Tunnel
Overview
 
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.

A tunnel may be for foot or vehicular road traffic
Traffic
Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...

, for rail
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...

 traffic, or for a canal
Canal
Canals are man-made channels for water. There are two types of canal:#Waterways: navigable transportation canals used for carrying ships and boats shipping goods and conveying people, further subdivided into two kinds:...

. Some tunnels are aqueduct
Aqueduct
An aqueduct is a water supply or navigable channel constructed to convey water. In modern engineering, the term is used for any system of pipes, ditches, canals, tunnels, and other structures used for this purpose....

s to supply water for consumption or for hydroelectric stations or are sewers
Sanitary sewer
A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...

. Other uses include routing power or telecommunication cables, some are to permit wildlife such as European badgers to cross highways.
Encyclopedia
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.

A tunnel may be for foot or vehicular road traffic
Traffic
Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...

, for rail
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...

 traffic, or for a canal
Canal
Canals are man-made channels for water. There are two types of canal:#Waterways: navigable transportation canals used for carrying ships and boats shipping goods and conveying people, further subdivided into two kinds:...

. Some tunnels are aqueduct
Aqueduct
An aqueduct is a water supply or navigable channel constructed to convey water. In modern engineering, the term is used for any system of pipes, ditches, canals, tunnels, and other structures used for this purpose....

s to supply water for consumption or for hydroelectric stations or are sewers
Sanitary sewer
A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...

. Other uses include routing power or telecommunication cables, some are to permit wildlife such as European badgers to cross highways. Secret tunnels
Secret passage
Secret passages, also commonly referred to as hidden passages or secret tunnels, are hidden routes used for stealthy travel. Such passageways are sometimes inside buildings leading to secret rooms. Others allow occupants to enter or exit buildings without being seen...

 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 Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

. Some tunnels are not for transport at all but rather, are fortifications, for example Mittelwerk
Mittelwerk
Central Works was a World War II factory that used Mittelbau-Dora forced labor in 2 main tunnels in the Kohnstein. The underground facility produced V-2 rockets, V-1 flying bombs, and other Nazi weapons.-Mittelwerk GmbH:...

 and Cheyenne Mountain
Cheyenne Mountain
Cheyenne Mountain is a mountain located just outside the southwest side of Colorado Springs, Colorado, U.S., and is home to the Cheyenne Mountain Air Force Station and its Cheyenne Mountain Directorate, formerly known as the Cheyenne Mountain Operations Center .Throughout the Cold War and...

.

In the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, a pedestrian tunnel or other underpass beneath a road is called a underpass subway
Subway (underpass)
In England and Wales, the Republic of Ireland, Hong Kong and some Commonwealth countries , the term subway normally refers to a specially constructed underpass for pedestrians and/or cyclists beneath a road or railway, allowing them to reach the other side in safety.The term is also used in the...

. In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 that term now means an underground rapid transit
Rapid transit
A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...

 system.

The central part of a rapid transit network is usually built in tunnels. Rail station platforms may be connected by pedestrian tunnels or by foot bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

s.

Usage limitations

A tunnel is relatively long and narrow; in general the length is more (usually much more) than twice the diameter. Some hold a tunnel to be at least 0.16 kilometre (0.0994196378639691 mi) long and call shorter passageways by such terms as an "underpass" or a "chute". For example, the underpass beneath Yahata Station
Kagoshima Main Line
The is a major railway line operated by the Kyushu Railway Company between Mojikō in Kitakyūshū, and Kagoshima Station in Kagoshima City, at the southern end of Kyushu...

 in Kitakyushu, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 is 0.13 km (0.0807784557644749 mi) and so might not be considered a tunnel.

Geotechnical investigation

A tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from boreholes 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 will make use of the best ground and water conditions.

In some cases conventional desk and site studies 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, may be driven ahead of the main drive. This smaller diameter tunnel will be easier to support should unexpected conditions be met, and will be incorporated in the final tunnel. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.

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, constructed in a shallow trench and then covered over.
  • Bored tunnels, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section.
  • Immersed tube tunnels, sunk into a body of water and sit on, or are buried just under, its bed.

Cut-and-cover

Cut-and-cover is a simple method of construction for shallow tunnels where a trench
Trench
A trench is a type of excavation or depression in the ground. Trenches are generally defined by being deeper than they are wide , and by being narrow compared to their length ....

 is excavated and roof
Roof
A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather. Structures that require roofs range from a letter box to a cathedral or stadium, dwellings being the most numerous....

ed 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: Here side support walls and capping beams are constructed from ground level by such methods as slurry walling, 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
Tunnelling shield
A tunnelling shield is a protective structure used in the excavation of tunnels through soil that is too soft or fluid to remain stable during the time it takes to line the tunnel with a support structure of concrete, cast iron or steel...

. For intermediate levels, both methods are possible.

Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube station
Canary Wharf tube station
Canary Wharf tube station is a London Underground station on the Jubilee Line, between and . It is in Travelcard Zone 2 and was opened by Ken Livingstone setting an escalator in motion on 17 September 1999 as part of the Jubilee Line Extension. It is maintained by Tube Lines...

 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
London Underground
The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...

, where bored tunnels were used for stations and passenger access.

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 combination's 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.

Clay-kicking

Clay-kicking is a specialised method developed in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, of manually digging tunnels in strong clay-based soil structures. Unlike previous manual methods of using mattocks 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 his feet. Turning the tool with his hands, he extracts a section of soil, which is then placed on the waste extract.

Regularly used in Victorian
Victorian era
The Victorian era of British history was the period of Queen Victoria's reign from 20 June 1837 until her death on 22 January 1901. It was a long period of peace, prosperity, refined sensibilities and national self-confidence...

 civil engineering, the methods found favour in the renewal of the United Kingdom's then ancient sewerage
Sewerage
Sewerage refers to the infrastructure that conveys sewage. It encompasses receiving drains, manholes, pumping stations, storm overflows, screening chambers, etc. of the sanitary sewer...

 systems, by not having to remove all property or infrastructure to create an effective small tunnel system. During the First World War, the system was successfully deployed by the Royal Engineer tunnelling companies
Royal Engineer tunnelling companies
Royal Engineer tunnelling companies were specialist units of the Corps of Royal Engineers within the British Army, formed to dig attacking tunnels under enemy lines during the First World War....

 to deploy large military mines beneath enemy German Empire
German Empire
The German Empire refers to Germany during the "Second Reich" period from the unification of Germany and proclamation of Wilhelm I as German Emperor on 18 January 1871, to 1918, when it became a federal republic after defeat in World War I and the abdication of the Emperor, Wilhelm II.The German...

 lines. The method was virtually silent not susceptible to listening methods of detection.

Boring machines

Tunnel boring machines (TBMs) and associated back-up systems are used to highly automate the entire tunneling process, reducing tunneling costs.

