Editing Tunnel (section)

== Construction ==
{{Main|Tunnel construction}}
{{anchor|Construction}}
{{anchor|Cut-and-cover}}
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the groundwater conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and the shape of the tunnel and appropriate risk management.

There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally, a tube can be sunk into a body of water, which is called an immersed tunnel.

=== Cut-and-cover ===
<!--links from Charles Pearson --><!--linked from [[Cut-and-cover]]-->
[[File:Paris Metro construction 03300288-3.jpg|thumb|Cut-and-cover construction at [[Saint-Michel (Paris Métro)|Saint-Michel]] on [[Paris Métro]] [[Paris Métro Line 4|Line 4]] (c. 1910)]]

'''Cut-and-cover''' is a simple method of construction for shallow tunnels where a [[trench]] is excavated and [[roof]]ed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.{{sfn|Ellis|2015|p=118}}

There are two basic forms of cut-and-cover tunnelling:
* ''Bottom-up method'': A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated.
* ''Top-down method'': Side support walls and capping beams are constructed from ground level by such methods as [[slurry wall]]ing or contiguous bored piling. Only a shallow excavation is needed to construct the tunnel roof using precast beams or in situ concrete sitting on the walls. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services, and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed.

Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a [[tunnelling shield]]. For intermediate levels, both methods are possible.

Large cut-and-cover boxes are often used for underground [[Rapid transit|metro]] stations, such as [[Canary Wharf tube station]] in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral, owing to the sheer size of the excavation. This contrasts with many traditional stations on [[London Underground]], where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the London Underground network, the Metropolitan and District Railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam.

A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction.<ref>{{Cite journal |last=Konyukhov |first=D. S. |date=12 April 2022 |title=Analysis of mechanized tunneling parameters to determine the overcutting characteristics |url=https://mst.misis.ru/jour/article/view/330 |journal=Gornye Nauki I Tekhnologii = Mining Science and Technology (Russia) |volume=7 |issue=1 |pages=49–56 |doi=10.17073/2500-0632-2022-1-49-56 |s2cid=248136002 |issn=2500-0632|doi-access=free }}</ref> This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century.

Prior to the replacement of manual excavation by the use of boring machines, [[Victorian era|Victorian]] tunnel excavators developed a specialized method called clay-kicking for digging tunnels in clay-based soils. The clay-kicker lies on a plank at a 45-degree angle away from the working face and rather than a [[mattock]] with his hands, inserts with his feet a tool with a cup-like rounded end, then turns the tool with his hands to extract a section of soil, which is then placed on the waste extract.
Clay-kicking is a specialized method developed in the [[United Kingdom]] of digging tunnels in strong clay-based soil structures. This method of cut and cover construction required relatively little disturbance of property during the renewal of the United Kingdom's then ancient [[sewerage]] systems. It was also used during the [[First World War]] by [[Royal Engineer tunnelling companies]] placing mines beneath [[German Empire|German]] lines, because it was almost silent and so not susceptible to listening methods of detection.<ref>{{cite web |date=2009 |title=Tunnelling |url=http://tunnellersmemorial.com/Tunnelling.htm |url-status=dead |archive-url=https://web.archive.org/web/20100823081917/http://tunnellersmemorial.com/Tunnelling.htm |archive-date=23 August 2010 |access-date=20 June 2010 |website=The Tunneller's Memorial, Givenchy |publisher=}}</ref>

=== 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>

=== Shafts ===
[[File:Mersey Railway Tunnel - ventilation and drainage machinery.png|right|thumb|1886 illustration showing the ventilation and drainage system of the Mersey railway tunnel]]
A temporary access [[Shaft (civil engineering)|shaft]] is sometimes necessary during the excavation of a tunnel. They are usually circular and go straight down until they reach the level at which the tunnel is going to be built. A shaft normally has concrete walls and is usually built to be permanent. Once the access shafts are complete, TBMs are lowered to the bottom and excavation can start. Shafts are the main entrance in and out of the tunnel until the project is completed. If a tunnel is going to be long, multiple shafts at various locations may be bored so that entrance to the tunnel is closer to the unexcavated area.<ref name="Engineers. 1978">United States Army Corps of Engineers. (1978). Tunnels and shafts in rock. Washington, DC: Department of the Army.</ref>

Once construction is complete, construction access shafts are often used as [[ventilation shaft]]s, and may also be used as emergency exits.

