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A tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used construction techniques rather than traditional tunnel boring methods.

A tunnel may be for foot or vehicular road , for traffic, or for a . The central portions of a network are usually in tunnel. Some tunnels are aqueducts to supply water for consumption or for stations or are . are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.

Secret tunnels are built for military purposes, or by civilians for of , , or . Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely. Tunnels can be connected together in .


Terminology
A tunnel is relatively long and narrow; the length is often much greater than twice the , although similar shorter excavations can be constructed, such as cross passages between tunnels.

The definition of what constitutes a tunnel can vary widely from source to source. For example, the definition of a road tunnel in the is defined as "a subsurface highway structure enclosed for a length of or more." In the United States, the NFPA definition of a tunnel is "An underground structure with a design length greater than and a diameter greater than ."

In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called a subway, while an underground railway system is differently named in different cities, the "Underground" or the "Tube" in , the "" in Glasgow, and the "Metro" in Newcastle. The place where a road, railway, canal or watercourse passes under a footpath, cycleway, or another road or railway is most commonly called a or, if passing under a canal, an aqueduct. Where it is important to stress that it is passing underneath, it may be called an underpass, though the official term when passing under a railway is an . A longer underpass containing a road, canal or railway is normally called a "tunnel", whether or not it passes under another item of infrastructure. An underpass of any length under a river is also usually called a "tunnel", whatever mode of transport it is for.

In the US, the term "subway" means an underground rapid transit system, and the term pedestrian underpass is used for a passage beneath a barrier. Rail station platforms may be connected by pedestrian tunnels or .


History
Much of the early technology of tunneling evolved from and military engineering. The of the terms "mining" (for mineral extraction or for siege attacks), "military engineering", and "civil engineering" reveals these deep historic connections.


Antiquity and early middle ages
Predecessors of modern tunnels were to transport water for or drinking, and . The first are known from before 2000 B.C.


Geotechnical investigation and design
A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from and by other geophysical techniques. An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction.

Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel (or "drift tunnel") may be driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.

Other key geotechnical factors:

  • "Stand-up time" is the amount of time a newly excavated cavity can support itself without any added structures. Knowing this parameter allows the engineers to determine how far an excavation can proceed before support is needed, which in turn affects the speed, efficiency, and cost of construction. Generally, certain configurations of rock and clay will have the greatest stand-up time, while sand and fine soils will have a much lower stand-up time.
    (2019). 9781461380535, Kluwer Academic Publishers.
  • control is very important in tunnel construction. Water leaking into a tunnel or vertical shaft will greatly decrease stand-up time, causing the excavation to become unstable and risking collapse. The most common way to control groundwater is to install dewatering pipes into the ground and to simply pump the water out.Powers, P.J. (2007). Construction de-watering and groundwater control. Hoboken, NJ: John Wiley & Sons Inc. A very effective but expensive technology is , using pipes which are inserted into the ground surrounding the excavation, which are then cooled with special refrigerant fluids. This freezes the ground around each pipe until the whole space is surrounded with frozen soil, keeping water out until a permanent structure can be built.
  • Tunnel cross-sectional shape is also very important in determining stand-up time. If a tunnel excavation is wider than it is high, it will have a harder time supporting itself, decreasing its stand-up time. A square or rectangular excavation is more difficult to make self-supporting, because of a concentration of stress at the corners.


Choice of tunnels versus 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 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 and urban , this is a strong factor in favor of a tunnel. Boston's 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 under the at was chosen over a massively high bridge for defense reasons; it was feared that aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 under the Mersey. In Hampton Roads, Virginia, tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges might prevent vessels from leaving Naval Station Norfolk.

Water-crossing tunnels built instead of bridges include the and between and Manhattan in New York City; the Queens-Midtown Tunnel between Manhattan and the borough of on ; the Detroit-Windsor Tunnel between and Ontario; and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia; the 1934 road ; the Western Scheldt Tunnel, Zeeland, Netherlands; and the North Shore Connector tunnel in Pittsburgh, Pennsylvania.

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the ), 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 and the Chesapeake Bay Bridge-Tunnel in .

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can users, as happened at the Gotthard Road Tunnel in in 2001. One of the worst railway disasters ever, the Balvano train disaster, was caused by a train stalling in the Armi tunnel in in 1944, killing 426 passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.


Project planning and cost estimates
Government funds are often required for the creation of tunnels. When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use project management techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of bonds.

The costs and benefits for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the Port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project. Increased taxes to finance a large project may cause opposition.


Construction
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.

There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally a tube can be sunk into a body of water, which is called an immersed tunnel.


