Dictionary:

tunnel

  (tŭn'əl)

1. An underground or underwater passage.
2. A passage through or under a barrier.
3. Obsolete. The main flue on a chimney.

1. To make a tunnel through or under.
2. To produce, shape, or dig in the form of a tunnel.

To make a tunnel.

[Middle English tonel, tubular net, from Old French tonnelle, diminutive of tonne, tun, possibly of Celtic origin.]

Background

A tunnel is an underground or underwater passage that is primarily horizontal. Relatively small-diameter ones carry utility lines or function as pipelines. Tunnels that transport people by rail or by automobile often comprise two or three large, parallel passages for opposite-direction traffic, service vehicles, and emergency exit routes.

The world's longest tunnel carries water 105 mi (170 km) to New York City from the Delaware River. The lengthiest person-carrying tunnel is the Seikan Railroad Tunnel. It is a 33-mi (53-km) long, 32-ft (9.7-m) diameter railroad connection between Japan's two largest islands, Honshu and Hokkaido.

One of the most anticipated tunnels was the Channel Tunnel. Completed in 1994, this tunnel connects Great Britain to Europe through three, 31-mi (50-km) long tunnels (two one-way and one service tunnel). Twenty-three miles (37 km) of this tunnel are underwater.

History

Tunnels were hand-dug by several ancient civilizations in the Indian and Mediterranean regions. In addition to digging tools and copper rock saws, fire was sometimes used to heat a rock obstruction before dousing it with water to crack it apart. The cut-and-cover method—digging a deep trench, constructing a roof at an appropriate height within the trench, and covering the trench above the roof (a tunneling technique still employed today)—was used in Babylon 4,000 years ago.

The first advance beyond hand-digging was the use of gunpowder to blast a 515-ft (160-m) long canal tunnel in France in 1681. The next two major advances came about 1850. Nitroglycerine (stabilized in the form of dynamite) replaced the less powerful black powder in tunnel blasting. Steam and compressed air were used to power drills to create holes for the explosive charges. This mechanization eventually replaced the manual process made famous by John Henry, the "steel-driving man," who swung a 10-lb (4.4-kg) sledge hammer with each hand for 12 hours a day, pounding steel chisels as deep as 14 ft (4.2 m) into solid rock.

Between 1820 and 1865, British engineers Marc Brunel and James Greathead developed several models of a tunneling shield that enabled them to construct two tunnels under the Thames River. A rectangular or circular enclosure (the shield) was divided horizontally and vertically into several compartments. A man working in each compartment could remove one plank at a time from the face of the shield, dig ahead a few inches, and replace the plank. When space had been dug away from the entire front surface, the shield was pushed forward, and the digging process was repeated. Workers at the rear of the shield lined the tunnel with bricks or cast iron rings.

In 1873, American tunneler Clinton Haskins kept water from seeping into a railroad tunnel under construction below the Hudson River by filling it with compressed air. The technique is still used today, although it presents several dangers. Workers must spend time in decompression chambers at the end of their shift—a requirement that limits emergency exits from the tunnel. The pressure within the tunnel must be carefully balanced with the surrounding earth and water pressure; an imbalance causes the tunnel either to collapse or burst (which subsequently allows flooding).

Soft soil is prone to collapse and it can clog digging equipment. One way to stabilize the soil is to freeze it by circulating coolant through pipes embedded at intervals throughout the area. This technique has been used in the United States since the early 1900s. Another stabilization and waterproofing technique—widely used since the 1970s—is to inject grout (liquid bonding agent) into soil or fractured rock surrounding the tunnel route.

Shotcrete is a liquid concrete that is sprayed on surfaces. Invented in 1907, it has been used as both a preliminary and a final lining for tunnels since the 1920s.

