunnel, a 520-metre stretch below the Hooghly river on the East-West Metro (Howrah Maidan to Esplanade section), was inaugurated on 6 March 2024. Howrah station, about 32 metres below ground, is India's deepest metro station.
• Mumbai-Ahmedabad bullet train: The 21 km underground stretch between Bandra Kurla Complex and Shilphata includes about 7 km below Thane Creek, India's first undersea rail tunnel. About 16 km is being dug by TBMs with a 13.6 m diameter cutterhead (Mixshield type), and about 5 km by NATM.
• Supply: The main global makers are Herrenknecht (Germany), The Robbins Company (USA) and Chinese firms such as China Railway Engineering Equipment Group (CREG) and China Railway Construction Heavy Industry (CRCHI). Many machines, including Herrenknecht ones, are built in Chinese factories. This makes India's tunnel projects sensitive to Chinese export decisions.

Commonly confused concepts

• TBM vs NATM: The New Austrian Tunnelling Method (NATM) was developed in Austria between 1957 and 1965 by Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher. It digs in stages (often by drilling and blasting or excavators) and supports the walls with sprayed concrete (shotcrete), rock bolts and steel ribs, using the surrounding rock's own strength. NATM is flexible for short tunnels, changing shapes and mixed ground, and is used for many Himalayan road tunnels such as the Atal (Rohtang) Tunnel. TBMs are better for long, uniform, round tunnels, especially in cities.
• TBM vs drill-and-blast: Drill-and-blast drills holes, fills them with explosives and blasts the rock. It is cheap to start but slow and causes vibration, so it is unsuited below crowded cities.
• TBM vs cut-and-cover: Cut-and-cover digs an open trench from the surface, builds the tunnel and then covers it. It is used for shallow stations, but blocks roads during construction.
• EPB vs slurry TBM: EPB uses the soil paste itself to hold the face; slurry uses pumped bentonite liquid and suits higher water pressure, such as under rivers and seas.

Issues, criticism and the way forward

• Import dependence: India imports almost all its TBMs. Delays in supply directly delay metros, high-speed rail and strategic tunnels.
• Cost and time: Each machine is custom-built, and making one can take a year or more.
• Difficult geology: The young Himalaya has weak, folded and water-filled rock, where TBMs can get stuck. This is why many hill tunnels still use NATM.
• Skilled workforce: Operating and maintaining TBMs needs specialised engineers and technicians.
• Way forward: Experts suggest building TBMs and their key parts (cutters, bearings, segments) in India under Make in India, attracting global makers to set up Indian factories, sourcing from multiple countries, and refurbishing and reusing machines across projects.

Concepts to Know

• Cutterhead: The large rotating front wheel of a TBM that cuts the rock or soil.
• Disc cutter: A hard steel disc fixed on the cutterhead that rolls against the rock and cracks it under huge pressure.
• Precast segment: A curved concrete block made in a factory before use. Several segments join to make one ring of the tunnel wall.
• Bentonite: A type of clay that, mixed with water, forms a thick slurry. It helps hold up loose ground during digging.
• Shotcrete: Concrete sprayed at high speed onto a surface, such as a freshly dug tunnel wall.