Tunnel boring in certain predominantly urban applications, is viewed as quick and cost effective alternative to laying surface rails and roads. Expensive compulsory purchase of buildings and land with potentially lengthy planning inquiries is eliminated.

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
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

. 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.

In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of Linth Limmern Power Plant in Switzerland. The borehole has a diameter of 8.03 metres (26.3 ft).
The TBM used for digging the 57 kilometres (35.4 mi) Gotthard Base Tunnel
Gotthard Base Tunnel
The Gotthard Base Tunnel is a new railway tunnel beneath the Swiss Alps, expected to open in 2016. With a route length of and a total of of tunnels, shafts and passages, it is the world's longest rail tunnel, surpassing the Japanese Seikan Tunnel....

, in Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

, has a diameter of about 9 metres (29.5 ft). A larger TBM was built to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid
HSL-Zuid
HSL-Zuid , is a 125 km-long high-speed railway line in the Netherlands to the Belgian border, with a branch to Breda. Together with the Belgian HSL 4 it forms the Schiphol–Antwerp high-speed railway...

 in the Netherlands, with a diameter of 14.87 metres (48.8 ft). This in turn was superseded by the Madrid M30 ringroad
Autopista de Circunvalación M-30
The M-30 orbital motorway circles the central districts of Madrid, the capital city of Spain. It is the innermost ring road of the Spanish city. Outer rings are named M-40, M-45 and M-50....

, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

, and the Chong Ming tunnels in Shanghai
Shanghai
Shanghai is the largest city by population in China and the largest city proper in the world. It is one of the four province-level municipalities in the People's Republic of China, with a total population of over 23 million as of 2010...

, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

. All of these machines were built at least partly by Herrenknecht
Herrenknecht
Herrenknecht AG is a German manufacturer of tunnel boring machines of all sizes. Herrenknecht is headquartered in Allmannsweier near Schwanau and is the world market leader for heavy tunnel boring machines...

.

Shafts

A shaft
Shaft (civil engineering)
In civil engineering a shaft is an underground vertical or inclined passageway. Shafts are constructed for a number of reasons including:* For the construction of a tunnel* For ventilation of a tunnel or underground structure...

 is sometimes necessary for a tunnel project. 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 built to be permanent. Once they are built, Tunnel Boring Machines 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. Sometimes if a tunnel is going to be long, multiple shafts at various locations will be bored so that entrance into the tunnel is closer to the unexcavated area.

Other key factors

  • Stand-up time is the amount of time a tunnel will support itself without any added structures. Knowing this time allows the engineers to determine how much can be excavated before support is needed. The longer the stand-up time is the faster the excavating will go. Generally certain configurations of rock and clay will have the greatest stand-up time, and sand and fine soils will have a much lower stand-up time.
  • Groundwater control is very important in tunnel construction. If there is water leaking into the tunnel stand-up time will be greatly decreased. If there is water leaking into the shaft it will become unstable and will not be safe to work in. To stop this from happening there are a few common methods. One of the most effective is ground freezing. To do this pipes are inserted into the ground surrounding the shaft and are cooled until they freeze. This freezes the ground around each pipe until the whole shaft is surrounded frozen soil, keeping water out. The most common method is to install pipes into the ground and to simply pump the water out. This works for tunnels and shafts.
  • Tunnel shape is very important in determining stand-up time. The force from gravity is straight down on a tunnel, so if the tunnel is wider than it is high it will have a harder time supporting itself decreasing its stand-up time. If a tunnel is higher than it is wide the stand up time will increase making the project easier. The hardest shape to support itself is a square or rectangular tunnel. The forces have a harder time being redirected around the tunnel making it extremely hard to support itself. This of course all depends what the material of the ground is.

Sprayed concrete techniques

The New Austrian Tunneling Method
New Austrian Tunnelling method
The New Austrian Tunnelling method was developed between 1957 and 1965 in Austria. It was given its name in Salzburg in 1962 to distinguish it from old Austrian tunnelling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher...

(NATM) was developed in the 1960s, and is the best known of a number of engineering solutions that use calculated and empirical real-time measurements to provide optimised safe support to the tunnel lining. The main idea of this method is to use the geological stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...

 of the surrounding rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 to stabilize the tunnel itself, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the introduced support measures. Based on geotechnical measurements, an optimal cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

 is computed. The excavation is immediately protected by a layer of sprayed concrete, commonly referred to as shotcrete
Shotcrete
Shotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique....

, after excavation. Other support measures could 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
Deformation (mechanics)
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...

.

By special monitoring
Condition monitoring
Condition monitoring is the process of monitoring a parameter of condition in machinery, such that a significant change is indicative of a developing failure. It is a major component of predictive maintenance. The use of conditional monitoring allows maintenance to be scheduled, or other actions...

 the NATM method is very flexible, even at surprising changes of the geomechanical
Geomechanics
Geomechanics is the geologic study of the behavior of soil and rock. The two main disciplines of geomechanics are soil mechanics and rock mechanics. The former deals with the behaviour of soil from a small scale to a landslide scale...

 rock consistency during the tunneling work. The measured rock properties lead to appropriate tool
Tool
A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

s for tunnel strengthening
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...

. In the last decades also soft ground excavations up to 10 kilometres (6.2 mi) 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. Tunnels constructed by pipe jacking are normally small diameter tunnels with a maximum size of around 2.4m.

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 20m. A cutting head is normally used at the front of the box being jacked and excavation is normally by excavator from within the box.

Underwater tunnels

There are also several approaches to underwater tunnels, the two most common being bored tunnels or immersed tube
Immersed tube
An immersed tube is a kind of underwater tunnel composed of segments, constructed elsewhere and floated to the tunnel site to be sunk into place and then linked together. They are commonly used for road and rail crossings of rivers, estuaries and sea channels/harbours...

s. Submerged floating tunnels are another approach that has not been constructed.

Other

Other tunneling methods include:
  • Drilling and blasting
    Drilling and blasting
    Before the advent of tunnel boring machines, drilling and blasting was the only economical way of excavating long tunnels through hard rock, where digging is not possible. Even today, the method is still used in the construction of tunnels, such as in the construction of the Lötschberg Base Tunnel...

  • Hydraulic splitter
    Hydraulic splitter
    The hydraulic splitter, also known as rock splitter and darda splitter, is a type of portable hydraulic tool that is used in demolition jobs which involve breaking large blocks of concrete and rocks....

  • Slurry-shield machine
  • Wall-cover construction method.

Costs and cost overruns of tunnels

Tunnels are costly and generally more costly than bridges. Large cost overrun
Cost overrun
A cost overrun, also known as a cost increase or budget overrun, is an unexpected cost incurred in excess of a budgeted amount due to an under-estimation of the actual cost during budgeting...

s are common in tunnel construction.

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
Drawbridge
A drawbridge is a type of movable bridge typically associated with the entrance of a castle surrounded by a moat. The term is often used to describe all different types of movable bridges, like bascule bridges and lift bridges.-Castle drawbridges:...

 spans intersecting with shipping channels, necessitating a tunnel.