In 1931, the first drilling jumbos were devised to dig tunnels that would divert the Colorado River around the construction site for Hoover Dam. These jumbos consisted of 24-30 pneumatic drills mounted on a frame welded to the bed of a truck. Modern jumbos allow a single operator to control several drills mounted on hydraulically controlled arms. In 1954, while building diversion tunnels for construction of a dam in South Dakota, James Robbins invented the tunnel boring machine (TBM), a cylindrical device with digging or cutting heads mounted on a rotating front face that grinds away rock and soil as the machine creeps forward. Modern TBMs are customized for each project by matching the types and arrangement of the cutting heads to the site geology; also, the diameter of TBM must be equal to the diameter of the designed tunnel (including its lining).

Raw Materials

Materials used in tunnels vary with the design and construction methods chosen for each project. Grout used to stabilize soil or fill voids behind the tunnel lining may contain various materials, including sodium silicate, lime, silica fume, cement, and bentonite (a highly absorbent volcanic clay). Bentonite-and-water slurry is also used as a suspension and transportation medium for muck (debris excavated from the tunnel) and as a lubricant for objects being pushed through the tunnel (e.g., TBMs, shields). Water is used to control dust during drilling and after blasting, which is often done with a low-freezing gelatine explosive. Water-and-salt brine or liquid nitrogen are common refrigerants for stabilizing soft ground by freezing. The most common modern lining material, concrete reinforced by either steel or fiber, may be sprayed on, cast in place, or prefabricated in panels.

Choice of method

A tunnel's construction method is determined by several factors, including geology, cost, and potential disruption of other activities. Different methods may be used on individual tunnels that are part of the same larger project; for example, four separate methods are being used on portions of Boston's Central Artery/Tunnel project.

The Manufacturing Process

Preparing

• Site geology is evaluated by examining surface features and subsurface core samples. A pilot tunnel about one-third the diameter of the planned main tunnel may be constructed along the entire route to further evaluate the geology and to test the selected construction method. The pilot tunnel may run alongside the main tunnel's path and eventually be connected to it at intervals to provide ventilation, service access, and an escape route. Or the pilot tunnel may be enlarged to produce the main tunnel.
• If soil stabilization is required, it may be done by injecting grout through small pipes placed in the ground at intervals. Alternatively, a refrigerant may be circulated through pipes embedded in the ground to freeze the soil.

Mining

• 3 There are seven different methods used to remove material from the tunnel path. The first is the immersed tube method. Workers prepare an underwater tunnel site by digging a trench at the bottom of the waterway. Steel or reinforced concrete sections of tunnel shell are constructed on dry land. Each section may be several hundred feet (100 m or more) long. The ends of the section are sealed, and the section is floated to the tunnel site. The section is tied to anchors adjacent to the trench, and ballast tanks built into the section are flooded. As the section sinks, it is guided into place in the trench. The section is connected to the adjoining, previously placed section, and the plates sealing that end of each section are removed. A rubber seal between the two sections ensures a watertight connection.

  In the cut-and-cover method workers dig a trench large enough to contain the tunnel and its shell. A box-shaped tube is constructed, often by in-place casting of reinforced concrete. In certain types of soil or in close proximity to other structures, tunnel walls may be built before digging begins in order to keep the trench from collapsing during excavation. This may be done by driving steel sheets into the ground or building a slurry wall (a deep trench that is filled with watery clay as dirt is removed). When the desired size is attained for a section of wall, a cage of steel reinforcing rods is lowered into it and concrete is pumped in to displace the wet-clay slurry. As digging progresses enough for the excavation machinery to be below grade, temporary surface panels may be laid across the trench to allow traffic to move across it. When the tunnel shell has been completed, it is covered by replacing excavated soil.

  The third method is the top-down method. A parallel pair of walls are embedded into the ground along the tunnel's route by driving steel sheet piles or constructing slurry walls. A trench is dug between the walls to a depth equal to the planned distance from the surface to the inside of the tunnel roof. The tunnel roof is formed between the walls by framing and pouring reinforced concrete on the bottom of the shallow trench. After the tunnel roof has cured, it is covered with a waterproofing membrane and excavated soil is replaced above it. Conventional excavating machinery, such as a front-end loader, is used to dig out the soil between the diaphragm walls and under the tunnel roof. When sufficient depth has been reached, a reinforced concrete floor is poured to complete the tunnel shell.

