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A tunnel is an underground passageway. 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. Some civic planners define a tunnel as in length or longer, while anything shorter than this should be called an underpass or a chute. For example, the underpass beneath Yahata Station in Kitakyushu, Japanmarker is only long and therefore should not be considered a tunnel.

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 Tunnelsmarker or the tunnels connecting the Gaza Stripmarker to Egyptmarker. Some tunnels are not for transport at all but are fortifications, for example Mittelwerk and Cheyenne Mountainmarker.

In the United Kingdommarker a pedestrian tunnel or other underpass beneath a road is called a subway. This term was used in the past in the United Statesmarker, but now refers to underground rapid transit systems.

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. Rail stations with much traffic usually provide pedestrian tunnels from one platform to another, though others use bridges.

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 unforeseen ground conditions. In the early stages, the horizontal and vertical alignment will be optimized 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. A strong overhead support system is required to carry the load of the covering material, roads, streets or other transportation systems.

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 in situ 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 in situ 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.

Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube stationmarker in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral, owing to the sheer size of the excavation. This contrasts with most traditional stations on London Underground, where bored tunnels were used for stations and passenger access.

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 of TBMs, bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, 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 largest TBM built was used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. It had a diameter of 14.87 m.

Nowadays even larger machines exist, for example the machines used for the Madrid M30 ringroad, Spainmarker, and the Chong Ming tunnels in Shanghai, Chinamarker. The TBM used for digging the 57 km Gotthard Base Tunnel in Switzerlandmarker has a diameter of about 9 m. All of these machines were built at least partly 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, the two most common being bored tunnels or immersed tubes. Submerged floating tunnels are another approach that has not been constructed.

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 Manhattanmarker and urban Hong Kongmarker, this is a strong factor in tunnels' favor. Boston's Big Digmarker project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.

The 1934 Queensway Road Tunnel under the River Merseymarker at Liverpool, was chosen over a massively high bridge for defence reasons. It was feared aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships navigate under was considered higher than a tunnel. Similar conclusions were met for the 1971 Kingsway Tunnel under the River Mersey.

Examples of water-crossing tunnels built instead of bridges include the Holland Tunnelmarker and Lincoln Tunnel between New Jerseymarker and Manhattan in New York Citymarker, and the Elizabeth River tunnels between Norfolkmarker and Portsmouth, Virginiamarker, the 1934 River Merseymarker road Queensway Tunnelmarker and the Westerschelde tunnelmarker, 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 Tunnelmarker), 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 linkmarker and the Chesapeake Bay Bridge-Tunnel in the eastern United Statesmarker.

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in Switzerlandmarker in 2001. One of the worst railway disasters ever, the Balvano train disaster, was caused by a train stalling in the Armi tunnel in Italymarker in 1944, killing 426 passengers.

Variant tunnel types

Double-deck tunnel

Some tunnels are double-deck, for example the two major segments of the San Francisco – Oakland Bay Bridgemarker (completed in 1936) are linked by a double-deck tunnel, once the largest diameter tunnel in the world. At construction this was a combination bidirectional rail and truck pathway on the lower deck with automobiles above, now converted to one-way road vehicle traffic on each deck.

A recent double-decker tunnel with both decks for motor vehicles is the Fuxing Road Tunnel in Shanghai, China. Cars travel on the two-lane upper deck and heavier vehicles on the single-lane lower.

Multipurpose tunnel are tunnels that have more than one purpose. The SMART Tunnel in Malaysiamarker is the first multipurpose tunnel in the world, as it is used both to control traffic and flood in Kuala Lumpurmarker.

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.

Snow sheds are a kind of artificial tunnel built to protect a railway from avalanches of snow. Similarly the Stanwell Parkmarker, New South Walesmarker steel tunnel, on the South Coast railway line, 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.

Hazards

Owing to the enclosed space of a tunnel, fires can have very serious effects on users. The main dangers are gas and smoke production, with low concentrations of carbon monoxide being highly toxic. Fires killed 11 people in the Gotthard tunnel fire of 2001 for example, all of the victims succumbing to smoke and gas inhalation. Over 400 passengers died in the Balvano train disaster in Italy in 1944, when the locomotive halted in a long tunnel. Carbon monoxide poisoning was the main cause of the horrifying death rate. Fires have also occurred in the Channel Tunnelmarker, leading to great delays for users.

