TBMs

TBMs, or tunnel boring machines, are large engineering machines used to excavate tunnels through a variety of ground conditions. They combine earth excavation, muck removal, and structural support in a single autonomous operation. A TBM consists of a rotating cutterhead at the front, a shield that supports the tunnel wall, a screw or belt conveyor system to remove excavated material, and propulsion units that push the machine through the ground. The cutterhead is equipped with discs or picks that break rock or soil; the ground is supported by segmental lining or sprayed concrete as the machine advances, sometimes with precast concrete segments installed by a separate erector.

TBMs are often categorized by the ground conditions and the method of ground support they employ. Hard rock TBMs, disc cutters, and rigid shields are used where ground is strong; earth pressure balance TBMs and slurry TBMs are used in soft ground or water-bearing conditions. EPB TBMs balance earth pressure at the facing to prevent collapse, while slurry TBMs rely on a pressurized slurry to stabilize the face and remove spoil. The choice depends on geology, groundwater, ground movement risk, and project economics.

Benefits include faster excavation, improved worker safety, reduced surface settlement, and better ground control relative to drill-and-blast methods. Drawbacks include high capital costs, risk of mechanical failure in challenging ground, and complex operations for muck handling and segment installation.

TBMs have been used for major metropolitan tunnels and long underground passages worldwide, including subway lines, water conveyance tunnels, and utility tunnels. Their development continues with hybrid designs and innovations in cutterhead design, lubrication, and automation.