  With the drill-and-blast method a drilling jumbo is used to drill a predetermined pattern of holes in the rock along the tunnel's path. Carefully planned charges of dynamite are inserted in the drilled holes. The charges are detonated in a sequence designed to break away material from the tunnel's path without unduly damaging the surrounding rock. Air is circulated through the blast area to remove explosion gases and dust. Rubble dislodged by the blast is hauled away. Pneumatic drills and hand tools are used to smooth the surface of the blasted section and remove loose pieces of rock.

  It is usually necessary to stabilize and reinforce the surface of the newly blasted section with a preliminary lining. One technique involves inserting a series of steel ribs connected by wood or steel braces. Another technique, called the new Austrian tunneling method (NATM), involves spraying the surface with a few inches (several centimeters) of concrete. In appropriate geologic conditions, this "shotcrete" lining may be supplemented by inserting long steel rods (rock bolts) into the rock and tightening nuts against steel plates surrounding the head of each bolt.

  A fifth method to remove material from the tunnel is the shield driving or tunnel jacking method. Some tunnels are still dug using a Greathead-style shield. The top of the shield extends beyond the sides and bottom, providing a protective roof for workers digging in advance of the shield. The leading edge of the shield top is sharp so it can cut through the soil. Excavation may be done by hand or with power tools. Excess material is passed back through the shield on a convey or belt, loaded into carts, and hauled out of the tunnel. When workers have dug out material in front of the shield as far as the top extends, jacks at the rear of the shield are braced against the most recently installed section of tunnel lining. Activating the jacks pushes the shield forward so workers can begin digging another section. After the shield has moved forward, the jacks are retracted, and steel or reinforced concrete ring segments are bolted into place to form a section of permanent lining for the tunnel.

  Tunnel jacking is a similar technique, but the shield being driven through the ground is actually a prefabricated section of tunnel lining.

  In the parallel drift method a series of parallel, horizontal holes (drifts) are bored using microtunneling machinery (microtunnels are too small for human miners to work inside of) such as augers or small versions of TMBs. These drifts are filled; for example, steel pipes may be driven into them and then the pipes packed with grout. The filled drifts form a protective arch around the tunnel path. Excavation machinery is used to remove the soil from inside the arch.

  The final method is the tunnel boring machine method. The types and arrangement of cutting devices on the face of the TBM are determined by the geology at the tunnel site. The face slowly rotates and grinds away the rock and soil in front of it (e.g., the TBMs used to build the Channel Tunnel could rotate up to 12 revolutions per minute in optimal soil). The TBM is constantly pushed forward to keep the face in contact with its target. Forward pressure may be exerted by jacks at the rear of the TBM pushing against the most recently installed section of tunnel lining. Alternatively, gripper arms may extend outward from the sides of the TBM and push against rocky tunnel walls to hold the machine in place while the face is pushed forward. Muck is passed through holes in the face and carried by conveyor belt to the rear of the TBM, where it drops into carts that transport it out of the tunnel. Bentonite may be pumped through the TBM face to make the soil surface more workable and to carry away the muck. Some TBMs are equipped at the rear with robotic arms that position and attach segments of tunnel lining as soon as the machine has moved forward a sufficient distance. In other cases, the NATM is used to create a preliminary lining as the TBM progresses.

  Especially in cases where two TBMs dig toward each other from opposite ends of a tunnel, it may be too difficult or expensive to remove them when the digging is completed. As it nears the end of its mission, the TBM may be steered away from the tunnel's path to dig a short spur in which it is permanently sealed.

Final lining

• 4 In some cases, the final lining is placed during the excavation process. Two examples are TBMs that install lining segments and prefabricated tunnels that are jacked into place. In other cases, a final lining must be constructed after the entire tunnel is excavated. One option is to pour a reinforced concrete lining in place. Slipforming is an efficient technique in which a section of form is slowly moved forward as the concrete is poured between it and the tunnel wall; the concrete hardens quickly enough to support itself by the time the form moves on.

