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

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

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

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

The definition of what constitutes a tunnel can vary widely from source to source.

For example, the definition of a road tunnel in the United Kingdom is defined as "a subsurface highway structure enclosed for a length of 150 metres (490 ft) or more."^[1] In the United States, the NFPA definition of a tunnel is "An underground structure with a design length greater than 23 m (75 ft) and a diameter greater than 1,800 millimetres (5.9 ft)."^[2]

In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called asubway, while an underground railway system is differently named in different cities, the "Underground" or the "Tube" in London, the "Subway" in Glasgow, and the "Metro" in Newcastle. The place where a road, railway, canal or watercourse passes under a footpath, cycleway, or another road or railway is most commonly called abridge or, if passing under a canal, anaqueduct. Where it is important to stress that it is passing underneath, it may be called an underpass, though the official term when passing under a railway is anunderbridge

In the US, the term "subway" means an underground rapid transit system, and the termpedestrian underpass is used for a passage beneath a barrier.

Rail station platforms may be connected by pedestrian tunnels or footbridges.

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

Predecessors of modern tunnels were adits to transport water for irrigation or drinking, and sewerage. The first Qanats are known from before 2000 B.C.

The Tunnel of Eupalinos is a tunnel aqueduct 1,036 m (3,399 ft) long running through Mount Kastro in Samos, Greece, built in the 6th century BC to serve as an aqueduct. It is the second known tunnel to have been excavated from both ends, after the Siloam tunnel in the Palestinian neighbourhood of Silwan in eastern Jerusalem.

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:

For water crossings, a tunnel is generally more costly to construct than a bridge.

However, navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel.

Bridges usually require a larger footprint on each shore than tunnels.

In areas with expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in 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 was chosen over a massively high bridge for defense reasons; it was feared that aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 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 vessels from leaving Naval Station Norfolk.

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

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the 51.5-kilometre or 32.0-mile Channel Tunnel), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).

Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in Virginia.

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 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 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.

Government funds are often required for the creation of tunnels.^[6] When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process.

Civil engineers usually use project management techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of bonds.

The costs and benefits for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the Port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project.^[7] Increased taxes to finance a large project may cause opposition.^[8]

Tunnels are dug in types of materials varying from soft clay to hard rock.

The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.

There are three basic types of tunnel construction in common use.

Cut-and-cover tunnels are constructed in a shallow trench and then covered over.

Bored tunnels are constructed in situ, without removing the ground above.

Finally a tube can be sunk into a body of water, which is called an immersed tunnel.

Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.^[9] Two basic forms of cut-and-cover tunnelling are available:

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, 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.

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

In certain predominantly urban applications, tunnel boring is viewed as quick and cost effective alternative to laying surface rails and roads.

Expensive 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 types of TBMs, the bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, allowing them to be used in difficult conditions below the water table. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurised compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. Despite these difficulties, TBMs are now preferred over the older method of tunnelling in compressed air, with an air lock/decompression chamber some way back from the TBM, which required operators to work in high pressure and go through decompression procedures at the end of their shifts, much like deep-sea divers.

In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of the Linth–Limmern Power Stations located south of Linthal in the canton of Glarus. The borehole has a diameter of 8.03 metres (26.3 ft).^[10] The four TBMs used for excavating the 57-kilometre (35 mi) Gotthard Base Tunnel, in Switzerland, had a diameter of about 9 metres (30 ft). 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 14.87 metres (48.8 ft).^[11] This in turn was superseded by the Madrid M30 ringroad, Spain, and the Chong Ming tunnels in Shanghai, China. All of these machines were built at least partly by Herrenknecht. As of August 2013, the world's largest TBM is "Big Bertha", a 57.5-foot (17.5 m) diameter machine built by Hitachi Zosen Corporation, which is digging the Alaskan Way Viaduct replacement tunnel in Seattle, Washington (US).^[12]

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

Used in Victorian civil engineering, the method found favour in the renewal of Britain's ancient sewerage systems, by not having to remove all property or infrastructure to create a small tunnel system. During the First World War, the system was used by Royal Engineer tunnelling companies to put mines beneath the German Empire lines. The method was virtually silent and so not susceptible to listening methods of detection.^[13]

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.^[5]

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

The New Austrian Tunnelling method (NATM)—also referred to as the Sequential Excavation Method (SEM)^[14]—was developed in the 1960s. The main idea of this method is to use the geological stress of the surrounding rock mass to stabilize the tunnel, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the supports. Based on geotechnical 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, rockbolts and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibres being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's deformation.^[14]

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 tools for tunnel strengthening.^[14]

Inpipe jacking, hydraulic jacks 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 3.2 metres (10 ft).

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 20 metres (66 ft).

A cutting head is normally used at the front of the box being jacked, and spoil removal is normally by excavator from within the box.Recent

developments of the Jacked Arch and Jacked deck have enabled longer and larger structures to be installed to close accuracy. The

126m long 20m clear span underpass below the high speed rail lines at Cliffsend in Kent, UK is an example of this technique.

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

During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove excavated spoil, often narrow gauge 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".

The vehicles or traffic using a tunnel can outgrow it, requiring replacement or enlargement:

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.

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 540-foot (160 m) double-deck tunnel section through Yerba Buena Island, the largest-diameter bored tunnel in the world.^[19] 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 5.4 km (3.4 mi) two-deck road tunnel with two lanes on each deck.^[20] Additionally, in 2015 the Turkish government announced that it will build the world's first three-level tunnel, also under the Bosporus.^[21] The tunnel is intended to carry both the Istanbul metro and a two-level highway, over a length of 6.5 km (4.0 mi).

