Learn more about Tunnel
A tunnel is an underground passage. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels have a ratio of the length of the passage to the width of at least 2 to 1. In addition, they should be completely enclosed on all sides, save the openings for the length of the covered area.
A tunnel may be for pedestrians or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are aqueducts, constructed purely for carrying water — for consumption, for hydroelectric purposes or as sewers — while others carry other services such as telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species. Some secret tunnels have also been made as a method of entrance or escape from an area, such as the Cu Chi Tunnels.
In the United Kingdom a pedestrian tunnel or other underpass beneath a road is called a subway. This term was also used in the past in the United States, but is now used to refer to underground rapid transit systems.
The longest canal tunnel is the Standedge Tunnel in the United Kingdom, which stretches over three miles.
The central part of a rapid transit network is usually built in tunnels. To allow non-level crossings, some lines are in deeper tunnels than others. At metro stations there are usually also pedestrian tunnels from one platform to another. Often, ground-level railway stations also have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without having to walk across the tracks. However, in the United Kingdom bridges are an equally popular method of pedestrian access between two or more different railway station platforms.
Tunnels are dug in various types of materials, from soft clays to hard rocks, and the method of excavation heavily depends on the ground conditions.
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.
 Boring machine
Tunnel-boring machines (TBMs) can be used to automate the entire tunneling process. There are a variety of TBMs that can operate in a variety of conditions. One type of TBM, called an earth-pressure balance machine, can be used deep below the water table. This pressurizes the cutter head with air or another fluid in order to balance the water pressure. As a result operators of the TBM must go through decompression chambers, much like divers.
The biggest TBM built was used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. Its diameter is 14.85 m. 
The New Austrian Tunneling Method (NATM) was developed in the 1960s. The main idea of this method is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself. Based on geotechnical measurements, an optimal cross section is computed. The excavation is immediately protected by thin shotcrete, just behind the excavation. This creates a natural load-bearing ring, which minimizes the rock's deformation.
By special monitoring the NATM method is very flexible, even at surprising changes of the geomechanical rock consistency during the tunneling work. The measured rock properties lead to appropriate tools for tunnel strengthening. In the last decades also soft ground excavations up to 10 km became usual.
 Underwater tunnels
Other tunneling methods are:
 Choice of tunnels vs. bridges
For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or drawbridge spans when intersecting with shipping channels at some locations, necessitating use of a tunnel. Additionally, bridges usually require a larger footprint on each shore than tunnels; in areas with particularly expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in tunnels' favor. Boston's Big Dig project replaced elevated roadways with a tunnel system in order to increase traffic capacity, hide traffic, reclaim land, redecoration, and reunite the city with the waterfront. Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia and the Westerscheldetunnel, Zeeland, Netherlands. Other reasons for choosing a tunnel instead of a bridge are aesthetic reasons (i.e. preserve 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).
 Very short tunnels
 Artificial Tunnels
Overbridges can sometimes be built by covering a road or river or railway with brick or still arches, and then levelling the surface with earth. In railway parlance, a surface-level track which has been built or covered over is normally called a covered way.
Snowsheds are a kind of artificial tunnel built to protect a railway from avalanches of snow. Similarly the Stanwell Park, New South Wales steel tunnel, on the Nowra, New South Wales railway line, which protects the line from rockfalls.
Common utility ducts are man-made tunnels created to carry two or more utility lines underground. Through colocation of different utilities in one tunnel, governments and companies are able to reduce the costs of building and maintining the utilities.
 Examples of tunnels
 In history
- The qanat or kareez of Persia is a water management system used to provide a reliable supply of water to human settlements or for irrigation in hot, arid and semi-arid climates. The oldest and largest known qanat is in the Iranian city of Gonabad which after 2700 years still provides drinking and agricultural water to nearly 40,000 people. Its main well depth is more than 360 meters and its length is 45 kilometers.
- The Eupalinian aqueduct on the island Samos (Ionia). Built 520 BC by the Ionian engineer Eupalinos. Eupalinos organised the work so that the tunnel was begun from both sides of the hill and the two teams met in the middle. The estimates for the time required range from 5 to 15 years: the mountain is solid limestone and one has to suppose that many of the slaves doing the work died. The tunnel's existence was recorded by Herodotus (as was the mole and harbour, and the third wonder of the island, the great temple to Hera, thought by many to be the largest in the Greek world). The precise location of the tunnel was only re-established in the 19th century by German archaeologists. The tunnel proper is 1030 metres - 3432 feet - long and visitors can still enter it Eupalinos tunnel.
- Sapperton Tunnel on the Thames & Severn Canal in England, dug through hills, which opened in 1789, was 3.5 km long and allowed ship transport of coal. Above it runs the Sapperton Long Tunnel which carries the "Golden Valley" railway line between Swindon and Gloucester.
