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 transportationincluding road vehicles, trains, subways, and canalsand 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 insidewith the overlying material left in placeand 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. Additional reading Ancient and modern tunnels in Europe, Japan, and the U.S. are described in the well-illustrated study by Patrick Beaver, A History of Tunnels (1972). T.M. Megaw and J.V. Bartlett, TunnelsPlanning, Design, Construction, 2 vol. (198182), is a substantial study. Graham West, Innovation and the Rise of the Tunnelling Industry (1988), focuses on both hard- and soft-rock projects. Agricola, De Re Metallica, trans. from Latin by Herbert Clark Hoover and Lou Henry Hoover (1912, reprinted 1950), is a classic work on early mining in Europe. Frederick Walter Simms, Practical Tunneling, 4th ed., rev. and expanded by D. Kinnear Clark (1896), on early public works, recounts the difficulties overcome by pioneers. C.A. Pequignot (ed.), Tunnels and Tunnelling (1963), on English practice, includes comprehensive tables comparing tunnels of the world. Gsta E. Sandstrm, Tunnels (also published as The History of Tunneling, 1963), is a historical survey that summarizes Sweden's contributions to underground engineering. K.G. Stagg and O.C. Zienkiewicz (eds.), Rock Mechanics in Engineering Practice (1968), an introductory work, contains 12 chapters, each written by a noted authority. Harold W. Richardson and Robert S. Mayo, Practical Tunnel Driving (1941, reissued 1975), a history of U.S. practice to 1940, emphasizes tunnel equipment. Albert D. Parker, Planning and Estimating Underground Construction (1970), also on U.S. practice, emphasizes construction engineering and estimating. Microtunneling methods are discussed in Dietrich Stein, Klemens Mllers, and Rolf Bielecki, Microtunnelling: Installation and Renewal of Nonman-size Supply and Sewage Lines by the Trenchless Construction Method (1989; originally published in German, 1988). Contemporary developments are detailed in the periodicals ENR (weekly), an engineering news record; and Tunnels & Tunnelling (monthly). Kenneth S. Lane The Editors of the Encyclopdia Britannica Tunneling techniques Basic tunneling system Tunnels are generally grouped in four broad categories, depending on the material through which they pass: soft ground, consisting of soil and very weak rock; hard rock; soft rock, such as shale, chalk, and friable sandstone; and subaqueous. While these four broad types of ground condition require very different methods of excavation and ground support, nearly all tunneling operations nevertheless involve certain basic procedures: investigation, excavation and materials transport, ground support, and environmental control. Similarly, tunnels for mining and for civil-engineering projects share the basic procedures but differ greatly in the design approach toward permanence, owing to their differing purposes. Many mining tunnels have been planned only for minimum-cost temporary use during ore extraction, although the growing desire of surface owners for legal protection against subsequent tunnel collapse may cause this to change. By contrast, most civil-engineering or public-works tunnels involve continued human occupancy plus full protection of adjacent owners and are much more conservatively designed for permanent safety. In all tunnels, geologic conditions play the dominant role in governing the acceptability of construction methods and the practicality of different designs. Indeed, tunneling history is filled with instances in which a sudden encounter with unanticipated conditions caused long stoppages for changes in construction methods, in design, or in both, with resulting great increases in cost and time. At the Awali Tunnel in Lebanon in 1960, for example, a huge flow of water and sand filled over 2 miles of the bore and more than doubled construction time to eight years for its 10-mile length. Geologic investigation Thorough geologic analysis is essential in order to assess the relative risks of different locations and to reduce the uncertainties of ground and water conditions at the location chosen. In addition to soil and rock types, key factors include the initial defects controlling behaviour of the rock mass; size of rock block between joints; weak beds and zones, including faults, shear zones, and altered areas weakened by weathering or thermal action; groundwater, including flow pattern and pressure; plus several special hazards, such as heat, gas, and earthquake risk. For mountain regions the large cost and long time required for deep borings generally limit their number; but much can be learned from thorough aerial and surface surveys, plus well-logging and geophysical techniques developed in the oil industry. Often the problem is approached with flexibility toward changes in design and in construction methods and with continuous exploration ahead of the tunnel face, done in older tunnels by mining a pilot bore ahead and now by drilling. Japanese engineers have pioneered methods for prelocating troublesome rock and water conditions. For large rock chambers and also particularly large tunnels, the problems increase so rapidly with increasing opening size that adverse geology can make the project impractical or at least tremendously costly. Hence, the