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 100–500 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 rate—averaging 240 feet per day with a maximum of 420 feet per day.
Excavation and materials handling
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 and blasting methods for harder rock. Here each cycle involves drilling, loading explosive, blasting, ventilating fumes, and excavation of the blasted rock (called mucking). Commonly, the mucker 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 often facilitated by mining several headings simultaneously, as opening up intermediate headings from shafts or from adits driven 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 40–60 feet (12–18 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 systems—continuous-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 by landfill.
For survey control, 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 the laser, 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.
Ground support
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-called stand-up time—i.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 as cohesive clay 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 the ground-arch effect. 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 is infinite, 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 termed structure-medium interaction. The support load increases greatly when the inherent ground 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, the full-face method of advance, 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 the top heading and bench method of advance. For the extreme case of very soft ground, this approach results in the multiple-drift method of advance, 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 the Straight Creek interstate highway tunnel 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. Since steel became 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 bottom facilitates hauling. By contrast, the stronger and more structurally efficient circular shape is generally required to support the greater loads from soft ground. 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.
Environmental control
In all but the shortest tunnels, control of the environment is essential to provide safe working conditions. Ventilation is 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-level noise generated at the heading by drilling equipment and throughout the tunnel by high-velocity air in the vent lines frequently requires the use of earplugs with sign language for 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.
Dust is 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 as silicosis, 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-mile Telecote 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-mile Graton Tunnel, driven under the Andes to drain a copper mine in Peru.
Modern soft-ground tunneling
Settlement damage and lost ground
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 in such cases is generally designed to support the entire load of the ground above it, in part because the ground arch in soil deteriorates with time and in part as an allowance for load changes resulting from future construction of buildings or tunnels. Soft-ground tunnels are typically circular in shape because of this shape’s inherently greater strength and ability to readjust to future load changes. In locations within street rights-of-way, the dominant concern in urban tunneling is the need to avoid intolerable settlement damage to adjoining buildings. While this is rarely a problem in the case of modern skyscrapers, which usually have foundations extending to rock and deep basements often extending below the tunnel, it can be a decisive consideration in the presence of moderate-height buildings, whose foundations are usually shallow. In this case the tunnel engineer must choose between underpinning or employing a tunneling method that is sufficiently foolproof that it will prevent settlement damage.
Surface settlement results from lost ground—i.e., ground that moves into the tunnel in excess of the tunnel’s actual volume. All soft-ground tunneling methods result in a certain amount of lost ground. Some is inevitable, such as the slow lateral squeeze of plastic clay that occurs ahead of the tunnel face as new stresses from doming at the heading cause the clay to move toward the face before the tunnel even reaches its location. Most lost ground, however, results from improper construction methods and careless workmanship. Hence the following emphasizes reasonably conservative tunneling methods, which offer the best chance for holding lost ground to an acceptable level of approximately 1 percent.
Hand-mined tunnels
The ancient practice of hand mining is still economical for some conditions (shorter and smaller tunnels) and may illustrate particular techniques better than its mechanized counterpart. Examples are forepoling and breasting techniques as developed for the hazardous case of running (unstable) ground. shows the essentials of the process: heading advanced under a roof of forepole planks that are driven ahead at the crown (and at the sides in severe cases) plus continuous planking or breasting at the heading. With careful work the method permits advance with very little lost ground. The top breastboard may be removed, a small advance excavated, this breastboard replaced, and progress continued by working down one board at a time. While solid wall forepoling is nearly a lost art, an adaptation of it is termed spiling. In spiling the forepoles are intermittent with gaps between. Crown spiling is still resorted to for passing bad ground; in this case spiles may consist of rails driven ahead, or even steel bars set in holes drilled into crushed rock.
In ground providing a reasonable stand-up time, a modern support system uses steel liner-plate sections placed against the soil and bolted into a solid sheeted complete circle and, in larger tunnels, strengthened inside by circular steel ribs. Individual liner plates are light in weight and are easily erected by hand. By employing small drifts (horizontal passageways), braced to a central core, liner-plate technique has been successful in larger tunnels— shows 1940 practice on the 20-foot tunnels of the Chicago subway. The top heading is carried ahead, preceded slightly by a “monkey drift” in which the wall plate is set and serves as a footing for the arch ribs, also to span over as the wall plate is underpinned by erecting posts in small notches at each side of the lower bench. As the ribs and liner plate provide only a light support, they are stiffened by installation of a concrete lining about one day behind the mining. While liner-plate tunnels are more economical than shield tunnels, the risks of lost ground are somewhat greater and require not only very careful workmanship but also thorough soil-mechanics investigation in advance, pioneered in Chicago by Karl V. Terzaghi.
