A tunnel is an underground passageway, completely enclosed except for openings for entrance and exit, commonly at each end.
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 built in tunnels. 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.
A tunnel is relatively long and narrow; in general the length is more (usually much more) 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 150m or more.". 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 1800 mm (6 ft).".
In the UK, a pedestrian tunnel or other underpass beneath a road is called an underpass subway. In the US, the term "subway" means an underground rapid transit system, and the term "pedestrian underpass" is used instead. Rail station platforms may be connected by pedestrian tunnels or by footbridges.
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Much of the early technology of tunneling evolved from the techniques of 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.
Clay-kicking is a specialised method developed in the United Kingdom, of manually digging tunnels in strong clay-based soil structures. Unlike previous manual methods of using mattocks which relied on the soil structure to be hard, clay-kicking was relatively silent and hence did not harm soft clay based structures.
The clay-kicker lies on a plank at a 45-degree angle away from the working face, and inserts a tool with a cup-like rounded end with his feet. Turning the tool with his hands, he extracts a section of soil, which is then placed on the waste extract.
Regularly used in Victorian civil engineering, the methods found favour in the renewal of the United Kingdom's then ancient sewerage systems, by not having to remove all property or infrastructure to create an effective small tunnel system. During the First World War, the system was successfully deployed by the Royal Engineer tunnelling companies to deploy large military mines beneath enemy German Empire lines. The method was virtually silent, and so not susceptible to listening methods of detection.
Geotechnical investigation and design 
A tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from boreholes and by other geophysical techniques. An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route the horizontal and vertical alignments will make use of the best ground and water conditions.
In some cases, conventional desk and site studies yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large diameter tunnels. To give more information a pilot tunnel, or drift, may be driven ahead of the main drive. This smaller diameter tunnel will be easier to support should unexpected conditions be met, and will be incorporated in the final tunnel. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.
Other key geotechnical factors include:
- Stand-up time is the amount of time a tunnel will support itself without any added structures. Knowing this time allows the engineers to determine how much can be excavated before support is needed. The longer the stand-up time is the faster the excavating will go. Generally certain configurations of rock and clay will have the greatest stand-up time, and sand and fine soils will have a much lower stand-up time.
- Groundwater control is very important in tunnel construction. If there is water leaking into the tunnel stand-up time will be greatly decreased. If there is water leaking into the shaft it will become unstable and will not be safe to work in. To stop this from happening there are a few common methods. One of the most effective is ground freezing. To do this pipes are inserted into the ground surrounding the shaft and are cooled until they freeze. This freezes the ground around each pipe until the whole shaft is surrounded frozen soil, keeping water out. The most common method is to install pipes into the ground and to simply pump the water out. This works for tunnels and shafts.
- Tunnel shape is very important in determining stand-up time. The force from gravity is straight down on a tunnel, so if the tunnel is wider than it is high it will have a harder time supporting itself, decreasing its stand-up time. If a tunnel is higher than it is wide the stand up time will increase making the project easier. The hardest shape to support itself is a square or rectangular tunnel. The forces have a harder time being redirected around the tunnel making it extremely hard to support itself. This of course all depends what the material of the ground is.
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. In Hampton Roads, tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges would prevent U.S. Navy vessels from leaving Naval Station Norfolk.
The 1934 Queensway Road Tunnel under the River Mersey at Liverpool was chosen over a massively high bridge for defence reasons. It was feared aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships to navigate under was considered higher than a tunnel. Similar conclusions were met for the 1971 Kingsway Tunnel under the River Mersey.
Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel 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, 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).
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.
Cost estimates and overruns 
Government funds are a major factor in the creation of tunnels. When a tunnel is in the process of being constructed, economics and politics play a large factor in the decision making process. This division of the project is part of the construction/project management aspect of civil engineering. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Understanding the amount of time the project requires, the amount of labors and materials needed is a crucial part of the project. Also, the amount of land that will need to be excavated and the proper machinery that is needed is also very important. Since infrastructures require millions, or even billions of dollars, acquiring these funds can be challenging.