Cut-and-cover
Cut-and-cover is a simple method of construction for shallow tunnels where a is excavated and over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. 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 in it. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated.
  • Top-down method: Side support walls and capping beams are constructed from ground level by such methods as or contiguous bored piling. Then a shallow excavation allows making the tunnel roof of 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 then takes place under the permanent tunnel roof, and the base slab is constructed.

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 stations, such as Canary Wharf tube station 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 many traditional stations on London Underground, where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the London Underground network, the Metropolitan and District Railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam.

A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction. This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century.


Boring machines
Tunnel boring machines (TBMs) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as quick and cost effective alternative to laying surface rails and roads. Expensive of buildings and land, with potentially lengthy planning inquiries, is eliminated. Disadvantages of TBMs arise from their usually large size – the difficulty of transporting the large TBM to the site of tunnel construction, or (alternatively) the high cost of assembling the TBM on-site, often within the confines of the tunnel being constructed.

There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, the bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, allowing them to be used in difficult conditions below the . 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 over the older method of tunnelling 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 deep-sea divers.

In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of the Linth–Limmern Power Stations located south of Linthal in the canton of Glarus. The borehole has a diameter of . The four TBMs used for excavating the Gotthard Base Tunnel, in , had a diameter of about . A larger TBM was built to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the in the Netherlands, with a diameter of . This in turn was superseded by the Madrid M30 ringroad, , and the Chong Ming tunnels in , . All of these machines were built at least partly by . , the world's largest TBM is "Big Bertha", a diameter machine built by Hitachi Zosen Corporation, which is digging the Alaskan Way Viaduct replacement tunnel in Seattle, Washington (US).


Clay-kicking
Clay-kicking is a specialised method developed in the of digging tunnels in strong clay-based soil structures. Unlike previous manual methods of using which relied on the soil structure to be hard, clay-kicking was relatively silent and hence did not harm soft clay-based structures. The clay-kicker lies on a plank at a 45-degree angle away from the working face and inserts a tool with a cup-like rounded end with the feet. Turning the tool manually, the kicker extracts a section of soil, which is then placed on the waste extract.

Used in civil engineering, the method found favour in the renewal of Britain's ancient systems, by not having to remove all property or infrastructure to create a small tunnel system. During the First World War, the system was used by Royal Engineer tunnelling companies to put mines beneath the lines. The method was virtually silent and so not susceptible to listening methods of detection.


Shafts
A temporary access shaft is sometimes necessary during the excavation of a tunnel. They are usually circular and go straight down until they reach the level at which the tunnel is going to be built. A shaft normally has concrete walls and is usually built to be permanent. Once the access shafts are complete, TBMs are lowered to the bottom and excavation can start. Shafts are the main entrance in and out of the tunnel until the project is completed. If a tunnel is going to be long, multiple shafts at various locations may be bored so that entrance to the tunnel is closer to the unexcavated area.United States Army Corps of Engineers. (1978). Tunnels and shafts in rock. Washington, DC: Department of the Army.

Once construction is complete, construction access shafts are often used as ventilation shafts, and may also be used as emergency exits.


Sprayed concrete techniques
The New Austrian Tunnelling method (NATM)—also referred to as the Sequential Excavation Method (SEM)—was developed in the 1960s. The main idea of this method is to use the geological stress of the surrounding rock to stabilize the tunnel, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the supports. Based on measurements, an optimal cross section is computed. The excavation is protected by a layer of sprayed concrete, commonly referred to as . Other support measures can include steel arches, rockbolts and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibres being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's deformation.

By special monitoring the NATM method is flexible, even at surprising changes of the rock consistency during the tunneling work. The measured rock properties lead to appropriate for tunnel strengthening.


Pipe jacking
In , are used to push specially made pipes through the ground behind a TBM or shield. This method is commonly used to create tunnels under existing structures, such as roads or railways. Tunnels constructed by pipe jacking are normally small diameter bores with a maximum size of around .


Box jacking
Box jacking is similar to pipe jacking, but instead of jacking tubes, a box-shaped tunnel is used. Jacked boxes can be a much larger span than a pipe jack, with the span of some box jacks in excess of . A cutting head is normally used at the front of the box being jacked, and spoil removal is normally by excavator from within the box.Recent developments of the Jacked Arch and Jacked deck have enabled longer and larger structures to be installed to close accuracy. The 126m long 20m clear span underpass below the high speed rail lines at Cliffsend in Kent, UK is an example of this technique.


Underwater tunnels
There are also several approaches to underwater tunnels, the two most common being bored tunnels or , examples are Bjørvika Tunnel and . Submerged floating tunnels are a novel approach under consideration; however, no such tunnels have been constructed to date.


Temporary way
During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove , often so that it can be to allow the operation of empty and loaded trains at the same time. The temporary way is replaced by the at completion, thus explaining the term "Perway".