Shield tunnels
The risk of lost ground can also be reduced by using a shield with individual pockets from which workers can mine ahead; these can quickly be closed to stop a run-in. In extremely soft ground the shield may be simply shoved ahead with all its pockets closed, completely displacing the soil ahead of it; or it may be shoved with some of the pockets open, through which the soft soil extrudes like a sausage, cut into chunks for removal by a belt conveyor. The first of these methods was used on the Lincoln Tunnel in Hudson River silt.
Support erected inside the tail of the shield consists of large segments, so heavy that they require a power erector arm for positioning while being bolted together. Because of its high resistance to corrosion, cast iron has been the most commonly used material for segments, thus eliminating the need for a secondary lining of concrete. Today, lighter segments are employed. In 1968, for example, the San Francisco subway used welded steel-plate segments, protected outside by a bituminous coating and galvanized inside. British engineers have developed precast concrete segments that are proving popular in Europe.
An inherent problem with the shield method is the existence of a 2- to 5-inch (5- to 13-centimetre) ring-shaped void left outside the segments as the result of the thickness of the skin plate and the clearance needed for segment erection. Movement of soil into this void could result in up to 5 percent lost ground, an amount intolerable in urban work. Lost ground is held to reasonable levels by promptly blowing small-sized gravel into the void, then injecting cement grout (sand-cement-water mixture).
Water control
A soft-ground tunnel below the water table involves a constant risk of a run-in—i.e., soil and water flowing into the tunnel, which often results in complete loss of the heading. One solution is to lower the water table below the tunnel bottom before construction begins. This can be accomplished by pumping from deep wells ahead and from well points within the tunnel. While this benefits the tunneling, dropping the water table increases the loading on deeper soil layers. If these are relatively compressible, the result can be a major settlement of adjacent buildings on shallow foundations, an extreme example being a 15- to 20-foot subsidence in Mexico City due to overpumping.
When soil conditions make it undesirable to drop the water table, compressed air inside the tunnel may offset the outside water pressure. In larger tunnels, air pressure is generally set to balance the water pressure in the lower part of the tunnel, with the result that it then exceeds the smaller water pressure at the crown (upper part). Since air tends to escape through the upper part of the tunnel, constant inspection and repair of leaks with straw and mud are required. Otherwise, a blowout could occur, depressurizing the tunnel and possibly losing the heading as soil enters. Compressed air greatly increases operating costs, partly because a large compressor plant is needed, with standby equipment to insure against loss of pressure and partly because of the slow movement of workers and muck trains through the air locks. The dominant factor, however, is the huge reduction in productive time and lengthy decompression time required for people working under air to prevent the crippling disease known as the bends (or caisson disease), also encountered by divers. Regulations stiffen as pressure increases up to the usual maximum of 45 pounds per square inch (3 atmospheres) where daily time is limited to one hour working and six hours for decompression. This, plus higher hazard pay, makes tunneling under high air pressure very costly. In consequence, many tunneling operations attempt to lower the operating air pressure, either by partially dropping the water table or, especially in Europe, by strengthening the ground through the injection of solidifying chemical grouts. French and British grouting-specialist companies have developed a number of highly engineered chemical grouts, and these are achieving considerable success in advance cementing of weak soil.
Soft-ground moles
Since their first success in 1954, moles (mining machines) have been rapidly adopted worldwide. Close copies of the Oahe moles were used for similar large-diameter tunnels in clay shale at Gardiner Dam in Canada and at Mangla Dam in Pakistan during the mid-1960s, and subsequent moles have succeeded at many other locations involving tunneling through soft rocks. Of the several hundred moles built, most have been designed for the more easily excavated soil tunnel and are now beginning to divide into four broad types (all are similar in that they excavate the earth with drag teeth and discharge the muck onto a belt conveyor, and most operate inside a shield).
The open-face-wheel type is probably the most common. In the wheel the cutter arm rotates in one direction; in a variant model it oscillates back and forth in a windshield-wiper action that is most suitable in wet, sticky ground. While suitable for firm ground, the open-face mole has sometimes been buried by running or loose ground.
The closed-faced-wheel mole partly offsets this problem, since it can be kept pressed against the face while taking in muck through slots. Since the cutters are changed from the face, changing must be done in firm ground. This kind of mole performed well, beginning in the late 1960s, on the San Francisco subway project in soft to medium clay with some sand layers, averaging 30 feet per day. In this project, mole operation made it cheaper and safer to drive two single-track tunnels than one large double-track tunnel. When adjacent buildings had deep foundations, a partial lowering of the water table permitted operations under low pressure, which succeeded in limiting surface settlement to about one inch. In areas of shallow building foundations, dewatering was not permitted; air pressure was then doubled to 28 pounds per square inch, and settlements were slightly smaller.
