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 used in the past in the United States, but now refers to underground rapid transit
systems.
The longest canal tunnel is the Standedge Tunnel in the United Kingdom, over three
miles long.
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 walking across the
tracks. In the United Kingdom bridges are an equally popular for pedestrian access between two or more railway station platforms.
Geotechnical investigation
-
It is essential that any tunnel project starts with a comprehensive investigation of ground conditions. The results of the
investigation will allow proper choice of machinery and methods for excavation and ground support, and will reduce the risk of
encountering unforseen ground conditions. In the early stages, the horizontal and vertical alignment will be optimised to make
use of the best ground and water conditions.
In some cases, conventional desk and site studies will not produce sufficient information to assess, for example, the blocky
nature of rocks, the exact location of fault zones, or stand-up times of softer ground. This may be a particular concern in large
diameter tunnels. To overcome these problems, a pilot tunnel, or drift, may be driven ahead of the main drive. This smaller
diameter tunnel will be easier to support when unexpected conditions occur, and will be incorporated in the final tunnel.
Alternatively, horizontal boreholes may sometimes be used ahead of the advancing tunnel face.
Construction
Tunnels are dug in various types of materials, from soft clay to hard rock, and the method of excavation 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 due to over head pressure.
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. The
tunnel may be of insitu concrete, precast concrete, precast arches, corrugated steel arches and such, with brickwork used in
early days. The trench is then backfilled, with 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, followed by 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 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 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:
Choice of tunnels vs. bridges
For water crossings, a tunnel is generally more costly to construct than a bridge. Navigational considerations may limit the
use of high bridges or drawbridge spans 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
tunnels' favor. 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.
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, organizations 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 Canal Tunnel on the Thames and Severn Canal in England, dug through hills, which
opened in 1789, was 3.5 km long and allowed boat transport of coal and other goods. 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
-
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
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
Wikimedia Commons has media related to:
References
External links
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)