Tunnels and underground excavations, horizontal underground passageway produced by excavation or occasionally by nature’s action in dissolving a soluble rock, such as limestone. A vertical opening is usually called a shaft. Tunnels have many uses: for mining ores, for transportation—including road vehicles, trains, subways, and canals—and for conducting water and sewage. Underground chambers, often associated with a complex of connecting tunnels and shafts, increasingly are being used for such things as underground hydroelectric-power plants, ore-processing plants, pumping stations, vehicle parking, storage of oil and water, water-treatment plants, warehouses, and light manufacturing; also command centres and other special military needs.

True tunnels and chambers are excavated from the inside—with the overlying material left in place—and then lined as necessary to support the adjacent ground. A hillside tunnel entrance is called a portal; tunnels may also be started from the bottom of a vertical shaft or from the end of a horizontal tunnel driven principally for construction access and called an adit. So-called cut-and-cover tunnels (more correctly called conduits) are built by excavating from the surface, constructing the structure, and then covering with backfill. Tunnels underwater are now commonly built by the use of an immersed tube: long, prefabricated tube sections are floated to the site, sunk in a prepared trench, and covered with backfill. For all underground work, difficulties increase with the size of the opening and are greatly dependent upon weaknesses of the natural ground and the extent of the water inflow.

History

Ancient tunnels

It is probable that the first tunneling was done by prehistoric people seeking to enlarge their caves. All major ancient civilizations developed tunneling methods. In Babylonia, tunnels were used extensively for irrigation; and a brick-lined pedestrian passage some 3,000 feet (900 metres) long was built about 2180 to 2160 BC under the Euphrates River to connect the royal palace with the temple. Construction was accomplished by diverting the river during the dry season. The Egyptians developed techniques for cutting soft rocks with copper saws and hollow reed drills, both surrounded by an abrasive, a technique probably used first for quarrying stone blocks and later in excavating temple rooms inside rock cliffs. Abu Simbel Temple on the Nile, for instance, was built in sandstone about 1250 BC for Ramses II (in the 1960s it was cut apart and moved to higher ground for preservation before flooding from the Aswān High Dam). Even more elaborate temples were later excavated within solid rock in Ethiopia and India.

The Greeks and Romans both made extensive use of tunnels: to reclaim marshes by drainage and for water aqueducts, such as the 6th-century-BC Greek water tunnel on the isle of Samos driven some 3,400 feet through limestone with a cross section about 6 feet square. Perhaps the largest tunnel in ancient times was a 4,800-foot-long, 25-foot-wide, 30-foot-high road tunnel (the Pausilippo) between Naples and Pozzuoli, executed in 36 BC. By that time surveying methods (commonly by string line and plumb bobs) had been introduced, and tunnels were advanced from a succession of closely spaced shafts to provide ventilation. To save the need for a lining, most ancient tunnels were located in reasonably strong rock, which was broken off (spalled) by so-called fire quenching, a method involving heating the rock with fire and suddenly cooling it by dousing with water. Ventilation methods were primitive, often limited to waving a canvas at the mouth of the shaft, and most tunnels claimed the lives of hundreds or even thousands of the slaves used as workers. In AD 41 the Romans used some 30,000 men for 10 years to push a 3.5-mile (6-kilometre) tunnel to drain Lacus Fucinus. They worked from shafts 120 feet apart and up to 400 feet deep. Far more attention was paid to ventilation and safety measures when workers were freemen, as shown by archaeological diggings at Hallstatt, Austria, where salt-mine tunnels have been worked since 2500 BC.