A third type is the pressure-on-face mole. Here, only the face is pressurized, and the tunnel proper operates in free air—thus avoiding the high costs of labour under pressure. In 1969 a first major attempt used air pressure on the face of a mole operating in sands and silts for the Paris Metro. A 1970 attempt in volcanic clays of Mexico City used a clay-water mixture as a pressurized slurry (liquid mixture); the technique was novel in that the slurry muck was removed by pipeline, a procedure simultaneously also used in Japan with a 23-foot-diameter pressure-on-face mole. The concept has been further developed in England, where an experimental mole of this type was first constructed in 1971.
The digger-shield type of machine is essentially a hydraulic-powered digger arm excavating ahead of a shield, whose protection can be extended forward by hydraulically operated poling plates, acting as retractable spiles. In 1967–70 in the 26-foot-diameter Saugus-Castaic Tunnel near Los Angeles, a mole of this type produced daily progress in clayey sandstone averaging 113 feet per day and 202 feet maximum, completing five miles of tunnel one-half year ahead of schedule. In 1968 an independently developed device of similar design also worked well in compacted silt for a 12-foot-diameter sewer tunnel in Seattle.
Pipe jacking
For small tunnels in a five- to eight-foot size range, small moles of the open-face-wheel type have been effectively combined with an older technique known as pipe jacking, in which a final lining of precast concrete pipe is jacked forward in sections. The system used in 1969 on two miles of sewer in Chicago clay had jacking runs up to 1,400 feet between shafts. A laser-aligned wheel mole cut a bore slightly larger than the lining pipe. Friction was reduced by bentonite lubricant added outside through holes drilled from the surface, which were later used for grouting any voids outside the pipe lining. The original pipe-jacking technique was developed particularly for crossing under railroads and highways as a means of avoiding traffic interruption from the alternate of construction in open trench. Since the Chicago project showed a potential for progress of a few hundred feet per day, the technique has become attractive for small tunnels.
Modern rock tunneling
Nature of the rock mass
It is important to distinguish between the high strength of a block of solid or intact rock and the much lower strength of the rock mass consisting of strong rock blocks separated by much weaker joints and other rock defects. While the nature of intact rock is significant in quarrying, drilling, and cutting by moles, tunneling and other areas of rock engineering are concerned with the properties of the rock mass. These properties are controlled by the spacing and nature of the defects, including joints (generally fractures caused by tension and sometimes filled with weaker material), faults (shear fractures frequently filled with claylike material called gouge), shear zones (crushed from shear displacement), altered zones (in which heat or chemical action have largely destroyed the original bond cementing the rock crystals), bedding planes, and weak seams (in shale, often altered to clay). Since these geologic details (or hazards) usually can only be generalized in advance predictions, rock-tunneling methods require flexibility for handling conditions as they are encountered. Any of these defects can convert the rock to the more hazardous soft-ground case.
Also important is the geostress—i.e., the state of stress existing in situ prior to tunneling. Though conditions are fairly simple in soil, geostress in rock has a wide range because it is influenced by the stresses remaining from past geologic events: mountain building, crustal movements, or load subsequently removed (melting of glacial ice or erosion of former sediment cover). Evaluation of the geostress effects and the rock mass properties are primary objectives of the relatively new field of rock mechanics and are dealt with below with underground chambers since their significance increases with opening size. This section therefore emphasizes the usual rock tunnel, in the size range of 15 to 25 feet.
Conventional blasting
Blasting is carried on in a cycle of drilling, loading, blasting, ventilating fumes, and removing muck. Since only one of these five operations can be conducted at a time in the confined space at the heading, concentrated efforts to improve each have resulted in raising the rate of advance to a range of 40–60 feet per day, or probably near the limit for such a cyclic system. Drilling, which consumes a major part of the time cycle, has been intensely mechanized in the United States. High-speed drills with renewable bits of hard tungsten carbide are positioned by power-operated jib booms located at each platform level of the drilling jumbo (a mounted platform for carrying drills). Truck-mounted jumbos are used in larger tunnels. When rail-mounted, the drilling jumbo is arranged to straddle the mucker so that drilling can resume during the last phase of the mucking operation.
By experimenting with various drill-hole patterns and the sequence of firing explosives in the holes, Swedish engineers have been able to blast a nearly clean cylinder in each cycle, while minimizing use of explosives.
Dynamite, the usual explosive, is fired by electric blasting caps, energized from a separate firing circuit with locked switches. Cartridges are generally loaded individually and seated with a wooden tamping rod; Swedish efforts to expedite loading often employ a pneumatic cartridge loader. American efforts toward reduced loading time have tended to replace dynamite with a free-running blasting agent, such as a mixture of ammonium nitrate and fuel oil (called AN-FO), which in granular form (prills) can be blown into the drill hole by compressed air. While AN-FO-type agents are cheaper, their lower power increases the quantity required, and their fumes usually increase ventilating requirements. For wet holes, the prills must be changed to a slurry requiring special processing and pumping equipment.
