Tunnel

Tunnel, underground passageway usually designed to carry a highway or railroad, to serve as a conduit for water or sewage, or to provide access to an underground working face. Although tunnels have been built since prehistoric times, tunneling methods remained primitive and hazardous until the 19th century. Modern softground tunneling was pioneered by Marc Brunel, who invented the “tunneling shield” (1824), a device subsequently improved (1869–86) by James Greathead, whose shield is basically a large steel cylinder with a sharp cutting edge driven forward by hydraulic rams. Tunneling through hard rock is facilitated by an array of pneumatic drills mounted on a “jumbo” carriage running on rails. Explosives are inserted in a pattern of holes drilled in the rock face and then detonated. Increasingly, however, automatic tunneling machines called moles, with cutting heads consisting of a rotating or oscillating wheel that digs, grinds, or chisels away the working face, are employed. Another common tunnel-building method—used in constructing the New York subway—is “cut-and-cover,” which involves excavating a trench, building the tunnel lining, and then covering it. The world's longest vehicular or railroad tunnel is the 12.3-mi (19.8-km) Simplon II in the Alps, completed in 1922.