Editing Tunnel (section)

=== Boring machines ===
{{Main|Tunnel boring machine}}
[[File:TBM S-210 Alptransit Faido East.jpg|thumb|A workman is dwarfed by the cutting end of a [[tunnel boring machine]] used to excavate the [[Gotthard Base Tunnel]] ([[Switzerland]]), the world's longest railway tunnel.]]

[[Tunnel boring machine]]s (TBMs) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as a quick and cost-effective alternative to laying surface rails and roads. Expensive [[eminent domain|compulsory purchase]] of buildings and land, with potentially lengthy planning inquiries, is eliminated. Disadvantages of TBMs arise from their usually large size – the difficulty of transporting the large TBM to the site of tunnel construction, or (alternatively) the high cost of assembling the TBM on-site, often within the confines of the tunnel being constructed.

There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some TBMs, the bentonite slurry and earth-pressure balance types, have pressurized compartments at the front end, allowing them to be used in difficult conditions below the [[water table]]. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurized compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. Despite these difficulties, TBMs are now preferred over the older method of tunnelling in compressed air, with an airlock/decompression chamber some way back from the TBM, which required operators to [[work in compressed air|work in high pressure]] and go through decompression procedures at the end of their shifts, much like [[underwater diving|deep-sea divers]].

In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of the [[Linth–Limmern Power Stations]] located south of [[Linthal, Glarus|Linthal]] in the [[canton of Glarus]]. The borehole has a diameter of {{convert|8.03|m|ft}}.<ref>{{cite web |title=Tunnels & Tunnelling International |url=http://www.tunnelsonline.info/story.asp?sectioncode=1&storycode=61846 |url-status=dead |archive-url=https://web.archive.org/web/20120316162001/http://www.tunnelsonline.info/story.asp?sectioncode=1&storycode=61846 |archive-date=16 March 2012 |access-date=19 April 2013 |website=Tunnelsonline.info |publisher=}}</ref> The four TBMs used for excavating the {{convert|57|km|mi|adj=on}} [[Gotthard Base Tunnel]], in [[Switzerland]], had a diameter of about {{convert|9|m|ft}}. A larger TBM was built to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the [[HSL-Zuid]] in the Netherlands, with a diameter of {{convert|14.87|m|ft|1}}.<ref>{{cite web |title=The Groene Hart Tunnel |url=http://www.hslzuid.nl/hsl/uk/bouw/ment/Bored_Tunnel_Groene_Hart/index.jsp |url-status=dead |archive-url=https://web.archive.org/web/20090925160241/http://www.hslzuid.nl/hsl/uk/bouw/ment/Bored_Tunnel_Groene_Hart/index.jsp |archive-date=25 September 2009 |access-date=19 April 2013 |website=Hslzuid.nl |publisher=}}</ref> This in turn was superseded by the [[Autopista de Circunvalación M-30|Madrid M30 ringroad]], Spain, and the [[Shanghai Yangtze River Tunnel and Bridge|Chong Ming]] tunnels in [[Shanghai]], China. All of these machines were built at least partly by [[Herrenknecht]]. {{As of|2013|August|}}, the world's largest TBM was "[[Bertha (tunnel boring machine)|Big Bertha]]", a {{convert|57.5|ft|adj=on|order=flip}} diameter machine built by [[Hitachi Zosen Corporation]], which dug the [[Alaskan Way Viaduct replacement tunnel]] in [[Seattle, Washington]] (US).<ref name="NYT Dec 2012">{{cite news |last=Johnson |first=Kirk |date=5 December 2012 |title=Engineering Projects Will Transform Seattle, All Along the Waterfront |url=https://www.nytimes.com/2012/12/05/us/projects-to-transform-seattle-all-along-the-waterfront.html |url-access=subscription |access-date=2024-01-23 |newspaper=[[The New York Times]]}}</ref>