The French A86 Duplex Tunnel in west Paris consists of two bored tunnel tubes, the eastern one of which has two levels for light motorized vehicles, over a length of 10 km (6.2 mi). Although each level offers a physical height of 2.54 m (8.3 ft), only traffic up to 2 m (6.6 ft) 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, China, a 2.8 km (1.7 mi) two-tube double-deck tunnel was built starting in 2002. In each tube of the Fuxing Road Tunnel both decks are for motor vehicles. In each direction, only cars and taxis travel on the 2.6 m (8.5 ft) high two-lane upper deck, and heavier vehicles, like trucks and buses, as well as cars, may use the 4.0 m (13 ft) high single-lane lower level.^[22]

In the Netherlands, a 2.3 km (1.4 mi) two-storey, eight lane, cut-and-cover road tunnel under the city of Maastricht was opened in 2016.^[23] 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.^[24]

The Alaskan Way Viaduct replacement tunnel is a bored road tunnel that is under construction since 2013 in the city of Seattle in the U.S. state of Washington. The 2-mile (3.2 km) tunnel will carry State Route 99 under Downtown Seattle from the SoDo neighborhood to South Lake Union in the north on two levels with two lanes each.

New York City's 63rd Street Tunnel under the East River, between the boroughs of Manhattan and Queens, was intended to carry subway trains on the upper level and Long Island Rail Road commuter trains on the lower level. Construction started in 1969,^[25] and the two sides of the tunnel were bored through in 1972.^[26] The upper level, used by the IND 63rd Street Line (F and trains) of the New York City Subway, was not opened for passenger service until 1989.^[27] The lower level, intended for commuter rail, will not see passenger service until completion of the East Side Access project, expected in late 2022.^[28]

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

Hong Kong's Lion Rock Tunnel, built in the mid 1960s, connecting 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 1.2m and 1.5m below the road section of the tunnel.^[29]

Some tunnels have more than one purpose.

The SMART Tunnel in Malaysia is the first multipurpose "Stormwater Management And Road Tunnel" in the world, created to convey both traffic and occasional flood waters in Kuala Lumpur. When necessary, floodwater is first diverted into a separate bypass tunnel located underneath the 2.5 mi (4.0 km) 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.^[30]

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

Over-bridges can sometimes be built by covering a road or river or railway with brick or steel arches, and then leveling the surface with earth.

In railway parlance, a surface-level track which has been built or covered over is normally called a "covered way".

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

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 being highly toxic. Fires killed 11 people in the Gotthard tunnel fire of 2001 for example, all of the victims succumbing to smoke and gas inhalation. Over 400 passengers died in the Balvano train disaster in Italy in 1944, when the locomotive halted in a long tunnel. Carbon monoxide poisoning was the main cause of death. In the Caldecott Tunnel fire of 1982, the majority of fatalities were caused by toxic smoke, rather than by the initial crash.

Motor vehicle tunnels usually require ventilation shafts and powered fans to remove toxic exhaust gases during routine operation.^[31]

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.

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.^[32]

Larger, heavily used tunnels, such as the Big Dig tunnel in Boston, Massachusetts, may have a dedicated 24-hour manned operations center which monitors and reports on traffic conditions, and responds to emergencies.^[33] Video surveillance equipment is often used, and real-time pictures of traffic conditions for some highways may be viewable by the general public via the Internet.

Earthquakes are one of nature’s most formidable threats.

A magnitude 6.7 earthquake shook the San Fernando valley in Los Angeles in 1994.

The earthquake caused extensive damage to various structures including buildings, freeway overpasses and road systems throughout the area.

The National Center for Environmental Information estimates total damages to be 40 billion dollars.^[34] According to an article issued by Steve Hymon of TheSource – Transportation News and Views, there was no serious damage sustained by the LA subway system.

Metro, the owner of the LA subway system, issued a statement through their engineering staff about the design and consideration that goes into a tunnel system.

Engineers and architects perform extensive analysis as to how hard they expect earthquakes to hit that area.

All of this goes into the overall design and flexibility of the tunnel.

This same trend of limited subway damage following an earthquake can be seen in many other places.

In 1985 a magnitude 8.1 earthquake shakes Mexico City; there is no damage to the subway system, and in fact the subway systems serve as a lifeline for emergency personnel and evacuations.

A magnitude 7.2 rips through Kobe Japan in 1995, leaving no damage to the tunnels themselves.

Entry portals sustained minor damages, however these damages were attributed to inadequate earthquake design that originated from the original construction date of 1965.

In 2010 a magnitude 8.8, massive by any scale, absolutely annihilates Chile.

Entrance stations to subway systems suffered minor damages, and the subway system was down for the rest of the day.

By the next afternoon, the subway system was operational again.^[35]

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.^[36][37] Some well known ancient and modern tunnels are briefly introduced below:

The use of tunnels for mining is called drift mining.

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 military use during armed conflict, or civilian responses to threat of attack. Another use for tunnels was for the storage of chemical weapons^[61][62] [2] ^[76].

Secret tunnels have given entrance to or escape from an area, such as the Cu Chi Tunnels or the smuggling tunnels in the Gaza Strip which connect it to 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, such as illegal drugs and weapons. Elaborately engineered 1,000-foot (300 m) 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.^[63] 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.^[63] Secret tunnels have also been used by thieves to break into bank vaults and retail stores after hours.^[64][65] Several tunnels have been discovered by the Border Security Forces across the Line of Control along the India-Pakistan border, mainly to allow terrorists access to the Indian state of Jammu and Kashmir.^[66][67]

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