Tunnels and undergrod excavations

, horizontal underground passageway produced by excavation or occasionally by natures action in dissolving a soluble rock, such as limestone. A vertical opening is usually called ashaft. Tunnels have many uses: for mining ores, for transportationincludingroadvehicles, 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 theadjacentground. A hillside tunnel entrance is called aportal; 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 anadit. So-called cut-and-cover tunnels (more correctly calledconduits) 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 animmersed 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.

Although very expensive, tunneling provides the most economical means for railroads to traverse mountainous terrain, to gain access to the heart of a crowded city, or, more recently in Japan and Europe, to project a railway across a maritime strait below its seabed. Railroad

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

TheGreeksandRomansboth made extensive use of tunnels: to reclaim marshes by drainage and for water aqueducts, such as the 6th-century-bcGreek water tunnel on the isle ofSamosdriven some 3,400 feet through limestone with across sectionabout 6 feet square. Perhaps the largest tunnel in ancient times was a 4,800-foot-long, 25-foot-wide, 30-foot-high road tunnel (thePausilippo) between Naples and Pozzuoli, executed in 36bc. By that timesurveyingmethods (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-calledfire quenching, a method involving heating the rock with fire and suddenly cooling it by dousing with water.Ventilationmethods 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. Inad41 the Romans used some 30,000 men for 10 years to push a 3.5-mile (6-kilometre) tunnel to drainLacus 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 atHallstatt, Austria, where salt-mine tunnels have been worked since 2500bc.

Because the limited tunneling in the Middle Ages was principally for mining andmilitary engineering, the next major advance was to meet Europes growing transportation needs in the 17th century. The first of many majorcanaltunnels was theCanal du Midi(also known as Languedoc) tunnel inFrance, built in 166681 byPierre Riquetas 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 ofexplosivesin public-works tunneling, gunpowder placed in holes drilled by handheld iron drills. A notable canal tunnel inEnglandwas theBridgewater CanalTunnel, built in 1761 byJames Brindleyto carry coal to Manchester from the Worsley mine. Many more canal tunnels were dug in Europe andNorth Americain the 18th and early 19th centuries. Though the canals fell into disuse with the introduction ofrailroadsabout 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 (theWoodhead) of the Manchester-Sheffield Railroad (183945) was driven from five shafts up to 600 feet deep. In theUnited States, the first railroad tunnel was a 701-foot construction on theAllegheny Portage Railroad. Built in 183133, it was a combination of canal and railroad systems, carrying canal barges over a summit. Though plans for a transport link from Boston to theHudson Riverhad first called for a canal tunnel to pass under the Berkshire Mountains, by 1855, when theHoosac Tunnelwas 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 ofdynamite, the first use of electric firing of explosives, and the introduction of powerdrills, initially steam and later air, from which there ultimately developed acompressed-airindustry.

Simultaneously, more spectacular railroad tunnels were being started through theAlps. The first of these, theMont Cenis Tunnel(also known as Frjus), required 14 years (185771) 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 anair drillthat 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-mileSt. Gotthard(187282), which introduced compressed-air locomotives and suffered major problems with water inflow, weak rock, and bankrupt contractors; the 12-mileSimplon(18981906); and the 9-mileLötschberg(190611), 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-thickmasonrylining 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 mostnotoriouswas the firstJapaneseTanna 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-airtunneling with shieldandair lock, a technique almost unheard-of in mountain tunneling.

Tunneling under rivers was considered impossible until the protective shield was developed in England byMarc Brunel, a French migr engineer. The first use of the shield, by Brunel and his son Isambard, was in 1825 on theWapping-Rotherhithe Tunnelthrough clay under theThames River. The tunnel was of horseshoe section 221/4by 371/2feet 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 worlds 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. Barlowand 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 Yorks 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 modernGreathead shieldillustrates 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 andsubwaycrossings 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 byClifford Hollandfor the worlds first vehiculartunnel, 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 methodsincludingLincolnand Queens tunnels inNew York City, Sumner and Callahan in Boston, and Mersey in Liverpool. Since 1950, however, most subaqueous tunnelers preferred theimmersed-tubemethod, 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 theDetroit River Railroad Tunnelbetween Detroit and Windsor, Ontario (190610). 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 atatmospheric pressure(free air).