Rock support
Most common loading on the support of a tunnel in hard rock is due to the weight of loosened rock below the ground arch, where designers rely particularly on experience with Alpine tunnels as evaluated by two Austrians, Karl V. Terzaghi, the founder of soil mechanics, and Josef Stini, a pioneer in engineering geology. The support load is greatly increased by factors weakening the rock mass, particularly blasting damage. Furthermore, if a delay in placing support allows the zone of rock loosening to propagate upward (i.e., rock falls from the tunnel roof), the rock-mass strength is reduced, and the ground arch is raised. Obviously, the loosened rock load can be greatly altered by a change in joint inclination (orientation of rock fractures) or by the presence of one or more of the rock defects previously mentioned. Less frequent but more severe is the case of high geostress, which in hard, brittle rock may result in dangerous rock bursts (explosive spalling off from the tunnel side) or in a more plastic rock mass may exhibit a slow squeezing into the tunnel. In extreme cases, squeezing ground has been handled by allowing the rock to yield while keeping the process under control, then remining and resetting initial support several times, plus deferring concrete lining until the ground arch becomes stabilized.
For many years steel rib sets were the usual first-stage support for rock tunnels, with close spacing of the wood blocking against the rock being important to reduce bending stress in the rib. Advantages are increased flexibility in changing rib spacing plus the ability to handle squeezing ground by resetting the ribs after remining. A disadvantage is that in many cases the system yields excessively, thus inviting weakening of the rock mass. Finally, the rib system serves only as a first-stage or temporary support, requiring a second-stage encasement in a concrete lining for corrosion protection.
Concrete lining
Concrete linings aid fluid flow by providing a smooth surface and insure against rock fragment falling on vehicles using the tunnel. While shallow tunnels are often lined by dropping concrete down holes drilled from the surface, the greater depth of most rock tunnels requires concreting entirely within the tunnel. Operations in such congested space involve special equipment, including agitator cars for transport, pumps or compressed-air devices for placing the concrete, and telescoping arch forms that can be collapsed to move forward inside forms remaining in place. The invert is generally concreted first, followed by the arch where forms must be left in place from 14 to 18 hours for the concrete to gain necessary strength. Voids at the crown are minimized by keeping the discharge pipe buried in fresh concrete. The final operation consists of contact grouting, in which a sand-cement grout is injected to fill any voids and to establish full contact between lining and ground. The method usually produces progress in the range of 40 to 120 feet per day. In the 1960s there was a trend toward an advancing-slope method of continuous concreting, as originally devised for embedding the steel cylinder of a hydropower penstock. In this procedure, several hundred feet of forms are initially set, then collapsed in short sections and moved forward after the concrete has gained necessary strength, thus keeping ahead of the continuously advancing slope of fresh concrete. As a 1968 example, Libby Dam’s Flathead Tunnel in Montana attained a concreting rate of 300 feet (90 metres) per day by using the advancing slope method.
Rock bolts
Rock bolts are used to reinforce jointed rock much as reinforcing bars supply tensile resistance in reinforced concrete. After early trials about 1920, they were developed in the 1940s for strengthening laminated roof strata in mines. For public works their use has increased rapidly since 1955, as confidence has developed from two independent pioneering applications, both in the early 1950s. One was the successful change from steel rib sets to cheaper rock bolts on major portions of the 85 miles of tunnels forming New York City’s Delaware River Aqueduct. The other was the success of such bolts as the sole rock support in large underground powerhouse chambers of Australia’s Snowy Mountains project. Since about 1960, rock bolts have had major success in providing the sole support for large tunnels and rock chambers with spans up to 100 feet. Bolts are commonly sized from 0.75 to 1.5 inches and function to create a compression across rock fissures, both to prevent the joints opening and to create resistance to sliding along the joints. For this they are placed promptly after blasting, anchored at the end, tensioned, and then grouted to resist corrosion and to prevent anchor creep. Rock tendons (prestressed cables or bundled rods, providing higher capacity than rock bolts) up to 250 feet long and prestressed to several hundred tons each have succeeded in stabilizing many sliding rock masses in rock chambers, dam abutments, and high rock slopes. A noted example is their use in reinforcing the abutments of Vaiont Dam in Italy. In 1963 this project experienced disaster when a giant landslide filled the reservoir, causing a huge wave to overtop the dam, with large loss of life. Remarkably, the 875-foot-high arch dam survived this huge overloading; the rock tendons are believed to have supplied a major strengthening.
