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A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two portals common at each end, though there may be access and ventilation openings at various points along the length. A
pipeline A pipeline is a system of Pipe (fluid conveyance), pipes for long-distance transportation of a liquid or gas, typically to a market area for consumption. The latest data from 2014 gives a total of slightly less than of pipeline in 120 countries ...
differs significantly from a tunnel, though some recent tunnels have used
immersed tube An immersed tube (or immersed tunnel) is a kind of undersea tunnel composed of segments, Modular construction, constructed elsewhere and floated to the tunnel site to be sunk into place and then linked together. They are commonly used for road an ...
construction techniques rather than traditional tunnel boring methods. A tunnel may be for foot or vehicular
road A road is a thoroughfare used primarily for movement of traffic. Roads differ from streets, whose primary use is local access. They also differ from stroads, which combine the features of streets and roads. Most modern roads are paved. Th ...
traffic Traffic is the movement of vehicles and pedestrians along land routes. Traffic laws govern and regulate traffic, while rules of the road include traffic laws and informal rules that may have developed over time to facilitate the orderly an ...
, for rail traffic, or for a
canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface ...
. The central portions of a
rapid transit Rapid transit or mass rapid transit (MRT) or heavy rail, commonly referred to as metro, is a type of high-capacity public transport that is generally built in urban areas. A grade separation, grade separated rapid transit line below ground su ...
network are usually in the tunnel. Some tunnels are used as sewers or aqueducts to supply water for consumption or for
hydroelectric Hydroelectricity, or hydroelectric power, is Electricity generation, electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other Renewable energ ...
stations.
Utility tunnel A utility tunnel, utility corridor, or utilidor is a passage built underground or above ground to carry utility lines such as electricity, steam, water supply pipes, and sewer pipes. Communications utilities like fiber optics, cable television, ...
s are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.Salazar, Waneta. ''Tunnels in Civil Engineering''. Delhi, India : White Word Publications, 2012. Secret tunnels are built for military purposes, or by civilians for smuggling of
weapon A weapon, arm, or armament is any implement or device that is used to deter, threaten, inflict physical damage, harm, or kill. Weapons are used to increase the efficacy and efficiency of activities such as hunting, crime (e.g., murder), law ...
s,
contraband Contraband (from Medieval French ''contrebande'' "smuggling") is any item that, relating to its nature, is illegal to be possessed or sold. It comprises goods that by their nature are considered too dangerous or offensive in the eyes of the leg ...
, or
people The term "the people" refers to the public or Common people, common mass of people of a polity. As such it is a concept of human rights law, international law as well as constitutional law, particularly used for claims of popular sovereignty. I ...
. Special tunnels, such as
wildlife crossing Wildlife crossings are structures that allow animals to cross human-made barriers safely. Wildlife crossings may include underpass tunnels or wildlife tunnels, viaducts, and overpasses or green bridges (mainly for large or herd-type animals ...
s, are built to allow wildlife to cross human-made barriers safely. Tunnels can be connected together in
tunnel network In transport, tunnels can be connected together to form a tunnel network. These can be used in mining to reach ore below ground, in cities for underground rapid transit systems, in sewer systems, in warfare to avoid enemy detection or attacks, ...
s. A tunnel is relatively long and narrow; the length is often much greater than twice the
diameter In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest Chord (geometry), chord of the circle. Both definitions a ...
, 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, in the United Kingdom, a road tunnel 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 ."


Etymology

The word "tunnel" comes from the
Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman Conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English pe ...
''tonnelle'', meaning "a net", derived from
Old French Old French (, , ; ) was the language spoken in most of the northern half of France approximately between the late 8th [2-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ...
''tonnel'', a diminutive of ''tonne'' ("cask"). The modern meaning, referring to an underground passageway, evolved in the 16th century as a metaphor for a narrow, confined space like the inside of a cask.


History

It's more likely that the first tunneling was done made by prehistoric people seeking to enlarge their Cave, caves. Babylon, about 2200 B.C., it is believed that the first artificial tunnel was constructed. To join the temple of Belos with the palace, this was built with the aid of the cut and cover technique. Some of the earliest tunnels used by humans were paleoburrows excavated by prehistoric mammals. Much of the early technology of tunnelling evolved from
mining Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...
and military engineering. The etymology of the terms "mining" (for mineral extraction or for siege attacks), " military engineering", and "
civil engineering Civil engineering is a regulation and licensure in engineering, professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads ...
" reveals these deep historic connections.


Antiquity and early middle ages

Predecessors of modern tunnels were adits that transported water for
irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...
, drinking, or sewerage. The first qanats are known from before 2000 BC. The earliest tunnel known to have been excavated from both ends is the Siloam Tunnel, built in
Jerusalem Jerusalem is a city in the Southern Levant, on a plateau in the Judaean Mountains between the Mediterranean Sea, Mediterranean and the Dead Sea. It is one of the List of oldest continuously inhabited cities, oldest cities in the world, and ...
by the kings of Judah around the 8th century BC. Another tunnel excavated from both ends, maybe the second known, is the Tunnel of Eupalinos, which is a tunnel aqueduct long running through Mount Kastro in Samos, Greece. It was built in the 6th century BC to serve as an aqueduct. In
Pakistan Pakistan, officially the Islamic Republic of Pakistan, is a country in South Asia. It is the List of countries and dependencies by population, fifth-most populous country, with a population of over 241.5 million, having the Islam by country# ...
, the mughal era tunnel has been restored in the
Lahore Lahore ( ; ; ) is the capital and largest city of the Administrative units of Pakistan, Pakistani province of Punjab, Pakistan, Punjab. It is the List of cities in Pakistan by population, second-largest city in Pakistan, after Karachi, and ...
. In
Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...
, the Siqurto foot tunnel, hand-hewn in the Middle Ages, crosses a mountain ridge. In the
Gaza Strip The Gaza Strip, also known simply as Gaza, is a small territory located on the eastern coast of the Mediterranean Sea; it is the smaller of the two Palestinian territories, the other being the West Bank, that make up the State of Palestine. I ...
, the network of tunnels was used by Jewish strategists as rock-cut shelters, in first links to Judean resistance against Roman rule in the
Bar Kokhba revolt The Bar Kokhba revolt (132–136 AD) was a major uprising by the Jews of Judaea (Roman province), Judaea against the Roman Empire, marking the final and most devastating of the Jewish–Roman wars. Led by Simon bar Kokhba, the rebels succeeded ...
during the 2nd century AD.