Enlargement
The vehicles or traffic using a tunnel can outgrow it, requiring replacement or enlargement:
  • The original single line near was replaced with a double-track tunnel, with the original tunnel used for growing mushrooms.
  • The 1832 double-track mile-long tunnel from Edge Hill to Lime Street in was near totally removed, apart from a 50-metre section at Edge Hill and a section nearer to Lime Street, as four tracks were required. The tunnel was dug out into a very deep four-track cutting, with short tunnels in places along the cutting. Train services were not interrupted as the work progressed. There are other occurrences of tunnels being replaced by open cuts, for example, the .
  • The in England was enlarged using a tunnel boring machine (TBM) in 2015.The Railway Magazine August 2015, p12. The Rhyndaston Tunnel was enlarged using a borrowed TBM so as to be able to take .
  • Tunnels can also be enlarged by lowering the floor.


Open building pit
An open building pit consists of a horizontal and a vertical boundary that keeps groundwater and soil out of the pit. There are several potential alternatives and combinations for (horizontal and vertical) building pit boundaries. The most important difference with cut-and-cover is that the open building pit is muted after tunnel construction; no roof is placed.


Other construction methods
  • Drilling and blasting
  • Hydraulic splitter
  • Slurry-shield machine
  • Wall-cover construction method.


Variant tunnel types

Double-deck and multipurpose tunnels
Some tunnels are double-deck, for example the two major segments of the San Francisco–Oakland Bay Bridge (completed in 1936) are linked by a double-deck tunnel section through Yerba Buena Island, the largest-diameter bored 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.

In Turkey, the under the , opened in 2016, has at its core a two-deck road tunnel with two lanes on each deck.
Additionally, in 2015 the Turkish government announced that it will build the world's first three-level tunnel, also under the Bosporus. The tunnel is intended to carry both the Istanbul metro and a two-level highway, over a length of .

The French A86 Duplex Tunnel in west Paris consists of two bored tunnel tubes, the eastern one of which has two levels for light motorized vehicles, over a length of . Although each level offers a physical height of , only traffic up to tall is allowed in this tunnel tube, and motorcyclists are directed to the other tube. Each level was built with a three-lane roadway, but only two lanes per level are used – the third serves as a hard shoulder within the tunnel. The A86 Duplex is Europe's longest double-deck tunnel.

In , China, a two-tube double-deck tunnel was built starting in 2002. In each tube of the Fuxing Road Tunnel both decks are for motor vehicles. In each direction, only cars and taxis travel on the high two-lane upper deck, and heavier vehicles, like trucks and buses, as well as cars, may use the high single-lane lower level.

In the Netherlands, a two-storey, eight lane, cut-and-cover road tunnel under the city of was opened in 2016. Each level accommodates a full height, two by two lane highway. The two lower tubes of the tunnel carry the A2 motorway, which originates in Amsterdam, through the city; and the two upper tubes take the N2 regional highway for local traffic. Unique stacked tunnel under Maastricht (archived) Traffic & Infra

The Alaskan Way Viaduct replacement tunnel is a bored road tunnel that is under construction since 2013 in the city of in the U.S. state of Washington. The tunnel will carry State Route 99 under from the SoDo neighborhood to South Lake Union in the north on two levels with two lanes each.

New York City's 63rd Street Tunnel under the , between the boroughs of and , was intended to carry subway trains on the upper level and Long Island Rail Road commuter trains on the lower level. Construction started in 1969, and the two sides of the tunnel were bored through in 1972. The upper level, used by the IND 63rd Street Line () of the New York City Subway, was not opened for passenger service until 1989. The lower level, intended for commuter rail, will not see passenger service until completion of the East Side Access project, expected in late 2022. MTA East Side Access program

In the UK, the 1934 under the between and was originally to have road vehicles running on the upper deck and trams on the lower. During construction the tram usage was cancelled. The lower section is only used for cables, pipes and emergency accident refuge enclosures.

Hong Kong's Lion Rock Tunnel, built in the mid 1960s, connecting and Sha Tin, carries a motorway but also serves as an aqueduct, featuring a gallery containing five water mains lines with diameters between 1.2m and 1.5m below the road section of the tunnel.

Some tunnels have more than one purpose. The in is the first multipurpose " Stormwater Management And Road Tunnel" in the world, created to convey both traffic and occasional waters in . When necessary, floodwater is first diverted into a separate bypass tunnel located underneath the double-deck roadway tunnel. In this scenario, traffic continues normally. Only during heavy, prolonged rains when the threat of extreme flooding is high, the upper tunnel tube is closed off to vehicles and automated flood control gates are opened so that water can be diverted through both tunnels.

Common utility ducts or utility tunnels carry two or more utility lines. Through co-location of different utilities in one tunnel, organizations are able to reduce the costs of building and maintaining utilities.


