tunnel

A tunnel is a man-made passage that runs underground or underwater, bored through surrounding rock or soil. Tunnels are built to carry transportation routes, such as roads and railways, to house utilities including water pipes, sewage, and electrical cables, or to provide access for mining, drainage, or municipal services. They can be bored through rock or excavated using cut-and-cover methods, and may include ventilation and safety systems along their length.

Construction methods vary. Cut-and-cover builds begin near the surface: a trench is excavated, the tunnel shape is formed, and the cover is added after lining. Boring uses tunnel boring machines (TBMs) or drilling and blasting to create a circular or horseshoe-shaped void that is then reinforced with concrete lining, steel segments, or shotcrete. The New Austrian Tunneling Method emphasizes flexible sequence and ground support.

Design and safety considerations address structural integrity, ventilation, drainage, lighting, and fire protection. Long tunnels include ventilation shafts, emergency exits, and monitoring systems for ground movement and gas. Maintenance involves inspection of linings, joints, and drainage, and rehabilitation to address leaks or material degradation.

Types and examples. Road and rail tunnels form major urban routes; pedestrian and utility tunnels support transit and infrastructure. Notable examples include the Channel Tunnel, the Gotthard Base Tunnel, and the Severn Tunnel, illustrating long-distance bore and large-diameter construction.

History. Early tunnels were widened natural caves used for mining or drainage. Modern construction began in the 19th century with rail and road projects and advanced with concrete and steel in the 20th century.