The need for an infrastructure such as a tunnel must be identified. Political issues are bound to occur as it was shown in 2005 when the US House of Representatives approved a $100 million dollar federal grant to build a tunnel in the New York Harbor. However, the Port Authority of New York and New Jersey was aware of this bill and had never asked for a grant or for such a project. The current state of the economy reflects on the amount of money the government can give for public projects. Since taxpayers money goes to projects such as the creation of tunnels, or any other infrastructures, increasing taxes may cause issues.
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
There are three basic types of tunnel construction in common use:
- Cut-and-cover tunnels, constructed in a shallow trench and then covered over.
- Bored tunnels, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section.
- Immersed tube tunnels, sunk into a body of water and sit on, or are buried just under, its bed.
Cut-and-cover 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. Two basic forms of cut-and-cover tunnelling are available:
- Bottom-up method: A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ concrete, precast concrete, precast arches,or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated.
- Top-down method: Side support walls and capping beams are constructed from ground level by such methods as slurry walling, or contiguous bored piling. Then a shallow excavation allows making the tunnel roof of precast beams or in situ concrete. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed.
Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a tunnelling shield. 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 most traditional stations on London Underground, where bored tunnels were used for stations and passenger access.
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 quick and cost effective alternative to laying surface rails and roads. Expensive compulsory purchase of buildings and land with potentially lengthy planning inquiries is eliminated.
There are a variety of TBMs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, allowing them to be used in difficult conditions below the water table. 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 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 divers.
In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of the Linth–Limmern Power Stations in Switzerland. The borehole has a diameter of 8.03 metres (26.3 ft). 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). 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.
A temporary access shaft is sometimes necessary during the excavation of a tunnel project. They are usually circular and go straight down until they reach the level at which the tunnel is going to be built. A shaft normally has concrete walls and is usually built to be permanent. Once the access shafts are complete, Tunnel Boring Machines are lowered to the bottom and excavation can start. Shafts are the main entrance in and out of the tunnel until the project is completed. Sometimes if a tunnel is going to be long, multiple shafts at various locations will be bored so that entrance into the tunnel is closer to the unexcavated area.
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 Tunneling Method (NATM) was developed in the 1960s, and is the best known of a number of engineering solutions that use calculated and empirical real-time measurements to provide optimised safe support to the tunnel lining. The main idea of this method is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the introduced support measures. Based on geotechnical measurements, an optimal cross section is computed. The excavation is immediately protected by a layer of sprayed concrete, commonly referred to as shotcrete, after excavation. Other support measures could include steel arches, rockbolts and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibres being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's deformation.
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 kilometres (6.2 mi) became usual.
Pipe jacking 
Pipe Jacking, also known as pipejacking or pipe-jacking, is a method of tunnel construction where hydraulic jacks are used to push specially made pipes through the ground behind a tunnel boring machine or shield. This technique is commonly used to create tunnels under existing structures, such as roads or railways. Tunnels constructed by pipe jacking are normally small diameter tunnels with a maximum size of around 3.2m.
Box jacking 
Box jacking is similar to pipe jacking, but instead of jacking tubes, a box shaped tunnel is used. Jacked boxes can be a much larger span than a pipe jack with the span of some box jacks in excess of 20m. A cutting head is normally used at the front of the box being jacked and excavation is normally by excavator from within the box.
Underwater tunnels 
There are also several approaches to underwater tunnels, the two most common being bored tunnels or immersed tubes. 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. This temporary railway is often narrow gauge so that it can be double tracked, which facilitates 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. The original single line Gib Tunnel near Mittagong was replaced with a double line tunnel, with the original tunnel used for growing mushrooms. The Rhyndaston Tunnel was enlarged using a borrowed Tunnel Boring Machine so as to be able to take ISO containers.
The 1836 Lime Street two track 1 mile tunnel from Edge Hill to Lime Street in Liverpool was totally removed, apart from a short 50 metre section at Edge Hill. Four tracks were required. The tunnel was converted into a very deep 4 track open cutting. However, short larger 4 track tunnels were left in some parts of the run. Train services were not interrupted as the work progressed. Photos of the work in progress:  There are other occurrences of tunnels being replaced by open cuts, for example, the Auburn Tunnel.