Shotcrete
Shotcrete is small-aggregate concrete conveyed through a hose and shot from an air gun onto a backup surface on which it is built up in thin layers. Though sand mixes had been so applied for many years, new equipment in the late 1940s made it possible to improve the product by including coarse aggregate up to one inch; strengths of 6,000 to 10,000 pounds per square inch (400 to 700 kilograms per square centimetre) became common. Following initial success as rock-tunnel support in 1951–55 on the Maggia Hydro Project in Switzerland, the technique was further developed in Austria and Sweden. The remarkable ability of a thin shotcrete layer (one to three inches) to bond to and knit fissured rock into a strong arch and to stop raveling of loose pieces soon led to shotcrete largely superseding steel rib support in many European rock tunnels. By 1962 the practice had spread to South America. From this experience plus limited trial at the Hecla Mine in Idaho, the first major use of coarse-aggregate shotcrete for tunnel support in North America developed in 1967 on the Vancouver Railroad Tunnel, with a cross section 20 by 29 feet high and a length of two miles. Here an initial two- to four-inch coat proved so successful in stabilizing hard, blocky shale and in preventing raveling in friable (crumbly) conglomerate and sandstone that the shotcrete was thickened to six inches in the arch and four inches on the walls to form the permanent support, saving about 75 percent of the cost of the original steel ribs and concrete lining.
A key to shotcreting’s success is its prompt application before loosening starts to reduce the strength of the rock mass. In Swedish practice this is accomplished by applying immediately after blasting and, while mucking is in progress, utilizing the “Swedish robot,” which allows the operator to remain under the protection of the previously supported roof. On the Vancouver tunnel, shotcrete was applied from a platform extending forward from the jumbo while the mucking machine operated below. By taking advantage of several unique properties of shotcrete (flexibility, high bending strength, and ability to increase thickness by successive layers), Swedish practice has developed shotcreting into a single-support system that is strengthened progressively as needed for conversion into the final support.
Preserving rock strength
In rock tunnels, the requirements for support can be significantly decreased to the extent that the construction method can preserve the inherent strength of the rock mass. The opinion has been often expressed that a high percentage of support in rock tunnels in the United States (perhaps over half) has been needed to stabilize rock damaged by blasting rather than because of an inherently low strength of the rock. As a remedy, two techniques are currently available. First is the Swedish development of sound-wall blasting (to preserve rock strength), treated below under rock chambers, since its importance increases with size of the opening. The second is the American development of rock moles that cut a smooth surface in the tunnel, thus minimizing rock damage and support needs—here limited to rock bolts connected by steel straps for this sandstone tunnel. In stronger rocks (as the 1970 Chicago sewers in dolomite) mole excavation not only largely eliminated need for support but also produced a surface of adequate smoothness for sewer flow, which permitted a major saving by omitting the concrete lining. Since their initial success in clay shale, the use of rock moles has expanded rapidly and has achieved significant success in medium-strength rock such as sandstone, siltstone, limestone, dolomite, rhyolite, and schist. The advance rate has ranged up to 300 to 400 feet per day and has often outpaced other operations in the tunneling system. While experimental moles were used successfully to cut hard rock such as granite and quartzite, such devices were not economical, because cutter life was short, and frequent cutter replacement was costly. This was likely to change, however, as mole manufacturers sought to extend the range of application. Improvement in cutters and progress in reducing the time lost from equipment breakages were producing consistent improvements.
American moles have developed two types of cutters: disk cutters that wedge out the rock between initial grooves cut by the hard-faced rolling disks, and roller-bit cutters using bits initially developed for fast drilling of oil wells. As later entrants in the field, European manufacturers have generally tried a different approach—milling-type cutters that mill or plane away part of the rock, then shear off undercut areas. Attention is also focusing on broadening the moles’ capabilities to function as the primary machine of the whole tunneling system. Thus, future moles are expected not only to cut rock but also to explore ahead for dangerous ground; handle and treat bad ground; provide a capability for prompt erection of support, rock bolting, or shotcreting; change cutters from the rear in loose ground; and produce rock fragments of a size appropriate to capability of the muck removal system. As these problems are solved, the continuous-tunneling system by mole is expected largely to replace the cyclic drilling and blasting system.
Water inflows
Exploring ahead of the path of a tunnel is particularly necessary for location of possible high water inflows and permitting their pretreatment by drainage or grouting. When high-pressure flows occur unexpectedly, they result in long stoppages. When huge flows are encountered, one approach is to drive parallel tunnels, advancing them alternately so that one relieves pressure in front of the other. This was done in 1898 in work on the Simplon Tunnel and in 1969 on the Graton Tunnel in Peru, where flow reached 60,000 gallons (230,000 litres) per minute. Another technique is to depressurize ahead by drain holes (or small drainage drifts on each side), an extreme example being the 1968 Japanese handling of extraordinarily difficult water and rock conditions on the Rokko Railroad Tunnel, using approximately three-quarters of a mile of drainage drifts and five miles of drain holes in a one-quarter-mile length of the main tunnel.