- The tunnel created for the first true steam locomotive, the Penydarren locomotive, was built prior to Richard Trevithick was able to make his historic journey from Penydarren to Abercynon in 1804. Part of this tunnel can still be seen at Pentrebach, Merthyr Tydfil. This is arguably the oldest railway tunnel in the world, for self-propelled steam engines on rails.
- Box Tunnel in England, which opened in 1841, was the longest railway tunnel in the world at the time of construction. It was dug and has a length of 2.9 km.
- The Thames Tunnel, built by Marc Isambard Brunel and his son Isambard Kingdom Brunel and opened in 1843, was the first underwater tunnel and the first to use a tunnelling shield. Originally used as a foot-tunnel, it is now part of the East London Line of the London Underground.
- The Cobble Hill Tunnel and Murray Hill Tunnel in New York City are the world's oldest railway tunnels lying below streets, roofed over in 1850 and the 1850s, respectively.
- The oldest sections of the London Underground were built using the cut-and-cover method in the 1860s. The Metropolitan, Hammersmith & City, Circle and District lines were the first to prove the success of a metro or subway system.
- Col de Tende Road Tunnel, one of the first longer road tunnels under a pass
 The longest
- The Seikan Tunnel in Japan is the longest rail tunnel in the world at 53.9 km (33.4 miles), of which 23.3 km (14.5 miles) is under the sea.
- The Channel Tunnel between France and England under the English Channel is the second-longest, with a total length of 50 km (31 miles), of which 39 km (24 miles) is under the sea.
- The Lærdal Tunnel in Norway from Lærdal to Aurland is the world's longest road tunnel, intended for cars and similar vehicles, at 24.5 km (15.2 miles).
- The St. Gotthard Tunnel from Göschenen to Airolo in Switzerland, opened on September 5, 1980, is the world's second longest highway tunnel at 16.32 km (10.14 miles).
- The Ryfast road program in Stavanger, Norway includes the tunnel Solbakktunnelen, which is scheduled to be opened within 2015. This tunnel will be 14 km long, making it both the world's longest underwater road tunnel and longest underwater highway tunnel. The tunnel will have four driving lanes in total, and a speed limit of 90 km/h.
- The Hsuehshan Tunnel in northern Taiwan opened on June 16, 2006 with a length of 12.955 km (8.05 miles). This tunnel is the longest highway tunnel in Asia and the 3rd in the world.
- The Rennsteig Tunnel in middle Germany runs under the Thuringian Forest over a length of 8.5 km (5 1/5 miles) and is currently Germany's longest tunnel. The highway A71 connects Erfurt with Schweinfurt.
- The North Cape Tunnel in northern Norway, connecting the island of Magerøya with the mainland, was the world's longest undersea road tunnel when opened in 1999, at about 7 km. It reaches a depth of 212 m below sea level.
- Päijänne-tunneli is the world's longest complete tunnel that is bored into cliff. It is located in southern Finland and it is 120 km long. Its purpose is to provide the Greater Helsinki metropolitan area with fresh water.
- The Lincoln Tunnel between New Jersey and New York is one of the busiest vehicular tunnels in the world, at 120,000 vehicles/day.
- The Fredhälls Tunnel in Stockholm, Sweden is busier yet (150,000)
- Williamson's tunnels in Liverpool, built by a wealthy eccentric are probably the largest underground folly in the world.
- New York City Water Tunnel No. 3, started in 1970, has an expected completion date of 2020.
- The Chicago Deep Tunnel Project is a network of 109 miles (197 km) of tunnels designed to reduce flooding in the Chicago area. Started in the mid 1970s, the project is due to be completed in 2019.
- The Fenghuoshan tunnel on Qinghai-Tibet railway is the world's highest railway tunnel.
 Other uses
Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with military use during armed conflict, or civilian responses to threat of attack.
 Natural tunnel
 See also
- List of tunnels
- Underground city
- Urban exploration
- Roof and tunnel hacking
- Smuggling tunnel
- List of rathole tunnels
- Wind tunnel
- World's longest tunnels
 External links
- Trans Global Highway and proposed tunnels.
- Royal Engineers Museum British Army First World War Tunnelling.
- Directory of the world's longest tunnels by category
- ITA-AITES International Tunnelling Associationbg:Тунел
ca:Túnel cs:Tunel da:Tunnel de:Tunnel es:Túnel eo:Tunelo fa:تونل fr:Tunnel it:Tunnel he:מנהרה nl:Tunnel ja:トンネル no:Tunnel pl:Tunel (budownictwo) ro:Tunel ru:Тоннель simple:Tunnel sl:Predor fi:Tunneli sv:Tunnel tr:Tünel (trafik) uk:Тунель zh:隧道