Editing Tunnel (section)

== Geotechnical investigation and design ==
{{Main|Geotechnical investigation}}
A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from [[borehole]]s and by other geophysical techniques.<ref>{{Cite web| title=Guide for tunnelling work | url=https://www.safeworkaustralia.gov.au/system/files/documents/1702/guide-tunnelling.pdf | archive-url=https://web.archive.org/web/20180408083316/https://www.safeworkaustralia.gov.au/system/files/documents/1702/guide-tunnelling.pdf | archive-date=2018-04-08}}</ref> An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction.

Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel (or "drift tunnel") may be driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.

Other key geotechnical factors:
* '''Stand-up time''' is the amount of time a newly excavated cavity can support itself without any added structures. Knowing this parameter allows the engineers to determine how far an excavation can proceed before support is needed, which in turn affects the speed, efficiency, and cost of construction. Generally, certain configurations of rock and clay will have the greatest stand-up time, while sand and fine soils will have a much lower stand-up time.<ref>{{cite book |editor1-last= Bickel|editor1-first= John O.|editor2-first =Thomas R.|editor2-last = Kuesel |editor3-first=Elwyn H.|editor3-last =King|page = 210|date = 2004|title= Tunnel Engineering Handbook|edition = 2nd|publisher =Kluwer Academic Publishers|chapter = Tunnel Boring Machines|first = Harry|last = Sutcliffe|chapter-url = https://books.google.com/books?id=59V5BgAAQBAJ&pg=PA210|isbn = 978-1-4613-8053-5}}</ref>
* [[Groundwater]] control is very important in tunnel construction. Water leaking into a tunnel or vertical shaft will greatly decrease stand-up time, causing the excavation to become unstable and risking collapse. The most common way to control groundwater is to install dewatering pipes into the ground and to simply pump the water out.<ref>Powers, P.J. (2007). Construction dewatering and groundwater control. Hoboken, NJ: John Wiley & Sons Inc.</ref> A very effective but expensive technology is [[ground freezing]], using pipes which are inserted into the ground surrounding the excavation, which are then cooled with special refrigerant fluids. This freezes the ground around each pipe until the whole space is surrounded with frozen soil, keeping water out until a permanent structure can be built.
* Tunnel [[Cross section (geometry)|cross-sectional shape]] is also very important in determining stand-up time. If a tunnel excavation is wider than it is high, it will have a harder time supporting itself, decreasing its stand-up time. A square or rectangular excavation is more difficult to make self-supporting, because of a concentration of [[Stress (mechanics)|stress]] at the corners.<ref name="Engineers. 1978"/>

=== Choice of tunnels versus bridges ===
{{unreferenced section|date=February 2021}}
{{Anchor|Choice of tunnels vs bridges}}
[[File:2007 09 19 - 895tunnel - WB 3.JPG|right|thumb|The [[Harbor Tunnel (Baltimore)|Harbor Tunnel]] in [[Baltimore]], USA, which carries [[Interstate 895 (Maryland)|I-895]], serves as an example of a water-crossing tunnel built instead of a bridge.]]
For water crossings, a tunnel is generally more costly to construct than a bridge.<ref>{{Cite web |title=Tunnels - an environmentally attractive option? |url=https://www.worldhighways.com/wh10/feature/tunnels-environmentally-attractive-option |access-date=2024-10-08 |website=World Highways |language=en}}</ref> However, both navigational and traffic considerations may limit the use of high bridges or [[drawbridge]]s intersecting with shipping channels, necessitating a tunnel.

Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as [[Manhattan]] and urban [[Hong Kong]], this is a strong factor in favor of a tunnel. Boston's [[Big Dig]] project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.<ref>{{Cite web |title=FHWA - Center for Innovative Finance Support - Project Profiles |url=https://www.fhwa.dot.gov/ipd/project_profiles/ma_boston_central_artery.aspx |access-date=2024-11-08 |website=www.fhwa.dot.gov}}</ref>

The 1934 [[Queensway Tunnel]] under the [[River Mersey]] at [[Liverpool]] was chosen over a massively high bridge partly for defence reasons; it was feared that aircraft could destroy a bridge in times of war, not merely impairing road traffic but blocking the river to navigation.<ref>{{Cite web |title=Queensway Tunnel which links Liverpool to Birkenhead marks 90th anniversary |url=https://www.bbc.com/news/articles/c7283vplp56o |access-date=2024-10-08 |website=www.bbc.com |date=20 July 2024 |language=en-GB}}</ref> Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 [[Kingsway Tunnel]] under the Mersey. In [[Hampton Roads, Virginia]], tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges might prevent [[US Navy]] vessels from leaving [[Naval Station Norfolk]].

Water-crossing tunnels built instead of bridges include the [[Seikan Tunnel]] in Japan; the [[Holland Tunnel]] and [[Lincoln Tunnel]] between [[New Jersey]] and Manhattan in [[New York City]]; the [[Queens-Midtown Tunnel]] between Manhattan and the [[Borough (New York City)|borough]] of [[Queens]] on [[Long Island]]; the [[Detroit-Windsor Tunnel]] between [[Michigan]] and [[Ontario, Canada|Ontario]]; and the [[Elizabeth River (Virginia)|Elizabeth River]] tunnels between [[Norfolk, Virginia|Norfolk]] and [[Portsmouth, Virginia]]; the 1934 [[River Mersey]] road [[Queensway Tunnel]]; the [[Western Scheldt Tunnel]], Zeeland, Netherlands; and the [[North Shore Connector]] tunnel in [[Pittsburgh, Pennsylvania]]. The [[Sydney Harbour Tunnel]] was constructed to provide a second harbour crossing and to alleviate traffic congestion on the [[Sydney Harbour Bridge]], without spoiling the iconic view.

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the {{convert|51.5|km|mi|adj=on|disp=or}} [[Channel Tunnel]]), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).

Some water crossings are a mixture of bridges and tunnels, such as the [[Oresund Bridge|Denmark to Sweden link]] and the [[Chesapeake Bay Bridge-Tunnel]] in [[Virginia]].

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can [[asphyxia]]te users, as happened at the [[Gotthard Road Tunnel#2001 collision and fire|Gotthard Road Tunnel]] in [[Switzerland]] in 2001. One of the worst railway disasters ever, the [[Balvano train disaster]], was caused by a train stalling in the Armi tunnel in [[Italy]] in 1944, killing 426 passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.

=== Project planning and cost estimates ===
Government funds are often required for the creation of tunnels.<ref>{{cite web|url=http://www.cord.edu/faculty/bfoss/ADVC20.htm |title=Capital Projects Funds |publisher=Cord.edu |access-date=19 April 2013 |url-status=dead |archive-url=https://web.archive.org/web/20111217080357/http://www.cord.edu/faculty/bfoss/ADVC20.htm |archive-date=17 December 2011 }}</ref> When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use [[project management]] techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a [[work breakdown structure]] and [[critical path method]]. Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of [[Bond (finance)|bonds]].

The [[Cost–benefit analysis|costs and benefits]] for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the [[Port Authority of New York and New Jersey]] was not aware of this bill and had not asked for a grant for such a project.<ref>{{cite news| url=https://www.nytimes.com/2005/08/03/nyregion/03rail.html?_r=0 | work=The New York Times | first=Sewell | last=Chan | title=$100 Million for a Tunnel. What Tunnel? | date=3 August 2005}}</ref> Increased taxes to finance a large project may cause opposition.<ref>{{cite web |url=http://www.cfr.org/infrastructure/encouraging-us-infrastructure-investment/p27771 |title=Encouraging U.S. Infrastructure Investment – Council on Foreign Relations |publisher=Cfr.org |access-date=19 April 2013 |archive-date=23 May 2013 |archive-url=https://web.archive.org/web/20130523092826/http://www.cfr.org/infrastructure/encouraging-us-infrastructure-investment/p27771 |url-status=dead }}</ref>