  A second option is to install segments of preformed concrete or steel lining, much as some TBMs do. Lining segments are constructed so that several of them can be joined to form a complete ring a few feet (a meter or two) wide. Once a ring has been bolted into place, grout is injected between it and the tunnel wall.

  A third option is to spray a layer of shotcrete several inches (70 mm or more) thick onto the tunnel walls. One or two layers of wire mesh might be placed first to reinforce the shotcrete, or reinforcing fibers might be added to the concrete mixture to increase its strength.

Byproducts/Waste

Sometimes the earth removed from a tunnel is simply discarded into a landfill. In other cases, however, it becomes raw material for other projects. For example, it may be used to form the base course for an approach roadway or to create roadway embankments for wider shoulders or erosion control.

Quality Control

Besides maintaining ground stability around the tunnel and ensuring structural integrity of the tunnel lining, proper alignment of the excavation path must be achieved. Two valuable tools are global positioning system (GPS) sensors that receive precise locational data via satellite signals and guidance systems that project and detect a laser beam within the tunnel.

The Future

Exploration methods, materials, and machinery are possible areas of improvement. Sound waves transmitted through the earth can now generate a virtual CAT scan of the tunnel path, reducing the need to drill core samples and pilot tunnels. Some examples of materials research involve cutting tools that are more effective and durable, concrete with more precisely controlled hardening rates, and better processes for modifying soil to make it easier to cut, dig, or remove. Recent developments in machine technology include multiple-headed TBMs that can bore two or three parallel tunnels simultaneously and a TBM that can turn a corner up to 90° while cutting. Better remote control capabilities for digging machinery would improve safety by reducing the amount of time people have to be underground during the digging process.

Where to Learn More

Periodicals

Burroughs, Dan, et al. "Depressing Traffic Top-Down." Civil Engineering (January 1994): 62.

Campo, David W., and Donald P. Richards. "Tunneling Beneath Cairo." Civil Engineering (January 2000): 36.

Iseley, Tom. "Microtunneling MARTA." Engineering (December 1991): 50.

O'Connor, Leo. "Tunneling Under the Channel." Mechanical Engineering (December 1993): 60.

Other

The Cumberland Gap Tunnel. http://www.efl.fha.dot.gov/cumgap/tunnel.htm (January 2000).

"A Short History of Tunnelling." http://pisces.sbu.ac.uk/BE/CECM/Civ-eng/tunhist.html (January 2000).

"Tunnel Jacking." Central Artery/Tunnel Project. http://www.bigdig.com (January 2001).

[Article by: Loretta Hall]

An underground space of substantial length, usually having a tubular shape. Tunnels can be either constructed or natural and are used as passageways, storage areas, carriageways, and utility ducts. They may also be used for mining, water supply, sewerage, flood prevention, and civil defense.

Tunnels are constructed in numerous ways. Shallow tunnels are usually constructed by burying sections of tunnel structures in trenches dug from the surface. This is a preferred method of tunneling as long as space is available and the operation will not cause disturbance to surface activities. Otherwise, tunnels can be constructed by boring underground. Short tunnels are usually bored manually or by using light machines. If the ground is too hard to bore, a drill-and-blast method is frequently used. For long tunnels, it is more economical and much faster to use tunneling boring machines which work on the full face (complete diameter of the opening) at the same time. In uniform massive rock formations without fissures or joints, tunnels can be bored without any temporary supports to hold up the tunnel crowns. However, temporary supports are usually required because of the presence of destabilizing fissures and joints in the rock mass.

For tunnels to be constructed across bodies of water, an alternative to boring is to lay tunnel boxes directly on the prepared seabed. These boxes, made of either steel or reinforced concrete, are usually buried in shallow trenches dug for this purpose and covered by ballast so they will not be affected by the movement of the water. The joints between tunnel sections are made watertight by using rubber gaskets, and water is pumped out of the tunnel to make it ready for service. See also Concrete; Steel.

For more information on tunnel, visit Britannica.com.