Covered passageways
Over-bridges can sometimes be built by covering a road or river or railway with brick or steel arches, and then leveling the surface with earth. In railway parlance, a surface-level track which has been built or covered over is normally called a "covered way".

are a kind of artificial tunnel built to protect a railway from of snow. Similarly the , New South Wales "steel tunnel", on the Illawarra railway line, protects the line from rockfalls.


Safety and security
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 even low concentrations of 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 death. In the Caldecott Tunnel fire of 1982, the majority of fatalities were caused by toxic smoke, rather than by the initial crash.

Motor vehicle tunnels usually require ventilation shafts and powered fans to remove toxic during routine operation.

Rail tunnels usually require fewer air changes per hour, but still may require forced-air ventilation. Both types of tunnels often have provisions to increase ventilation under emergency conditions, such as a fire. Although there is a risk of increasing the rate of combustion through increased airflow, the primary focus is on providing breathable air to persons trapped in the tunnel, as well as .

When there is a parallel, separate tunnel available, airtight but unlocked emergency doors are usually provided which allow trapped personnel to escape from a smoke-filled tunnel to the parallel tube.

Larger, heavily used tunnels, such as the tunnel in Boston, Massachusetts, may have a dedicated 24-hour manned operations center which monitors and reports on traffic conditions, and responds to emergencies. Video surveillance equipment is often used, and real-time pictures of traffic conditions for some highways may be viewable by the general public via the Internet.

Earthquakes are one of nature’s most formidable threats. A magnitude 6.7 earthquake shook the San Fernando valley in Los Angeles in 1994. The earthquake caused extensive damage to various structures including buildings, freeway overpasses and road systems throughout the area. The National Center for Environmental Information estimates total damages to be 40 billion dollars. According to an article issued by Steve Hymon of TheSource – Transportation News and Views, there was no serious damage sustained by the LA subway system. Metro, the owner of the LA subway system, issued a statement through their engineering staff about the design and consideration that goes into a tunnel system. Engineers and architects perform extensive analysis as to how hard they expect earthquakes to hit that area. All of this goes into the overall design and flexibility of the tunnel.

This same trend of limited subway damage following an earthquake can be seen in many other places. In 1985 a magnitude 8.1 earthquake shakes Mexico City; there is no damage to the subway system, and in fact the subway systems serve as a lifeline for emergency personnel and evacuations. A magnitude 7.2 rips through Kobe Japan in 1995, leaving no damage to the tunnels themselves. Entry portals sustained minor damages, however these damages were attributed to inadequate earthquake design that originated from the original construction date of 1965. In 2010 a magnitude 8.8, massive by any scale, absolutely annihilates Chile. Entrance stations to subway systems suffered minor damages, and the subway system was down for the rest of the day. By the next afternoon, the subway system was operational again.Hymon, Steve. "Designing A Subway to Withstand an Earthquake." The Source. N.p., 2017. Web. 11 November 2017. http://thesource.metro.net/2012/08/10/designing-a-subway-to-withstand-an-earthquake/