Examples of tunnels

In history

  • World's oldest underwater tunnel is rumored to be the Terelek kaya tüneli under Kızıl Rivermarker, a little south of the towns of Boyabatmarker and Duragan in Turkeymarker. Estimated to have been built more than 2000 years ago (possibly 5000) it is assumed to have had a defence purpose.
  • The qanat or kareez of Persiamarker 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 and its length is .


  • The Eupalinian aqueductmarker on the island of Samosmarker (North Aegean, Greecemarker). Built in 520 BC by the ancient Greek engineer Eupalinos of Megaramarker. Eupalinos organised the work so that the tunnel was begun from both sides of mount Kastro. The two teams advanced simultaneously and met in the middle with excellent accuracy, something that was extremely difficult in that time. The aqueduct was of utmost defensive importance, since it ran underground and it was not easily found by an enemy who could otherwise cut off the water supply to Pythagoreionmarker, the ancient capital of Samosmarker. 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 long and visitors can still enter it Eupalinos tunnel.
  • The Via Flaminia, an important Roman road, penetrated the Furlomarker pass in the Apenninesmarker through a tunnel which emperor Vespasian had ordered built in 76-77. A modern road, the SS 3 Flaminia, still uses this tunnel, which had an precursor dating back to the 3rd century BC; remnants of this earlier tunnel (one of the first road tunnels) are also still visible.
  • Sapperton Canal Tunnelmarker on the Thames and Severn Canal in Englandmarker, dug through hills, which opened in 1789, was long and allowed boat transport of coal and other goods. Above it runs the Sapperton Long Tunnelmarker which carries the "Golden Valley" railway line between Swindonmarker and Gloucestermarker.
  • The 1796 Stoddart Tunnel in Chapel-en-le-Frith in Derbyshire is reputed to be the oldest rail tunnel in the world. Rail wagons were horse drawn.
  • The tunnel created for the first true steam locomotive, the Penydarrenmarker locomotive, built by Richard Trevithick. The locomotive made an historic journey from Penydarrenmarker to Abercynonmarker in 1804. Part of this tunnel can still be seen at Pentrebach, Merthyr Tydfilmarker. This is arguably the oldest railway tunnel in the world, for self-propelled steam engines on rails.
  • The Montgomery Bell Tunnelmarker in Tennessee, a 290 foot (88 metres) long, 15 foot wide by 8 foot high water diversion tunnel to power a water wheel, was built by slave labor in 1819, being the first full-scale tunnel in North America.
  • Crown Street Stationmarker, Liverpoolmarker, 1829. Built by George Stephenson, a single track tunnel 291 yards long was bored from Edge Hill to Crown Street to serve the world's first passenger railway station. The station was abandoned in 1836 being too far from Liverpool city centre, with the area converted for freight use. Closed down in 1972, the tunnel is disused. However it is the oldest rail tunnel running under streets in the world. [24312]
  • The 1.26 mile (2.03 km) 1830 Wapping Tunnelmarker in Liverpool, England, was the first rail tunnel bored under a metropolis. Currently disused since 1972. Having two tracks, the tunnel runs from Edge Hill in the east of the city to the south end Liverpool docks being used only for freight. The tunnel is still in excellent condition and is being considered for reuse by Merseyrail rapid transit rail system, with maybe an underground station cut into the tunnel. The river portal is opposite the new Liverpool Arena being ideal for a serving station. If reused it will be the oldest used underground rail tunnel in the world and oldest part of any underground metro system.
  • 1836, Lime St Station tunnel, Liverpool. A two track rail tunnel, 1.13 miles (1,811 m) long was bored under a metropolis from Edge Hill in the east of the city to Lime Street. In the 1880s the tunnel was converted to a deep cutting four tracks wide. The only occurrence of a tunnel being removed. A very short section of the original tunnel still exists at Edge Hill station making this the oldest rail tunnel in the world still in use, and the oldest in use under a street, albeit only one street and one building.
  • Box Tunnelmarker 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 .
  • The 0.75 mile long 1842 Shildon tunnel near Darlington, England, is the oldest sizable tunnel in the world still in use under a settlement.
  • The Thames Tunnelmarker, 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 was a part of the East London Line of the London Underground until 2007, being the oldest section of the system. From 2010 the tunnel becomes a part of the London Overground system.
  • The 2.07 miles (3.34 km) Waterloo Tunnelmarker in Liverpoolmarker, opened in 1849, was bored under a metropolis. Initially used only for rail freight and later freight and passengers serving the Liverpool ship liner terminal, the tunnel runs from Edge Hill in the east of the city to the north end Liverpool docks. Used until 1972 it is still in excellent condition, being considered for reuse by the Merseyrail rapid transit rail system. Stations being cut into the tunnel are being considered. Also, reuse by a monorail system from the proposed Liverpool Watersmarker redevelopment of Liverpool's Central Docks has been proposed.
  • 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. Dating from 1863, Baker Street stationmarker is the oldest underground station in the world.
  • The 1882 Col de Tende Road Tunnelmarker, at 3182 metres long, was one of the first long road tunnels under a pass, running between France and Italy.
  • The Mersey Railway tunnel opened in 1886 running from Liverpool to Birkenhead under the River Mersey. By 1892 the extensions on land from Birkenhead Park station to Liverpool Central Low level station gave a tunnel 3.12 miles (5029 m) in length. The under river section is 0.75 miles in length, being the longest underwater tunnel in world in January 1886.
  • The rail Severn Tunnel was opened in late 1886, at 4 miles 624 yd (7,008 m) long, although only 2¼ miles (3.62 km) of the tunnel is actually under the river. The tunnel replaced the Mersey Railway tunnel's longest under water record, which it held for less than a year.
  • James Greathead, in constructing the City & South London Railway tunnel beneath the Thames, opened in 1890, brought together three key elements of tunnel construction under water: 1) shield method of excavation; 2) permanent cast iron tunnel lining; 3) construction in a compressed air environment to inhibit water flowing through soft ground material into the tunnel heading.
  • St. Clair Tunnelmarker, also opened later in 1890, linked the elements of the Greathead tunnels on a larger scale.
  • The 1927 Holland Tunnelmarker was the first underwater tunnel designed for automobiles. This fact required a novel ventilation system.
See also the History of rapid transit.

