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 are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, save for the openings at each end.
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 or the
tunnels connecting the
Gaza Strip to
Egypt.
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.
In the Czech republic, the verb to 'tunnel' is a synonym for to embezzle. For example: the manager 'tunnelled' the company and now lives on the Bahamas; or, many banks collapsed because they were 'tunnelled'.
[1]
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 run in deeper tunnels than others. At metro stations there are usually 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.
Construction
under construction]]
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
'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.
Two basic forms of cut-and-cover tunnelling are available:
★ ''Bottom-up method'': A trench is excavated, with ground support as necessary, and the tunnel is constructed within this. The tunnel may be of insitu concrete, precast concrete, precast arches, corrugated steel arches, etc. (Early examples used brickwork.) The trench is then backfilled, with suitable precautions regarding balancing compaction of the backfill material, and the surface is reinstated.
★ ''Top-down method'': In this method, side support walls and capping beams are constructed from ground level, using slurry walling, contiguous bored piles, or some other method. A shallow excavation is then made to allow the tunnel roof to be constructed using precast beams or insitu concrete. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation machinery is then lowered into the access openings, and the main excavation is carried out under the permanent tunnel roof, before constructing the base slab.
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 machines
'
Tunnel boring machines' (TBMs) and associated back-up systems can be used to highly automate the entire tunneling process. There are a variety of TBMs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types, (bentonite slurry and earth-pressure balance machines), have pressurised compartments at the front, allowing them to be used in difficult conditions below the
water table. This pressurizes the ground ahead of the TBM cutter head in order to balance the water pressure. The operators work in normal air pressure behind the pressurised 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 locally free from water. Despite these difficulties, TBMs are now preferred to the older method of tunneling in compressed air, with an air lock/decompression chamber some way back from the TBM, which required operators to work in high pressure and go through decompression procedures at the end of their shifts, much like divers.
Until recently 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 14.87 m.
[1]
Nowadays 4 even larger machines exist: 2 for the M30 ringroad in Madrid, Spain, 2 for the Chong Ming tunnels in Shanghai, China. These machines are 15,2 m and 15,4m in diameter respectively. The two machines for Spain were built by Mitsubishi/Dura Fuelgo and Herrenknecht
[2]. The TBMs for China were built by Herrenknecht.
NATM
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.
Pipe jacking
'Pipe Jacking', also known as 'pipejacking' or 'pipe-jacking', is a method of tunnel construction where hydraulic jacks are used to push specially made pipes through the ground behind a tunnel boring machine or shield. This technique is commonly used to create tunnels under existing structures, such as roads or railways.
Underwater tunnels
There are also several approaches to underwater tunnels, for instance an
immersed tube as in the
Sydney Harbour, and the
Posey and Webster Street Tubes which connect the cities of
Oakland and
Alameda, California, running beneath the
Alameda-Oakland Estuary.
Other
'Other tunneling methods' include:
★
Drilling and blasting
★ Slurry-
shield machine
★
Wall-cover construction method.
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, redecorate, 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 include avoiding difficulties with tides, weather and shipping during construction (as in the 51.5 km
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
Denmark to Sweden link and the
Chesapeake Bay Bridge-Tunnel in the eastern
United States.
Short tunnels
A short tunnel can be built as an alternative to an
overpass. One example of a short tunnel is the
Croom Tunnel on the
South Coast railway line.
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.
Snow sheds 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
South Coast 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 co-location of different utilities in one tunnel, governments and companies are able to reduce the costs of building and maintaining 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 (3,430 ft) 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.
Longest
Main articles: List of tunnels by length
★ 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 the
United Kingdom 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
Lötschberg Base Tunnel opened in June 2007 in
Switzerland is the longest land tunnel, with a total of 34.5 km (21.5 miles).
★ 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 4th in the world (in 2006).
★ 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.
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
★
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[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.
★
Moffat Tunnel in
Colorado straddles the
Continental Divide. The tunnel is 6.2 mi (10.0 km) long and at 9,239 feet (2,816 m) above sea level is the highest railroad tunnel in the
United States.
★ The
Fenghuoshan tunnel on
Qinghai-Tibet railway is the world's highest railway tunnel.
★ The
Houston Downtown Tunnel System is a system of tunnels about twenty feet below
Houston's downtown street system. The system forms a network of subterranean, climate-controlled, pedestrian walkways that link twenty-five full city blocks.
★ The
Sydney Harbour Tunnel in
Sydney,
Australia was built in 1992 to augment the
Sydney Harbour Bridge.
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.
Media
Natural tunnel
★
Punarjani Guha
'Snow tunnels' are created by voles, chipmunks and other rodents for protection and access to food sources. Larger versions are created by humans, usually for fun.
For more information regarding tunnels built by animals, see
Burrow
See also
★
List of tunnels
★
Undersea tunnel
★
Underground city
★
Urban exploration
★
Roof and tunnel hacking
★
Smuggling tunnel
★
List of rathole tunnels
★
Wind tunnel
★
World's longest tunnels
★
Megaprojects
★
Lava tube
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 Association
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