Examples

In history
The history of ancient tunnels and tunneling in the world is reviewed in various sources which include many examples of these structures that were built for different purposes.Klaus Grewe, 1998, Licht am Ende des Tunnels – Planung und Trassierung im antiken Tunnelbau, Verlag Philipp von Zabern, Mainz am Rhein.Siamak Hashemi, 2013, The Magnificence of Civilization in Depths of Ground (A Review of Underground Structures in Iran – Past to Present), Shadrang Printing and Publishing Co., Tehran. Some well known ancient and modern tunnels are briefly introduced below:
  • The or of are water management systems used to provide a reliable supply of water to human settlements or for irrigation in hot, arid and semi-arid climates. The deepest known qanat is in the Iranian city of , 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 .UNESCO World Heritage Centre – World Heritage List: Qanats of Gonabad, Date of Inscription 2007, Reference No. 5207, At:
  • The was built before 701 for a reliable supply of water, to withstand attacks.
  • The Eupalinian aqueduct on the island of (, ) was built in 520 BCE by the ancient Greek engineer of under a contract with the local community. 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 , the ancient capital of . 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 and visitors can still enter it.
  • One of the first known drainage and sewage networks in form of was constructed at Persepolis in Iran at the same time as the construction of its foundation in 518 BCE. In most places the network was dug in the sound rock of the mountain and then covered by large pieces of rock and stone followed by earth and piles of rubble to level the ground. During investigations and surveys, long sections of similar rock tunnels extending beneath the palace area were traced by Herzfeld and later by Schmidt and their archeological teams.Schmidt, E.F., 1953, Persepolis I – Structures, Reliefs, Inscriptions; The University of Chicago Oriental Institute Publications, Volume LXVIII, The University of Chicago Press.
  • The , an important , penetrated the pass in the through a tunnel which emperor had ordered built in 76–77 CE. A modern road, the SS 3 Flaminia, still uses this tunnel, which had a precursor dating back to the 3rd century BCE; remnants of this earlier tunnel (one of the first road tunnels) are also still visible.
  • The world's oldest tunnel traversing under a water body is claimedBlogcu to be the Terelek kaya tüneli under Kızıl River, a little south of the towns of and Durağan in , just downstream from where joins its tributary Gökırmak. The tunnel is presently under a narrow part of a lake formed by a dam some kilometers further downstream. Estimated to have been built more than 2000 years ago, possibly by the same civilization that also built the royal tombs in a rock face nearby, it is assumed to have had a defensive purpose.
  • Sapperton Canal Tunnel on the Thames and Severn Canal in , dug through hills, which opened in 1789, was long and allowed transport of and other goods. Above it the Sapperton Long Tunnel was constructed which carries the "Golden Valley" railway line between and .
  • The 1791 is on the , in , . The tunnel is long. Closed in 1962 the tunnel was reopened in 1973. The series of tunnels was extended in 1984 and 1989. Map of Dudley Canals | Discover Black Country Canals
  • , constructed in 1793 in Derbyshire by the Butterley Company to transport limestone to its ironworks factory. The Butterley company engineered and built its own railway a victim of the depression the company closed after 219 years in 2009. The tunnel is the world's oldest railway tunnel traversed by rail wagons using gravity and horse haulage. The railway was converted to steam locomotion in 1813 using a Steam Horse locomotive engineered and built by the Butterley company, however reverted to horses. Steam trains used the tunnel continuously from the 1840s when the railway was converted to a narrow gauge. The line closed in 1933. In the Second World War, the tunnel was used as an air raid shelter. Sealed up in 1977 it was rediscovered in 2013 and inspected. The tunnel was resealed to preserved the construction as it was designated an ancient monument.
  • The 1794 canal tunnel is 3,083 yards (2,819m) in length on the in Ripley, Derbyshire, England. The tunnel was built simultaneously with the 1773 . The tunnel partially collapsed in 1900 splitting the Cromford Canal, and has not been used since. The Friends of Cromford Canal, a group of volunteers, are working at fully restoring the Cromford Canal and the Butterley Tunnel. Friends of the Cromford Canal – HOME
  • The 1796 Stoddart Tunnel in Chapel-en-le-Frith in is reputed to be the oldest rail tunnel in the world. The rail wagons were originally horse-drawn.
