What is a Tunnel Boring Machine (TBM)? The “Mole” of Railway Construction

A Tunnel Boring Machine (TBM), often referred to as a “mole,” is a massive machine used to excavate tunnels with a circular cross-section through a variety of soil and rock strata. They can bore through everything from hard rock to sand.
TBMs are the backbone of modern railway infrastructure, used extensively in high-speed rail projects and urban metro systems where surface disruption must be minimized.

How Does a TBM Work?

A TBM is essentially a moving factory. As the front of the machine grinds away the earth, the rear of the machine builds the tunnel.
The Cutterhead: The front of the TBM features a rotating cutterhead equipped with disc cutters. It grinds the rock or soil.
The Shield: A steel cylinder that protects the machine and workers from the pressure of the surrounding ground.
Segment Erector: As the machine moves forward, a robotic arm installs pre-cast concrete segments to form the tunnel lining (the walls of the tunnel).
Muck Removal: The excavated earth (muck) is transported out of the tunnel via a conveyor belt or slurry pipes.

TBM vs. Drill and Blast Method

While TBMs are faster for long tunnels, traditional “Drill and Blast” methods are still used for shorter distances.

Types of TBMs in Rail Projects

Hard Rock TBMs: Used for mountain tunnels (e.g., Gotthard Base Tunnel). They use grippers to push against the tunnel walls.
Earth Pressure Balance (EPB): Used in soft ground (common in Metro projects). The excavated earth is used to balance the pressure at the tunnel face.
Slurry Shield TBMs: Used when tunneling under high water pressure or under rivers.
TBMs have revolutionized railway construction, allowing engineers to build longer and deeper tunnels like the Channel Tunnel (connecting UK and France) and the Marmaray project.