A tunnel is an underground passageway. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, save for the openings at each end. Some civic planners define a tunnel as 0.1 miles (0.16 kilometers) in length or longer, while anything shorter than this should be called a chute. For example, the underpass beneath Yahata Station in Kitakyushu, Japan is only long and therefore should not be considered a tunnel.
A tunnel may be for pedestrians or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are aqueducts, constructed purely for carrying water — for consumption, for hydroelectric purposes or as sewers — while others carry other services such as telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species. Some secret tunnels have also been made as a method of entrance or escape from an area, such as the Cu Chi Tunnels or the tunnels connecting the Gaza Strip to Egypt. Some tunnels are not for transport at all but are fortifications, for example Mittelwerk and Cheyenne Mountain.
The longest canal tunnel is the Standedge Tunnel in the United Kingdom, over three miles (5 km) long.
In the United Kingdom a pedestrian tunnel or other underpass beneath a road is called a subway. This term was used in the past in the United States, but now refers to underground rapid transit systems.
The central part of a rapid transit network is usually built in tunnels. To allow non-level crossings, some lines run in deeper tunnels than others. Rail stations with much traffic usually provide pedestrian tunnels from one platform to another, though others use bridges.
It is essential that any tunnel project starts with a comprehensive investigation of ground conditions. The results of the investigation will allow proper choice of machinery and methods for excavation and ground support, and will reduce the risk of encountering unforeseen ground conditions. In the early stages, the horizontal and vertical alignment will be optimised to make use of the best ground and water conditions.
In some cases, conventional desk and site studies will not produce sufficient information to assess, for example, the blocky nature of rocks, the exact location of fault zones, or stand-up times of softer ground. This may be a particular concern in large diameter tunnels. To overcome these problems, a pilot tunnel, or drift, may be driven ahead of the main drive. This smaller diameter tunnel will be easier to support when unexpected conditions occur, and will be incorporated in the final tunnel. Alternatively, horizontal boreholes may sometimes be used ahead of the advancing tunnel face.
Tunnels are dug in various types of materials, from soft clay to hard rock, and the method of excavation depends on the ground conditions.
is a simple method of construction for shallow tunnels where a trench
is excavated and roofed
over. A strong overhead support system is required to carry the load of the covering material.
Two basic forms of cut-and-cover tunnelling are available:
- Bottom-up method: A trench is excavated, with ground support as necessary, and the tunnel is constructed within. The tunnel may be of in situ concrete, precast concrete, precast arches, corrugated steel arches and such, with brickwork used in early days. The trench is then backfilled, with precautions regarding balancing compaction of the backfill material, and the surface is reinstated.
- Top-down method: In this method, side support walls and capping beams are constructed from ground level, using slurry walling, contiguous bored piles, or some other method. A shallow excavation is then made to allow the tunnel roof to be constructed using precast beams or in situ concrete. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation machinery is then lowered into the access openings, and the main excavation is carried out under the permanent tunnel roof, followed by constructing the base slab.
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 due to the sheer size of the excavation. This contrasts with most traditional stations on London Underground, where bored tunnels were used for stations and passenger access.
Tunnel boring machines (TBMs) and associated back-up systems can be used to highly automate the entire tunneling process. There are a variety of TBMs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, allowing them to be used in difficult conditions below the water table. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurised compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. Despite these difficulties, TBMs are now preferred to the older method of tunneling in compressed air, with an air lock/decompression chamber some way back from the TBM, which required operators to work in high pressure and go through decompression procedures at the end of their shifts, much like divers.
Until recently the largest TBM built was used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. It had a diameter of 14.87 m.
Nowadays even larger machines exist: two for the M30 ringroad in Madrid, Spain, and two for the Chong Ming tunnels in Shanghai, China. These machines are 15,2 m and 15,4 m in diameter respectively. The two machines for Spain were built by Mitsubishi/Duro Felguera and Herrenknecht. The TBMs for China were built by Herrenknecht.
The New Austrian Tunneling Method
(NATM) 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 itself. Based on geotechnical
measurements, an optimal cross section
is computed. The excavation is immediately protected by thin shotcrete
, just behind the excavation. This creates a natural load-bearing ring, which minimizes the rock's deformation
By special monitoring the NATM method is very 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. In the last decades also soft ground excavations up to 10 km became usual.
, also known as pipejacking
, is a method of tunnel construction where hydraulic jacks are used to push specially made pipes through the ground behind a tunnel boring machine or shield. This technique is commonly used to create tunnels under existing structures, such as roads or railways.
There are also several approaches to underwater tunnels
, for instance an immersed tube
as in the Sydney Harbour
, and the Posey and Webster Street Tubes
which connect the cities of Oakland
and Alameda, California
, running beneath the Alameda-Oakland Estuary
Other tunneling methods
Choice of tunnels vs. bridges
For water crossings, a tunnel is generally more costly to construct than a bridge. Navigational considerations may limit the use of high bridges or drawbridge 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 tunnels' favor. Boston's Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront. Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia and the Westerschelde tunnel, Zeeland, Netherlands. Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather and shipping during construction (as in the 51.5 km 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 the eastern United States.
There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard tunnel in Switzerland in 2001. One of the worst railway disasters ever occurred when a train stalled in the Armi tunnel in Italy in 1944, killing 426 passengers.
Variant tunnel types
Some tunnels are double-deck, for example the Eastern Harbour Crossing
in Hong Kong
, where roads and rails (the MTR
metro) occupy different decks of the tunnel.
The two major segments of the San Francisco – Oakland Bay Bridge (completed in 1936) are linked by a double-deck tunnel, once the largest diameter 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.