Bridges usually require a larger footprint on each shore than tunnels. There are actually more codes to follow with bridges than with tunnels. In areas with expensive real estate, such as Manhattan
Manhattan
Manhattan is the oldest and the most densely populated of the five boroughs of New York City. Located primarily on the island of Manhattan at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York...

 and urban Hong Kong
Hong Kong
Hong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...

, 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.

The 1934 Queensway Road Tunnel under the River Mersey
River Mersey
The River Mersey is a river in North West England. It is around long, stretching from Stockport, Greater Manchester, and ending at Liverpool Bay, Merseyside. For centuries, it formed part of the ancient county divide between Lancashire and Cheshire....

 at Liverpool
Liverpool
Liverpool is a city and metropolitan borough of Merseyside, England, along the eastern side of the Mersey Estuary. It was founded as a borough in 1207 and was granted city status in 1880...

, 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 Tunnel
Holland Tunnel
The Holland Tunnel is a highway tunnel under the Hudson River connecting the island of Manhattan in New York City with Jersey City, New Jersey at Interstate 78 on the mainland. Unusual for an American public works project, it is not named for a government official, politician, or local hero or...

 and Lincoln Tunnel
Lincoln Tunnel
The Lincoln Tunnel is a long tunnel under the Hudson River, connecting Weehawken, New Jersey and the borough of Manhattan in New York City.-History:...

 between New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...

 and Manhattan in New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

, the Queens-Midtown Tunnel between Manhattan and the borough
Borough (New York City)
New York City, one of the largest cities in the world, is composed of five boroughs. Each borough now has the same boundaries as the county it is in. County governments were dissolved when the city consolidated in 1898, along with all city, town, and village governments within each county...

 of Queens
Queens
Queens is the easternmost of the five boroughs of New York City. The largest borough in area and the second-largest in population, it is coextensive with Queens County, an administrative division of New York state, in the United States....

 on Long Island
Long Island
Long Island is an island located in the southeast part of the U.S. state of New York, just east of Manhattan. Stretching northeast into the Atlantic Ocean, Long Island contains four counties, two of which are boroughs of New York City , and two of which are mainly suburban...

, and the Elizabeth River
Elizabeth River (Virginia)
The Elizabeth River is a tidal estuary forming an arm of Hampton Roads harbor at the southern end of Chesapeake Bay in southeast Virginia in the United States. It is located along the southern side of the mouth of the James River, between the cities of Portsmouth and Norfolk...

 tunnels between Norfolk
Norfolk, Virginia
Norfolk is an independent city in the Commonwealth of Virginia in the United States. With a population of 242,803 as of the 2010 Census, it is Virginia's second-largest city behind neighboring Virginia Beach....

 and Portsmouth, Virginia
Portsmouth, Virginia
Portsmouth is located in the Hampton Roads metropolitan area of the U.S. Commonwealth of Virginia. As of 2010, the city had a total population of 95,535.The Norfolk Naval Shipyard, often called the Norfolk Navy Yard, is a historic and active U.S...

, the 1934 River Mersey
River Mersey
The River Mersey is a river in North West England. It is around long, stretching from Stockport, Greater Manchester, and ending at Liverpool Bay, Merseyside. For centuries, it formed part of the ancient county divide between Lancashire and Cheshire....

 road Queensway Tunnel
Queensway Tunnel
The Queensway Tunnel is a road tunnel under the River Mersey, in the north west of England, between Liverpool and Birkenhead. It is often called the Birkenhead Tunnel, to distinguish it from the Kingsway Tunnel, which serves Wallasey.-History:...

 and the Western Scheldt Tunnel
Western Scheldt Tunnel
Western Scheldt Tunnel is a tunnel in The Netherlands on highway N62 under the Western Scheldt estuary between Ellewoutsdijk and Terneuzen. It is the longest tunnel for highway traffic in The Netherlands.-History:...

, 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 kilometres or 32 mi Channel Tunnel
Channel Tunnel
The Channel Tunnel is a undersea rail tunnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is deep...

), 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
Oresund Bridge
The Øresund or Öresund Bridge is a combined twin-track railway and dual carriageway bridge-tunnel across the Øresund strait.The bridge connects Sweden and Denmark, and it is the longest road and rail bridge in Europe. The Øresund Bridge also connects two major Metropolitan Areas: those of the...

 and the Chesapeake Bay Bridge-Tunnel
Chesapeake Bay Bridge-Tunnel
The Chesapeake Bay Bridge-Tunnel is a long fixed link crossing the mouth of the Chesapeake Bay and connecting the Delmarva Peninsula's Eastern Shore of Virginia with Virginia Beach and the metropolitan area of Hampton Roads, Virginia...

 in the eastern United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

.

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate
Asphyxia
Asphyxia or asphyxiation is a condition of severely deficient supply of oxygen to the body that arises from being unable to breathe normally. An example of asphyxia is choking. Asphyxia causes generalized hypoxia, which primarily affects the tissues and organs...

 users, as happened at the Gotthard Road Tunnel in Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

 in 2001. One of the worst railway disasters ever, the Balvano train disaster
Balvano train disaster
In the Balvano train disaster of March 2/3, 1944, some 426 people illegally riding a steam-hauled freight train died of carbon monoxide poisoning when the train stalled on a steep gradient in the Armi tunnel. The accident occurred in southern Italy, near Balvano .-Circumstances:Naples suffered...

, was caused by a train stalling in the Armi tunnel in Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

 in 1944, killing 426 passengers.

Double-deck tunnel

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, the largest diameter bore 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.

The Lion Rock Tunnel
Lion Rock Tunnel
The Lion Rock Tunnel, being the first tunnel in Hong Kong, is a twin-bored toll tunnel, connecting Sha Tin in the New Territories and New Kowloon near Kowloon Tong. It has two lanes in each direction, with toll booths located at the Sha Tin end. It is a vital component of Route 1.The Lion Rock...

, built in the mid-1960s connecting New Kowloon
New Kowloon
New Kowloon is an area in Kowloon, Hong Kong, bounded in the south by Boundary Street, and in the north by the ranges of the Lion Rock, Beacon Hill, Tate's Cairn and Kowloon Peak...

 and Sha Tin
Sha Tin New Town
Sha Tin New Town is one of the new towns in Hong Kong. It is within the Sha Tin District, which covers the areas of Sha Tin, Tai Wai, Fo Tan, Tai Shui Hang, Ma On Shan, etc. With development started in the 1970s, it covers an area of 35.87 square kilometres . It presently has a population over...

 within the territory of Hong Kong, carries a motorway and an aqueduct.

A recent double-decker tunnel with both decks for motor vehicles is the Fuxing Road Tunnel in Shanghai
Shanghai
Shanghai is the largest city by population in China and the largest city proper in the world. It is one of the four province-level municipalities in the People's Republic of China, with a total population of over 23 million as of 2010...

