TBMs are large machines that tunnel through ground, progressively installing concrete linings to support the excavated tunnel. Excavated material is transported through the machine to the surface for removal by trucks.
source/image(PrtSc): Crossrail Project
These huge machines work 24 hours a day excavating large volumes of ground and erecting the concrete tunnel lining.Each machine has a rotating cutter head at the front, and a series of trailers behind, housing all the mechanical and electrical equipment required for the excavation of material. Watch the video from Crossrail Project for more info:
Each TBM weighs approximately 1000 tonnes and can be up to 140m in length with an external diameter of 7.1m. This allows for an inside tunnel diameter of 6.2m once the concrete tunnel segments are in place.
Tunnel diameters can range from one metre 3.3 ft done with micro-TBMs to 17.6 metres (58 ft) to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs. . TBMs can also be designed to excavate non-circular tunnels, including u-shaped or horseshoe and square or rectangular tunnels./wikipedia