=== Sprayed concrete techniques ===
The [[New Austrian tunneling method|new Austrian tunnelling method]] (NATM)—also referred to as the Sequential Excavation Method (SEM)<ref name=tbm20181205/>—was developed in the 1960s.
The main idea of this method is to use the geological [[stress (physics)|stress]] of the surrounding [[Rock (geology)|rock]] [[mass]] to stabilize the tunnel, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the supports. Based on [[geotechnical]] measurements, an optimal [[cross section (geometry)|cross section]] is computed. The excavation is protected by a layer of sprayed concrete, commonly referred to as [[shotcrete]]. Other support measures can include steel arches, rock bolts, and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibers being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's [[Deformation (mechanics)|deformation]].<ref name=tbm20181205/>

[[File:Hill 60 illowra battery port kembla.jpg|thumb|[[Illowra Battery]] utility tunnel, Port Kembla. One of many [[Template:Barracks Batteries Bunkers and Forts in Sydney|bunkers south of Sydney]].]]

By special [[Condition monitoring|monitoring]] the NATM method is flexible, even at surprising changes of the [[geomechanics|geomechanical]] rock consistency during the tunneling work. The measured rock properties lead to appropriate [[tool]]s for tunnel [[Strength of materials|strengthening]].<ref name="tbm20181205">
{{cite news |date=5 December 2018 |title=Understanding the New Austrian Tunnel Method (NATM) |url=https://tunnelingonline.com/understanding-the-new-austrian-tunnel-method-natm/ |access-date=27 December 2018 |work=Tunnel Business Magazine |publisher=Benjamin Media}}</ref>

=== Pipe jacking ===
{{Main|Pipe jacking}}

In '''[[pipe jacking]]''', [[hydraulic jack]]s are used to push specially made pipes through the ground behind a TBM or shield. This method is commonly used to create tunnels under existing structures, such as roads or railways. Tunnels constructed by pipe jacking are normally small diameter bores with a maximum size of around {{convert|3.2|m}}.

=== Box jacking ===
Box jacking is similar to pipe jacking, but instead of jacking tubes, a box-shaped tunnel is used. Jacked boxes can be a much larger span than a pipe jack, with the span of some box jacks in excess of {{convert|20|m}}. A cutting head is normally used at the front of the box being jacked, and spoil removal is normally by excavator from within the box. Recent developments of the Jacked Arch and Jacked deck have enabled longer and larger structures to be installed to close accuracy.

=== Underwater tunnels ===
[[File:Georgiaaquariumtunnel.jpg|thumb|[[Shark tunnel]] at the [[Georgia Aquarium]]]]
{{Main|Undersea tunnel}}

There are also several approaches to underwater tunnels, the two most common being bored tunnels or [[immersed tube]]s, examples are [[Bjørvika Tunnel]] and [[Marmaray]]. [[Submerged floating tunnel]]s are a novel approach under consideration; however, no such tunnels have been constructed to date.

=== Temporary way ===
During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove [[spoil tip|excavated spoil]], often [[narrow gauge]] so that it can be [[double track]] to allow the operation of empty and loaded trains at the same time. The temporary way is replaced by the [[permanent way]] at completion, thus explaining the term "[[Perway]]".