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 
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 through Treasure Island, San Francisco, the largest diameter bore tunnel in the world. At construction this was a combination bidirectional rail and truck pathway on the lower deck with automobiles above, now converted to one-way road vehicle traffic on each deck.
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 used for cables, pipes and emergency accident refuge enclosures.
In China, the Lion Rock Tunnel, built in the mid-1960s connecting New Kowloon and Sha Tin within the territory of Hong Kong, carries a motorway and an aqueduct[disambiguation needed]. A recent double-decker tunnel with both decks for motor vehicles is the Fuxing Road Tunnel in Shanghai. Cars travel on the two-lane upper deck, and heavier vehicles on the single-lane lower level.
Multipurpose tunnels exist that have more than one purpose. The SMART Tunnel in Malaysia is the first multipurpose flood control tunnel in the world, as it is used both to convey traffic and occasional flood waters in Kuala Lumpur.
Common utility ducts or utility tunnels 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.
Covered passageways 
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 South Coast railway line, protects the line from rockfalls.
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 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 the horrifying death rate. 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. 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.
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. 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.
Examples of tunnels 
In history 
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- The world's oldest underwater tunnel is claimed 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, it is assumed to have had a defence purpose.
- The qanat or kareez of Persia 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 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 m (1,180 ft), and its length is 45 km (28 mi).
- Hezekiah's Tunnel was built before 701 BCE for water as a defense against siege attacks.
- The Eupalinian aqueduct on the island of Samos (North Aegean, Greece) was built in 520 BCE 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 1.030 km long (3,380 ft) and visitors can still enter it Eupalinos tunnel.
- The Via Flaminia, an important Roman road, penetrated the Furlo pass in the Apennines through a tunnel which emperor Vespasian had ordered built in 76-77 CE. A modern road, the SS 3 Flaminia, still uses this tunnel, which had a precursor dating back to the 3rd century BCE; remnants of this earlier tunnel (one of the first road tunnels) are also still visible.
- Sapperton Canal Tunnel on the Thames and Severn Canal in England, dug through hills, which opened in 1789, was 3.5 km (2.2 mi) 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 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.
- A tunnel was created for the first true steam locomotive, from Penydarren to Abercynon. 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, Wales. This is arguably the oldest railway tunnel in the world, for self-propelled steam engines on rails.
- The Montgomery Bell Tunnel in Tennessee, an 88 m long (289 ft) water diversion tunnel, 4.50 m × 2.45 m high (15 ft × 8.0 ft), 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, England. 0.0321 km (105 ft) long. Built in the late 1820's, the exact date is unknown. This is the first tunnel in the world that runs under a railway line. The construction of the Liverpool to Manchester Railway ran over a horse drawn tramway from the Sutton collieries to the Liverpool to Warrington turnpike road. It was decided to bore a tunnel 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, England, 1829. Built by George Stephenson, a single track tunnel 266 m long (873 ft), 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 rail tunnel running under streets in the world. 
- The 2.03 km (1.26 mi) 1829 twin track Wapping Tunnel in Liverpool, England, was the first rail tunnel bored under a metropolis. Currently disused since 1972, the tunnel runs from Edge Hill in the east of the city to the south end Liverpool docks being used only for freight. The tunnel was to be a part of the Merseyrail network, with work started and abandoned because of costs. The tunnel is still in excellent condition and is still being considered for reuse by Merseyrail system, maybe with an underground station cut into the tunnel for Liverpool university. The river portal is opposite the new Liverpool Arena being ideal for a serving station. If reused it will be the oldest used underground rail tunnel in the world and oldest part of any underground metro system.