, 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
SMART Tunnel
The "Stormwater Management and Road Tunnel" or "SMART Tunnel", is a storm drainage and road structure in Kuala Lumpur, Malaysia, a major national project in the country...

 in Malaysia is the first multipurpose tunnel in the world, as it is used both to control traffic and flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

 in Kuala Lumpur
Kuala Lumpur
Kuala Lumpur is the capital and the second largest city in Malaysia by population. The city proper, making up an area of , has a population of 1.4 million as of 2010. Greater Kuala Lumpur, also known as the Klang Valley, is an urban agglomeration of 7.2 million...

.

Artificial tunnels

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.

Snow shed
Snow shed
An avalanche snow bridge or simply snow bridge is a type of rigid snow-supporting structure for avalanche control . Avalanche bridges can be made of steel, prestressed concrete frames, or timber....

s are a kind of artificial tunnel built to protect a railway from avalanche
Avalanche
An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...

s of snow. Similarly the Stanwell Park, New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

 steel tunnel, on the South Coast railway line
South Coast railway line, New South Wales
The South Coast Line is in the intercity region of Sydney's CityRail services. It serves the coastal region to the south of the Sydney metropolitan area including the Illawarra region, most notably the regional city of Wollongong, and extended services reach as far as Nowra in Shoalhaven.- Line...

, protects the line from rockfalls.

Common utility duct
Common utility duct
A common utility duct, sometimes called a common utility conduit, is any structure – above, on, or below ground – that carries more than two types of public utility lines...

s are human-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
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 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
Balvano train disaster
In the Balvano train disaster of March 2/3, 1944, some 426 people illegally riding a steam-hauled freight train died of carbon monoxide poisoning when the train stalled on a steep gradient in the Armi tunnel. The accident occurred in southern Italy, near Balvano .-Circumstances:Naples suffered...

 in Italy in 1944, when the locomotive halted in a long tunnel. Carbon monoxide poisoning
Carbon monoxide poisoning
Carbon monoxide poisoning occurs after enough inhalation of carbon monoxide . Carbon monoxide is a toxic gas, but, being colorless, odorless, tasteless, and initially non-irritating, it is very difficult for people to detect...

 was the main cause of the horrifying death rate.

In history

  • The World's oldest underwater tunnel is rumored to be the Terelek kaya tüneli under Kızıl River, a little south of the towns of Boyabat
    Boyabat
    Boyabat is a town and district of Sinop Province in the Black Sea region of Turkey. The mayor is Mehmet Ermiş .Boyabat has a population of 50,000 in the town itself. The town is in the Gökırmak valley located 100 km south of Sinop over the mountain range along the Black Sea coast.The town is...

     and Duragan in Turkey
    Turkey
    Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...

    . Estimated to have been built more than 2000 years ago (possibly 5000), it is assumed to have had a defence purpose.
  • The qanat
    Qanat
    A qanāt is a water management system used to provide a reliable supply of water for human settlements and irrigation in hot, arid and semi-arid climates...

     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
    Gonabad
    Gonabad is a city in and the capital of Gonabad County, in Razavi Khorasan Province, Iran. At the 2006 census, its population was 34,563, in 9,789 families.It is mostly well-known because of the Gonabadi Dervishes and for its qanats, also known as kareez...

    , which after 2700 years, still provides drinking and agricultural water to nearly 40,000 people. Its main well depth is more than 360 m (1,181.1 ft), and its length is 45 km (28 mi).
  • The Eupalinian aqueduct
    Eupalinian aqueduct
    The Tunnel of Eupalinos or Eupalinian aqueduct is a tunnel of length in Samos, Greece, built in the 6th century BC to serve as an aqueduct. The tunnel is the second known tunnel in history which was excavated from both ends , and the first with a geometry-based approach in doing so...

     on the island of Samos
    Samos Island
    Samos is a Greek island in the eastern Aegean Sea, south of Chios, north of Patmos and the Dodecanese, and off the coast of Asia Minor, from which it is separated by the -wide Mycale Strait. It is also a separate regional unit of the North Aegean region, and the only municipality of the regional...

     (North Aegean
    North Aegean
    The North Aegean is one of the thirteen regions of Greece. It comprises the islands of the north-eastern Aegean Sea, except for Samothrace, which belongs to the region of East Macedonia and Thrace, and Imbros and Tenedos which belong to Turkey....

    , Greece
    Greece
    Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

    ). Built in 520 BC by the ancient Greek engineer Eupalinos
    Eupalinos
    Eupalinos or Eupalinus of Megara was an ancient Greek engineer who built the Tunnel of Eupalinos on Samos Island in the 6th century BC....

     of Megara
    Megara
    Megara is an ancient city in Attica, Greece. It lies in the northern section of the Isthmus of Corinth opposite the island of Salamis, which belonged to Megara in archaic times, before being taken by Athens. Megara was one of the four districts of Attica, embodied in the four mythic sons of King...

    . 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 Pythagoreion
    Pythagoreion
    The remains of the Pythagoreion, an ancient fortified port with Greek and Roman monuments and a spectacular tunnel, the Tunnel of Eupalinos or Eupalinian aqueduct, along with the Heraion of Samos were jointly registered as a UNESCO World Heritage Site in 1992....

    , the ancient capital of Samos
    Samos Island
    Samos is a Greek island in the eastern Aegean Sea, south of Chios, north of Patmos and the Dodecanese, and off the coast of Asia Minor, from which it is separated by the -wide Mycale Strait. It is also a separate regional unit of the North Aegean region, and the only municipality of the regional...

    . 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 m (3,379.3 ft) and visitors can still enter it Eupalinos tunnel.
  • The Via Flaminia
    Via Flaminia
    The Via Flaminia was an ancient Roman road leading from Rome over the Apennine Mountains to Ariminum on the coast of the Adriatic Sea, and due to the ruggedness of the mountains was the major option the Romans had for travel between Etruria, Latium and Campania and the Po Valley...

    , an important Roman road
    Roman road
    The Roman roads were a vital part of the development of the Roman state, from about 500 BC through the expansion during the Roman Republic and the Roman Empire. Roman roads enabled the Romans to move armies and trade goods and to communicate. The Roman road system spanned more than 400,000 km...