Geotechnical investigation and design

A major 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 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. *
Groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
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. A very effective but expensive technology is ground freezing, 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, both navigational and traffic considerations may limit the use of high bridges or
drawbridge A drawbridge or draw-bridge is a type of moveable bridge typically at the entrance to a castle or tower surrounded by a moat. In some forms of English, including American English, the word ''drawbridge'' commonly refers to all types of moveable b ...
s intersecting with shipping channels, necessitating a tunnel. Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as
Manhattan Manhattan ( ) is the most densely populated and geographically smallest of the Boroughs of New York City, five boroughs of New York City. Coextensive with New York County, Manhattan is the County statistics of the United States#Smallest, larg ...
and urban
Hong Kong Hong Kong)., Legally Hong Kong, China in international treaties and organizations. is a special administrative region of China. With 7.5 million residents in a territory, Hong Kong is the fourth most densely populated region in the wor ...
, this is a strong factor in favor of a tunnel. 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 Tunnel under the River Mersey at
Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...
was chosen over a massively high bridge partly for defence reasons; it was feared that aircraft could destroy a bridge in times of war, not merely impairing road traffic but blocking the river to navigation. 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 Kingsway Tunnel under the Mersey. In Hampton Roads, Virginia, tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges might prevent
US Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...
vessels from leaving
Naval Station Norfolk Naval Station Norfolk is a United States Navy base in Norfolk, Virginia, that is the headquarters and home port of the U.S. Navy's Fleet Forces Command. The installation occupies about of waterfront space and of pier and wharf space of the Ham ...
. Water-crossing tunnels built instead of bridges include the Seikan Tunnel in Japan; the Holland Tunnel and Lincoln Tunnel between
New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...
and Manhattan in
New York City New York, often called New York City (NYC), is the most populous city in the United States, located at the southern tip of New York State on one of the world's largest natural harbors. The city comprises five boroughs, each coextensive w ...
; the Queens-Midtown Tunnel between Manhattan and the
borough A borough is an administrative division in various English language, English-speaking countries. In principle, the term ''borough'' designates a self-governing walled town, although in practice, official use of the term varies widely. History ...
of
Queens Queens is the largest by area of the Boroughs of New York City, five boroughs of New York City, coextensive with Queens County, in the U.S. state of New York (state), New York. Located near the western end of Long Island, it is bordered by the ...
on
Long Island Long Island is a densely populated continental island in southeastern New York (state), New York state, extending into the Atlantic Ocean. It constitutes a significant share of the New York metropolitan area in both population and land are ...
; the Detroit-Windsor Tunnel between
Michigan Michigan ( ) is a peninsular U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, Upper Midwestern United States. It shares water and land boundaries with Minnesota to the northwest, Wisconsin to the west, ...
and
Ontario Ontario is the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...
; and the Elizabeth River tunnels between
Norfolk Norfolk ( ) is a Ceremonial counties of England, ceremonial county in England, located in East Anglia and officially part of the East of England region. It borders Lincolnshire and The Wash to the north-west, the North Sea to the north and eas ...
and
Portsmouth, Virginia Portsmouth is an Independent city (United States), independent city in southeastern Virginia, United States. It lies across the Elizabeth River (Virginia), Elizabeth River from Norfolk, Virginia, Norfolk. As of the 2020 United States census, 2020 ...
; the 1934 River Mersey road Queensway Tunnel; the Western Scheldt Tunnel, Zeeland, Netherlands; and the North Shore Connector tunnel in
Pittsburgh, Pennsylvania Pittsburgh ( ) is a city in Allegheny County, Pennsylvania, United States, and its county seat. It is the List of municipalities in Pennsylvania#Municipalities, second-most populous city in Pennsylvania (after Philadelphia) and the List of Un ...
. The Sydney Harbour Tunnel was constructed to provide a second harbour crossing and to alleviate traffic congestion on the Sydney Harbour Bridge, without spoiling the iconic view. Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the
Channel Tunnel The Channel Tunnel (), sometimes referred to by the Portmanteau, portmanteau Chunnel, is a undersea railway tunnel, opened in 1994, that connects Folkestone (Kent, England) with Coquelles (Pas-de-Calais, France) beneath the English Channel at ...
), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge). Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in
Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...
. There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in
Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...
in 2001. One of the worst railway disasters ever, the Balvano train disaster, was caused by a train stalling in the Armi tunnel in
Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...
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 Project management is the process of supervising the work of a Project team, team to achieve all project goals within the given constraints. This information is usually described in project initiation documentation, project documentation, crea ...
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 A work-breakdown structure (WBS) in project management and systems engineering is a breakdown of a project into smaller components. It is a key project management element that organizes the team's work into manageable sections. The Project Mana ...
and
critical path method The critical path method (CPM), or critical path analysis (CPA), is an algorithm for schedule (project management), scheduling a set of project activities. A critical path is determined by identifying the longest stretch of dependent activiti ...
. 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 The Port Authority of New York and New Jersey, (PANYNJ; stylized, in logo since 2020, as Port Authority NY NJ) is a joint venture between the U.S. states of New York (state), New York and New Jersey, established in 1921 through an interstate c ...
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 groundwater 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 the 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
trench A trench is a type of digging, excavation or depression in the ground that is generally deeper than it is wide (as opposed to a swale (landform), swale or a bar ditch), and narrow compared with its length (as opposed to a simple hole or trapping ...
is excavated and
roof A roof (: roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of tempera ...
ed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. There are two basic forms of cut-and-cover tunnelling: * ''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
slurry wall A slurry wall is a civil engineering technique used to build reinforced concrete walls in areas of soft earth close to open water, or with a high groundwater table. This technique is typically used to build diaphragm (water-blocking) walls surro ...
ing or contiguous bored piling. Only a shallow excavation is needed to construct the tunnel roof using precast beams or in situ concrete sitting on the walls. 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 metro 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 The London Underground (also known simply as the Underground or as the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England. The Undergro ...
, 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. Prior to the replacement of manual excavation by the use of boring machines, Victorian tunnel excavators developed a specialized method called clay-kicking for digging tunnels in clay-based soils. The clay-kicker lies on a plank at a 45-degree angle away from the working face and rather than a mattock with his hands, inserts with his feet a tool with a cup-like rounded end, then turns the tool with his hands to extract a section of soil, which is then placed on the waste extract. Clay-kicking is a specialized method developed in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
of digging tunnels in strong clay-based soil structures. This method of cut and cover construction required relatively little disturbance of property during the renewal of the United Kingdom's then ancient sewerage systems. It was also used during the
First World War World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
by Royal Engineer tunnelling companies placing mines beneath German lines, because it was almost silent and so 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 tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as a 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. 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 TBMs, the bentonite slurry and earth-pressure balance types, have pressurized compartments at the front end, allowing them to be used in difficult conditions below the
water table The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...
. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurized 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 airlock/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 canton of Glarus ( ; ; ; ) is a cantons of Switzerland, canton in east-central Switzerland. The capital is Glarus. The population speaks a variety of Alemannic German. The majority of the population (81%) identifies as Christianity in Switzer ...
. The borehole has a diameter of . The four TBMs used for excavating the Gotthard Base Tunnel, in
Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...
, had a diameter of about . A larger TBM was built to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands, with a diameter of . This in turn was superseded by the Madrid M30 ringroad, Spain, and the Chong Ming tunnels in
Shanghai Shanghai, Shanghainese: , Standard Chinese pronunciation: is a direct-administered municipality and the most populous urban area in China. The city is located on the Chinese shoreline on the southern estuary of the Yangtze River, with the ...
, China. All of these machines were built at least partly by Herrenknecht. , the world's largest TBM was " Big Bertha", a diameter machine built by Hitachi Zosen Corporation, which dug the Alaskan Way Viaduct replacement tunnel in
Seattle, Washington Seattle ( ) is the List of municipalities in Washington, most populous city in the U.S. state of Washington (state), Washington and in the Pacific Northwest region of North America. With a population of 780,995 in 2024, it is the List of Unit ...
(US).


