For example, in the United Kingdom a road tunnel 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. Terminology An entrance of the Rantaväylä Tunnel in the northern part of Tampere, Pirkanmaa, Finland A 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. Tunnels can be connected together in tunnel networks. 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. Tunnel in Col du Galibier, France Tunnel in Fort de Mutzig, France Decorated entrance to a road tunnel in Guanajuato, Mexico Utility tunnel for heating pipes between Rigshospitalet and Amagerværket in Copenhagen, Denmark Tunnel on the Taipei Metro in Taiwan Southern portal of the 421 m long (1,381 ft) Chirk canal tunnel, WalesĪ tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. For a tunnel for pedestrians, see Subway (underpass). For the John Foxx song, see Underpass (song).
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