Tunnels

A tunnel is an artificial passage that is excavated or bored through surrounding soil or rock, or constructed underwater, for use by vehicles, trains, pedestrians, or utilities. Tunnels enable routes where building above-ground can be impractical, expensive, or environmentally disruptive.

Common types include road tunnels, rail tunnels, pedestrian or cycling tunnels, and utility tunnels that carry water, sewage, electricity, or fiber optic cables. Large-scale road and rail tunnels are central to urban transport networks and long-distance infrastructure. Immersed tunnels and subsea tunnels cross bodies of water.

Construction methods: cut-and-cover, where a trench is excavated and then covered; bored tunnels made with tunnel boring machines; and immersed-tube tunnels built from segments laid underwater and connected offshore. Selection depends on ground conditions, depth, cross-section, and budget.

Design and safety: tunnel linings may be concrete, shotcrete, or precast segments. Waterproofing, drainage, ventilation, and lighting are essential. Fire safety relies on compartmentalization, standpipes, and emergency exits. Maintenance includes pump stations, inspections, and monitoring of ground movement.

History and examples: early tunnels date to mining and drainage projects. The Channel Tunnel between the United Kingdom and France, the Gotthard Base Tunnel in Switzerland, the Seikan Tunnel in Japan, and other major projects illustrate modern tunneling capability. Tunnels also enable utilities networks in dense urban areas.

Impact: tunnels affect land use, urban development, and environment; their construction can disrupt communities and ecosystems, but they reduce surface traffic and hazards by routing flows underground. Advances in boring technology and intelligence in maintenance are improving safety and efficiency.