    , penetrated the Furlo pass in the Apennines through a tunnel which emperor Vespasian
    Vespasian
    Vespasian , was Roman Emperor from 69 AD to 79 AD. Vespasian was the founder of the Flavian dynasty, which ruled the Empire for a quarter century. Vespasian was descended from a family of equestrians, who rose into the senatorial rank under the Emperors of the Julio-Claudian dynasty...

     had ordered built in 76-77. A modern road, the SS 3 Flaminia, still uses this tunnel, which had a 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 Tunnel
    Sapperton Canal Tunnel
    The Sapperton Canal Tunnel is a tunnel on the Thames and Severn Canal near Cirencester in Gloucestershire, England. It was the longest canal tunnel, and the longest tunnel of any kind, in England from 1789 to 1811....

     on the Thames and Severn Canal
    Thames and Severn Canal
    The Thames and Severn Canal is a canal in Gloucestershire in the south of England, which was completed in 1789. It was conceived as part of a canal route from Bristol to London. At its eastern end, it connects to the River Thames at Inglesham Lock near Lechlade, while at its western end, it...

     in England
    England
    England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

    , dug through hills, which opened in 1789, was 3.5 km (2.2 mi) long and allowed boat
    Canal boat
    There are three articles associated with canal watercraft:* The Volunteer - A replica 1848 canal boat docked on the Illinois and Michigan Canal at LaSalle, Illinois* Narrowboat - a specialized craft for operation in early narrow canals...

     transport of coal
    Coal
    Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

     and other goods. Above it runs the Sapperton Long Tunnel which carries the "Golden Valley" railway line between Swindon
    Swindon
    Swindon is a large town within the borough of Swindon and ceremonial county of Wiltshire, in South West England. It is midway between Bristol, west and Reading, east. London is east...

     and Gloucester
    Gloucester
    Gloucester is a city, district and county town of Gloucestershire in the South West region of England. Gloucester lies close to the Welsh border, and on the River Severn, approximately north-east of Bristol, and south-southwest of Birmingham....

    .
  • 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 was created for the first true steam locomotive, from Penydarren
    Penydarren
    Penydarren Ironworks was the fourth of the great ironworks established at Merthyr Tydfil in South Wales.Built in 1784 by the brothers Samuel Homfray, Jeremiah Homfray, and Thomas Homfray, all sons of Francis Homfray of Stourbridge. Their father, Francis, for a time managed a nail warehouse there...

     to Abercynon
    Abercynon
    Abercynon is a small village in the Cynon Valley in Mid Glamorgan, Wales. The unitary authority is now known as Rhondda Cynon Taff. It is composed of the village of Abercynon itself,Carnetown,Glancynon,Park View and Pontcynon. However, in recent years the sign to show motorists they are entering...

    . The Penydarren
    Penydarren
    Penydarren Ironworks was the fourth of the great ironworks established at Merthyr Tydfil in South Wales.Built in 1784 by the brothers Samuel Homfray, Jeremiah Homfray, and Thomas Homfray, all sons of Francis Homfray of Stourbridge. Their father, Francis, for a time managed a nail warehouse there...

     locomotive was built by Richard Trevithick
    Richard Trevithick
    Richard Trevithick was a British inventor and mining engineer from Cornwall. His most significant success was the high pressure steam engine and he also built the first full-scale working railway steam locomotive...

    . The locomotive made the historic journey from Penydarren
    Penydarren
    Penydarren Ironworks was the fourth of the great ironworks established at Merthyr Tydfil in South Wales.Built in 1784 by the brothers Samuel Homfray, Jeremiah Homfray, and Thomas Homfray, all sons of Francis Homfray of Stourbridge. Their father, Francis, for a time managed a nail warehouse there...

     to Abercynon
    Abercynon
    Abercynon is a small village in the Cynon Valley in Mid Glamorgan, Wales. The unitary authority is now known as Rhondda Cynon Taff. It is composed of the village of Abercynon itself,Carnetown,Glancynon,Park View and Pontcynon. However, in recent years the sign to show motorists they are entering...

     in 1804. Part of this tunnel can still be seen at Pentrebach
    Pentrebach
    Pentrebach is a village in the County Borough of Merthyr Tydfil, Wales. It lies on the east side of the River Taff opposite Abercanaid, South of Merthyr and North of Troedyrhiw. To the east of the village lies the Mynydd Cilfach-yr-Encil which rises up to 445 m...

    , Merthyr Tydfil
    Merthyr Tydfil
    Merthyr Tydfil is a town in Wales, with a population of about 30,000. Although once the largest town in Wales, it is now ranked as the 15th largest urban area in Wales. It also gives its name to a county borough, which has a population of around 55,000. It is located in the historic county of...

    , Wales
    Wales
    Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

    . This is arguably the oldest railway tunnel in the world, for self-propelled steam engines on rails.
  • The Montgomery Bell Tunnel
    Montgomery Bell Tunnel
    The Montgomery Bell Tunnel, also known as the Patterson Forge Tunnel, in Cheatham County, Tennessee, is a long tunnel through limestone rock which was the first "full-scale" water diversion tunnel built in the United States. It is also apparently the first "full-scale" tunnel of any type in the...

     in Tennessee, a 88 m (288.7 ft), high water diversion tunnel, 4.5 by 2.45 m (14.8 by 8 ft), to power a water wheel, was built by slave labour in 1819, being the first full-scale tunnel in North America.
  • Crown Street Station, Liverpool
    Liverpool
    Liverpool is a city and metropolitan borough of Merseyside, England, along the eastern side of the Mersey Estuary. It was founded as a borough in 1207 and was granted city status in 1880...

    , 1829. Built by George Stephenson
    George Stephenson
    George Stephenson was an English civil engineer and mechanical engineer who built the first public railway line in the world to use steam locomotives...

    , a single track tunnel 291 yd (266.1 m) 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. http://www.liverpoolwiki.org/Liverpool%27s_Historic_Rail_Tunnels
  • The 1.26 mile (2.03 km) 1829 Wapping Tunnel
    Wapping Tunnel
    Wapping or Edge Hill Tunnel in Liverpool, England, was constructed to enable goods services to operate between Liverpool docks and Manchester, and all points between, as part of the planned Liverpool and Manchester Railway. It was the first tunnel in the world to be bored under a metropolis...

     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 Tunnel
    Box Tunnel
    Box Tunnel is a railway tunnel in Western England, between Bath and Chippenham, dug through Box Hill, and is one of the most significant structures on the Great Western Main Line...

     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 (1.8 mi).
  • The 0.75 mile long 1842 Prince of Wales Tunnel, in Shildon
    Shildon
    Shildon is a town in County Durham, in England. It is situated 2 miles to the south east of Bishop Auckland and 11 miles north of Darlington. It is 13 miles away from Durham, 23 miles from Sunderland and 23 miles from Newcastle-upon-Tyne...

     near Darlington, England, is the oldest sizable tunnel in the world still in use under a settlement.
  • The Thames Tunnel
    Thames Tunnel
    The Thames Tunnel is an underwater tunnel, built beneath the River Thames in London, United Kingdom, connecting Rotherhithe and Wapping. It measures 35 feet wide by 20 feet high and is 1,300 feet long, running at a depth of 75 feet below the river's surface...

