TBMs are suited to longer tunnel drives where the cross sectional profile remains constant and can erect the finished precast concrete lining directly behind the machine. They also limit the disturbance to the surrounding ground and the surface above, making them ideal for use in heavily urbanized areas.

The tunnel boring machine can be considered as a large diameter horizontal metal cylinder, a 'shield'. At the front is a full face cutter head which rotates at 1 to 3 rpm. As the TBM advances forward the cutter head excavates the ground. The excavated material is then removed from the tunnel face by a screw conveyor at a controlled rate and deposited onto a series of belt conveyors (pipes for a slurry machine) which remove the excavated material through the TBM and out of the tunnel. Foam or slurry can be injected into the face of the TBM to help 'condition' the excavated material and assist its handling properties

The TBM advances forward by using a series of hydraulic rams at the back of the shield which push forward from the last precast concrete ring erected

Once the shield has advanced forward by a defined distance the next precast concrete ring can be erected within the tail of the shield, in the space provided when the hydraulic rams are retracted.

A series of trailers housing all the necessary electrical and mechanical support equipment are towed behind the shield.

Materials, including tunnel segments will be brought to the TBM by a construction railway.