concentrated opening areas of these projects are invariably investigated during the design stage by a series of small exploratory tunnels called drifts, which also provide for in-place field tests to investigate engineering properties of the rock mass and can often be located so their later enlargement affords access for construction. Since shallow tunnels are more often in soft ground, borings become more practical. Hence, most subways involve borings at intervals of 100500 feet to observe the water table and to obtain undisturbed samples for testing strength, permeability, and other engineering properties of the soil. Portals of rock tunnels are often in soil or in rock weakened by weathering. Being shallow, they are readily investigated by borings, but, unfortunately, portal problems have frequently been treated lightly. Often they are only marginally explored or the design is left to the contractor, with the result that a high percentage of tunnels, especially in the United States, have experienced portal failures. Failure to locate buried valleys has also caused a number of costly surprises. The five-mile Oso Tunnel in New Mexico offers one example. There, in 1967, a mole had begun to progress well in hard shale, until 1,000 feet from the portal it hit a buried valley filled with water-bearing sand and gravel, which buried the mole. After six months' delay for hand mining, the mole was repaired and soon set new world records for advance rateaveraging 240 feet per day with a maximum of 420 feet per day. Underground excavations and structures Rock chambers While chambers in 1971 were being excavated in rock to fulfill a wide variety of functions, the main stimulus to their development had come from hydroelectric-power-plant requirements. Though the basic concept originated in the United States, where the world's first underground hydroplants were built in enlarged tunnels at Snoqualme Falls near Seattle, Wash., in 1898 and at Fairfax Falls, Vt., in 1904, Swedish engineers developed the idea into excavating large chambers to accommodate hydraulic machinery. After an initial trial in 191014 at the Porjus Plant north of the Arctic Circle, many underground power plants were subsequently built by the Swedish State Power Board. Swedish success soon popularized the idea through Europe and over the world, particularly to Australia, Scotland, Canada, Mexico, and Japan, where several hundred underground hydroplants have been built since 1950. Sweden, having a long experience with explosives and rock work, with generally favourable strong rock, and with energetic research and development, has even been able to lower the costs for underground work to approximate those for surface construction of such facilities as power plants, warehouses, pumping plants, oil-storage tanks, and water-treatment plants. With costs in the United States being 5 to 10 times greater underground, new construction of underground chambers was not significantly resumed there until 1958, when the Haas underground hydroplant was built in California and the Norad underground air force command centre in Colorado. By 1970 the United States had begun to adopt the Swedish concept and had completed three more hydroplants with several more under construction or being planned. Favourably located, an underground hydroplant can have several advantages over a surface plant, including lower costs, because certain plant elements are built more simply underground: less risk from avalanches, earthquakes, and bombing; cheaper year-round construction and operation (in cold climates); and preservation of a scenic environmenta dominant factor in Scotland's tourist area and now receiving recognition worldwide. A typical layout involves a complex assembly of tunnels, chambers, and shafts. The world's largest underground powerhouse, Churchill Falls in the Labrador wilderness of Canada, with a capacity of five million kilowatts, has been under construction since 1967 at a total project cost of about $1 billion. By building a dam of modest height well above the falls and by locating the powerhouse at 1,000 feet depth with a one-mile tunnel (the tailrace tunnel) to discharge water from the turbines below downstream rapids, the designers have been able to develop a head (water height) of 1,060 feet while at the same time preserving the scenic 250-foot-high waterfall, expected to be a major tourist attraction once several hundred miles of wilderness-road improvement permits public access. Openings here are of impressive size: machine hall (powerhouse proper), 81-foot span by 154 feet high by 972 feet long; surge chamber, 60 feet by 148 feet high by 763 feet; and two tailrace tunnels, 45 by 60 feet high. Large rock chambers are economical only when the rock can essentially support itself through a durable ground arch with the addition of only a modest amount of artificial support. Otherwise, major structural support for a large opening in weak rock is very costly. The Norad project, for example, included an intersecting grid of chambers in granite 45 by 60 feet high, supported by rock bolts except in one local area. Here, one of the chamber intersections coincided with the intersection of two curving shear zones of fractured rocka happening which added $3.5 million extra cost for a perforated concrete dome 100 feet in diameter to secure this local