  • Derby Tunnels in Salem, Massachusetts were built in 1801 to smuggle imports affected by President 's new customs duties. Jefferson had ordered local militias to help the Custom House in each port collect these dues, but the smugglers, led by Elias Derby, hired the Salem militia to dig the tunnels and hide the spoil. The tunnels ran 3 miles connecting the wharfs in town to an underground train station. Along the way they connected prominent businessmen and politicians homes to their stores, bank, and museums. Members of the Salem Commons Fund hid the tunnels behind a project to fill in the ponds and grade the local common. Tunnel dirt was hidden in those ponds and was used to fill in rivers to create new wharfs to connect the tunnels to. Many politicians were involved including a Superior Court Justice, a Secretary of the Navy, and many Senators in the Federalist Party.
  • A tunnel was created for the first true steam locomotive, from to . The Penydarren locomotive was built by Richard Trevithick. The locomotive made the historic journey from Penydarren to Abercynon in 1804. Part of this tunnel can still be seen at , , . This is arguably the oldest railway tunnel in the world, dedicated only to self-propelled steam engines on rails.
  • The Montgomery Bell Tunnel in Tennessee, an water diversion tunnel, , to power a water wheel, was built by slave labour in 1819, being the first full-scale tunnel in North America.
  • Bourne's Tunnel, , near , England. long. Built in the late 1820s, the exact date is unknown, however probably built in 1828 or 1829. This is the first tunnel in the world constructed under a railway line. The construction of the Liverpool to Manchester Railway ran over a horse-drawn tramway that ran from the Sutton collieries to the Liverpool-Warrington turnpike road. A tunnel was bored under the railway for the tramway. As the railway was being constructed the tunnel was made operational, opening prior to the Liverpool tunnels on the Liverpool to Manchester line. The tunnel was made redundant in 1844 when the tramway was dismantled.
  • Crown Street station, , England, 1829. Built by George Stephenson, a single track railway tunnel , was bored from Edge Hill to Crown Street to serve the world's first intercity passenger railway terminus 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 passenger rail tunnel running under streets in the world.
  • The 1829 in Liverpool, England at long on a twin track railway, was the first rail tunnel bored under a metropolis. The tunnel's path is from Edge Hill in the east of the city to in the south end Liverpool docks. The tunnel was used only for freight terminating at the Park Lane goods terminal. Currently disused since 1972, the tunnel was to be a part of the metro network, with work started and abandoned because of costs. The tunnel is in excellent condition and is still being considered for reuse by Merseyrail, maybe with an underground station cut into the tunnel for Liverpool university. The river portal is opposite the new King's Dock Liverpool Arena being an ideal location for a serving station. If reused the tunnel will be the oldest used underground rail tunnel in the world and oldest section of any underground metro system.
  • 1832, Lime Street railway station tunnel, Liverpool. A two track rail tunnel, long was bored under the metropolis from Edge Hill in the east of the city to Lime Street in Liverpool's city centre. The tunnel was in use from 1832 being used to transport building materials to the new Lime St station while under construction. The station and tunnel was opened to passengers in 1836. In the 1880s the tunnel was converted to a deep cutting, open to the atmosphere, being four tracks wide. This is the only occurrence of a major tunnel being removed. Two short sections of the original tunnel still exist at Edge Hill station and further towards Lime Street, giving the two tunnels the distinction of being the oldest rail tunnels in the world still in use, and the oldest in use under streets. Liverpool Lime St Over time a section of the deep cutting has been converted back into tunnel due to sections having buildings built over.
  • in England, which opened in 1841, was the longest railway tunnel in the world at the time of construction. It was dug by hand, and has a length of .
  • The 1842 Prince of Wales Tunnel, in near Darlington, England, is the oldest sizeable tunnel in the world still in use under a settlement.
  • The Victoria Tunnel Newcastle opened in 1842, is a 2.4 mile subterranean wagonway with a maximum depth of that drops from entrance to exit. The tunnel runs under Newcastle upon Tyne, England, and originally exited at the River Tyne. It remains largely intact. Originally designed to carry coal from to the river, in WW2 part of the tunnel was used as a shelter. Under the management of a charitable foundation called the Ouseburn Trust it is currently used for heritage tours.