Heavy ground
The miner’s term for very weak or high geostress ground that causes repeated failures and replacement of support is heavy ground. Ingenuity, patience, and large increases of time and funds are invariably required to deal with it. Special techniques have generally been evolved on the job, as indicated by a few of the numerous examples. On the 7.2-mile Mont Blanc Vehicular Tunnel of 32-foot size under the Alps in 1959–63, a pilot bore ahead helped greatly to reduce rock bursts by relieving the high geostress. The 5-mile, 14-foot El Colegio Penstock Tunnel in Colombia was completed in 1965 in bituminous shale, requiring the replacement and resetting of more than 2,000 rib sets, which buckled as the invert (bottom supports) and sides gradually squeezed in up to 3 feet, and by deferring concreting until the ground arch stabilized.
While the ground arch eventually stabilized in these and numerous similar examples, knowledge is inadequate to establish the point between desirable deformation (to mobilize ground strength) and excessive deformation (which reduces its strength), and improvement is most likely to come from carefully planned and observed field-test sections at prototype scale, but these have been so costly that very few have actually been executed, notably the 1940 test sections in clay on the Chicago subway and the 1950 Garrison Dam test tunnel in the clay-shale of North Dakota. Such prototype field testing has resulted, however, in substantial savings in eventual tunnel cost. For harder rock, reliable results are even more fragmentary.
Unlined tunnels
Numerous modest-size conventionally blasted tunnels have been left unlined if human occupancy was to be rare and the rock was generally good. Initially, only weak zones are lined, and marginal areas are left for later maintenance. Most common is the case of a water tunnel that is built oversized to offset the friction increase from the rough sides and, if a penstock tunnel, is equipped with a rock trap to catch loose rock pieces before they can enter the turbines. Most of these have been successful, particularly if operations could be scheduled for periodic shutdowns for maintenance repair of rockfalls; the Laramie-Poudre Irrigation Tunnel in northern Colorado experienced only two significant rockfalls in 60 years, each easily repaired during a nonirrigation period. In contrast, a progressive rockfall on the 14-mile Kemano penstock tunnel in Canada resulted in shutting down the whole town of Kitimat in British Columbia, and vacationing workers for nine months in 1961 since there were no other electric sources to operate the smelter. Thus, the choice of an unlined tunnel involves a compromise between initial saving and deferred maintenance plus evaluation of the consequences of a tunnel shutdown.
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 1910–14 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 environment—a 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 rock—a 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 openings—as at Churchill Falls—to 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.
Chamber excavation and support
Excavation for rock chambers generally starts with a horizontal tunnel at the top of the area to be excavated and progresses down in steps. Rock is excavated by drilling and blasting, carried on simultaneously in several headings. This procedure may give way, however, as moles gain in their ability to cut hard rock economically and as a rock saw or other device is developed for squaring up the circular surface normally cut by the mole. High geostress can be a real problem (causing inward movement of the chamber walls) unless handled by a careful sequence of partial excavations designed to relieve it gradually.
Many of the earlier underground hydroplants were roofed with a concrete arch, often designed for a major load, as in some Italian projects in weak rocks or where blast damage was considerable, as at a few projects in Scotland. Since about 1960, however, most have relied solely on rock bolts for support (sometimes supplemented with shotcrete). That such a light support has been widely successful can be attributed to careful investigation resulting in locations with strong rock, employment of techniques to relieve high geostress, and controlled blasting to preserve rock strength.
Sound-wall blasting
Sound-wall blasting is a technique, primarily developed in Sweden, that preserves the finished rock surfaces in sound condition by careful design of the blasting charges to fit the rock conditions. In underground work, Swedish practice has often produced remarkable results almost like rock sculpturing in which the excellent shaping and preservation of the rock surfaces often permit omitting concrete lining at savings greater than the extra cost of the engineered blasting. While Swedish success is due partly to the generally strong rock in that country, it is due even more to energetic research and development programs to develop (1) theoretical methods for blasting design plus field blast tests to determine pertinent rock properties, (2) special explosives for different rock conditions, and (3) institutes for the training of specialized blasting engineers to apply these procedures in the field construction.
In the United States, sound-wall blasting has enjoyed only indifferent success underground. Reluctance of the blasting industry to change from its customary empirical approach and the lack of specialized blasting engineers trained in Swedish practices have led to a return to the more costly technique of mining an initial pilot bore to afford stress relief, followed by blasting successively thinner slabs toward the free face of the pilot bore.
