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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 B.C.) 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 Hugh L. Carey (formerly Brooklyn-Battery) Tunnel, which connects Manhattan Island with Brooklyn and is the longest vehicular tunnel (1.7 mi/2.7 km) in the United States. The Anton Anderson Memorial Tunnel, also known as the Whittier Tunnel (2.5 mi/4 km), which opened in 1943 to rail traffic and in 2000 to vehicular traffic, connects Whittier, Alaska, to Anchorage and other cities; the unique single-lane tunnel allows rail or road traffic in one direction only at a time. 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 (opened 1906; see under Simplon) through the Alps was for many years the longest railway tunnel (12.3 mi/19.8 km) in the world. The Gotthard Base Tunnel (2016; see under Saint Gotthard), also in the Alps, is now the world's longest tunnel (35.4 mi/57 km), and the Seikan Tunnel (1988), connecting Honshu and Hokkaido, Japan, is the world's longest underwater tunnel (33.5 mi/53.6 km). The Channel Tunnel (1994; 31 mi/50 km) under the English Channel, however, has the longest underwater section. 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.
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).
a horizontal or sloping underground structure used for transportation, the moving of water, the laying of underground utility systems, and other purposes. According to use, the following types of tunnels are distinguished: railroad tunnels, vehicular tunnels, subway tunnels, tunnels at intersections of city streets and traffic arteries (seePEDESTRIAN CROSSING and ), canal tunnels, tunnels for the simultaneous passage of several types of transportation, water tunnels, municipal-service tunnels (for urban water-supply, heat- and gas-supply, sewer, and other systems), and special-purpose tunnels (for example, tunnels that are part of the underground structures of hydroelectric power plants, warehouses, and garages). According to location (Figure 1), tunnels are classified as mountain, subaqueous, and urban. Mountain tunnels are driven through ranges, divides, and individual mountains in mountainous regions; urban tunnels include subway tunnels.
History. The origins of tunnel building go back to ancient times. Long before the Common Era underground work was carried out in Babylon, Egypt, Greece, and Rome, first in the mining of minerals and the construction of tombs and temples and then for water supply and transportation. Road, water-supply, and drainage tunnels—mostly arched—were driven in stable rock without reinforcement of the rock. The tunnels were cut with primitive tools. Upon the fall of the Roman Empire a period of relative stagnation in tunnel building ensued, and tunnels were built primarily for military purposes. At the end of the Middle Ages, as a result of the expansion of international trade, the building of canal tunnels that linked waterways was begun. The use of black powder to blast the rock was a prerequisite for such tunnel building.
The first railroad tunnel (1.19 km long) was built in Great Britain for the Liverpool and Manchester Railway between 1826 and 1830. The invention of pyroxylin and dynamite and the successful use of drilling machines in mining made it possible to build the great Alpine tunnels between France, Italy, and Switzerland. Before World War I, 26 tunnels, each longer than 5 km, had been built, including the Simplón Tunnel (about 20 km long) between Italy and Switzerland. Among the tunnels built in the 1920’s and early 1930’s were the Apennine Tunnel (18.5 km long), which is a double-track railroad tunnel on the Florence-Bologna line in Italy, and the Rove Tunnel (more than 7 km long), which carries the Marseille-Rhône Canal in France.
Together with the building of mountain tunnels, the building of subaqueous tunnels was also developed. Subaqueous tunnel building became possible through the use of tunneling shields (in combination with compressed air) and prefabricated lining. A number of major subaqueous tunnels have been driven by the shield method, for example, two vehicular tunnels (each 2.6 km long) under the Hudson River in the USA and the railroad tunnel (over 6 km long) driven under the Straits of Shimonoseki in Japan between 1936 and 1941. An important advance in the building of subaqueous tunnels was the use of sunken tube sections up to 150 m long.
The first railroad tunnel in Russia was the double-track Kovno Tunnel (1.28 km long), which was built in 1862. In the late 19th century many tunnels were built for railroads in the Urals, the Crimea, and the Caucasus. The largest was the Surami Tunnel (about 4 km long), which was built between 1886 and 1890. In the early 20th century a number of tunnels were built in Siberia and the Far East. Tunnel building developed considerably in the USSR as a result of intensive railroad building, the construction of a network of hydroelectric power plants, and the construction of subways and underground urban installations.
Main elements of tunnels. The building of a tunnel requires an excavation, which is a cavity dug or cut in the earth’s crust in one or more stages, beginning from an adit, which is usually of trapezoidal cross section. In strong unweathered rock of uniform structure the tunnel excavation may be unsupported, but in unstable rock temporary mine supports must be installed. The temporary supports are later replaced by a permanent structure, which is called a lining and may be cast in situ or prefabricated. The lining is the most important element of a tunnel; it forms the tunnel’s inner surface, absorbs the rock pressure, and protects the tunnel against groundwater. The entrance section of a tunnel is called the portal; it assures the stability of the front and side slopes, or the preportal excavation, and gives the tunnel entrance its architectural form. Reinforced-concrete ramps, trough-shaped in cross section, are used both to prevent flooding of the entrance sections of subaqueous tunnels and to cope with mountainous conditions.