    , built by Marc Isambard Brunel
    Marc Isambard Brunel
    Sir Marc Isambard Brunel, FRS FRSE was a French-born engineer who settled in England. He preferred the name Isambard, but is generally known to history as Marc to avoid confusion with his more famous son Isambard Kingdom Brunel...

     and his son Isambard Kingdom Brunel
    Isambard Kingdom Brunel
    Isambard Kingdom Brunel, FRS , was a British civil engineer who built bridges and dockyards including the construction of the first major British railway, the Great Western Railway; a series of steamships, including the first propeller-driven transatlantic steamship; and numerous important bridges...

     and opened in 1843, was the first underwater tunnel and the first to use a tunnelling shield
    Tunnelling shield
    A tunnelling shield is a protective structure used in the excavation of tunnels through soil that is too soft or fluid to remain stable during the time it takes to line the tunnel with a support structure of concrete, cast iron or steel...

    . Originally used as a foot-tunnel, it was a part of the East London Line
    East London Line
    The East London Line is a London Overground line which runs north to south through the East End, Docklands and South areas of London.Built in 1869 by the East London Railway Company, which reused the Thames Tunnel, originally intended for horse-drawn carriages, the line became part of the London...

     of the London Underground
    London Underground
    The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...

     until 2007, being the oldest section of the system. From 2010 the tunnel becomes a part of the London Overground
    London Overground
    London Overground is a suburban rail network in London and Hertfordshire. It has been operated by London Overground Rail Operations since 2007 as part of the National Rail network, under the franchise control and branding of Transport for London...

     system.
  • The 2.07 miles (3.34 km) Victoria Tunnel
    Victoria Tunnel (Liverpool)
    -History:The Victoria Tunnel in Liverpool, England is a 2,475 metre long rail tunnel. Opened in 1849, its eastern portal is adjacent to Edge Hill station. The western end opens into a short cutting, between Byrom Street and Fontenoy Street, the short Waterloo Tunnel exits the cutting terminating...

     in Liverpool
    Liverpool
    Liverpool is a city and metropolitan borough of Merseyside, England, along the eastern side of the Mersey Estuary. It was founded as a borough in 1207 and was granted city status in 1880...

    , opened in 1848, 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
    Merseyrail
    Merseyrail is a train operating company and commuter rail network in the United Kingdom, centred on Liverpool, Merseyside. The network is predominantly electric with diesel trains running on the City Line. Two City Line branches are currently being electrified on the overhead wire AC system with...

     rapid transit rail system. Stations being cut into the tunnel are being considered. Also, reuse by a monorail system from the proposed Liverpool Waters
    Liverpool Waters
    Liverpool Waters is a large scale £5.5bn development that has been proposed by the company Peel Holdings in the Vauxhall area of Liverpool, Merseyside, England. The development will make use of a series of presently derelict dock spaces at Central Docks, with much of the docks in a World Heritage...

     redevelopment of Liverpool's Central Docks has been proposed.
  • The oldest underground sections of the London Underground
    London Underground
    The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...

     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
    Rapid transit
    A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...

     or subway system. Dating from 1863, Baker Street station is the oldest underground station in the world.
  • On June 18, 1868, the Central Pacific Railroad's
    Central Pacific Railroad
    The Central Pacific Railroad is the former name of the railroad network built between California and Utah, USA that formed part of the "First Transcontinental Railroad" in North America. It is now part of the Union Pacific Railroad. Many 19th century national proposals to build a transcontinental...

     1,659-foot (506 m) Summit Tunnel (Tunnel #6) at Donner Pass
    Donner Pass
    Donner Pass is a mountain pass in the northern Sierra Nevada, located above Donner Lake about nine miles west of Truckee, California. It has a steep approach from the east and a gradual approach from the west....

     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
    Southern Pacific Railroad
    The Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company, and usually simply called the Southern Pacific or Espee, was an American railroad....

     closed it in favor of sending all rail traffic through the 10,322-foot (3,146 m) long Tunnel #41 (aka "The Big Hole") built a mile to the south in 1925.
  • The 1882 Col de Tende Road Tunnel
    Col de Tende Road Tunnel
    thumb|Portal on Italian sideCol de Tende Road Tunnel is a 3182 metre long road tunnel running under Col de Tende between France and Italy. It was inaugurated in 1882 and it was until 1964 the longest road tunnel in the Alps. At its opening time it may have been one of world's longest road tunnels....

    , at 3182 metres long, was one of the first long road tunnels under a pass, running between France and Italy.
  • The Mersey Railway
    Mersey Railway
    The Mersey Railway connected Liverpool and Birkenhead, England, via the Mersey Railway Tunnel under the River Mersey. Opened in 1886, it was the second oldest urban underground railway network in the world. The railway contained the first tunnel built under the River Mersey. It was constructed by...

     tunnel opened in 1886 running from Liverpool to Birkenhead under the River Mersey. The Mersey Railway
    Mersey Railway
    The Mersey Railway connected Liverpool and Birkenhead, England, via the Mersey Railway Tunnel under the River Mersey. Opened in 1886, it was the second oldest urban underground railway network in the world. The railway contained the first tunnel built under the River Mersey. It was constructed by...

     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 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
    Severn Tunnel
    The Severn Tunnel is a railway tunnel in the United Kingdom, linking South Gloucestershire in the west of England to Monmouthshire in south Wales under the estuary of the River Severn....

     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 Tunnel
    St. Clair Tunnel
    The St. Clair Tunnel is the name for two separate rail tunnels which were built under the St. Clair River between Sarnia, Ontario and Port Huron, Michigan. It was the first full-size subaqueous tunnel built in North America. -First tunnel :The St. Clair Tunnel Company opened the first tunnel in...

    , also opened later in 1890, linked the elements of the Greathead tunnels on a larger scale.
  • The 1927 Holland Tunnel
    Holland Tunnel
    The Holland Tunnel is a highway tunnel under the Hudson River connecting the island of Manhattan in New York City with Jersey City, New Jersey at Interstate 78 on the mainland. Unusual for an American public works project, it is not named for a government official, politician, or local hero or...

     was the first underwater tunnel designed for automobiles. This fact required a novel ventilation system.

See also the rapid transit history.