area. In some Italian and Portuguese underground powerhouses, weak-rock areas have necessitated comparable costly lining. While significant rock defects are more manageable in the usual 10- to 20-foot rock tunnel, the problem so increases with increasing size of opening that the presence of extensive weak rock can easily place a large-chamber project outside the range of economic practicality. Hence, geologic conditions are very carefully investigated for rock-chamber projects, using many borings plus exploratory drifts to locate rock defects, with a three-dimensional geologic model to aid in visualizing conditions. A chamber location is selected that offers the least risk of support problems. This objective was largely attained in the granite gneiss at Churchill Falls, where the location and chamber configuration were changed several times to avoid rock defects. Rock-chamber projects, furthermore, rely heavily on the relatively new field of rock mechanics to evaluate the engineering properties of the rock mass, in which exploratory drifts are particularly important in affording access for in-place field testing. Rock-mechanics investigation The young field of rock mechanics was beginning, early in the 1970s, to develop a rational basis of design for projects in rock; much is already developed for projects in soil by the older field of soil mechanics. Initially, the discipline had been stimulated by such complex projects as arch dams and underground chambers and then increasingly with similar problems with tunnels, rock slopes, and building foundations. In treating the rock mass with its defects as an engineering material, the science of rock mechanics utilizes numerous techniques such as theoretical analysis, laboratory testing, field testing on-site, and instrumentation to monitor performance during construction and operation. Since rock mechanics is a discipline in itself, only the most common field tests are briefly outlined below to give some concept of its role in design, particularly for a rock-chamber project. Geostress, which can be a significant factor in choice of chamber orientation, shape, and support design, is usually determined in exploratory drifts. Two methods are common, although each is still in the development stage. One is an overcoring method (developed in Sweden and South Africa) used for ranges up to about 100 feet out from the drift and employing a cylindrical instrument known as a borehole deformeter. A small hole is drilled into the rock and the deformeter inserted. Diameter changes of the borehole are measured and recorded by the deformeter as the geostress is relieved by overcoring (cutting a circular core around the small hole) with a six-inch bit. Measurements at several depths in at least three borings at different orientations furnish the data needed for computing the existing geostress. When measurement is desired only at the surface of the drift, the so-called French flat-jack method is preferred. In this, a slot is cut at the surface, and its closure is measured as the geostress is relieved by the slot. Next, a flat hydraulic jack is inserted in the rock. The jack pressure necessary to restore closure of the slot (to the condition before its cutting) is considered to equal the original geostress. As these methods require a long drift or shaft for access to the area of measurement, development is under way (particularly in the United States) to extend the range of depth to a few thousand feet. Such will aid in comparing geostress at alternate sites and hopefully avoid locations with high geostress, which has proved very troublesome in several past chamber projects. Shear strength of a joint, fault, or other rock defect is a controlling factor in appraising strength of the rock mass in terms of its resistance to sliding along the defect. Although partly determinable in the laboratory, it is best investigated in the field by a direct shear test at the work site. While this test has long been used for soil and soft rock, its adaptation to hard rock is due largely to work performed in Portugal. Shear strength is important in all problems of sliding; at Morrow Point Dam, in Colorado, for example, a large rock wedge between two faults started to move into the underground powerhouse and was stabilized by large tendons anchored back in a drainage tunnel plus strut action provided by the concrete structure that supported the generator machinery. The modulus of deformation (that is, the stiffness of the rock) is significant in problems involving movement under stress and in sharing of load between rock and structure, as in a tunnel lining, embedded steel penstock, or foundation of a dam or heavy building. The simplest field test is the plate-jacking method, in which the rock in a test drift is loaded by hydraulic jacks acting on a plate two to three feet in diameter. Larger areas can be tested either by radially loading the internal surface of a test tunnel or by pressurizing a membrane-lined chamber. Analysis methods in rock mechanics have helped in appraising stress conditions around openingsas at Churchill Fallsto identify and then correct zones of tension and stress concentration. Related work with rock block models is contributing to understanding the failure mechanism of the rock mass, notable work being under way in Austria, Yugoslavia, and the United States.