Sporadic attempts to realize the tunnel engineers dream of a mechanicalrotaryexcavatorculminated in 1954 at Oahe Dam on theMissouri Rivernear Pierre, inSouth 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 machinethe Mittry Mole. Later contracts developed three other Oahe-type moles, so that all the various tunnels here were machine-minedtotaling 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 worlds 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. TheOahe mole was partly inspired by work on a pilot tunnel in chalk started under theEnglish Channelfor which an air-powered rotary cutting arm, theBeaumont 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.

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. Manyminingtunnels 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, mostcivil-engineeringor 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 theAwali 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.

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 largerock chambersand 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 calleddrifts, 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 thewater tableand to obtain undisturbed samples for testing strength, permeability, and other engineering properties of the soil.Portalsof 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-mileOso Tunnel inNew Mexicooffers 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.

Excavation of the ground within the tunnel bore may be either semicontinuous, as by handheld power tools or mining machine, or cyclic, as by drilling andblastingmethods for harder rock. Here each cycle involves drilling, loading explosive, blasting, ventilating fumes, and excavation of the blasted rock (called mucking). Commonly, themucker is a type of front-end loader that moves the broken rock onto a belt conveyor that dumps it into a hauling system of cars or trucks. As all operations are concentrated at the heading, congestion is chronic, and much ingenuity has gone into designing equipment able to work in a small space. Since progress depends on the rate of heading advance, it is oftenfacilitatedby mining several headings simultaneously, as opening up intermediate headings from shafts or fromaditsdriven to provide extra points of access for longer tunnels.

For smaller diameters and longer tunnels, a narrow-gauge railroad is commonly employed to take out the muck and bring in workers and construction material. For larger-size bores of short to moderate length, trucks are generally preferred. For underground use these require diesel engines with scrubbers to eliminate dangerous gases from the exhaust. While existing truck and rail systems are adequate for tunnels progressing in the range of 4060 feet (1218 metres) per day, their capacity is inadequate to keep up with fast-moving moles progressing at the rate of several hundred feet per day. Hence, considerable attention is being devoted to developing high-capacity transport systemscontinuous-belt conveyors,pipelines, and innovative rail systems (high-capacity cars on high-speed trains). Muck disposal and its transport on the surface can also be a problem in congested urban areas. One solution successfully applied in Japan is to convey it by pipeline to sites where it can be used for reclamation bylandfill.

Forsurveycontrol, high-accuracy transit-level work (from base lines established by mountaintop triangulation) has generally been adequate; long tunnels from opposite sides of the mountain commonly meet with an error of one foot or less. Further improvements are likely from the recent introduction of thelaser, the pencil-size light beam of which supplies a reference line readily interpreted by workers. Most moles in the United States now use a laser beam to guide steering, and some experimental machines employ electronic steering actuated by the laser beam.

The dominant factor in all phases of the tunneling system is the extent of support needed to hold the surrounding ground safely. Engineers must consider the type of support, its strength, and how soon it must be installed after excavation. The key factor in timing support installation is so-calledstand-up timei.e.,how long the ground will safely stand by itself at the heading, thus providing a period for installing supports. In soft ground, stand-up time can vary from seconds in such soils as loose sand up to hours in such ground ascohesiveclay and even drops to zero in flowing ground below the water table, where inward seepage moves loose sand into the tunnel. Stand-up time in rock may vary from minutes in raveling ground (closely fractured rock where pieces gradually loosen and fall) up to days in moderately jointed rock (joint spacing in feet) and may even be measured in centuries in nearly intact rock, where the rock-block size (between joints) equals or exceeds size of the tunnel opening, thus requiring no support. While a miner generally prefers rock to soft ground, local occurrences of major defects within the rock can effectively produce a soft-ground situation; passage through such areas generally requires radical change to the use of a soft-ground type of support.

Under most conditions, tunneling causes a transfer of the ground load by arching to sides of the opening, termed theground-arch effect(Figure 1, top). At the heading the effect is three-dimensional, locally creating a ground dome in which the load is arched not only to the sides but also forward and back. If permanence of the ground arch is completely assured, stand-up time isinfinite, and no support is required. Ground-arch strength usually deteriorates with time, however, increasing the load on the support. Thus, the total load is shared between support and ground arch in proportion to their relative stiffness by a physical mechanism termedstructure-medium interaction. The support load increases greatly when theinherentground strength is much reduced by allowing excessive yield to loosen the rock mass. Because this may occur when installation of support is delayed too long, or because it may result from blast damage, good practice is based on the need to preserve the strength of the ground arch as the strongest load-carrying member of the system, by prompt installation of proper support and by preventing blast damage and movement from water inflow that has a tendency to loosen the ground.