Longest

  • The Delaware Aqueduct
    Delaware Aqueduct
    The Delaware Aqueduct is the newest of the New York City aqueducts. It takes water from the Rondout Reservoir through the Chelsea Pump Station, the West Branch Reservoir, and the Kensico Reservoir, ending at the Hillview Reservoir in Yonkers, New York....

     in New York USA is the longest tunnel, of any type, in the world at 137 km (85.1 mi). It is drilled through solid rock.
  • The Gotthard Base Tunnel
    Gotthard Base Tunnel
    The Gotthard Base Tunnel is a new railway tunnel beneath the Swiss Alps, expected to open in 2016. With a route length of and a total of of tunnels, shafts and passages, it is the world's longest rail tunnel, surpassing the Japanese Seikan Tunnel....

     will be the longest rail tunnel in the world at 57 km (35.4 mi). It will be totally completed in 2017.
  • The Seikan Tunnel
    Seikan Tunnel
    The Seikan Tunnel is a railway tunnel in Japan, with a long portion under the seabed. Track level is about below seabed and below sea level. It travels beneath the Tsugaru Strait—connecting Aomori Prefecture on the Japanese island of Honshu and the island of Hokkaido—as part of the Kaikyo Line...

     in Japan
    Japan
    Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

     is the longest undersea rail tunnel in the world at 53.9 km (33.5 mi), of which 23.3 km (14.5 mi) is under the sea.
  • The Channel Tunnel
    Channel Tunnel
    The Channel Tunnel is a undersea rail tunnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is deep...

     between France
    France
    The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

     and the United Kingdom
    United Kingdom
    The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

     under the English Channel
    English Channel
    The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...

     is the second-longest, with a total length of 50 km (31.1 mi), of which 39 km (24.2 mi) is under the sea.
  • The Lötschberg Base Tunnel
    Lötschberg Base Tunnel
    The Lötschberg Base Tunnel is a railway tunnel on the BLS Lötschbergbahn's Lötschberg Line cutting through the Alps of Switzerland some below the existing Lötschberg Tunnel. It is currently the world's longest land tunnel and accommodates passenger and freight trains. It runs between Frutigen,...

     opened in June 2007 in Switzerland
    Switzerland
    Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

     was the longest land rail tunnel, with a total of 34.5 km (21.4 mi).
  • The Lærdal Tunnel in Norway from Lærdal
    Lærdal
    Lærdal is a municipality in the southwestern part of Sogn og Fjordane county, Norway. It is part of the traditional district of Sogn. The administrative center of the municipality is the village of Lærdalsøyri...

     to Aurland
    Aurland
    Aurland is a municipality in the county of Sogn og Fjordane, Norway. It is located in the traditional district of Sogn. The administrative center is the village of Aurlandsvangen. Other villages include Bakka, Flåm, Undredal, and Gudvangen....

     is the world's longest road tunnel, intended for cars and similar vehicles, at 24.5 km (15.2 mi).
  • The Zhongnanshan Tunnel
    Zhongnanshan Tunnel
    Zhongnanshan Tunnel, or Qinling Zhongnanshan Tunnel in Shaanxi province, China, is the longest two-tube road tunnel in the world. It is also the second longest road tunnel overall in the world, after the Lærdal Tunnel in Norway. The long tunnel, crosses under the Zhongnan Mountain...

     in People's Republic of China
    People's Republic of China
    China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...

     opened in January 2007 is the world's second longest highway
    Highway
    A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...

     tunnel and the longest road tunnel in Asia, at 18 km (11.2 mi).

  • The longest canal tunnel is the Rove Tunnel
    Rove Tunnel
    The Rove Tunnel is a canal tunnel in France. It is about 4.5 mi long and 72 ft wide and opened in 1927. The Rove Tunnel remains the biggest canal tunnel in the world. The tunnel was closed in 1963 after its collapse....

     in France, over 7.12 km (4.4 mi) long.

Notable

  • The Central Artery Tunnel in Boston
    Boston
    Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...

     carries approximately 200,000 vehicles/day.
  • The Fredhälls Tunnel
    Essingeleden
    Essingeleden is a motorway that goes from Solna to Stockholm, Sweden, crossing the westmost parts of central Stockholm, by going over Kungsholmen, Lilla Essingen, and Stora Essingen....

     in Stockholm
    Stockholm
    Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...

    , Sweden
    Sweden
    Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

    , and the New Elbe Tunnel in Hamburg
    Hamburg
    -History:The first historic name for the city was, according to Claudius Ptolemy's reports, Treva.But the city takes its modern name, Hamburg, from the first permanent building on the site, a castle whose construction was ordered by the Emperor Charlemagne in AD 808...

    , Germany
    Germany
    Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

    , both with around 150,000 vehicles a day, two of the most trafficked tunnels in the world.
  • Gerrards Cross tunnel
    Gerrards Cross railway station
    Gerrards Cross railway station serves the villages of Gerrards Cross and Chalfont St Peter in Buckinghamshire. It is in a cutting on the Chiltern Main Line between and ....

     in Britain is notable in that it is being built over a railway cutting that was dug in the early part of the 20th Century. Thus, arguably, making it the tunnel longest in construction by the cut and cover method. When complete a branch of the Tesco
    Tesco
    Tesco plc is a global grocery and general merchandise retailer headquartered in Cheshunt, United Kingdom. It is the third-largest retailer in the world measured by revenues and the second-largest measured by profits...

     supermarket chain will occupy the space above the railway tunnel.
  • Williamson's tunnels
    Williamson's tunnels
    The Williamson Tunnels consist of a labyrinth of tunnels in the Edge Hill area of Liverpool, England, which were built under the direction of the eccentric businessman Joseph Williamson between the early 19th century and 1840. They remained derelict, filled with rubble and refuse, until...

     in Liverpool
    Liverpool
    Liverpool is a city and metropolitan borough of Merseyside, England, along the eastern side of the Mersey Estuary. It was founded as a borough in 1207 and was granted city status in 1880...

    , built by a wealthy eccentric are probably the largest underground folly in the world.
  • New York City Water Tunnel No. 3
    New York City Water Tunnel No. 3
    New York City Water Tunnel No. 3 is the largest capital construction project in New York state's history and among the most complex engineering projects in the world today. It is being constructed by the New York City Department of Environmental Protection , and is intended to provide the City with...

    http://www.nyc.gov/html/dep/html/news/3rdtunnel.html, started in 1970, has an expected completion date of 2020.
  • The Chicago Deep Tunnel Project
    Tunnel and Reservoir Plan
    The Tunnel and Reservoir Plan is a large civil engineering project that aims to reduce flooding in the metropolitan Chicago area, and to reduce the harmful effects of flushing raw sewage into Lake Michigan by diverting storm water and sewage into temporary holding reservoirs...

     is a network of 175 km (108.7 mi) 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
    Moffat Tunnel
    The Moffat Tunnel is a railroad and water tunnel that cuts through the Continental Divide in north-central Colorado. Named after Colorado railroad pioneer David Moffat, the tunnel's first railroad traffic passed through in February 1928....

     in Colorado
    Colorado
    Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...

     straddles the Continental Divide
    Continental Divide
    The Continental Divide of the Americas, or merely the Continental Gulf of Division or Great Divide, is the name given to the principal, and largely mountainous, hydrological divide of the Americas that separates the watersheds that drain into the Pacific Ocean from those river systems that drain...

    . The tunnel is 6.2 mi (10 km) long and at 9239 ft (2,816 m) above sea level is the highest railroad tunnel in the United States
    United States
    The United States of America is a federal constitutional republic comprising fifty states and a federal district...

