Tunnel

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

• 1 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.
• 2 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.

  Construction on the English Channel Tunnel between England and France, a dream for centuries envisioned and encouraged by Napoleon, was begun in 1987. Originally referred to as the Chunnel and now known as the Eurotunnel, it was completed in 1994 at a cost of $13 billion. The two rail tunnels (one for northbound and one for southbound traffic) and one service tunnel are each 31 mi (50 km) in length and have an average depth of 150 ft (46 m) under the seabed. It is the first physical link between Britain and the European continent. Passenger rail service is provided, as well as the ferrying of automobiles and trucks. Travel times from London to Paris have been reduced from more than five hours (over sea) to three hours via the Eurotunnel.

  The Seikan Tunnel in Japan was placed in service in 1988. The 33mi-(53km-)long tunnel connects the northern tip of Japan's main island of Honshu with the island of Hokkaido, passing under the Tsugaru Strait. The Seikan Tunnel is the world's longest submarine tunnel, involving excavation 330 ft (100 m) below the seabed across a strait where the sea is up to 460 ft (140 m) in depth.

  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).

—LorettaHall

TUNNELS

TUNNELS. 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.

Carl W.Condit/a. r.

See alsoHoosac Tunnel ; Interstate Highway System ; Railways, Urban, and Rapid Transit .

tunnel underground passage usually made without removing the overlying rock or soil. Although tunnels are approximately horizontal, they must be built with sufficient gradient for proper drainage. Tunnels may be ventilated by shafts leading to the surface or by exhaust fans at the ends.

Design and Construction Techniques

Methods of tunneling vary with the nature of the material to be cut through. When soft earth is encountered, the excavation is timbered for support as the work advances; the timbers are sometimes left as a permanent lining for the tunnel. Another method is to cut two parallel excavations in which the side walls are constructed first. Arches connecting them are then built as the material between them is extracted. Portions of the unexcavated center, left temporarily for support, may be removed later. A tunnel cut through rock frequently requires no lining. Hard rock is removed by blasting.

In constructing tunnels under rivers, the ordinary methods can be used as long as a stratum of impermeable material lies between the tunnel and the riverbed. In all cases, however, pumping equipment must be installed. Where mud, quicksand, or permeable earth is present in underwater tunneling, it becomes necessary to provide some means of holding back the water while the enclosing sections of the tunnel are placed in position. For this purpose the shield was devised and first used in 1825 by the French-born engineer Sir Marc I. Brunel when boring between Wapping and Rotherhithe, in England. Considered unsuccessful, the device was not employed again until 1869, when the British engineer James H. Greathead and the American inventor Alfred E. Beach developed improvements at about the same time. Their shields were metal cylinders fitting around the outside of the tunnel, the forward end closed by a diaphragm plate. As the rock or earth was cut away, the shield was shoved forward into the earth by hydraulic rams, compressed air being used to keep seepage to a minimum. The use of the pneumatic shield is now universal in tunneling under rivers. The actual cutting is performed by huge rotating cutter heads, each with up to fifty separate cutters, capable of penetrating 10 mm (1/2 in.) per revolution.

River-crossing tunnels are also constructed by dredging a trench in the riverbed and then lowering prefabricated tunnel sections through the water into the trench, where they are connected to each other. The trench and tunnel are then covered over. In 1969, a tunnel was constructed across the Schelde River in Belgium, using sections 330 ft (100 m) long. Often, to speed construction, work is started at both ends. This poses no problem with the cut-and-cover method, but when the tunnel is bored from within, it must be assured that the tubes will actually meet in the center. Modern methods accomplish this with high precision.

Significant Historic and Modern Tunnels

The origin of tunnel building is disputed. The Egyptians built tunnels as entrances to tombs. The Babylonians built (c.2180 BC) a tunnel under the Euphrates using what is now called the "cut-and-cover" method; the river was diverted, a wide trench was dug across its bed, and a brick tube was constructed in it and covered up. The ancient Greeks and Romans built tunnels for carrying water and for mining purposes; some of the Roman tunnels are still in use. One of the first notable tunnels in Great Britain was part of the Grand Trunk Canal. It was nearly 2 mi (3.2 km) long and was completed in 1777. The Mont Cénis Tunnel, a railroad tunnel in the French Alps that opened in 1871 and is now 8.5 mi (13.7 km) long, was probably the first tunnel built using compressed-air drills.