- 1836, Lime St Station tunnel, Liverpool. A two track rail tunnel, 1.811 km (1.125 mi) long was bored under the metropolis from Edge Hill in the east of the city to Lime Street. In the 1880s the tunnel was converted to a deep open to the atmosphere cutting four tracks wide, the only occurrence of a major tunnel being removed. A short section of the original tunnel still exists at Edge Hill station, giving the tunnel the distinction of being the oldest rail tunnel in the world still in use, and the oldest in use under streets.
- The 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 2.9 km (1.8 mi).
- The 1.1 km (0.68 mi) 1842 Prince of Wales Tunnel, in Shildon near Darlington, England, is the oldest sizeable tunnel in the world still in use under a settlement.
- The Thames Tunnel, built by Marc Isambard Brunel and his son Isambard Kingdom Brunel opened in 1843, was the first underwater tunnel and the first to be built using a tunnelling shield. Originally used as a foot-tunnel, it was a part of the East London Line of the London Underground until 2007, then being the oldest section of the network. From 2010 the tunnel becomes a part of the London Overground system.
- The 3.34 km (2.08 mi) Victoria Tunnel/[Waterloo Tunnel]] in Liverpool, England, opened in 1848, being bored under a metropolis. The tunnel was initially used only for rail freight and later freight and passengers serving the Liverpool ship liner terminal, the tunnel runs from Edge Hill in the east of the city to the north end Liverpool docks. 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 2,375 m (1.476 mi) long Victoria section was originally cable hauled and the shorter 862 m (Template:Convert/yds) Waterloo section was locomotive hauled, two separate names were given, the short section being named the Waterloo tunnel. In 1895 the two tunnels were converted to locomotive haulage. Used until 1972, the tunnel is still in excellent condition, being considered for reuse by the Merseyrail system. Stations being cut into the tunnel are being considered. Also, reuse by a monorail system from the proposed Liverpool Waters redevelopment of Liverpool's Central Docks has been proposed.
- The vertex tunnel of the Semmering railway, the first Alpine tunnel, was opened in 1848 and was 1.431 km (0.889 mi) 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 3.25 km (2.02 mi) long.
- The oldest underground sections of the London Underground were built using the cut-and-cover method in the 1860s, and opened in January 1863. What are now the Metropolitan, Hammersmith & City and Circle lines were the first to prove the success of a metro or subway system.
- On June 18, 1868, the Central Pacific Railroad's 506 m (0.314 mi) Summit Tunnel (Tunnel #6) at Donner Pass in the California Sierra Nevada mountains was opened permitting the establishment of the commercial mass transportation of passengers and freight over the Sierras for the first time. It remained in daily use until 1993 when the Southern Pacific Railroad closed it in favour of sending all rail traffic through the 5.10 km ( 3.2 mi) Tunnel #41 (aka "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, 13.7 km (8.5 mi) long. At that time it was the longest in the world.
- The third Alpine tunnel, the Gotthard Rail Tunnel opened in 1882 and was the longest rail tunnel in the world, measuring 15 km (9.3 mi).
- The 1882 Col de Tende Road Tunnel, at 3.182 km (1.977 mi) 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 3.12 mi (5.02 km) in length. The under river section is 0.75 mi (1.21 km) in length, and was the longest underwater tunnel in world in January 1886.
- The rail Severn Tunnel was opened in late 1886, at 7.008 km (4.355 mi) long, although only 3.62 km (2.25 mi) 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: 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.
- Built from 1890 to 1940, the section of London Underground's Northern Line from Morden to East Finchley was the longest railway tunnel in the world at 27.8 km (17.3 mi) 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, linking Paris to Milan. It is 19.7 km (12.2 mi) long, and was the longest tunnel in the world until 1982.
- The 1927 Holland Tunnel was the first underwater tunnel designed for automobiles. The construction required a novel ventilation system.
- In 1945 the Delaware Aqueduct tunnel was completed, supplying water to New York City in the US. At 137 km (85 mi) it is the second longest tunnel in the world.
- In 1988 the 53.850 km (33.461 mi) long Seikan Tunnel in Japan was completed under the Tsugaru Strait, linking the islands of Honshu and Hokkaido. It is the longest railway tunnel in the world.