    .
  • The Fenghuoshan tunnel
    Fenghuoshan tunnel
    The Fenghuoshan Tunnel is the highest railway tunnel in the world. It is 1,338 metres long, and stands 4,905 meters above sea level...

     on Qinghai-Tibet railway is the world's highest railway tunnel, about 4905 m (16,092.5 ft) above sea level.
  • The La Linea Tunnel in Colombia
    Colombia
    Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...

    , will be (2013) the longest, 8.58 km (5.3 mi), mountain tunnel in South America. It crosses beneath a mountain at 2500 m (8,202.1 ft) above sea level with six lanes and it has a parallel emergency tunnel. The tunnel is subject to serious groundwater pressure. The tunnel, which is currently under construction, will link Bogotá
    Bogotá
    Bogotá, Distrito Capital , from 1991 to 2000 called Santa Fé de Bogotá, is the capital, and largest city, of Colombia. It is also designated by the national constitution as the capital of the department of Cundinamarca, even though the city of Bogotá now comprises an independent Capital district...

     and its urban area with the coffee-growing region and with the main port on the Colombian Pacific coast.
  • The Honningsvåg Tunnel
    Honningsvåg Tunnel
    The Honningsvåg Tunnel or Honningsvågtunnelen is a road tunnel on the island of Magerøya, in Finnmark, Norway.It is located slightly north from the North Cape Tunnel, and it is part of European route E69. The length is . It was opened in 1999, at the same time as the North Cape Tunnel. The tunnel...

     (4.443 km (2.8 mi) long) on European route E69
    European route E69
    European route E 69 is an E-road between Olderfjord and North Cape in northern Norway. The road is long. It contains five tunnels, totalling...

     in Norway is the world's northernmost road tunnel, except for mines (which exist on Svalbard
    Svalbard
    Svalbard is an archipelago in the Arctic, constituting the northernmost part of Norway. It is located north of mainland Europe, midway between mainland Norway and the North Pole. The group of islands range from 74° to 81° north latitude , and from 10° to 35° east longitude. Spitsbergen is the...

    ).
  • The Eiksund Tunnel
    Eiksund Tunnel
    The Eiksund tunnel is an undersea tunnel between the municipalities of Ørsta and Ulstein in Møre og Romsdal county, Norway. The Eiksund Bridge joins Hareidlandet island and the village of Eiksund with the nearby island Eika. The Eiksund Tunnel begins at the south end of the bridge and connects...

     http://www.tunnelintelligence.com/safety-in-detail-106.html on national road Rv 653 in Norway is the world's deepest subsea road tunnel (7,776 m long, with deepest point at -287 metres below the sea level, opened in feb. 2008)

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
Tunnel warfare
Tunnel warfare is a type of warfare that occurs in tunnels.It can be sometimes linked with urban warfare, as tunnels are often found in urban area and cities, although urban warfare as a whole usually dominates any tunnel considerations...

, or civilian responses to threat of attack. The use of tunnels for mining is called drift mining
Drift mining
Drift mining is either the mining of a placer deposit by underground methods, or the working of coal seams accessed by adits driven into the surface outcrop of the coal bed. Drift is a more general mining term, meaning a near-horizontal passageway in a mine, following the bed or vein of ore. A...

. One of the strangest uses of a tunnel was for the storage of chemical weapons http://www.mustardgas.org.

Natural tunnels

  • Lava tubes are partially empty, cave-like conduits underground, formed during volcanic eruptions by flowing and cooling lava.
  • Natural Tunnel State Park
    Natural Tunnel State Park
    Natural Tunnel State Park is a Virginia state park, centered around the Natural Tunnel, a massive naturally formed cave that is so large it is used as a railroad tunnel...

     (Virginia, USA) features an 850 feet (259.1 m) natural tunnel, really a limestone cave, that has been used as a railroad tunnel since 1890.
  • Punarjani Guha
    Punarjani Guha
    Punarjani Guha is a natural tunnel 150 meters long in a rocky cliff in Thrissur district of Kerala state, south India. Punarjani Guha is situated 2 kilometres from the Thiruvilwamala Temple where the chief presiding deity is Sri Rama. The idol is believed to be ‘swayamboo’ i.e. born on its own...

     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.
  • Small "snow tunnels" are created by voles, chipmunks and other rodents for protection and access to food sources. For more information regarding tunnels built by animals, see Burrow
    Burrow
    A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...


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
Permanent way
The permanent way is the elements of railway lines: generally the pairs of rails typically laid on the sleepers embedded in ballast, intended to carry the ordinary trains of a railway...

 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 Mittagong was replaced with a double line tunnel, with the original tunnel used for growing mushrooms. The Rhyndaston Tunnel
Rhyndaston Tunnel
The Rhyndaston Tunnel is a 955 m long 1 in 40 grade railway tunnel in southern Tasmania. The northern end of the tunnel is 72.5 km from the Hobart railway yards...

 was enlarged using a borrowed Tunnel Boring Machine
Tunnel boring machine
A tunnel boring machine also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre to almost 16 metres to date...

 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: http://i34.tinypic.com/23ixthy.jpg http://i36.tinypic.com/16k1ahx.jpg
There are other occurrences of tunnels being replaced by open cuts, for example, the Auburn Tunnel
Auburn Tunnel
Auburn Tunnel was a 19th century canal tunnel built for the Schuylkill Canal, near Auburn, Pennsylvania. Auburn Tunnel was the first transportation tunnel built in the United States....

.

Accidents

  • Clayton Tunnel rail crash
    Clayton Tunnel rail crash
    The Clayton Tunnel rail crash, which took place on Sunday 25 August 1861, five miles from Brighton on the south coast of England, was the worst accident of the British railway system to that time...

     - 1861 - confusion about block signals
  • Balvano train disaster
    Balvano train disaster
    In the Balvano train disaster of March 2/3, 1944, some 426 people illegally riding a steam-hauled freight train died of carbon monoxide poisoning when the train stalled on a steep gradient in the Armi tunnel. The accident occurred in southern Italy, near Balvano .-Circumstances:Naples suffered...

     - 1944 -
  • 1996 Channel Tunnel fire - 1996 -
  • Gotthard Road Tunnel
    Gotthard Road Tunnel
    The St. Gotthard Tunnel in Switzerland runs from Göschenen in the Canton of Uri at its northern portal, to Airolo in Ticino to the south, and is just under in length below the St. Gotthard Pass...

     - 2001 -
  • Bujanov (railway) Tunnel
    Bujanov Tunnel
    The Bujanov Tunnel is the longest double track railway tunnel in Slovakia, on the Margecany - Košice track. It is 3,411 m long and is the second longest railway tunnel in Slovakia . It was opened in 1955.-External links:...

  • Harmanec (railway) Tunnel
    Harmanec Tunnel
    The Harmanec Tunnel is a railway tunnel on the Banská Bystrica - Dolná Štubňa track. It is 4 698 m long and it is the longest railway tunnel in Slovakia. It was built from 1936 to 1940.-Sources:...

     -

Further reading


External links

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