There is nothing more common in local tradition than stories about secret tunnels; to give individual references is impossible, but most books on local history and lore include some. They are said to run from castles, inns, churches, manor houses, or virtually any old and notable building, and to lead to another building, or down to the sea, or occasionally into the depths of a hill. Most people who talk or write about them do so with belief, partly because sometimes short lengths of tunnelling are indeed found leading off from the cellars of old buildings, though so far these have always turned out, on investigation, to be merely ice-houses or drains. But the tunnels of rumour and tradition are far longer, often running for impossible distances, and through impossible terrain—under rivers, through swampy or sandy ground, where they would inevitably soon collapse or become flooded.

Most have some sort of explanation attached, drawing on real or imagined history. The tunnel may be said to have served as a secret passage for smugglers; or to enable priests or monks to leave their buildings unobserved (in order to meet their mistresses, especially if these were nuns); or as a secret entrance to a castle. A modern variation is to allege the existence of deep civil or military shelters, linked by a network of underground passages. Such places do exist (the cliffs of Dover are said to be honeycombed with them), but it strains credulity to be told, ‘When the new Public Library was being built at X, they put in a nuclear shelter for the Mayor at the same time.’

Tunnels can also be places where treasure is hidden, and where ghosts walk, either in the tunnel or above ground, following its track. A tragic story is told about eight or nine of them: two men who had seen one end of a tunnel wanted to know where it led to, or whether there was treasure in it. One of them was a musician; at Richmond Castle (Yorkshire) he is said to have been a drummer boy, but at Grantchester (Cambridgeshire) and elsewhere, a fiddler or a piper. This musician entered the tunnel and walked along it, playing as he went, while his friend followed above ground, tracking him by the sound; suddenly the music ceased, and the musician was never seen again. It has been pointed out that in some places where this story is told there is a small wood called Fiddler's Copse midway along the supposed course of the tunnel (Folk-Lore 34 (1923), 378-9).

A canal or hollow groove that may form a passageway for nerves or blood vessels through a body structure.

The digging of permanent tunnels is the most difficult, expensive, and hazardous of civil engineering works. Although extensive tunneling has characterized deep-level mining and the construction of water supply systems since ancient times, transportation tunnels have been largely the products of nineteenth-century technology. The earliest such tunnels in the United States were built for canals. The pioneer work was constructed in 1818–1821 to carry the Schuylkill Canal through a hill at Pottsville, Pa., and it was shortly followed by the tunnel of the Union Canal at Lebanon, Pa. (1825–1827). Possibly the first tunnel to exceed a length of 1,000 feet was excavated in 1843 for the passage of the Whitewater Canal through the ridge at Cleves, Ohio, near Cincinnati. The first U.S. railway tunnel was probably that of the New York and Harlem Railroad at Ninety-first Street in New York City (1837). Until 1866 all tunnels had to be laboriously carved out by hand techniques, with drills, picks, and shovels as the primary tools.

The beginning of modern rock tunneling in the United States came with the digging of the railroad tunnel through Hoosac Mountain, Mass., which required twenty-two years for completion (1854–1876). The enterprise was initially carried out by hammer drilling, hand shoveling, and hand setting of black-powder charges. This method was suddenly changed in 1866, when Charles Burleigh introduced the first successful pneumatic drill, and the chief engineer of the project, Thomas Doane, first used the newly invented nitroglycerin to shatter the rock. With a length of 4.73 miles, the Hoosac was the longest tunnel in the United States for half a century following its completion.

For tunneling through soft ground an entirely different technique is necessary, since the problem is more one of removing the muck and holding the earth in place than digging through it. An adequate solution to the problem involved the use of the tunnel-driving shield, originally patented in 1818 by the British engineer Marc Isambard Brunel for a tunnel under the Thames River. It was introduced in the United States by Alfred Ely Beach for the abortive Broadway subway in New York City (1869–1870), a successful work that was abandoned in the face of political opposition.