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
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
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
geotechnical Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. I ...
measurements, an optimal cross section is computed. The excavation is protected by a layer of sprayed concrete, commonly referred to as shotcrete. Other support measures can include steel arches, rock bolts, and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibers 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 geomechanical rock consistency during the tunneling work. The measured rock properties lead to appropriate
tool A tool is an Physical object, object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many Tool use by animals, animals use simple tools, only human bei ...
s for tunnel strengthening.


Pipe jacking

In pipe jacking,
hydraulic jack A jack is a mechanical lifting device used to apply great forces or lift heavy loads. A mechanical jack employs a screw thread for lifting heavy equipment. A hydraulic jack uses hydraulic machinery, hydraulic power. The most common form is a car ...
s 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.


Underwater tunnels

There are also several approaches to underwater tunnels, the two most common being bored tunnels or
immersed tube An immersed tube (or immersed tunnel) is a kind of undersea tunnel composed of segments, Modular construction, constructed elsewhere and floated to the tunnel site to be sunk into place and then linked together. They are commonly used for road an ...
s, examples are Bjørvika Tunnel and Marmaray. 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 excavated spoil, often
narrow gauge A narrow-gauge railway (narrow-gauge railroad in the US) is a railway with a track gauge (distance between the rails) narrower than . Most narrow-gauge railways are between and . Since narrow-gauge railways are usually built with Minimum railw ...
so that it can be double track to allow the operation of empty and loaded trains at the same time. The temporary way is replaced by the permanent way at completion, thus explaining the term " Perway".