- The Thirlmere Aqueduct in North West England, United Kingdom is the longest tunnel, of any type, in the world at 154 km (96 mi).
- The Gotthard Base Tunnel will be the longest rail tunnel in the world at 57 km (35 mi). It will be totally completed in 2017.
- The Seikan Tunnel in Japan is the longest undersea rail tunnel in the world at 53.9 km (33.5 mi), of which 23.3 km (14.5 mi) is under the sea.
- The Channel Tunnel between France and the United Kingdom under the English Channel is the second-longest, with a total length of 50 km (31 mi), of which 39 km (24 mi) is under the sea.
- The Lötschberg Base Tunnel opened in June 2007 in Switzerland was the longest land rail tunnel, with a total of 34.5 km (21.4 mi).
- The Lærdal Tunnel in Norway from Lærdal to Aurland is the world's longest road tunnel, intended for cars and similar vehicles, at 24.5 km (15.2 mi).
- The Zhongnanshan Tunnel in People's Republic of China opened in January 2007 is the world's second longest highway tunnel and the longest road tunnel in Asia, at 18 km (11 mi).
- The longest canal tunnel is the Rove Tunnel in France, over 7.12 km (4.42 mi) long.
- Williamson's tunnels in Liverpool, 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.
- Gerrards Cross railway tunnel, in England, opened in 2010, is notable in that it was built in a railway cutting, that was first opened around 1906. This arguably is the longest ever tunnel in construction taking 104 years. The tunnel was built using the cut-and-cover method with prefabricated forms in order to keep the busy railway operating. A branch of the Tesco supermarket chain occupies the space above the railway tunnel with an adjacent railway station. During construction, a portion of the tunnel collapsed when the soil cover was added. The prefabricated forms were covered with a layer of reinforced concrete after the collapse.
- Moffat Tunnel, opened in 1928 in Colorado, straddles the Continental Divide. The tunnel is 10.0 km (6.2 mi) long and at 2.816 km (1.750 mi) above sea level is the highest railroad tunnel in the US.
- The Fredhälls road tunnel was opened in 1966, in Stockholm, Sweden, and the 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 (4.443 km (2.76 mi) 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, USA, is a part of the larger Big Dig completed around 2007, carries approximately 200,000 vehicles/day under the city along Interstate 93, U.S. Route 1, and Massachusetts Route 3, which share a concurrency through the tunnels. The Big Dig replaced Boston's old badly deteriorated I-93 elevated highway.
- Stormwater Management And Road Tunnel or SMART Tunnel, is a storm drainage and road structure opened in 2007 in Kuala Lumpur, Malaysia. The 9.7 km (6.0 mi) tunnel is the longest stormwater tunnel in South East Asia and second longest in Asia.
- The Eiksund Tunnel  on national road Rv 653 in Norway is the world's deepest subsea road tunnel, measuring 7.776 km ( 4.8 mi), with deepest point at −287 m (−942 ft) below the sea level, opened in February 2008.
- The Fenghuoshan tunnel (date of completion unknown) on Qinghai-Tibet railway is the world's highest railway tunnel, about 4.905 km (3.05 mi) above sea level.
- The La Linea Tunnel in Colombia, will be (2013) the longest, 8.58 km (5.33 mi), mountain tunnel in South America. It crosses beneath a mountain at 2,500 m (8,202.1 ft) 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 Chicago Deep Tunnel Project is a network of 175 km (109 mi) of tunnels designed to reduce flooding in the Chicago area. Started in the mid 1970s, the project is due to be completed in 2019.
- New York City Water Tunnel No. 3, started in 1970, has an expected completion date of 2020, and will measure more than 97 km long (60 mi).
The use of tunnels for mining is called drift mining.
Military use 
Some tunnels are not for transport at all but rather, are fortifications, for example Mittelwerk and Cheyenne Mountain. 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. One of the strangest uses of a tunnel was for the storage of chemical weapons .
Secret tunnels 
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.
The actual usage of erdstall tunnels is unknown but theories connect it to a rebirth ritual.