Longest





Notable



Multiple headings

Long tunnels get increasingly slow to build as they get longer. A solution if the geography suits is to have intermediate access shaft so that excavation can take place on several faces. Surveyors have a good record of making the separate sections meet with errors in alignment of just a few inches. An exception would be one of the hundred tunnels on the Kalka-Shimla Railway.

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. The use of tunnels for mining is called drift mining. One of the strangest uses of a tunnel was for the storage of chemical weapons [24314].

Natural tunnel

  • Natural Tunnel State Parkmarker (Virginia, USA) features an natural tunnel, really a limestone cave, that has been used as a railroad tunnel since 1890.
  • Punarjani Guha Kerala, India. Hindus believe that crawling through the tunnel (which they believe was created by a Hindu god) from one end to the other will wash away all of one’s sins and thus attain rebirth, although only men are permitted to crawl through the cave.


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

Temporary Way

During construction of a tunnel it is often convenient to install a temporary railway particularly to remove spoil. This temporary railway is often narrow gauge so that it can be double track, which facilitates the operation of empty and loaded trains at the same time. The temporary way is replaced by the permanent way at completion, thus explaining the term Perway.

Enlargement

The vehicles using a tunnel can outgrow it, requiring replacement or enlargement. The original single line Gib Tunnel near Mittagongmarker was replaced with a double line tunnel, with the original tunnel used for growing mushrooms. The Rhyndaston Tunnel was enlarged using a borrowed Tunnel Boring Machine so as to be able to take ISO containers.

The 1836 Lime Street two track 1 mile tunnel from Edge Hill to Lime Street in Liverpool was totally removed, apart from a short 50 metre section at Edge Hill. Four tracks were required. The tunnel was converted into a very deep 4 track open cutting. However, short larger 4 track tunnels were left in some parts of the run. Train services were not interrupted as the work progressed. Photos of the work in progress: [24315] [24316]There are other occurrences of tunnels being replaced by open cuts, for example, the Auburn Tunnelmarker.

Accidents



References

Bibliography

  • Railway Tunnels in Queensland by Brian Webber, 1997, ISBN 0909937338


External links




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