The first tunnel of importance in the United States was the tunnel through the Hoosac Range in Massachusetts. There are hundreds of miles of tunnels in New York City and its vicinity, e.g., for subways, roads, water systems, and railroads. The Delaware Aqueduct , which provides part of New York City's water supply, is at 105 mi (168 km) the longest continuous tunnel in the world. Road tunnels include the Holland Tunnel and the Lincoln Tunnel, which connect New York City's Manhattan Island with New Jersey, and the Brooklyn-Battery Tunnel, which connects Manhattan Island with Brooklyn and is longest vehicular tunnel (1.7 mi/2.7 km) in the United States. The Chesapeake Bay Bridge-Tunnel in Virginia, opened in 1964, has a length of 17.6 mi (28.2 km) and includes two tunnel segments over a mile long.

The Simplon Tunnel (see under Simplon ) through the Alps, for many years the longest railway tunnel in the world, consists of two parallel single-line tunnels (both 12.3 mi/19.8 km) with connecting tunnels at short intervals. The Channel Tunnel (Chunnel) under the English Channel is much longer, at 31 mi (50 km), and the Seikan Tunnel in Japan, the world's deepest underwater tunnel, is also the longest railroad tunnel at 33.5 mi (53.6 km). The world's longest vehicular tunnel, the Lærdal Tunnel (15.2 mi/24.5 km long), connects Lærdal and Aurland, Norway, and is an important overland link between Oslo and Bergen. The St. Gotthard Tunnel (10.2 mi/16.4 km long), in the Swiss Alps, was formerly the longest vehicular tunnel.

Bibliography

See T. M. Megaw and J. V. Bartlett, Tunnels (1981-82); B. Stack, Handbook of Mining and Tunnelling Machinery (1982); Approaching the 21st Century (1987).

tunnels The earliest recorded tunnels are the quanats found in the Middle East and North Africa. These tunnels, some dating from 4000 bc, conducted water from springs in the foothills to communities in the desert. The use of tunnels, some of 60 km length, made it possible to even out gradients and prevented the evaporation that would have taken place in an open watercourse. Many quanats operate to this day.

The Romans developed tunnelling further, and many Roman water-supply systems used a combination of aqueducts and tunnels to negotiate uneven terrain at a constant gradient. The Romans also developed tunnelling for military purposes, either by breaking through behind enemy defences or by undermining fortifications to cause their collapse.

Much early tunnelling was carried out in association with mineral mining, either to provide access to the body of ore or to provide drainage as workings became deeper. Excavation was carried out using pick and shovel. Harder rocks were broken down by ‘fire-setting’, a process in which the rock face was heated by burning faggots against it, and then quenched with water, causing the rock to fracture.

Modern tunnelling developed with the canal age, which required tunnels of relatively large cross-section to pass beneath the watersheds between valley systems. Arched brickwork linings were developed and the use of gunpowder eased excavation in hard ground. Railways created a further demand for tunnelling, and by the mid-nineteenth century the construction of urban sewers, which now form the greatest length of tunnels constructed, began in earnest.

The nineteenth century saw the development of compressed air tunnelling, in which the entire tunnel was pressurized in order to hold back groundwater and support the face. The elder Brunel ( Marc Isambard) also developed the tunnelling shield, a steel cylinder from within which excavation could safely take place. These techniques enabled softer ground to be tackled.

Machines to excavate tunnels began to appear in the latter half of the nineteenth century. A machine drove over a mile of a Channel tunnel in the late 1800s before the work was stopped by political problems. Preformed linings for tunnels were first developed using cast iron. Later, concrete segments which could be bolted together to form a circular lining were used to provide support to ground and watertightness.

Modern tunnelling can take place in virtually any ground conditions. Machines 15 m in diameter bore through the hardest rock for two-lane highways. Sealed-face machines tackle soft silts, sands and water pressure beneath river crossings. Remotely controlled machines enable tunnels below man-entry size to be constructed, and at the smaller sizes the boundary between tunnelling and drilling techniques is becoming increasingly blurred. The growth of modern cities has greatly increased the demand for tunnelling for road transport and metro systems and for water, gas, electricity, and telecommunications routes. The high land costs of alternative routes more than offset the additional costs of tunnelling.