Various forms of pneumatic and shield tunneling were most extensively employed in the great subaqueous tunnel system of New York City. The first Hudson River tunnel, the trouble-plagued enterprise of the promoter De Witt C. Haskin, dragged on from 1873 to 1904. Haskin began operations by pneumatic excavation, the technique employed in the pressure caissons developed for building bridge piers, but a blowout in 1880 cost twenty lives and led to its abandonment. Work was resumed in 1889 by means of the shield invented by the English engineer James H. Great head for working in near-fluid alluvial sediments, but lack of capital held up completion for another fifteen years. The tunnel eventually became part of the Hudson and Manhattan Railroad system. The longest of all the tunnels underlying metropolitan New York is the second Croton Aqueduct (1885–1890), blasted largely through igneous rock for a total length of thirty-one miles. Still another variation on the Great head shield was introduced by James Hobson for mining the Grand Trunk Railroad's Saint Clair Tunnel (1886–1891) between Port Huron, Mich., and Sarnia, Ontario, the first to unite Canada with the United States.

The extensive tunnel system of the Pennsylvania Railroad's New York extension (1903–1910) required for its completion all the existing techniques of tunneling. The soft sediments of the Hudson River bed allowed the use of the shield; the igneous rock of Manhattan called for power drills and blasting or cut-and-cover methods, while the gravel underlying the East River necessitated mining in front of the shield under a vast blanket of clay laid down on the bed to prevent the blowouts that would have occurred in the porous material.

The safest and most economical method of tunneling—the trench method—was first used at the beginning of the twentieth century. The Detroit River Tunnel of the Michigan Central Railroad (1906–1910) was the first to be built by the trench method: cylindrical concrete sections with sealed ends were poured on land, towed to position, sunk into a trench previously dredged in the riverbed, and covered with gravel. The longest tunnel built by this method is the subaqueous portion of the Chesapeake Bay Bridge and Tunnel (1960–1964).

With all the techniques of excavation and lining well established, tunnel engineers were able to build the big rail and vehicular bores necessary to keep pace with the expanding traffic that followed World War I. The pioneer automotive tube was the Clifford M. Holland Tunnel under the Hudson River at New York City (1920–1927), which was followed by three others under the East and Hudson Rivers. The Moffat Tunnel (1923–1928) of the Denver and Salt Lake Railroad through James Peak in Colorado held the short-lived record for transportation tunnel length, 6.1 miles, and was the first long rail tunnel designed with a forced-draft ventilating system for the operation of steam locomotives. The Cascade Tunnel (1925–1929) of the Great Northern Railway in Washington, 7.79 miles long, is the longest tunnel in the United States. The complete mechanization of rock tunneling was finally achieved in 1952 by means of the mechanical mole, a cylindrical drilling machine as large as the tunnel interior equipped with rotating hardened-steel cutters that can grind through the densest rock.

American engineers led advances in tunnel technology in the nineteenth and early twentieth century. But the completion of the national highway grid and a tendency to rely on automobiles and aviation, rather than railways, resulted in fewer new tunnel projects. Probably the most extensive of the late twentieth century was the Central Artery/Tunnel Project in Boston, Mass. (dubbed by locals as the Big Dig). Begun in 1991 and scheduled to be finished in 2004, this massive project extended the Massachusetts Turnpike through a tunnel to Logan Airport while putting the elevated Central Artery underground, freeing hundreds of acres in downtown Boston for redevelopment.

The Big Dig notwithstanding, the most ambitious and technologically advanced tunnels in the early twenty-first century were being built in nations with growing public transportation systems in Europe and Asia. The Seikan railway tunnel under the Tsugaru Strait in Japan, built in 1988, is 33.5 miles in length, two miles longer than the Chunnel, the railway link under the English Channel linking England with Normandy, France, completed in 1994. Various other European countries planned Alpine tunnels that would span even longer distances.

Bibliography

Beaver, Patrick. A History of Tunnels. London: P. Davies, 1972.

Bickel, John O., and T. R. Kuesel, eds. Tunnel Engineering Handbook. New York: Van Nostrand Reinhold, 1982.