Enlargement

The vehicles or traffic using a tunnel can outgrow it, requiring replacement or enlargement: * The original single line Gib Tunnel near Mittagong was replaced with a double-track tunnel, with the original tunnel used for growing mushrooms. * The 1832 double-track -long tunnel from Edge Hill to Lime Street in
Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...
was near totally removed, apart from a 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 Auburn Tunnel. * The Farnworth Tunnel in England was enlarged using a tunnel boring machine (TBM) in 2015. The Rhyndaston Tunnel was enlarged using a borrowed TBM so as to be able to take ISO containers. * 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 Eurasia Tunnel under the Bosphorus, 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 ''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 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
Shanghai Shanghai, Shanghainese: , Standard Chinese pronunciation: is a direct-administered municipality and the most populous urban area in China. The city is located on the Chinese shoreline on the southern estuary of the Yangtze River, with the ...
, China, a two-tube double-deck tunnel was built starting in 2002. In each tube of the 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
Maastricht Maastricht ( , , ; ; ; ) is a city and a Municipalities of the Netherlands, municipality in the southeastern Netherlands. It is the capital city, capital and largest city of the province of Limburg (Netherlands), Limburg. Maastricht is loca ...
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. The Alaskan Way Viaduct replacement tunnel, is a $3.3 billion , double-decker bored highway tunnel under Downtown Seattle. Construction began in July 2013 using " Bertha", at the time the world's largest earth pressure balance tunnel boring machine, with a cutterhead diameter. After several delays, tunnel boring was completed in April 2017, and the tunnel opened to traffic on 4 February 2019.
New York City New York, often called New York City (NYC), is the most populous city in the United States, located at the southern tip of New York State on one of the world's largest natural harbors. The city comprises five boroughs, each coextensive w ...
's
63rd Street Tunnel The 63rd Street Tunnel is a double-deck subway and railroad tunnel under the East River between the Borough of New York City, boroughs of Manhattan and Queens in New York City. Opened in 1989, it is the newest of the East River tunnels, as we ...
under the
East River The East River is a saltwater Estuary, tidal estuary or strait in New York City. The waterway, which is not a river despite its name, connects Upper New York Bay on its south end to Long Island Sound on its north end. It separates Long Island, ...
, between the boroughs of
Manhattan Manhattan ( ) is the most densely populated and geographically smallest of the Boroughs of New York City, five boroughs of New York City. Coextensive with New York County, Manhattan is the County statistics of the United States#Smallest, larg ...
and
Queens Queens is the largest by area of the Boroughs of New York City, five boroughs of New York City, coextensive with Queens County, in the U.S. state of New York (state), New York. Located near the western end of Long Island, it is bordered by the ...
, was intended to carry subway trains on the upper level and
Long Island Rail Road The Long Island Rail Road , or LIRR, is a Rail transport, railroad in the southeastern part of the U.S. state of New York (state), New York, stretching from Manhattan to the eastern tip of Suffolk County, New York, Suffolk County on Long Islan ...
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, saw passenger service after completion of the
East Side Access East Side Access (ESA) is a public works project in New York City that extended the Long Island Rail Road (LIRR) two miles from its Main Line (Long Island Rail Road), Main Line in Queens to the new Grand Central Madison station under Grand Cent ...
project, in late 2022. In the UK, the 1934 Queensway Tunnel under the River Mersey between
Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...
and
Birkenhead Birkenhead () is a town in the Metropolitan Borough of Wirral, Merseyside, England. The town is on the Wirral Peninsula, along the west bank of the River Mersey, opposite Liverpool. It lies within the Historic counties of England, historic co ...
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 The Lion Rock Tunnel, the first major road tunnel in Hong Kong, is a twin-board toll tunnel, connecting Hin Tin, Sha Tin District, Sha Tin in the New Territories and New Kowloon near Kowloon Tong. It has two southbound lanes, and there are ...
, built in the mid 1960s, connecting New Kowloon and Sha Tin, carries a motorway but also serves as an aqueduct, featuring a gallery containing five water mains lines with diameters between below the road section of the tunnel.
Wuhan Wuhan; is the capital of Hubei, China. With a population of over eleven million, it is the most populous city in Hubei and the List of cities in China by population, eighth-most-populous city in China. It is also one of the nine National cent ...
's Yangtze River Highway and Railway Tunnel is a two-tube double-deck tunnel under the Yangtze River completed in 2018. Each tube carries three lanes of local traffic on the top deck with one track
Wuhan Metro Wuhan Metro is a rapid transit system serving the city of Wuhan, Hubei, China. Owned and operated by Wuhan Metro Group Co., Ltd., the network now includes 12 lines, 312 stations, and 518 kilometres of track length. With 1.35 billion annual passe ...
Line 7 on the lower deck. Mount Baker Tunnel has three levels. The bottom level is to be used by Sound Transit light rail. The middle level is used by car traffic, and the top layer is for bicycle and pedestrian access. Some tunnels have more than one purpose. The SMART Tunnel in
Malaysia Malaysia is a country in Southeast Asia. Featuring the Tanjung Piai, southernmost point of continental Eurasia, it is a federation, federal constitutional monarchy consisting of States and federal territories of Malaysia, 13 states and thre ...
is the first multipurpose "''Stormwater Management And Road Tunnel''" in the world, created to convey both traffic and occasional
flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...
waters in
Kuala Lumpur Kuala Lumpur (KL), officially the Federal Territory of Kuala Lumpur, is the capital city and a Federal Territories of Malaysia, federal territory of Malaysia. It is the largest city in the country, covering an area of with a census population ...
. 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.


Covered passageways

Over-bridges can sometimes be built by covering a road or river or railway with
brick A brick is a type of construction material used to build walls, pavements and other elements in masonry construction. Properly, the term ''brick'' denotes a unit primarily composed of clay. But is now also used informally to denote building un ...
or
steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
arch An arch is a curved vertical structure spanning an open space underneath it. Arches may support the load above them, or they may perform a purely decorative role. As a decorative element, the arch dates back to the 4th millennium BC, but stru ...
es, and then levelling the surface with earth. In railway parlance, a surface-level track which has been built or covered over is normally called a "covered way". Snow sheds are a kind of artificial tunnel built to protect a railway from
avalanche An avalanche is a rapid flow of snow down a Grade (slope), slope, such as a hill or mountain. Avalanches can be triggered spontaneously, by factors such as increased precipitation or snowpack weakening, or by external means such as humans, othe ...
s of snow. Similarly the Stanwell Park,
New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...
"steel tunnel", on the Illawarra railway line, protects the line from rockfalls.