Natural tunnels 
- Lava tubes are partially empty, cave-like conduits underground, formed during volcanic eruptions by flowing and cooling lava.
- Natural Tunnel State Park (Virginia, USA) features an 850-foot (259 m) natural tunnel, really a limestone cave, that has been used as a railroad tunnel since 1890.
- Punarjani Guha Kerala, India. Hindus believe that crawling through the tunnel (which they believe was created by a Hindu god) from one end to the other will wash away all of one’s sins and thus attain rebirth, although only men are permitted to crawl through the cave.
- Small "snow tunnels" are created by voles, chipmunks and other rodents for protection and access to food sources. For more information regarding tunnels built by animals, see Burrow
- Clayton Tunnel rail crash - 1861 - confusion about block signals
- Welwyn Tunnel rail crash - 1866 - train failed in tunnel, guard did not protect train
- Balvano train disaster - 1944 -
- Caldecott Tunnel fire – 1982 – major motor vehicle tunnel crash and fire
- 1996 Channel Tunnel fire - 1996
See also 
- DESIGN MANUAL FOR ROADS AND BRIDGES: VOLUME 2: SECTION 2: PART 9: BD 78/99: DESIGN OF ROAD TUNNELS. The Department for Transport. 1999.
- NFPA Standard for Safeguarding Construction, Alteration, and Demolition Operations. National Fire Protection Association.
- "Tunnelling". tunnellersmemorial.com. Retrieved 2010-06-20.
- Bickel. (1995). Tunnel engineering handbook, 2nd edition. CBS Publishers.
- Powers, P.J. (2007). Construction de-watering and groundwater control. Hoboken, NJ: John Wiley & Sons Inc.
- United States Army Corps of Engineers. (1978). Tunnels and shafts in rock. Washington, DC: Department of the Army.
- "Capital Projects Funds". Cord.edu. Retrieved 2013-04-19.
- "Encouraging U.S. Infrastructure Investment - Council on Foreign Relations". Cfr.org. Retrieved 2013-04-19.
- "Tunnels & Tunnelling International". Tunnelsonline.info. Retrieved 2013-04-19.
- "The Groene Hart Tunnel". Hslzuid.nl. Retrieved 2013-04-19.
- "San Francisco-Oakland Bay Bridge". Bay Area Toll Authority.
- "Boston Big Dig, Central Artery / Tunnel Project, Massachusetts, United States of America". roadtraffic-technology.com. Net Resources International. Retrieved 1 April 2013.
- "Liverpool's Historic Rail Tunnels". The Liverpool Wiki. 1999-02-22. Retrieved 2013-04-19.
- Maund, T.B. (2001). Merseyrail electrics: the inside story. Sheffield: NBC Books. OCLC 655126526.
- Robie S. Lange (February, 1993). National Register of Historic Places Inventory-Nomination: St. Clair River Tunnel / St. Clair Railroad Tunnel. National Park Service
- [dead link]
- "City Water Tunnel No. 3". Web.archive.org. Archived from the original on 2007-06-21. Retrieved 2013-04-19.
- "Glenbrook Tunnel - Alcatraz Down Under - History Channel". Youtube.com. Retrieved 2013-04-19.
- Author lifts lid on chemical wartime history - Local News - News - General - Blue Mountains Gazette
- Railway Tunnels in Queensland by Brian Webber, 1997, ISBN 0-909937-33-8
Further reading 
- Sullivan, Walter. Progress In Technology Revives Interest In Great Tunnels, New York Times, June 24, 1986. Retrieved 15 August 2010.
|Wikisource has the text of the 1879 American Cyclopædia article Tunnel.|
- Trans Global Highway and proposed tunnels.
- Royal Engineers Museum British Army First World War Tunnelling.
- Directory of the world's longest tunnels by category
- ITA-AITES International Tunnelling Association
- Tunnels & Tunnelling International magazine
- Project Triton - Trentino Research & Innovation for Tunnel Monitoring at "DISI" (Dipartimento di Ingegneria e Scienza dell'Informazione) (University of Trento) Italy
- Pipe Jacking