For excavation from the ground surface, the requirements of sound-wall blasting largely have been met by the technique of presplitting, developed in the United States in the late 1950s. Basically, this technique consists of creating a continuous crack (or presplit) at a desired finished excavation line by initially firing a line of closely spaced, lightly loaded holes drilled there. Next, the interior rock mass is drilled and blasted by conventional means. If a high horizontal geostress is present, it is important that it first be relieved (as by an initial cut a modest distance from the presplit line); otherwise, the presplit crack is not likely to occur in the direction desired. Stockton Dam, in Missouri, illustrates the benefit of presplitting. Here, vertical faces in dolomite up to 110 feet were successfully presplit and promptly rock-bolted; this permitted a major reduction in thickness of the concrete facing, resulting in a net saving of about $2.5 million.
Shafts
The mining industry has been the primary constructor of shafts, because at many locations these are essential for access to ore, for ventilation, and for material transport. Depths of several thousand feet are common. In public-works projects, such as sewer tunnels, shafts are usually only a few hundred feet deep and because of their high cost are avoided in the design stage wherever practical. Shallower shafts find many uses, however, for penstocks and access to underground hydroplants, for dropping aqueduct tunnels beneath rivers, for missile silos, and for oil and liquefied-gas storage. Being essentially vertical tunnels, shafts involve the same problems of different types of ground and water conditions but on an aggravated scale, since vertical transport makes the operation slower, more costly, and even more congested than with horizontal tunneling. Except when there is a high horizontal geostress in rock, the loading on a shaft support is generally less than for a tunnel. Inflowing water, however, is far more dangerous during construction and generally intolerable during operation. Hence, most shafts are concrete-lined and waterproofed, and the lining installation usually follows only a short distance behind excavation. The shape is usually circular, although, before current mechanized excavation methods, mining shafts were frequently rectangular. Shafts may be sunk from the surface (or drilled in smaller sizes), or, if an existing tunnel provides access, they may be raised from below.
Shaft sinking and drilling
Mining downward, generally from the surface, although occasionally from an underground chamber, is called shaft sinking. In soil, shallow shafts are frequently supported with interlocking steel sheetpiling held by ring beams (circular rib sets); or a concrete caisson may be built on the surface and sunk by excavating inside as weight is added by extending its walls. More recently, large-diameter shallow shafts have been constructed by the “slurry trench method,” in which a circular trench is excavated while filled with a heavy liquid (usually bentonite slurry), which supports its walls until it is finally displaced by filling the trench with concrete. For greater depth in soil, another method involves freezing a ring of soil around the shaft. In this method, a ring of closely spaced freezing holes is drilled outside the shaft. A refrigerated brine is circulated in double-wall pipes in the holes to freeze the soil before starting the shaft excavation. It is then kept frozen until the shaft is completed and lined with concrete. This freezing method was developed in Germany and the Netherlands, where it was used successfully to sink shafts through nearly 2,000 feet of alluvial soil to reach coal beds in the underlying rock. It has also been applied under similar conditions in Britain, Poland, and Belgium. Occasionally, the freezing technique has been used in soft rock to solidify a deep aquifer (layer of water-bearing rock). Because of the long time required for drilling the freezing holes and for freezing the ground (18 to 24 months for some deep shafts), the freezing method has not been popular on public-works projects except as a last resort, although it has been used in New York City for shallow shafts through soil to gain access for deep-water tunnels.
More efficient methods for sinking deep shafts in rock were developed in South African gold-mining operations, in which shafts 5,000 to 8,000 feet deep are common and are generally 20 to 30 feet in diameter. South African procedure has produced progress of about 30 feet per day by utilizing a sinking stage of multiple platforms, which permits concurrent excavation and concrete lining. Excavation is by drilling and blasting with muck loaded into large buckets, with larger shafts operating four buckets alternately in hoisting wells extending through the platforms. Grouting is carried a few hundred feet ahead to seal out water. Best progress is achieved when the rock is pregrouted from two or three holes drilled from the surface before the shaft is started. Since the shallower shafts on public-works projects cannot justify the investment in the large plant needed to operate a sinking stage, their progress in rock is much slower—in the range of 5 to 10 feet per day.
Occasionally, shafts have been sunk through soil by drilling methods. The technique was first used in British practice in 1930 and was subsequently further refined in the Netherlands and Germany. The procedure involves first advancing a pilot hole, then reaming in several stages of enlargement to final diameter, while the walls of the hole are supported by a heavy liquid (called drilling mud), with circulation of the mud serving to remove the cuttings. Then a double-wall steel casing is sunk by displacing the drilling mud, followed by injecting concrete outside the casing and within the annular space between its double walls. One use of this technique was in the 25-foot-diameter Statemine shaft in the Netherlands, 1,500 feet deep through soil that required about three and one-half years before completion in 1959. For the 1962 construction of some 200 missile shafts in Wyoming in soft rock (clay shale and friable sandstone), a giant auger proved effective for sinking these 65-foot-deep, 15-foot-diameter shafts, generally at the rate of two to three days per shaft. Perhaps the largest drilled shaft is one in the Soviet Union: 2,674 feet deep, which was enlarged in four stages of reaming to a final diameter of 28.7 feet, progressing at a reported rate of 15 feet per day.