Underpass

An underpass is a road or railway or other passageway passing ''under'' another road or railway, under an overpass. This is not strictly a tunnel.


Utility Tunnel

A
Utility Tunnel A utility tunnel, utility corridor, or utilidor is a passage built underground or above ground to carry utility lines such as electricity, steam, water supply pipes, and sewer pipes. Communications utilities like fiber optics, cable television, ...
is built for the purpose of carrying one or more
utilities A public utility company (usually just utility) is an organization that maintains the infrastructure for a public service (often also providing a service using that infrastructure). Public utilities are subject to forms of public control and r ...
in the same space, for this reason they are also referred to as Multi-Utility Tunnels or MUTs. Through co-location of different utilities in one tunnel, organizations are able to reduce the financial and environmental costs of building and maintaining utilities. These tunnels can be used for many types of utilities, routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.


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
carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
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 Carbon monoxide poisoning typically occurs from breathing in carbon monoxide (CO) at excessive levels. Symptoms are often described as " flu-like" and commonly include headache, dizziness, weakness, vomiting, chest pain, and confusion. Large ...
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. Likewise 84 people were killed in the Paris Métro train fire of 1904. Motor vehicle tunnels usually require ventilation shafts and powered fans to remove toxic exhaust gases 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 firefighters. The
aerodynamic Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...
pressure wave produced by high speed trains entering a tunnel reflect at its open ends and change sign ( compression wavefront changes to rarefaction wavefront and vice versa). When two wavefronts of the same sign meet the train, significant and rapid air pressure may cause ear discomfort for passengers and crew. When a high-speed train exits a tunnel, a loud " Tunnel boom" may occur, which can disturb residents near the mouth of the tunnel, and it is exacerbated in mountain valleys where the sound can echo. 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 Big Dig tunnel in
Boston, Massachusetts Boston is the capital and most populous city in the Commonwealth (U.S. state), Commonwealth of Massachusetts in the United States. The city serves as the cultural and Financial centre, financial center of New England, a region of the Northeas ...
, may have a dedicated 24-hour staffed 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. A database of seismic damage to underground structures using 217 case histories shows the following general observations can be made regarding the seismic performance of underground structures: * Underground structures suffer appreciably less damage than surface structures. * Reported damage decreases with increasing over burden depth. Deep tunnels seem to be safer and less vulnerable to earthquake shaking than are shallow tunnels. * Underground facilities constructed in soils can be expected to suffer more damage compared to openings constructed in competent rock. * Lined and grouted tunnels are safer than unlined tunnels in rock. Shaking damage can be reduced by stabilizing the ground around the tunnel and by improving the contact between the lining and the surrounding ground through grouting. * Tunnels are more stable under a symmetric load, which improves ground-lining interaction. Improving the tunnel lining by placing thicker and stiffer sections without stabilizing surrounding poor ground may result in excess seismic forces in the lining. Backfilling with non-cyclically mobile material and rock-stabilizing measures may improve the safety and stability of shallow tunnels. * Damage may be related to peak ground acceleration and velocity based on the magnitude and epicentral distance of the affected earthquake. * Duration of strong-motion shaking during earthquakes is of utmost importance because it may cause fatigue failure and therefore, large deformations. * High frequency motions may explain the local spalling of rock or concrete along planes of weakness. These frequencies, which rapidly attenuate with distance, may be expected mainly at small distances from the causative fault. * Ground motion may be amplified upon incidence with a tunnel if wavelengths are between one and four times the tunnel diameter. * Damage at and near tunnel portals may be significant due to slope instability. 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 shook Mexico City; there was no damage to the subway system, and in fact the subway systems served as a lifeline for emergency personnel and evacuations. A magnitude 7.2 ripped 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, afflicted 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.