A recent double-decker tunnel with both decks for motor vehicles is the Fuxing Road Tunnel in Shanghai, China. Cars travel on the two-lane upper deck and heavier vehicles on the single-lane lower.
Overbridges can sometimes be built by covering a road or river or railway with brick or still arches, 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 avalanches of snow. Similarly the Stanwell Park, New South Wales steel tunnel, on the South Coast railway line, protects the line from rockfalls.
Common utility ducts are man-made tunnels created to carry two or more utility lines underground. Through co-location of different utilities in one tunnel, organizations are able to reduce the costs of building and maintaining utilities.
Owing to the enclosed space of a tunnel, fires can have very serious effects on users. The main dangers are gas and smoke production, with low concentrations of carbon monoxide
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. Fires have also occurred in the channel tunnel
, leading to great delays for users.
Examples of tunnels
- World's oldest underwater tunnel is rumored to be the Terelek kaya tüneli under Kızıl River, a little south of the towns of Boyabat and Duragan in Turkey. Estimated to have been built more than 2000 years ago (possibly 5000) it is assumed to have had a defense purpose.
- The qanat or kareez of Persia is a water management system used to provide a reliable supply of water to human settlements or for irrigation in hot, arid and semi-arid climates. The oldest and largest known qanat is in the Iranian city of Gonabad which after 2700 years still provides drinking and agricultural water to nearly 40,000 people. Its main well depth is more than 360 meters and its length is 45 kilometers.
- The Eupalinian aqueduct on the island of Samos (North Aegean, Greece). Built in 520 BC by the ancient Greek engineer Eupalinos of Megara. 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 1030 metres (3,430 ft) long and visitors can still enter it Eupalinos tunnel
- Sapperton Canal Tunnel on the Thames and Severn Canal in England, dug through hills, which opened in 1789, was 3.5 km long and allowed boat transport of coal and other goods. Above it runs the Sapperton Long Tunnel which carries the "Golden Valley" railway line between Swindon and Gloucester.
- The tunnel created for the first true steam locomotive, the Penydarren locomotive, was built prior to Richard Trevithick was able to make his historic journey from Penydarren to Abercynon in 1804. Part of this tunnel can still be seen at Pentrebach, Merthyr Tydfil. This is arguably the oldest railway tunnel in the world, for self-propelled steam engines on rails.
- The Montgomery Bell Tunnel in Tennessee, a 290 water diversion tunnel built by slave labor in 1819, was the first "full-scale" tunnel in the United States.
- Box Tunnel in England, which opened in 1841, was the longest railway tunnel in the world at the time of construction. It was dug and has a length of 2.9 km.
- The Thames Tunnel, built by Marc Isambard Brunel and his son Isambard Kingdom Brunel and opened in 1843, was the first underwater tunnel and the first to use a tunnelling shield. Originally used as a foot-tunnel, it is now part of the East London Line of the London Underground.
- James Greathead, in constructing the City & South London Railway tunnel beneath the Thames, in the late 1880s, brought together three key elements of tunnel construction under water: 1) shield method of excavation; 2) permanent cast iron tunnel lining; 3) construction in a compressed air environment to inhibit water flowing through soft ground material into the tunnel heading.
- St. Clair Tunnel, opened in 1890, linked those elements on a much larger scale and was the first full-size subaqueous tunnel in North America
- The Cobble Hill Tunnel and Murray Hill Tunnel in New York City are the world's oldest railway tunnels lying below streets, roofed over in 1850 and the 1850s, respectively.
- The oldest sections of the London Underground were built using the cut-and-cover method in the 1860s. The Metropolitan, Hammersmith & City, Circle and District lines were the first to prove the success of a metro or subway system.
- St. Clair Tunnel, opened in 1890, was the first full-size subaqueous tunnel in North America
- Col de Tende Road Tunnel, one of the first longer road tunnels under a pass, runs between France and Italy. At its opening it may have been one of world's longest road tunnels.
- The Holland Tunnel was the first underwater tunnel designed for automobiles, which required a novel ventilation system.
See also the History of Rapid transit.
- Päijänne-tunneli is the world's longest complete tunnel that is bored into rock. It is located in southern Finland and it is 120 km long. Its purpose is to provide the Greater Helsinki metropolitan area with fresh water.
- The Lincoln Tunnel between New Jersey and New York is one of the busiest vehicular tunnels in the world, at 120,000 vehicles/day.
- The Fredhälls Tunnel in Stockholm, Sweden, with around 150,000 vehicles a day one of the most trafficked tunnels in Europe
- Williamson's tunnels in Liverpool, built by a wealthy eccentric are probably the largest underground folly in the world.
- New York City Water Tunnel No. 3 , started in 1970, has an expected completion date of 2020.
- The Chicago Deep Tunnel Project is a network of 109 miles (197 km) of tunnels designed to reduce flooding in the Chicago area. Started in the mid 1970s, the project is due to be completed in 2019.
- Moffat Tunnel in Colorado straddles the Continental Divide. The tunnel is 6.2 mi (10.0 km) long and at 9,239 feet (2,816 m) above sea level is the highest railroad tunnel in the United States.
- The Fenghuoshan tunnel on Qinghai-Tibet railway is the world's highest railway tunnel.
- The Sydney Harbour Tunnel in Sydney, Australia was built in 1992 to augment the Sydney Harbour Bridge.
- The Clyde Tunnel in Glasgow, Scotland, connects the districts of Whiteinch and Govan in the west of the city.
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.
The use of tunnels for mining is called drift mining.
Snow tunnels are created by voles, chipmunks and other rodents for protection and access to food sources. Larger versions are created by humans, usually for fun.
For more information regarding tunnels built by animals, see Burrow