Nowadays tunnels are mainly lined with concrete segments or with concrete pumped in between the excavated ground and internal shuttering. Any voids between the lining and the ground are filled by pressure grouting to improve watertightness and ensure that the loading on the lining is even. Excavated material is removed by rail or, in some systems, by mixing the spoil with a bentonite clay slurry, which is then pumped to the surface. After settling, the slurry is reused. Lining segments are brought in by rail and erected using mechanical handling systems. Excavation is carried out either by a cutting head the size of the finished tunnel or by boom cutters which can be operated over the complete tunnel face by the driver. In more variable rock conditions, drill and blast techniques are used for excavations. Steel arches or sprayed concrete keep the tunnel stable until a final lining is put in place.

The Channel Tunnel, with 102 km of main tunnels and 51 km of service tunnels, achieved progress rates of 400m a week on each tunnel face. Rates will continue to improve as machines and materials are developed further.

Hamish J. Orr-ewing

tun·nel / ˈtənl/ • n. an artificial underground passage, esp. one built through a hill or under a building, road, or river. ∎  an underground passage dug by a burrowing animal. ∎  [in sing.] a passage in a sports stadium by which players enter or leave the field. • v. (-neled, -nel·ing; Brit. -nelled , -nel·ling ) 1. [intr.] dig or force a passage underground or through something: he tunneled under the fence | (tunnel one's way) the insect tunnels its way out of the plant. 2. [intr.] Physics (of a particle) pass through a potential barrier. PHRASES: light at the end of the tunnelsee light¹ .DERIVATIVES: tun·nel·er n.

tunnel † tubular net for catching birds XV; † shaft, flue XVI; subterranean passage XVIII. — OF. tonel (mod. tonneau tun, cask), f. tonne TUN; see -EL2.

tunnel (tun-ĕl) n. (in anatomy) a canal or hollow groove.

tunnel •annal, channel, flannel, impanel, multichannel, panel •cracknel •grapnel, shrapnel •carnal •antennal, crenel, fennel, kennel •regnal •anal, decanal •adrenal, officinal, penal, renal, venal •signal, spignel •hymnal • cardinal • libidinal • ordinal •attitudinal, latitudinal, longitudinal •altitudinal •imaginal, paginal •marginal, submarginal •aboriginal • virginal • disciplinal •seminal •criminal, liminal, subliminal •abdominal, nominal, phenomenal, pronominal •noumenal •germinal, terminal •vaticinal, vicinal •sentinel • intestinal • Juvenal •doctrinal, final, semi-final, spinal, urinal, vaginal •quarterfinal •cantonal, O'Connell •cornel • nounal •atonal, Donal, hormonal, Monel, patronal, polytonal, tonal, zonal •motional •lagoonal, monsoonal, tribunal •communal •Chunnel, funnel, gunnel, gunwale, runnel, tunnel •autumnal • meridional •embryonal, Lionel •diagonal, heptagonal, hexagonal, octagonal, tetragonal •trigonal • orthogonal • occasional •divisional, provisional, visional •delusional, fusional, illusional •regional • original • coronal • arsenal •medicinal •impersonal, interpersonal, personal, transpersonal •irrational, national, passional, rational •factional, fractional, redactional, transactional •confessional, congressional, expressional, impressional, obsessional, processional, professional, progressional, recessional, secessional, sessional, successional •connectional, correctional, directional, interjectional, intersectional, sectional, unidirectional •ascensional, attentional, conventional, declensional, intentional, tensional, three-dimensional, two-dimensional •conceptional, exceptional, perceptional •durational, locational, oblational, relational, vocational •rotational •additional, positional, tuitional, volitional •fictional, jurisdictional •inscriptional • optional • proportional •devotional, emotional, notional, promotional •constitutional, evolutional, institutional, substitutional •constructional, fluxional, instructional •conjunctional, dysfunctional, functional, multifunctional •versional • seasonal •colonel, diurnal, eternal, external, fraternal, infernal, internal, journal, kernel, maternal, nocturnal, paternal, supernal, vernal