Sandström, Gösta E. Tunnels. New York: Holt, Rinehart and Winston, 1963.

West, Graham. Innovation and the Rise of the Tunnelling Industry. New York: Cambridge University Press, 1988.

A passageway of varying length through a solid body, completely enclosed except for the open ends, permitting entrance and exit.

• carpal t. — see carpal tunnel, carpal tunnel syndrome.
• t. of Corti — spiral passage in the organ of Corti.
• t. graft — see rope flap.
• tarsal t. — see tarsal tunnel.

This article is about underground passages. For uses of the word tunnel, see Tunnel (disambiguation).

A disused railway tunnel now converted to pedestrian and bicycle use, near Houyet, Belgium

A tunnel is an underground passage. 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.

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.

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 longest canal tunnel is the Standedge Tunnel in the United Kingdom, over three miles long.

In the Czech republic, the verb to 'tunnel' is a synonym for to embezzle. For example: the manager 'tunnelled' the company and now lives on the Bahamas; or, many banks collapsed because they were 'tunnelled'. ^[1]

Colorful pedestrian Light Tunnel connecting two terminals in Detroit's DTW airport.

The North East MRT Line in Singapore is a fully-underground rail line.

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. At metro stations there are usually pedestrian tunnels from one platform to another. Often, ground-level railway stations also have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without walking across the tracks. In the United Kingdom bridges are an equally popular for pedestrian access between two or more railway station platforms.

Geotechnical investigation

Main article: Geotechnical investigation

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 unforseen 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.

Construction

Gotthard Base Tunnel under construction

Tunnels are dug in various types of materials, from soft clay to hard rock, and the method of excavation depends on the ground conditions.

Cut-and-cover

Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing due to over head pressure.

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 insitu 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 insitu 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.

Boring machines

A tunnel boring machine that was used at Yucca Mountain, Nevada

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, bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front, 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 biggest TBM built was used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. Its diameter is 14.87 m. [1]

Nowadays 4 even larger machines exist: 2 for the M30 ringroad in Madrid, Spain, 2 for the Chong Ming tunnels in Shanghai, China. These machines are 15,2 m and 15,4m in diameter respectively. The two machines for Spain were built by Mitsubishi/Dura Fuelgo and Herrenknecht [2]. The TBMs for China were built by Herrenknecht.

NATM

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.

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.

Underwater tunnels

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

Other tunneling methods include:

• Drilling and blasting
• Slurry-shield machine
• Wall-cover construction method.

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 Westerscheldetunnel, 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.

Short tunnels

A short tunnel can be built as an alternative to an overpass. One example of a short tunnel is the Croom Tunnel on the South Coast railway line.

Artificial tunnels

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, which 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.

Examples of tunnels

In history

Interior of the Thames Tunnel, London, mid 19th century

• 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 Samos (Ionia). Built 520 BC by the Ionian engineer Eupalinos. Eupalinos organised the work so that the tunnel was begun from both sides of the hill and the two teams met in the middle. The estimates for the time required range from 5 to 15 years: the mountain is solid limestone and one has to suppose that many of the slaves doing the work died. 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.
• 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.
• 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.
• Col de Tende Road Tunnel, one of the first longer road tunnels under a pass

See also the History of Rapid transit.

Longest

Main article: List of tunnels by length

• The Seikan Tunnel in Japan is the longest rail tunnel in the world at 53.9 km (33.4 miles), of which 23.3 km (14.5 miles) 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 miles), of which 39 km (24 miles) is under the sea.
• The Lötschberg Base Tunnel opened in June 2007 in Switzerland is the longest land tunnel, with a total of 34.5 km (21.5 miles).
• 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 miles).
• The Zhongnanshan Tunnel in People's Republic of China opened in January 2007 is the world's longest highway tunnel and the longest road tunnel in Asia, at 18.0 km (11.3 miles).
• Päijänne-tunneli is the world's longest complete tunnel that is bored into cliff. 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.