Planning the route of a tunnel. The depth at which a tunnel lies, the length of the tunnel, its plan and profile, and the shape of its cross section depend on the purpose of the tunnel and on topographical, geological, and climatic conditions. During the planning and building of a tunnel, a series of surveys is carried out to select and fix the axis of the tunnel in plan and profile, to compute the geometric elements of the tunnel axis, to project the axis within the excavation, to determine the length of the axis, and to mark out the tunnel cross sections. Geological surveys are conducted along the route of the tunnel to determine, for example, the geological structure of the rock mass being cut, the nature of the rock stratification, the degree of stability and the physical and mechanical properties of the rock, the hydrogeological condition and the chemical composition of the groundwater, the presence of gases, the temperature in the excavation, and the expected rock pressure. Such data are obtained on the basis of geological exploration and hydrogeological studies using boreholes and geophysical methods; in some cases, the data are obtained from the results of exploratory tunneling.
In plan, a tunnel may entirely or partially follow a straight line or a curve. Straight tunnels are more advantageous than curved tunnels from the viewpoints of construction and operation, because curved tunnels require a substantially larger excavation, are more difficult to build, and have poorer ventilation and visibility. Looping and spiral tunnels are sometimes built, for example, when building a railroad line through a rock mass.
The longitudinal profile of a tunnel may have one or two inclines. A tunnel with two inclines slopes upward in both directions from the middle. Because of drainage requirements, tunnels cannot be strictly horizontal. When a tunnel is very long or curved, the inclination must be reduced.
Tunnel materials and structures. The principal materials used for tunnel linings are concrete cast in situ, reinforced concrete cast in situ and precast, cast iron, and steel. The choice of lining material depends on the conditions in the region where the tunnel is built and the method of tunneling. Cast concrete and reinforced concrete are used primarily when tunnels are driven in regions that are not easily accessible, where it is not economically feasible to develop an industry for the production of prefabricated structures, and also in soft and weak rock, which requires that the tunnel lining be emplaced in sections. Prefabricated lining, which consists of such factory-manufactured elements as cast-iron tubing and solid or ribbed reinforced concrete blocks, is used when the rate of tunnel driving or the labor productivity must be increased.
The structural shape and cross section of the lining are determined by geological conditions and the direction of the primary loads on the lining. In weak water-bearing rock and at high hydrostatic pressure a circular lining is recommended; in stable rock, where vertical stresses predominate, a horseshoe-shaped lining fully meets the requirements of clearance.
Design of tunnel structures. Tunnel linings are designed for the least favorable but realistic combinations of primary, secondary, and special loads and their effects on the construction materials. Primary loads, such as rock pressure, act on the lining constantly or regularly. Secondary loads act for short times or periodically, and special loads are mainly seismic in nature. Rock pressure is determined theoretically (with allowance being made for arch formation, the mass of the “rock column, ” and other factors) or from the results of measurements made with instruments in finished excavations.
Linings are designed according to limit states on the basis of the methods of structural mechanics, elasticity theory, and soil mechanics. In such designing the combined behavior of the lining and rock is regarded as that of a single elastic system. The structural model of the lining is chosen in accordance with the nature of the structure, the character of the surrounding rock, and the conditions under which the work is done; that is, the lining as a whole or its individual parts must have sufficient strength and stability in all stages of emplacement. The strength of preselected sections of concrete and cast-iron linings is tested with respect to bearing capacity in accordance with the requirements of the Construction Code.
Tunnel building. Depending on the depth at which it lies, a tunnel may be built bv the cut-and-cover method or excavated from the inside. In the cut-and-cover method an excavation is dug from the surface, and the tunnel structures are built in the excavation. The excavation is then backfilled, and the disturbed surface is restored. When a tunnel is excavated from the inside, the rock is excavated, and the lining emplaced, through shafts or the tunnel portals. Many different methods are used for excavating and tunneling; the principal techniques are rock tunneling and shield tunneling.