From the Middle Ages to the present

Canal and railroad tunnels

Because the limited tunneling in the Middle Ages was principally for mining and military engineering, the next major advance was to meet Europe’s growing transportation needs in the 17th century. The first of many major canal tunnels was the Canal du Midi (also known as Languedoc) tunnel in France, built in 1666–81 by Pierre Riquet as part of the first canal linking the Atlantic and the Mediterranean. With a length of 515 feet and a cross section of 22 by 27 feet, it involved what was probably the first major use of explosives in public-works tunneling, gunpowder placed in holes drilled by handheld iron drills. A notable canal tunnel in England was the Bridgewater Canal Tunnel, built in 1761 by James Brindley to carry coal to Manchester from the Worsley mine. Many more canal tunnels were dug in Europe and North America in the 18th and early 19th centuries. Though the canals fell into disuse with the introduction of railroads about 1830, the new form of transport produced a huge increase in tunneling, which continued for nearly 100 years as railroads expanded over the world. Much pioneer railroad tunneling developed in England. A 3.5-mile tunnel (the Woodhead) of the Manchester-Sheffield Railroad (1839–45) was driven from five shafts up to 600 feet deep. In the United States, the first railroad tunnel was a 701-foot construction on the Allegheny Portage Railroad. Built in 1831–33, it was a combination of canal and railroad systems, carrying canal barges over a summit. Though plans for a transport link from Boston to the Hudson River had first called for a canal tunnel to pass under the Berkshire Mountains, by 1855, when the Hoosac Tunnel was started, railroads had already established their worth, and the plans were changed to a double-track railroad bore 24 by 22 feet and 4.5 miles long. Initial estimates contemplated completion in 3 years; 21 were actually required, partly because the rock proved too hard for either hand drilling or a primitive power saw. When the state of Massachusetts finally took over the project, it completed it in 1876 at five times the originally estimated cost. Despite frustrations, the Hoosac Tunnel contributed notable advances in tunneling, including one of the first uses of dynamite, the first use of electric firing of explosives, and the introduction of power drills, initially steam and later air, from which there ultimately developed a compressed-air industry.

Simultaneously, more spectacular railroad tunnels were being started through the Alps. The first of these, the Mont Cenis Tunnel (also known as Fréjus), required 14 years (1857–71) to complete its 8.5-mile length. Its engineer, Germain Sommeiller, introduced many pioneering techniques, including rail-mounted drill carriages, hydraulic ram air compressors, and construction camps for workers complete with dormitories, family housing, schools, hospitals, a recreation building, and repair shops. Sommeiller also designed an air drill that eventually made it possible to move the tunnel ahead at the rate of 15 feet per day and was used in several later European tunnels until replaced by more durable drills developed in the United States by Simon Ingersoll and others on the Hoosac Tunnel. As this long tunnel was driven from two headings separated by 7.5 miles of mountainous terrain, surveying techniques had to be refined. Ventilation became a major problem, which was solved by the use of forced air from water-powered fans and a horizontal diaphragm at mid-height, forming an exhaust duct at top of the tunnel. Mont Cenis was soon followed by other notable Alpine railroad tunnels: the 9-mile St. Gotthard (1872–82), which introduced compressed-air locomotives and suffered major problems with water inflow, weak rock, and bankrupt contractors; the 12-mile Simplon (1898–1906); and the 9-mile Lötschberg (1906–11), on a northern continuation of the Simplon railroad line.

Nearly 7,000 feet below the mountain crest, Simplon encountered major problems from highly stressed rock flying off the walls in rock bursts; high pressure in weak schists and gypsum, requiring 10-foot-thick masonry lining to resist swelling tendencies in local areas; and from high-temperature water (130° F [54° C]), which was partly treated by spraying from cold springs. Driving Simplon as two parallel tunnels with frequent crosscut connections considerably aided ventilation and drainage.

Lötschberg was the site of a major disaster in 1908. When one heading was passing under the Kander River valley, a sudden inflow of water, gravel, and broken rock filled the tunnel for a length of 4,300 feet, burying the entire crew of 25 men. Though a geologic panel had predicted that the tunnel here would be in solid bedrock far below the bottom of the valley fill, subsequent investigation showed that bedrock lay at a depth of 940 feet, so that at 590 feet the tunnel tapped the Kander River, allowing it and soil of the valley fill to pour into the tunnel, creating a huge depression, or sink, at the surface. After this lesson in the need for improved geologic investigation, the tunnel was rerouted about one mile (1.6 kilometres) upstream, where it successfully crossed the Kander Valley in sound rock.

Most long-distance rock tunnels have encountered problems with water inflows. One of the most notorious was the first Japanese Tanna Tunnel, driven through the Takiji Peak in the 1920s. The engineers and crews had to cope with a long succession of extremely large inflows, the first of which killed 16 men and buried 17 others, who were rescued after seven days of tunneling through the debris. Three years later another major inflow drowned several workers. In the end, Japanese engineers hit on the expedient of digging a parallel drainage tunnel the entire length of the main tunnel. In addition, they resorted to compressed-air tunneling with shield and air lock, a technique almost unheard-of in mountain tunneling.