Notable

• 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
• 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[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 Houston Downtown Tunnel System is a system of tunnels about twenty feet below Houston's downtown street system. The system forms a network of subterranean, climate-controlled, pedestrian walkways that link twenty-five full city blocks.
• The Sydney Harbour Tunnel in Sydney, Australia was built in 1992 to augment the Sydney Harbour Bridge.

Other uses

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.

Media

• Fenghuoshan tunnel

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  A journey through the world's highest railway tunnel. (5.41 MB, ogg/Theora format).

  
  

• Problems seeing the videos? See media help.

Natural tunnel

• Punarjani Guha

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

See also

Wikimedia Commons has media related to:

Tunnel

• List of tunnels
• Undersea tunnel
• Underground city
• Urban exploration
• Roof and tunnel hacking
• Smuggling tunnel
• List of rathole tunnels
• Wind tunnel
• World's longest tunnels
• Megaprojects
• Lava tube

References

1. ^ Tunneling (fraud)

External links

• 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

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - tunnel, akselgang
v. tr. - bygge en tunnel
v. intr. - gå gennem en tunnel

idioms:

• tunnel vision    tunnelsyn

Nederlands (Dutch)
tunnel, een tunnel graven

Français (French)
n. - tunnel
v. tr. - creuser
v. intr. - creuser

idioms:

• tunnel vision    (Méd) rétrécissement (tubulaire) du champ visuel, (fig) (avoir) des ¯illères

Deutsch (German)
n. - Tunnel, Stollen, unterirdischer Gang
v. - einen Tunnel graben, sich einen Weg bahnen

idioms:

• tunnel vision    Röhrengesichtsfeld, enges Blickfeld

Ελληνική (Greek)
n. - σήραγγα, τούνελ, λαγούμι (ζώου)
v. - ανοίγω ή εξορύσσω σήραγγα, σκάβω λαγούμι

idioms:

• tunnel vision    κοντόφθαλμη ή στενή θεώρηση

Italiano (Italian)
galleria

idioms:

• tunnel vision    visione tubulare, miopia

Português (Portuguese)
n. - túnel (m), cano de chaminé (m)
v. - cavar ou abrir túnel

idioms:

• tunnel vision    sem visão periférica

Русский (Russian)
туннель/тоннель

idioms:

• tunnel vision    узкий кругозор

Español (Spanish)
n. - túnel, galería, tubo, cañón
v. tr. - abrir un túnel a través de o debajo de, excavar
v. intr. - abrir túnel o galería

idioms:

• tunnel vision    estrechez de miras, falta de visión lateral

Svenska (Swedish)
n. - tunnel, gång
v. - bygga (gräva etc.) en tunnel, borra igenom, underminera

中文（简体） (Chinese (Simplified))
隧道, 地下道, 在...挖掘隧道, 在...打开通道, 挖, 凿, 打开, 挖掘隧道, 隧穿, 打开通道

idioms:

• tunnel vision    视野狭窄, 目光狭窄, 管状视力, 以管窥天

中文（繁體） (Chinese (Traditional))
n. - 隧道, 地下道
v. tr. - 在...挖掘隧道, 在...打開通道, 挖, 鑿, 打開
v. intr. - 挖掘隧道, 隧穿, 打開通道

idioms:

• tunnel vision    視野狹窄, 目光狹窄, 管狀視力, 以管窺天

한국어 (Korean)
n. - 터널, 굴, 지하도
v. tr. - ~에 터널을 파다, 갱도를 파고 나아가다
v. intr. - ~에 터널을 파다, 잠복하다

日本語 (Japanese)
n. - トンネル, 坑道
v. - トンネルを掘る, 掘る

idioms:

• tunnel vision    棒視, 視野の狭さ

العربيه (Arabic)
‏(الاسم) نفق, قمع, انبوب (فعل) يشق نفقا‏

עברית (Hebrew)
n. - ‮מנהרה, ניקבה, מחילה, תקופת סבל ממושכת‬
v. tr. - ‮פילס דרך ע"י חפירת מנהרה‬
v. intr. - ‮חפר מנהרה‬

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