Rock tunneling entails two main stages, namely, the excavation and mucking of the rock and the emplacement of a permanent structure—the lining—in the resulting excavation. Depending on the properties of the rock, the excavation may be advanced in parts or simultaneously over the entire cross section (see Figure 2). In soft and partially consolidated rock the cross section of the excavation is divided into individual, relatively small parts that are supported by temporary, primarily wooden, bracing, which prevents the rock from caving in. In hard rock the cross section can be divided into larger units; temporary bracing is installed only along the circumference of the excavation, and the inside of the excavation is left clear. As a result, tunneling in hard rock can be mechanized to a considerable extent. As a rule, the rock is excavated by drilling and blasting with the use of high-power drilling machines and mechanized mucking; the operations along the length of the tunnel are organized in a flow-line system. Movable metal forms are used to emplace a concrete lining; such forms make it possible to employ concrete-emplacing machines.
The supported-arch method has become very popular. It is used in sufficiently stable rock that is capable of withstanding the pressure of the concrete arch of the lining. In this method the excavation is cut in parts; a concrete arch supported by the rock is built first and then, depending on the extent to which the sections beneath the arch are excavated, concrete walls cast in situ are placed under the abutment of the arch. The tunnel may be driven from one or two adits. In the full-face method, which is used in the stable hard rock, the entire diameter of the excavation is cut with the use of special tunneling equipment, such as drilling platforms, jumbos, rigs, and complex units. The concrete lining is emplaced by concrete pumps or concrete-emplacing machines (see Figure 3).
The shield method has been used primarily for driving tunnels in weak and unstable rock. It is based on the use of a tunneling shield (see Figure 4) as temporary bracing. The shield is a movable steel cylindrical shell that protects the workers who excavate the rock and erect the lining, which, as a rule, is circular and prefabricated.
The tunnel lining is assembled by erector arms or tubing- or block-emplacing machines located directly on the shield or behind it on special movable supports. In unstable water-saturated rock, shield tunneling is combined with the use of compressed air as a means of drying the face. When excavation and lining installation are carried out at elevated air pressure, movable watertight bulkheads are used to separate the face from the rest of the tunnel. The bulkheads have airlocks for the passage of people, for mucking, and for the delivery of materials and various types of equipment. Unlike the rock methods of tunnel driving, shield tunneling does not require temporary bracing; this circumstance increases the safety and reduces the cost of tunnel driving. The tunneling shield may be adapted, by using special mechanisms, to drive through different kinds of rock, for example, plastic or friable rock and running ground. Such adaptation makes possible the total mechanization of all tunneling processes and ensures high quality and high driving rates. In the USSR, shield tunneling experienced its greatest development in the driving of subway tunnels.
In addition to the rock and shield methods of tunneling, other methods, such as the immersed-tube and caisson techniques, are also used, primarily for subaqueous tunnels.
Drainage systems and waterproofing. Tunnels must be protected against the penetration of surface water and groundwater. The drainage of surface water is made possible by the appropriate grading of the surface above the tunnel and by a system of open catch drains and watertight conduits for streams flowing above the tunnel. A tunnel is protected against groundwater by draining the water from the rock mass being cut or by waterproofing the lining. The groundwater is drained by using a drainage system outside the lining or by drilling wells.
The most popular means of waterproofing are as follows: the use of glued waterproofing, which consists of several layers of rolled bituminized material; the pumping of cement or other solutions into the space behind the lining; and the grouting of the rock around the tunnel. Within a tunnel, chutes and pipes are provided to drain water to the portals and to discharge it outside the tunnel.
Main trends in the development of tunnel-building technology. For different geological conditions, existing tunnel linings are being improved, and new types of linings are being developed. A method of emplacing linings that does not require movable forms is being introduced, and the most effective means of protecting tunnels against groundwater, especially in regions with a harsh climate, are being developed. Other trends include the introduction of efficient systems for total mechanization of tunneling and the improvement of the method of tunnel driving by drilling and blasting.
REFERENCESVolkov, V. P. Tonneli, 3rd ed. Moscow, 1970.
Malevich, N. A. Gornoprokhodcheskie mashiny i kompleksy. Moscow, 1971.
Kompaniets, S. A., A. K. Popravko, and A. A. Bogorodetskii. Proektirovanie tonnelei. Moscow, 1973.
Mostkov, V. M. Podzemnye sooruzheniia bol’shogo secheniia, 2d ed. Moscow, 1974.
Tonneli i metropoliteny, 2nd ed. Edited by V. P. Volkov. Moscow, 1975.
Stroitel’nye normy ipravila, part 3, sec. B, ch. 8: “Tonneli zhelezno-dorozhnye, avtodorozhnye i gidrotekhnicheskie: Pravila organizatsii stroitel’stva proizvodstva i priemki rabot.” Moscow, 1968.
V. P. VOLKOV
What does it mean when you dream about a tunnel?
A tunnel represents transition from one set of conditions to another. The “light at the end of the tunnel” may represent relief from old conditions. With or without the presence of a train, this symbol can also have the Freudian sexual interpretation of tunnel-as-vagina.