  • The , built by Marc Isambard Brunel and his son Isambard Kingdom Brunel opened in 1843, was the first tunnel (after Terelek) traversing under a water body, and the first to be built using a tunnelling shield. Originally used as a foot-tunnel, the tunnel was converted to a railway tunnel in 1869 and was a part of the East London Line of the London Underground until 2007. It was the oldest section of the network, although not the oldest purpose built rail section. From 2010 the tunnel became a part of the London Overground network.
  • The Victoria Tunnel/ in , England, was bored under a metropolis opening in 1848. The tunnel was initially used only for rail freight serving the Waterloo Freight terminal, and later freight and passengers serving the Liverpool ship liner terminal. The tunnel's path is from Edge Hill in the east of the city to the north end Liverpool docks at Waterloo Dock. The tunnel is split into two tunnels with a short open air cutting linking the two. The cutting is where the cable hauled trains from Edge Hill were hitched and unhitched. The two tunnels are effectively one on the same centre line and are regarded as one. However, as initially the long Victoria section was originally cable hauled and the shorter Waterloo section was locomotive hauled, two separate names were given, the short section was named the . In 1895 the two tunnels were converted to locomotive haulage. Used until 1972, the tunnel is still in excellent condition. A short section of the Victoria tunnel at Edge Hill is still used for shunting trains. The tunnel is being considered for reuse by the network. Stations cut into the tunnel are being considered and also reuse by a monorail system from the proposed redevelopment of Liverpool's Central Docks has been proposed.
  • The vertex tunnel of the Semmering railway, the first Alpine tunnel, was opened in 1848 and was long. It connected rail traffic between , the capital of Austro-Hungarian Empire, and , its port.
  • The Giovi Rail Tunnel through the Mounts opened in 1854, linking the capital city of the Kingdom of Sardinia, , to its port, . The tunnel was long.
  • The oldest underground sections of the London Underground were built using the cut-and-cover method in the 1860s, and opened in January 1863. What are now the Metropolitan, Hammersmith & City and Circle lines were the first to prove the success of a or subway system.
  • On 18 June 1868, the Central Pacific Railroad's 1,659-foot (506 m) Summit Tunnel (Tunnel #6) at in the California Sierra Nevada mountains was opened, permitting the establishment of the commercial mass transportation of passengers and freight over the Sierras for the first time. It remained in daily use until 1993, when the Southern Pacific Railroad closed it and transferred all rail traffic through the 10,322-foot (3,146 m) long Tunnel #41 (a.k.a. "The Big Hole") built a mile to the south in 1925.
  • In 1870, after fourteen years of works, the Fréjus Rail Tunnel was completed between France and Italy, being the second-oldest Alpine tunnel, long. At that time it was the longest in the world.
  • The third Alpine tunnel, the Gotthard Rail Tunnel, between northern and southern Switzerland, opened in 1882 and was the longest rail tunnel in the world, measuring .
  • The 1882 Col de Tende Road Tunnel, at long, was one of the first long road tunnels under a pass, running between France and Italy.
  • As the last bit is drilled, on 26 October 2017, becomes the longest undersea road tunnel with its 14.3 km length surpassing that of the tunnel under Tokyo Bay, Japan (9,583 m.), and previously the Shanghai Yangtze River Tunnel (8,950 m.). The tunnel is projected to open for use in 2019.
  • The tunnel opened in 1886, running from Liverpool to Birkenhead under the River Mersey. The was the world's first deep-level underground railway. By 1892 the extensions on land from Birkenhead Park station to Liverpool Central Low level station gave a tunnel in length. The under river section is in length, and was the longest underwater tunnel in world in January 1886. Engineering Timelines – Mersey Railway
  • The rail was opened in late 1886, at long, although only of the tunnel is actually under the River Severn. The tunnel replaced the Mersey Railway tunnel's longest under water record, which was held for less than a year.
  • , 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.
  • Built in sections between 1890 and 1939, the section of London Underground's from Morden to East Finchley via Bank was the longest railway tunnel in the world at in length.
  • St. Clair Tunnel, also opened later in 1890, linked the elements of the Greathead tunnels on a larger scale.
  • In 1906 the fourth Alpine tunnel opened, the , between Switzerland and Italy. It is long, and was the longest tunnel in the world until 1982. It was also the deepest tunnel in the world, with a maximum rock overlay of approximately .
  • The 1927 was the first underwater tunnel designed for automobiles. The construction required a novel ventilation system.
  • In 1945 the Delaware Aqueduct tunnel was completed, supplying water to New York City in the US. At it is the longest tunnel in the world.
  • In 1988 the long in Japan was completed under the , linking the islands of and . It was longest railway tunnel in the world at that time.