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. Some well known ancient and modern tunnels are briefly introduced below: * The qanat or kareez of
Persia Iran, officially the Islamic Republic of Iran (IRI) and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the nort ...
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 Gonabad, which after 2700 years, still provides drinking and agricultural water to nearly 40,000 people. Its main well depth is more than , and its length is . * The Siloam Tunnel was built before 701 BC for a reliable supply of water, to withstand
siege A siege () . is a military blockade of a city, or fortress, with the intent of conquering by attrition, or by well-prepared assault. Siege warfare (also called siegecrafts or poliorcetics) is a form of constant, low-intensity conflict charact ...
attacks. * The Eupalinian aqueduct on the island of Samos ( North Aegean, Greece) was built in 520 BC by the ancient Greek engineer Eupalinos of Megara 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 Pythagoreion, the ancient capital of Samos. 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 tunnels was constructed at Persepolis in Iran at the same time as the construction of its foundation in 518 BC. 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 archaeological teams. * The
Via Flaminia The Via Flaminia () was an ancient Roman roads, Roman road leading from Rome over the Apennine Mountains to ''Ariminum'' (Rimini) on the coast of the Adriatic Sea, and due to the ruggedness of the mountains was the major option the Romans had f ...
, an important
Roman road Roman roads ( ; singular: ; meaning "Roman way") were physical infrastructure vital to the maintenance and development of the Roman state, built from about 300 BC through the expansion and consolidation of the Roman Republic and the Roman Em ...
, penetrated the Furlo pass in the Apennines through a tunnel which emperor
Vespasian Vespasian (; ; 17 November AD 9 – 23 June 79) was Roman emperor from 69 to 79. The last emperor to reign in the Year of the Four Emperors, he founded the Flavian dynasty, which ruled the Empire for 27 years. His fiscal reforms and consolida ...
had ordered built in 76–77 AD. 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. * The world's oldest tunnel traversing under a water body is claimed to be the ''Terelek kaya tüneli'' under Kızıl River, a little south of the towns of Boyabat and Durağan in
Turkey Turkey, officially the Republic of Türkiye, is a country mainly located in Anatolia in West Asia, with a relatively small part called East Thrace in Southeast Europe. It borders the Black Sea to the north; Georgia (country), Georgia, Armen ...
, just downstream from where Kizil River joins its tributary Gökırmak. The tunnel is presently under a narrow part of a lake formed by a dam some kilometres 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
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...
, dug through hills, which opened in 1789, was long and allowed
boat A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size or capacity, its shape, or its ability to carry boats. Small boats are typically used on inland waterways s ...
transport of
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...
and other goods. Above it the Sapperton Long Tunnel was constructed which carries the "Golden Valley" railway line between
Swindon Swindon () is a town in Wiltshire, England. At the time of the 2021 Census the population of the built-up area was 183,638, making it the largest settlement in the county. Located at the northeastern edge of the South West England region, Swi ...
and
Gloucester Gloucester ( ) is a cathedral city, non-metropolitan district and the county town of Gloucestershire in the South West England, South West of England. Gloucester lies on the River Severn, between the Cotswolds to the east and the Forest of Dean ...
. * The 1791 Dudley canal tunnel is on the Dudley Canal, in
Dudley Dudley ( , ) is a market town in the West Midlands, England, southeast of Wolverhampton and northwest of Birmingham. Historically part of Worcestershire, the town is the administrative centre of the Metropolitan Borough of Dudley. In the ...
,
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...
. The tunnel is long. Closed in 1962 the tunnel was reopened in 1973. The series of tunnels was extended in 1984 and 1989. *
Fritchley Tunnel Fritchley Tunnel is a disused railway tunnel at Fritchley in Derbyshire, England, which is believed to be the oldest surviving example in the world. The tunnel was constructed in 1793 by Benjamin Outram as part of the Butterley Gangroad, altered ...
, 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. Gravity and horse haulage was utilised. 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 Butterley canal tunnel canal tunnel is in length on the Cromford Canal in
Ripley, Derbyshire Ripley is a market town and civil parish in the Amber Valley district of Derbyshire, England. It is northeast of Derby, northwest of Heanor, southwest of Alfreton and northeast of Belper. The town is continuous with Heanor, Eastwood, Nottingham ...
, England. The tunnel was built simultaneously with the 1793 Fritchley railway tunnel. 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. * The 1796 Stoddart Tunnel in Chapel-en-le-Frith in Derbyshire is reputed to be the oldest rail tunnel in the world. The rail wagons were originally horse-drawn. * Derby Tunnels in
Salem, Massachusetts Salem ( ) is a historic coastal city in Essex County, Massachusetts, United States, located on the North Shore (Massachusetts), North Shore of Greater Boston. Continuous settlement by Europeans began in 1626 with English colonists. Salem was one ...
, were built in 1801 to smuggle imports affected by President
Thomas Jefferson Thomas Jefferson (, 1743July 4, 1826) was an American Founding Fathers of the United States, Founding Father and the third president of the United States from 1801 to 1809. He was the primary author of the United States Declaration of Indepe ...
'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. * A tunnel was created for the first true steam locomotive, from
Penydarren : ''For Trevithick's Pen-y-darren locomotive, see Richard Trevithick#"Pen-y-Darren" locomotive, Richard Trevithick.'' Penydarren is a Community (Wales), community and electoral ward in Merthyr Tydfil County Borough in Wales. Description The area ...
to
Abercynon Abercynon () is a village and community (Wales), community (and electoral ward) in the Cynon Valley within the unitary authority of Rhondda Cynon Taf, Wales. The community comprises the village and the districts of Carnetown and Grovers Field 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 Pentrebach,
Merthyr Tydfil Merthyr Tydfil () is the main town in Merthyr Tydfil County Borough, Wales, administered by Merthyr Tydfil County Borough Council. It is about north of Cardiff. Often called just Merthyr, it is said to be named after Tydfil, daughter of K ...
,
Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...
. 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, Rainhill, near
Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...
, England. It is 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,
Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...
, England, 1829. Built by
George Stephenson George Stephenson (9 June 1781 – 12 August 1848) was an English civil engineer and Mechanical engineering, mechanical engineer during the Industrial Revolution. Renowned as the "Father of Railways", Stephenson was considered by the Victoria ...
, 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 Wapping Tunnel 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 Wapping Dock 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 Merseyrail 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. Over time a section of the deep cutting has been converted back into tunnel due to sections having buildings built over. * Box Tunnel 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
Shildon Shildon is a town and civil parish in County Durham (district), County Durham, in England. The population taken at the 2011 Census was 9,976. The town has the Locomotion Museum, due to it having the first , built in 1825, and locomotive works on ...
near Darlington, England, is the oldest sizeable tunnel in the world still in use under a settlement. * The Victoria Tunnel (Newcastle), Victoria Tunnel Newcastle opened in 1842, is a 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 Spital Tongues 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 Thames Tunnel, 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 The London Underground (also known simply as the Underground or as the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England. The Undergro ...
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 (Liverpool), Victoria Tunnel/Waterloo Tunnel in
Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...
, 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 Riverside railway station, 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 (Liverpool), 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 Waterloo Tunnel. 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 Merseyrail network. Stations cut into the tunnel are being considered and also reuse by a monorail system from the proposed Liverpool Waters redevelopment of Liverpool's Central Docks has been proposed. * The summit tunnel of the Semmering railway, the first Alpine tunnel, was opened in 1848 and was long. It connected rail traffic between Vienna, the capital of Austro-Hungarian Empire, and Trieste, its port. * The Giovi Rail Tunnel through the Appennini Mounts opened in 1854, linking the capital city of the Kingdom of Sardinia, Turin, to its port, Genoa. The tunnel was long. * The oldest underground sections of the
London Underground The London Underground (also known simply as the Underground or as the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England. The Undergro ...
were built using the cut-and-cover method in the 1860s, and opened in January 1863. What are now the Metropolitan line, Metropolitan, Hammersmith & City and Circle lines were the first to prove the success of a rapid transit, metro or subway system. *On 18 June 1868, the Central Pacific Railroad's Summit Tunnel (Tunnel #6) at Donner Pass in the California Sierra Nevada (U.S.), 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 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. * The Mersey Railway tunnel opened in 1886, running from Liverpool to Birkenhead under the River Mersey. The Mersey Railway 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. * The rail Severn Tunnel 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. * 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: *#shield method of excavation; *#permanent cast iron tunnel lining; *#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 Northern line 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 Simplon Tunnel, 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 Holland Tunnel was the first underwater tunnel designed for automobiles. The construction required a novel Holland Tunnel#Ventilation system, ventilation system. * In 1945 the Delaware Aqueduct tunnel was completed, supplying water to New York City. At it is the longest tunnel in the world. * In 1988 the long Seikan Tunnel in Japan was completed under the Tsugaru Strait, linking the islands of Honshu and Hokkaido. It was the longest railway tunnel in the world at that time. * Ryfast is the longest undersea road tunnel. It is in length. The tunnel opened for use in 2020.