Because stand-up time drops rapidly as size of the opening increases, thefull-face methodof advance (Figure 1, centre), in which the entire diameter of the tunnel is excavated at one time, it is most suitable for strong ground or for smaller tunnels. The effect of weak ground can be offset by decreasing the size of opening initially mined and supported, as in thetop heading and bench methodof advance. For the extreme case of very soft ground, this approach results in the multiple-drift method of advance (Figure 2), in which the individual drifts are reduced to a small size that is safe for excavation and portions of the support are placed in each drift and progressively connected as the drifts are expanded. The central core is left unexcavated until sides and crown are safely supported, thus providing a convenient central buttress for bracing the temporary support in each individual drift. While this obviously slow multidrift method is an old technique for very weak ground, such conditions still force its adoption as a last resort in some modern tunnels. In 1971, for example, on theStraight Creek interstatehighwaytunnel in Colorado, a very complex pattern of multiple drifts was found necessary to advance this large horseshoe-shaped tunnel 42 by 45 feet high through a weak shear zone more than 1,000 feet wide, after unsuccessful trials with full-face operation of a shield.

In early tunnels, timber was used for the initial or temporary support, followed by a permanent lining of brick or stone masonry. Sincesteelbecame available, it has been widely used as the first temporary stage or primary support. For protection against corrosion, it is nearly always encased in concrete as a second stage or final lining. Steel-rib support with timber blocking outside has been widely employed in rock tunnels. The horseshoe shape is common for all but the weakest rocks, since the flat bottomfacilitateshauling. By contrast, the stronger and more structurally efficient circular shape is generally required to support the greater loads from soft ground.Figure 1, bottom, compares these two shapes and indicates a number of terms identifying various parts of the cross section and adjacent members for a steel-rib type of support. Here a wall plate is generally used only with a top heading method, where it serves to support arch ribs both in the top heading and also where the bench is being excavated by spanning over this length until posts can be inserted beneath. Newer types of supports are discussed below with more modern tunnel procedures, in which the trend is away from two stages of support toward a single support system, part installed early and gradually strengthened in increments for conversion to the final complete support system.

In all but the shortest tunnels, control of theenvironmentis essential to provide safe working conditions.Ventilationis vital, both to provide fresh air and to remove explosive gases such as methane and noxious gases, including blast fumes. While the problem is reduced by using diesel engines with exhaust scrubbers and by selecting only low-fume explosives for underground use, long tunnels involve a major ventilating plant that employs a forced draft through lightweight pipes up to three feet in diameter and with booster fans at intervals. In smaller tunnels, the fans are frequently reversible, exhausting fumes immediately after blasting, then reversing to supply fresh air to the heading where the work is now concentrated.

High-levelnoisegenerated at the heading by drilling equipment and throughout the tunnel by high-velocity air in the vent lines frequently requires the use of earplugs withsign languagefor communication. In the future, equipment operators may work in sealed cabs, but communication is an unsolved problem. Electronic equipment in tunnels is prohibited, since stray currents may activate blasting circuits. Thunderstorms may also produce stray currents and require special precautions.

Dustis controlled by water sprays, wet drilling, and the use of respirator masks. Since prolonged exposure to dust from rocks containing a high percentage of silica may cause a respiratory ailment known assilicosis, severe conditions require special precautions, such as a vacuum-exhaust hood for each drill.

While excess heat is more common in deep tunnels, it occasionally occurs in fairly shallow tunnels. In 1953, workers in the 6.4-mileTelecote Tunnel near Santa Barbara, California, were transported immersed in water-filled mine cars through the hot area (117 F [47 C]). In 1970 a complete refrigeration plant was required to progress through a huge inflow of hot water at 150 F (66 C) in the 7-mileGraton Tunnel, driven under the Andes to drain a copper mine inPeru.

Soft-ground tunnels most commonly are used for urban services (subways, sewers, and other utilities) for which the need for quick access by passengers or maintenance staff favours a shallow depth. In many cities this means that the tunnels are above bedrock, making tunneling easier but requiring continuous support. The tunnel structure