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{{short description|Underground passage made for traffic}} {{about|underground passages}} {{Use dmy dates|date=August 2023}} [[File:Signalisation routière entrée sud tunnel du Galibier.jpg|thumb|right|Tunnel in [[Col du Galibier]], France]] [[File:14-46-35-f-mutzig.jpg|thumb|right|Tunnel in [[Fort de Mutzig]], France]] [[File:Tunel en Guanajuato.jpg|thumb|Decorated portal to a road tunnel in [[Guanajuato]], Mexico]] [[File:Heatpipe tunnel copenhagen 2009.jpg|thumb|[[Utility tunnel]] for heating pipes between [[Rigshospitalet]] and Amagerværket in [[Copenhagen]], Denmark]] [[File:A crossover on the south side of Zhongxiao Xinsheng Station.JPG|thumb|Tunnel on the [[Taipei Metro]] in [[Taiwan]]]] [[File:Southern portal of Chirk canal tunnel - geograph.org.uk - 1293092.jpg|thumb|Southern portal of the {{convert|421|m|ft|abbr=on|adj=mid|long}} [[Chirk Tunnel|Chirk canal tunnel]], Wales]] A '''tunnel''' is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two [[Portal (architecture)|portals]] common at each end, though there may be access and ventilation openings at various points along the length. A [[Pipeline transport|pipeline]] differs significantly from a tunnel,<ref>{{Cite journal |last1=Zhou |first1=Man |last2=Su |first2=Xiaolong |last3=Chen |first3=Yaying |last4=An |first4=Lin |date=2022-10-02 |title=New Technologies and Challenges in the Construction of the Immersed Tube Tunnel of the Hong Kong-Zhuhai-Macao Link |url=https://www.tandfonline.com/doi/full/10.1080/10168664.2021.1904487 |journal=Structural Engineering International |language=en |volume=32 |issue=4 |pages=455–464 |doi=10.1080/10168664.2021.1904487 |issn=1016-8664|url-access=subscription }}</ref>{{Clarify|date=March 2025}} though some recent tunnels have used [[immersed tube]] construction techniques rather than traditional tunnel boring methods.<ref name=":1">{{Cite book |last=Takahashi |first=Yutaka |title=Water Storage, Transport, and Distribution |publisher=[[EOLSS Publications]] |year=2009 |isbn=9781848261761 |pages=318–319 |language=EN}}</ref> A tunnel may be for foot or vehicular [[road]] [[traffic]], for [[rail transport|rail]] traffic, or for a [[canal]]. The central portions of a [[rapid transit]] network are usually in the tunnel. Some tunnels are used as [[sanitary sewer|sewers]] or [[aqueduct (watercourse)|aqueducts]] to supply water for consumption or for [[hydroelectric]] stations. [[Utility tunnel]]s are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.<ref name=":0">Salazar, Waneta. ''Tunnels in Civil Engineering''. Delhi, India : White Word Publications, 2012.</ref> [[Secret passage|Secret tunnels]] are built for military purposes, or by civilians for [[smuggling]] of [[weapon]]s, [[contraband]], or [[people]].<ref>{{Cite journal |last=Sorrensen |first=Cynthia |date=2014-07-01 |title=Making the Subterranean Visible: Security, Tunnels, and the United States–Mexico Border |url=https://www.tandfonline.com/doi/full/10.1111/j.1931-0846.2014.12029.x |journal=Geographical Review |language=en |volume=104 |issue=3 |pages=328–345 |doi=10.1111/j.1931-0846.2014.12029.x |bibcode=2014GeoRv.104..328S |issn=0016-7428|url-access=subscription }}</ref> Special tunnels, such as [[wildlife crossing]]s, are built to allow wildlife to cross human-made barriers safely.<ref>Brodziewska, J. (2005). Wildlife tunnels and fauna bridges in Poland: past, present and future, 1997-2013. ''UC Davis: Road Ecology Center''. Retrieved from <nowiki>https://escholarship.org/uc/item/4wd0j27j</nowiki></ref> Tunnels can be connected together in [[tunnel network]]s. A 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. The definition of what constitutes a tunnel can vary widely from source to source. For example, in the United Kingdom, a road tunnel is defined as "a subsurface highway structure enclosed for a length of {{convert|150|m|ft}} or more."<ref>{{cite book |url=https://www.standardsforhighways.co.uk/tses/attachments/987a669b-13a1-40b9-94da-1ea4e4604fdd |title=Highway Structures & Bridges Design CD 352, Design of road tunnels (formerly BD 78/99) |publisher=The Department for Transport |year=2020}}</ref> In the United States, the [[National Fire Protection Association|NFPA]] definition of a tunnel is "An underground structure with a design length greater than {{convert|23|m|ft|abbr=on}} and a diameter greater than {{convert|1800|mm|ft}}."<ref>{{cite book |title=NFPA Standard for Safeguarding Construction, Alteration, and Demolition Operations |publisher=National Fire Protection Association}}</ref>
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