Ways to build a tunnel

There is no single way to build a tunnel. The method follows the ground, the depth, the length and what is above.

In hard rock, drill-and-blast is still common: drill a pattern of holes, charge them, blast, ventilate, remove the broken rock, then install support. The New Austrian Tunnelling Method (NATM) is a design philosophy as much as a method: excavate in stages, apply flexible support such as shotcrete and rock bolts soon after, and monitor deformation so that the ground carries much of its own load.

Tunnel boring machines (TBMs) cut the full face mechanically. In soft ground under cities, closed-face TBMs balance the earth and water pressure at the face and erect precast segment rings behind, limiting surface settlement. They suit long tunnels where their high set-up cost pays back.

Cut-and-cover builds a box in an open excavation and backfills over it; it suits shallow metro stations. Immersed tunnels are made of precast elements floated out and sunk into a dredged trench across a river or strait.