Longest
  • The Thirlmere Aqueduct in North West England, is sometimes considered the longest tunnel, of any type, in the world at , though the aqueduct's tunnel section is not continuous.
  • The Dahuofang Water Tunnel in , opened in 2009, is the third longest water tunnel in the world at length.
  • The Gotthard Base Tunnel in , opened in 2016, is the longest and deepest railway tunnel in the world at length and maximum depth below the Gotthard Massif. It provides a between the North and South of Europe under the , at a maximum elevation of .
  • The in connects the main island of with the northern island of by rail. It is long, of which are crossing the undersea.
  • The crosses the between and the . It has a total length of , of which are the world's longest undersea tunnel section.
  • The Lötschberg Base Tunnel in was the longest land rail tunnel, with a length of , from its inauguration in 2007 until the completion of the Gotthard Base Tunnel in 2016.
  • The Lærdal Tunnel in Norway from Lærdal to is the world's longest road tunnel, intended for cars and similar vehicles, at .
  • The Zhongnanshan Tunnel in People's Republic of China opened in January 2007 is the world's second longest tunnel and the longest mountain road tunnel in Asia, at .

  • The longest canal tunnel is the in France, over long.


Notable
  • Williamson's tunnels in , from 1804 and completed around 1840 by a wealthy eccentric, are probably the largest underground in the world. The tunnels were built with no functional purpose.
  • , opened in 1928 in , straddles the Continental Divide. The tunnel is long and at above sea level is the highest active railroad tunnel in the US (the Tennessee Pass Line, currently inactive, and are higher).
  • The Pennsylvania Turnpike opened in 1940 with seven tunnels, most of which were bored as part of the stillborn South Pennsylvania Railroad and giving the highway the nickname "Tunnel Highway". Four of the tunnels (Allegheny Mountain, Tuscarora Mountain, Kittatinny Mountain, and Blue Mountain) remain in active use, while the other three (Laurel Hill, Rays Hill, and Sideling Hill) were bypassed in the 1960s; the latter two tunnels are on a bypassed section of the Turnpike now commonly known as the Abandoned Pennsylvania Turnpike.
  • The was opened in 1966, in Stockholm, Sweden, and the New Elbe road tunnel opened in 1975 in Hamburg, Germany. Both tunnels handle around 150,000 vehicles a day, making them two of the most trafficked tunnels in the world.
  • The Honningsvåg Tunnel ( long) opened in 1999 on European route E69 in Norway as the world's northernmost road tunnel, except for mines (which exist on ).
  • The Central Artery road tunnel in Boston, Massachusetts, is a part of the larger Big Dig completed around 2007, and carries approximately 200,000 vehicles/day under the city along Interstate 93, US Route 1, and Massachusetts Route 3, which share a concurrency through the tunnels. The Big Dig replaced Boston's old badly deteriorated I-93 elevated highway.
  • The Stormwater Management And Road Tunnel or , is a combined storm drainage and road structure opened in 2007 in , . The tunnel is the longest stormwater in South East Asia and second longest in Asia. The facility can be operated as a simultaneous traffic and stormwater passage, or dedicated exclusively to stormwater when necessary.
  • The on national road Rv 653 in Norway is the world's deepest subsea road tunnel, measuring long, with deepest point at below the sea level, opened in February 2008.
  • Gerrards Cross railway tunnel, in England, opened in 2010, is notable in that it was built in a railway cutting, that was first opened around 1906. The tunnel was built using the cut-and-cover method with prefabricated forms in order to keep the busy railway operating. A branch of the supermarket chain occupies the space above the railway tunnel with an adjacent railway station. During construction, a portion of the tunnel collapsed when the soil cover was added. The prefabricated forms were covered with a layer of reinforced concrete after the collapse.
  • The Fenghuoshan tunnel, completed in 2005 on the Qinghai-Tibet railway is the world's highest railway tunnel, about above sea level and long.
  • The La Linea Tunnel in , 2016, is the longest, , mountain tunnel in South America. It crosses beneath a mountain at above sea level with six traffic lanes, and it has a parallel emergency tunnel. The tunnel is subject to serious pressure. The tunnel will link Bogotá and its urban area with the coffee-growing region, and with the main port on the Colombian Pacific coast.
  • The Chicago Deep Tunnel Project is a network of of designed to reduce flooding in the Chicago area. Started in the mid-1970s, the project is due to be completed in 2019.
  • New York City Water Tunnel No. 3, started in 1970, has an expected completion date of 2020, and will measure more than .


Mining
The use of tunnels for mining is called .


Military use
Some tunnels are not for transport at all but rather, are fortifications, for example and Cheyenne Mountain Complex. Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with , or civilian responses to threat of attack. Another use for tunnels was for the storage of chemical weapons Author lifts lid on chemical wartime history – Local News – News – General – Blue Mountains Gazette [9].


Secret tunnels
have given entrance to or escape from an area, such as the Cu Chi Tunnels or the smuggling tunnels in the Gaza Strip which connect it to . Although the Underground Railroad network used to transport escaped slaves was "underground" mostly in the sense of secrecy, hidden tunnels were occasionally used. Secret tunnels were also used during the , under the and elsewhere, to smuggle refugees, and for .

use secret tunnels to transport or store , such as and . Elaborately engineered tunnels built to smuggle drugs across the Mexico-US border were estimated to require up to 9 months to complete, and an expenditure of up to $1 million. Some of these tunnels were equipped with lighting, ventilation, telephones, drainage pumps, hydraulic elevators, and in at least one instance, an electrified rail transport system. Secret tunnels have also been used by thieves to break into bank vaults and retail stores after hours. Several tunnels have been discovered by the Border Security Forces across the Line of Control along the India-Pakistan border, mainly to allow terrorists access to the Indian state of Jammu and Kashmir.

The actual usage of tunnels is unknown but theories connect it to a rebirth ritual.


Natural tunnels
  • are emptied lava conduits, formed during volcanic eruptions by flowing and cooling lava.
  • Natural Tunnel State Park (Virginia, US) features an natural tunnel, really a limestone cave, that has been used as a railroad tunnel since 1890.
  • in 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 allow one to attain rebirth. Only men are permitted to crawl through the tunnel.
  • , a Norwegian island with a hat-shaped silhouette, has a natural tunnel in the middle of the hat, letting light come through. The long, high, and wide tunnel is said to be the hole made by an arrow of the angry Hestmannen, the hill being the hat of the troll-king of Sømna trying to save the beautiful Lekamøya. The tunnel is thought actually to be the work of ice. The sun shines through the tunnel during two few minutes long periods every year.The hobby photograph has waited three years for this rare shot of the sun shining through Torghatten mount
  • Small "snow tunnels" are created by , and other for protection and access to food sources. For more information regarding tunnels built by animals, see .


Major accidents
  • Clayton Tunnel rail crash (1861) – confusion about block signals
  • Welwyn Tunnel rail crash (1866) – train failed in tunnel, guard did not protect train
  • Balvano train disaster (1944) –
  • Caldecott Tunnel fire (1982) – major motor vehicle tunnel crash and fire
  • 1996 Channel Tunnel fire (1996) –
  • Mont Blanc Tunnel fire (1999) –
  • Princess Diana's death – Car crash in Pont D'alma tunnel, Paris. Princess Diana died here.


See also


Bibliography


External links

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