Longest

* The Thirlmere Aqueduct in North West England, United Kingdom 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 China, opened in 2009, is the third longest water tunnel in the world at length. * The Gotthard Base Tunnel in
Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...
, opened in 2016, is the longest and deepest railway tunnel in the world at length and maximum depth below the Saint-Gotthard Massif, Gotthard Massif. It provides a NRLA, flat transit route between the North and South of Europe under the Swiss Alps, at a maximum elevation of . * The Seikan Tunnel in Japan connects the main island of Honshu with the northern island of Hokkaido by rail. It is long, of which are crossing the Tsugaru Strait undersea. * The
Channel Tunnel The Channel Tunnel (), sometimes referred to by the Portmanteau, portmanteau Chunnel, is a undersea railway tunnel, opened in 1994, that connects Folkestone (Kent, England) with Coquelles (Pas-de-Calais, France) beneath the English Channel at ...
crosses the English Channel between France and the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
. It has a total length of , of which are the world's longest undersea tunnel section. * The Lötschberg Base Tunnel in
Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...
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 Aurland 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 highway tunnel and the longest mountain road tunnel in Asia, at . * The longest canal tunnel is the Rove Tunnel in France, over long.


Notable

* The Moffat Tunnel, opened in 1928, passes under the Continental Divide of the Americas in Colorado. The tunnel is long and at an elevation of is the highest active railroad tunnel in the U.S. (The inactive Tennessee Pass Line and the historic Alpine Tunnel are higher.) * Williamson's tunnels in
Liverpool Liverpool is a port City status in the United Kingdom, city and metropolitan borough in Merseyside, England. It is situated on the eastern side of the River Mersey, Mersey Estuary, near the Irish Sea, north-west of London. With a population ...
, from 1804 and completed around 1840 by a wealthy eccentric, are probably the largest underground folly in the world. The tunnels were built with no functional purpose. * The Chicago Tunnel Company, Chicago freight tunnel network is the largest urban street tunnel network, comprising of tunnels beneath the majority of downtown Chicago streets. It operated between 1906 and 1956 as a freight network, connecting building basements and railway stations. Following a Chicago flood, 1992 flood the network was sealed, although some parts still carry utility and communications infrastructure. * 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 Tunnel, Allegheny Mountain, Tuscarora Mountain Tunnel, Tuscarora Mountain, Kittatinny Mountain Tunnel, Kittatinny Mountain, and Blue Mountain Tunnel, Blue Mountain) remain in active use, while the other three (Laurel Hill Tunnel, Laurel Hill, Rays Hill Tunnel, Rays Hill, and Sideling Hill Tunnel, 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 Essingeleden, Fredhälls road tunnel was opened in 1966, in Stockholm, Sweden, and the Elbe Tunnel (1975), 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 Svalbard). * The Central Artery road tunnel in
Boston, Massachusetts Boston is the capital and most populous city in the Commonwealth (U.S. state), Commonwealth of Massachusetts in the United States. The city serves as the cultural and Financial centre, financial center of New England, a region of the Northeas ...
, is a part of the larger Big Dig (Boston, Massachusetts), Big Dig completed around 2007, and carries approximately 200,000 vehicles/day under the city along Interstate 93, U.S. Route 1 in Massachusetts, US Route 1, and Massachusetts Route 3, which share a Concurrency (road), concurrency through the tunnels. The Big Dig replaced Boston's old badly deteriorated I-93 elevated highway. * The Stormwater Management And Road Tunnel or SMART Tunnel, is a combined storm drainage and road structure opened in 2007 in
Kuala Lumpur Kuala Lumpur (KL), officially the Federal Territory of Kuala Lumpur, is the capital city and a Federal Territories of Malaysia, federal territory of Malaysia. It is the largest city in the country, covering an area of with a census population ...
, Malaysia. The tunnel is the longest stormwater drainage tunnel 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 Eiksund Tunnel 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 Tunnel, Gerrards Cross railway tunnel, in England, opened in 2010, is notable in that it converted an existing railway cutting into a tunnel to create ground to build a supermarket over the tunnel. The railway in the cutting was first opened around 1906, stretching over 104 years to complete a railway tunnel. The tunnel was built using the cover method with craned in prefabricated forms in order to keep the busy railway operating. A branch of the Tesco supermarket chain occupies the newly created ground above the railway tunnel, with an adjacent existing railway station at the end of the tunnel. During construction, a portion of the tunnel collapsed when 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 Línea (Road Pass), La Linea Tunnel in Colombia, 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 groundwater 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 Tunnel and Reservoir Plan, Chicago Deep Tunnel Project is a network of of drainage tunnels designed to reduce flooding in the Chicago metropolitan area, Chicago area. Started in the mid-1970s, the project is due to be completed in 2029. * New York City Water Tunnel No. 3, started in 1970, has an expected completion beyond 2026, and will measure more than .