Subaqueous tunnels

Tunneling under rivers was considered impossible until the protective shield was developed in England by Marc Brunel, a French émigré engineer. The first use of the shield, by Brunel and his son Isambard, was in 1825 on the Wapping-Rotherhithe Tunnel through clay under the Thames River. The tunnel was of horseshoe section 22 ¹/₄ by 37 ¹/₂ feet and brick-lined. After several floodings from hitting sand pockets and a seven-year shutdown for refinancing and building a second shield, the Brunels succeeded in completing the world’s first true subaqueous tunnel in 1841, essentially nine years’ work for a 1,200-foot-long tunnel. In 1869 by reducing to a small size (8 feet) and by changing to a circular shield plus a lining of cast-iron segments, Peter W. Barlow and his field engineer, James Henry Greathead, were able to complete a second Thames tunnel in only one year as a pedestrian walkway from Tower Hill. In 1874, Greathead made the subaqueous technique really practical by refinements and mechanization of the Brunel-Barlow shield and by adding compressed air pressure inside the tunnel to hold back the outside water pressure. Compressed air alone was used to hold back the water in 1880 in a first attempt to tunnel under New York’s Hudson River; major difficulties and the loss of 20 lives forced abandonment after only 1,600 feet had been excavated. The first major application of the shield-plus-compressed-air technique occurred in 1886 on the London subway with an 11-foot bore, where it accomplished the unheard-of record of seven miles of tunneling without a single fatality. So thoroughly did Greathead develop his procedure that it was used successfully for the next 75 years with no significant change. A modern Greathead shield illustrates his original developments: miners working under a hood in individual small pockets that can be quickly closed against inflow; shield propelled forward by jacks; permanent lining segments erected under protection of the shield tail; and the whole tunnel pressurized to resist water inflow.

Once subaqueous tunneling became practical, many railroad and subway crossings were constructed with the Greathead shield, and the technique later proved adaptable for the much larger tunnels required for automobiles. A new problem, noxious gases from internal-combustion engines, was successfully solved by Clifford Holland for the world’s first vehicular tunnel, completed in 1927 under the Hudson River and now bearing his name. Holland and his chief engineer, Ole Singstad, solved the ventilation problem with huge-capacity fans in ventilating buildings at each end, forcing air through a supply duct below the roadway, with an exhaust duct above the ceiling. Such ventilation provisions significantly increased the tunnel size, requiring about a 30-foot diameter for a two-lane vehicular tunnel.

Many similar vehicular tunnels were built by shield-and-compressed-air methods—including Lincoln and Queens tunnels in New York City, Sumner and Callahan in Boston, and Mersey in Liverpool. Since 1950, however, most subaqueous tunnelers preferred the immersed-tube method, in which long tube sections are prefabricated, towed to the site, sunk in a previously dredged trench, connected to sections already in place, and then covered with backfill. This basic procedure was first used in its present form on the Detroit River Railroad Tunnel between Detroit and Windsor, Ontario (1906–10). A prime advantage is the avoidance of high costs and the risks of operating a shield under high air pressure, since work inside the sunken tube is at atmospheric pressure (free air).

Machine-mined tunnels

Sporadic attempts to realize the tunnel engineer’s dream of a mechanical rotary excavator culminated in 1954 at Oahe Dam on the Missouri River near Pierre, in South Dakota. With ground conditions being favourable (a readily cuttable clay-shale), success resulted from a team effort: Jerome O. Ackerman as chief engineer, F.K. Mittry as initial contractor, and James S. Robbins as builder of the first machine—the “Mittry Mole.” Later contracts developed three other Oahe-type moles, so that all the various tunnels here were machine-mined—totaling eight miles of 25- to 30-foot diameter. These were the first of the modern moles that since 1960 have been rapidly adopted for many of the world’s tunnels as a means of increasing speeds from the previous range of 25 to 50 feet per day to a range of several hundred feet per day. The Oahe mole was partly inspired by work on a pilot tunnel in chalk started under the English Channel for which an air-powered rotary cutting arm, the Beaumont borer, had been invented. A 1947 coal-mining version followed, and in 1949 a coal saw was used to cut a circumferential slot in chalk for 33-foot-diameter tunnels at Fort Randall Dam in South Dakota. In 1962 a comparable breakthrough for the more difficult excavation of vertical shafts was achieved in the American development of the mechanical raise borer, profiting from earlier trials in Germany.