Tunnel
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A disused railway tunnel now converted to pedestrian and bicycle use, near Houyet, Belgium |
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 UK. It stretches over three miles and was opened by the Prince of Wales.
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Colorful pedestrian Light Tunnel connecting two terminals in Detroit's DTW airport. |
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.
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.
Cut-and-cover is a simple method of construction for shallow tunnels where a
trench is excavated and
roofed over. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing.
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.
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 either fluid or air 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. [
1]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 TBM. 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.
There are also several approaches to underwater tunnels, for instance an
immersed tube as in
Sydney Harbour.
Other tunneling methods are:
*
Drilling and blasting* Slurry-
shield machine
*
Wall-cover construction method.
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, reclaim land, 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).
Some water crossings are a mixture of bridges and tunnels, such as the
Denmark to Sweden link and the
Chesapeake Bay Bridge-Tunnel in the eastern
United States.
A short tunnel can be a convenient way to build an overbridge. Very short tunnels resemble overbridges. One example of a short tunnel is the
Croom Tunnel on the
Nowra, New South Wales railway line.
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.
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
See also the
History of
Rapid transit.
The longest
*The
Gotthard Base Tunnel in Switzerland, a new rail tunnel through the Gotthard massif with a length of 57 km (36 miles), is currently under construction.
*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 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.
Notable
*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) but there are probably even more busy tunnels.
*
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[
2], 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.
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.
*
List of tunnels*
Underground city*
Urban exploration*
Roof and tunnel hacking*
Smuggling tunnel*
List of rathole tunnels*
Wind tunnel*
World's longest tunnels*
Megaprojects*
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 Association
