In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called a subway, while an underground railway system is differently named in different cities, the " Underground" or the " Tube" in London, the " Subway" in Glasgow, and the " Metro" in Newcastle. For example, the definition of a road tunnel in the United Kingdom is defined as "a subsurface highway structure enclosed for a length of 150 metres (490 ft) or more." In the United States, the NFPA definition of a tunnel is "An underground structure with a design length greater than 23 m (75 ft) and a diameter greater than 1,800 millimetres (5.9 ft)." The definition of what constitutes a tunnel can vary widely from source to source. Tunnels can be connected together in tunnel networks.Ī fabric tunnel in Moulvibazar District, BangladeshĪ tunnel is relatively long and narrow the length is often much greater than twice the diameter, although similar shorter excavations can be constructed, such as cross passages between tunnels. Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely.
Secret tunnels are built for military purposes, or by civilians for smuggling of weapons, contraband, or people. Utility tunnels are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment. Some tunnels are used as sewers or aqueducts to supply water for consumption or for hydroelectric stations. The central portions of a rapid transit network are usually in the tunnel. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.Ī tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Southern portal of the 421 m long (1,381 ft) Chirk canal tunnelĪ tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end.
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