Mining

The use of tunnels for mining is called drift mining. Drift mining can help find coal, goal, iron, and other minerals, just like normal mining. Sub-surface mining consists of digging tunnels or shafts into the earth to reach buried ore deposits.


Military use

Some tunnels are not for transport at all but rather, are fortifications, for example Mittelwerk and Cheyenne Mountain Complex. Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with tunnel warfare, military use during armed conflict, or civilian responses to threat of attack. Another use for tunnels was for the storage of chemical weapon


Secret tunnels

secret passage, Secret tunnels have given entrance to or escape from an area, such as the Cu Chi Tunnels or the Gaza Strip smuggling tunnels, smuggling tunnels in the Gaza Strip which connect it to Egypt. 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 Cold War, under the Berlin Wall and elsewhere, to smuggle refugees, and for espionage. Smugglers use secret tunnels to transport or store
contraband Contraband (from Medieval French ''contrebande'' "smuggling") is any item that, relating to its nature, is illegal to be possessed or sold. It comprises goods that by their nature are considered too dangerous or offensive in the eyes of the leg ...
, such as illegal drugs and
weapon A weapon, arm, or armament is any implement or device that is used to deter, threaten, inflict physical damage, harm, or kill. Weapons are used to increase the efficacy and efficiency of activities such as hunting, crime (e.g., murder), law ...
s. 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 territory of Jammu and Kashmir (union territory), Jammu and Kashmir. Metro-2 is a clandestine deep underground metro system in the Moscow metropolitan area. It was designed to provide the Politics of the Soviet Union, Soviet leadership with secure wartime Emergency evacuation, evacuation routes, Communications protection, communication hubs, and Command and control#Command and control centers, command posts, including a dedicated bunker for the National Command Authority (United States), national command authority. The actual usage of erdstall tunnels is unknown but theories connect it to a rebirth ritual.


Natural tunnels

* Lava tubes 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. * Punarjani Guha 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. * Torghatten, 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 troll Hestmannen, the hill being the hat of the troll-king of Sømna Municipality, 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.


Major accidents

* Clayton Tunnel rail crash (1861) – confusion about block signals leading to collision, 23 killed. * Welwyn Tunnel rail crash (1866) – train failed in tunnel, guard did not protect train. * Paris Métro train fire (1904) – train fire in Couronnes underground station, 84 killed by smoke and gases. * Church Hill Tunnel, Church Hill Tunnel collapse (1925) – tunnel collapse on a work train during renovation, killing four men and trapping a steam locomotive and ten flat cars. * Balvano train disaster (1944) – asphyxiation of about 500 "unofficial" passengers on freight train. * Caldecott Tunnel fire (1982) – major motor vehicle tunnel crash and fire. * 1996 Channel Tunnel fire, Channel Tunnel fire (1996) – Train carrying Large goods vehicle, Heavy Good Vehicles (HGV) caught on fire. * Death of Diana, Princess of Wales, Princess Diana's death (1997) – Car crash in Pont de l'Alma tunnel, Paris, which killed Princess Diana. * Mont Blanc tunnel fire, Mont Blanc Tunnel fire (1999) – Transport truck caught on fire and combusted inside tunnel. * Big Dig ceiling collapse, Big Dig Ceiling collapse (2006) – Concrete ceiling panel falls in Fort Point Tunnel, Fort Point tunnel, Boston, which causes the Big Dig, Big Dig project to be closed for a year.


See also

*Euphrates Tunnel *Cattle creep *Counter-beam lighting *Culvert *Hobby tunneling *Megaproject * Rapid transit *Sequential Excavation Method *Structure gauge – measure of maximum physical clearance in a tunnel *Tree tunnel – tunnel-like effect from tree canopies above a road *Tunnel tree – tunnel bored through the trunk of a tree *Tunnels in popular culture * Underground living


References


Bibliography

* * Railway Tunnels in Queensland by Brian Webber, 1997, . * Sullivan, Walter
Progress In Technology Revives Interest In Great Tunnels
''The New York Times'', 24 June 1986. Retrieved 15 August 2010.


External links


ITA-AITES International Tunnelling Association

Tunnels & Tunnelling International magazine
{{Authority control Tunnels, Crossings Civil engineering Transport buildings and structures Earthworks (engineering) Subterranean buildings and structures