=== Enlargement ===
[[File:Kolektory Praha, 15.jpg|thumb|A utility tunnel in [[Prague]]]]

The vehicles or traffic using a tunnel can outgrow it, requiring replacement or enlargement:
* The original single line [[Mount Gibraltar|Gib Tunnel]] near [[Mittagong]] was replaced with a double-track tunnel, with the original tunnel used for growing mushrooms.<ref name="Company Website">{{cite web |last=Sun |first=Li |title=Mushrooms and Tours {{!}} Li-Sun Exotic Mushrooms |url=http://www.li-sunexoticmushrooms.com.au/ |url-status=unfit |archive-url=http://web.archive.org/web/20180316053305/http://www.li-sunexoticmushrooms.com.au/ |archive-date=2018-03-16 |access-date=2024-01-23 |website=www.li-sunexoticmushrooms.com.au}}</ref><ref name="Article on Closure">{{cite web |last1=Biscoe |first1=Emma |date=15 May 2014 |title=Mittagong mushroom business will close down |url=https://www.illawarramercury.com.au/story/2284594/mittagong-mushroom-business-will-close-down/ |url-access=subscription |website=Illawarra Mercury |language=en}}</ref>
* The 1832 double-track {{convert|1|mi|km|adj=on|order=flip}}-long tunnel from [[Edge Hill railway station|Edge Hill]] to [[Liverpool Lime Street railway station|Lime Street]] in [[Liverpool]] was near totally removed, apart from a {{convert|50|m|yd|adj=on}} section at Edge Hill and a section nearer to Lime Street, as four tracks were required. The tunnel was dug out into a very deep four-track cutting, with short tunnels in places along the cutting. Train services were not interrupted as the work progressed.<ref>{{cite web |date=1881 |title=National Railway Museum / Science & Society image # 10445941 |url=https://www.scienceandsociety.co.uk/results.asp?image=10445941 |url-status=live |archive-url=https://web.archive.org/web/20160304114308/http://i34.tinypic.com/23ixthy.jpg |archive-date=4 March 2016 |access-date=2024-01-23 |website=Science & Society Picture Library}}</ref><ref>{{cite web |date=1881 |title=National Railway Museum / Science & Society image # 10445944: "Building a railway bridge in Liverpool, 1881" |url=https://www.scienceandsociety.co.uk/results.asp?image=10445944 |url-status=live |archive-url=https://web.archive.org/web/20160304185533/http://i36.tinypic.com/16k1ahx.jpg |archive-date=4 March 2016 |access-date=2023-01-24 |website=Science & Society Picture Library}}</ref> There are other occurrences of tunnels being replaced by open cuts, for example, the [[Auburn Tunnel]].
* The [[Farnworth Tunnel]] in England was enlarged using a [[tunnel boring machine]] (TBM) in 2015.<ref>{{Cite web |last= |first= |date=2015-08-12 |title=UK's biggest TBM rebores Farnworth Tunnel |url=https://www.railwaygazette.com/uks-biggest-tbm-rebores-farnworth-tunnel/41248.article |access-date=2024-01-23 |website=Railway Gazette International |language=en}}</ref> The [[Rhyndaston Tunnel]] was enlarged using a borrowed TBM so as to be able to take [[ISO container]]s.
* Tunnels can also be enlarged by lowering the floor.<ref>{{cite web|url=http://www.tunnel-online.info/en/artikel/tunnel_2012-04_Report_on_Redeveloping_Railway_Tunnels_1433844.html|title=Report on Redeveloping Railway Tunnels|website=www.tunnel-online.info}}</ref>

=== Open building pit ===
An open building pit consists of a horizontal and a vertical boundary that keeps groundwater and soil out of the pit. There are several potential alternatives and combinations for (horizontal and vertical) building pit boundaries. The most important difference with cut-and-cover is that the open building pit is muted after tunnel construction; no roof is placed.

=== Other construction methods ===
[[File:Tunnel end Onkalo.jpg|thumb|Tunnel excavated with drilling and blasting method.]]

* [[Drilling and blasting]]
* [[Hydraulic splitter]]
* Slurry-shield machine
* Wall-cover construction method.