
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the 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
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 fl ...
. The central portions of a
rapid transit
Rapid transit or mass rapid transit (MRT), also known as heavy rail or metro, is a type of high-capacity public transport generally found in urban areas. A rapid transit system that primarily or traditionally runs below the surface may be c ...
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 generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined a ...
stations.
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
Smuggling is the illegal transportation of objects, substances, information or people, such as out of a house or buildings, into a prison, or across an international border, in violation of applicable laws or other regulations.
There are vario ...
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.
Terminology

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 center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...
, 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
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...
is defined as "a subsurface highway structure enclosed for a length of or more." In the United States, the
NFPA definition of a tunnel is "An underground structure with a design length greater than and a diameter greater than ."
In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called a
subway
Subway, Subways, The Subway, or The Subways may refer to:
Transportation
* Subway, a term for underground rapid transit rail systems
* Subway (underpass), a type of walkway that passes underneath an obstacle
* Subway (George Bush Interconti ...
, while an underground railway system is differently named in different cities, the "
Underground" or the "
Tube" in
London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
, the "
Subway
Subway, Subways, The Subway, or The Subways may refer to:
Transportation
* Subway, a term for underground rapid transit rail systems
* Subway (underpass), a type of walkway that passes underneath an obstacle
* Subway (George Bush Interconti ...
" in
Glasgow
Glasgow ( ; sco, Glesca or ; gd, Glaschu ) is the most populous city in Scotland and the fourth-most populous city in the United Kingdom, as well as being the 27th largest city by population in Europe. In 2020, it had an estimated pop ...
, and the "
Metro" in
Newcastle. The place where a road, railway, canal or watercourse passes under a footpath, cycleway, or another road or railway is most commonly called a
bridge
A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...
or, if passing under a canal, an
aqueduct. Where it is important to stress that it is passing underneath, it may be called an underpass, though the official term when passing under a railway is an
underbridge. A longer underpass containing a road, canal or railway is normally called a "tunnel", whether or not it passes under another item of infrastructure. An underpass of any length under a river is also usually called a "tunnel", whatever mode of transport it is for.
In the US, the term "subway" means an underground rapid transit system, and the term
pedestrian underpass is used for a passage beneath a barrier. Rail station platforms may be connected by pedestrian tunnels or
footbridges.
History

Much of the early technology of tunneling evolved from
mining and
military engineering
Military engineering is loosely defined as the art, science, and practice of designing and building military works and maintaining lines of military transport and military communications. Military engineers are also responsible for logistics be ...
. The
etymology
Etymology () The New Oxford Dictionary of English (1998) – p. 633 "Etymology /ˌɛtɪˈmɒlədʒi/ the study of the class in words and the way their meanings have changed throughout time". is the study of the history of the form of words ...
of the terms "mining" (for mineral extraction or for
siege attacks), "military engineering", and "
civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewa ...
" reveals these deep historic connections.
Antiquity and early middle ages
Predecessors of modern tunnels were
adits to transport water for
irrigation
Irrigation (also referred to as watering) 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 been dev ...
or drinking, and
sewerage
Sewerage (or sewage system) is the infrastructure that conveys sewage or surface runoff (stormwater, meltwater, rainwater) using sewers. It encompasses components such as receiving drains, manholes, pumping stations, storm overflows, and screeni ...
. The first
Qanats are known from before 2000 B.C.
The
Tunnel of Eupalinos is a tunnel
aqueduct long running through Mount Kastro in
Samos,
Greece
Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders wit ...
, 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 neighbourhood of
Silwan in eastern
Jerusalem
Jerusalem (; he, יְרוּשָׁלַיִם ; ar, القُدس ) (combining the Biblical and common usage Arabic names); grc, Ἱερουσαλήμ/Ἰεροσόλυμα, Hierousalḗm/Hierosóluma; hy, Երուսաղեմ, Erusałēm. i ...
.
In
Ethiopia
Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the Er ...
, the
Siqurto foot tunnel, hand-hewn in the Middle Ages, crosses a mountain ridge.
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 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, 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
Manhattan (), known regionally as the City, is the most densely populated and geographically smallest of the five Boroughs of New York City, boroughs of New York City. The borough is also coextensive with New York County, one of the List of co ...
and urban
Hong Kong
Hong Kong ( (US) or (UK); , ), officially the Hong Kong Special Administrative Region of the People's Republic of China (abbr. Hong Kong SAR or HKSAR), is a city and special administrative region of China on the eastern Pearl River Delta i ...
, 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 City status in the United Kingdom, city and metropolitan borough in Merseyside, England. With a population of in 2019, it is the List of English districts by population, 10th largest English district by population and its E ...
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
New Jersey is a U.S. state, state in the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. It is bordered on the north and east by the state of New York (state), New York; on the ea ...
and Manhattan in
New York City
New York, often called New York City or NYC, is the most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the most densely populated major city in the U ...
; the
Queens-Midtown Tunnel between Manhattan and the
borough
A borough is an administrative division in various 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
In the Middle ...
of
Queens
Queens is a borough of New York City, coextensive with Queens County, in the U.S. state of New York. Located on Long Island, it is the largest New York City borough by area. It is bordered by the borough of Brooklyn at the western tip of Long ...
on
Long Island
Long Island is a densely populated island in the southeastern region of the U.S. state of New York (state), New York, part of the New York metropolitan area. With over 8 million people, Long Island is the most populous island in the United Sta ...
; the
Detroit-Windsor Tunnel between
Michigan
Michigan () is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the List of U.S. states and ...
and
Ontario
Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...
; and the
Elizabeth River tunnels between
Norfolk and
Portsmouth, Virginia
Portsmouth is an independent city in southeast Virginia and across the Elizabeth River from Norfolk. As of the 2020 census, the population was 97,915. It is part of the Hampton Roads metropolitan area.
The Norfolk Naval Shipyard and Naval M ...
; 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 the Commonwealth of Pennsylvania, United States, and the county seat of Allegheny County. It is the most populous city in both Allegheny County and Western Pennsylvania, the second-most populous city in Pennsyl ...
.
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), 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 state in the Mid-Atlantic and Southeastern regions of the United States, between the East Coast of the United States, Atlantic Coast and the Appalachian Mountains. The geography an ...
.
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
; rm, citad federala, links=no). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzon ...
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 ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...
in 1944, killing 426 passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.
Project planning and cost estimates
Government funds are often required for the creation of tunnels. When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use
project management techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a
work breakdown structure (WBS) and
critical path method (CPM). Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of
bonds.
The
costs and benefits for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the
Port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project. Increased taxes to finance a large project may cause opposition.
Construction
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the 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 excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit).
In geology, trenches result from ero ...
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.
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 walling 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, where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the London Underground network, the Metropolitan and District Railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam.
A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction. This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century.
Boring machines
Tunnel boring machines (TBMs) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as 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 types of TBMs, the bentonite slurry, and earth-pressure balance machines have pressurized 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 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 borehole has a diameter of . The four TBMs used for excavating the
Gotthard Base Tunnel, in
Switzerland
; rm, citad federala, links=no). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzon ...
, 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
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = '' Plus ultra'' ( Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, ...
, and the
Chong Ming tunnels in
Shanghai
Shanghai (; , , Standard Mandarin pronunciation: ) is one of the four direct-administered municipalities of the People's Republic of China (PRC). The city is located on the southern estuary of the Yangtze River, with the Huangpu River flowin ...
,
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 (US).
Clay-kicking
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 Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...
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.
Use