More dramatic has been the adaptation in the United States of oil-well-drilling methods in a technique called big-hole drilling, used for constructing small shafts in the diameter range of three to six feet. Big-hole drilling was developed for deep emplacement in underground testing of nuclear devices, with more than 150 such big holes drilled in the 1960s up to 5,000 feet deep in Nevada in rocks ranging from soft tuff to granite. In big-hole drilling the hole is made in one pass only with an array of roller-bit cutters that are pressed against the rock by the weight of an assembly of lead-filled drill collars, sometimes totaling 300,000 pounds. The drill rig must be huge in size to handle such loads. The greatest impediment controlling progress has been the removal of drill cuttings, where an air lift is showing promise.
Shaft raising
Handling cuttings is simplified when the shaft can be raised from an existing tunnel, since the cuttings then merely fall to the tunnel, where they are easily loaded into mine cars or trucks. This advantage has long been recognized in mining; where once an initial shaft has been sunk to provide access to and an opportunity for horizontal tunnels, most subsequent shafts are then raised from these tunnels, often by upward mining with men working from a cage hung from a cable through a small pilot hole drilled downward from above. In 1957 this procedure was improved by Swedish development of the raise climber, whose working cage climbs a rail fastened to the shaft wall and extends backward into the horizontal access tunnel into which the cage is retracted during a blast. Simultaneously in the 1950s Germans began experimenting with several mechanized reamers, including a motor-cutter unit pulled upward by a cable in a previously down-drilled pilot hole. A more significant step toward mechanized shaft raising occurred in 1962 when American mole manufacturers developed a device called a raise borer, in which the cutting head is rotated and pulled upward by a drill shaft in a down-drilled pilot hole, with the power unit being located at top of the pilot hole. The capacity of this type of borer (or upward reamer) generally ranges from 3–8-foot diameters in lifts up to 1,000 feet with progress ranging up to 300 feet per day. Furthermore, available cutters when operating on raise borers can cut through rock often almost twice as hard as rock moles can deal with. For larger shafts, bigger-diameter reamers may be operated in an inverted position to ream downward, with the cuttings sluiced to the access tunnel below. A 12-foot-diameter, 1,600-foot-deep vent shaft was completed by this method in 1969 at the White Pine Copper Mine in Michigan. Starting from a 10-inch pilot hole, it was enlarged in three downreaming passes.
The introduction of a workable raise borer in the 1960s represented a breakthrough in shaft construction, cutting construction time to one-third and cost to less than one-half that for an upward-mined shaft. At the beginning of the 1970s, the procedure was being widely adopted for shaft raising, and some projects had been specifically designed to take advantage of this more efficient method. At a Northfield Mountain (Massachusetts) underground hydroplant (completed in 1971), the previously common large surge chamber was replaced by a series of horizontal tunnels at three levels, connected by vertical shafts. This layout permitted significant economy by the use of jumbos already available from other tunnels of the project and the use of a raise borer for starting the shafts. If very large shafts are involved, the raise borer is particularly useful in simplifying the so-called glory-hole method, in which the main shaft is sunk by blasting; the muck is then dumped in the central glory hole, previously constructed by a raise borer. The example is based on the construction of a 133-foot-diameter surge shaft above the Angeles penstock tunnel near Los Angeles. The glory-hole technique was also used in 1944 in constructing a series of 20 underground fuel-oil chambers in Hawaii, working from access tunnels driven initially at both top and bottom of the chambers and later used to house oil and vent piping. The advent of the raise borer should now make this and similar construction more economically attractive. Recently, some deep sewer projects have been redesigned to utilize the raise borer for shaft connections.
Immersed-tube tunnels
Development of method
The immersed-tube, or sunken-tube, method, used principally for underwater crossings, involves prefabricating long tube sections, floating them to the site, sinking each in a previously dredged trench, and then covering with backfill. While more correctly classified as a subaqueous adaptation of the dry-land cut-and-cover procedure often used for subways, the immersed-tube method warrants inclusion as a tunneling technique because it is becoming a preferred alternate to the older method of constructing a subaqueous tunnel under compressed air with a Greathead shield. A major advantage is that, once the new section has been connected, interior work is conducted in free air, thus avoiding the high cost and major risk of operating a large shield under high air pressure. Furthermore, the immersed-tube method is usable in water deeper than is possible with the shield method, which essentially is restricted to less than 100 feet of water by the maximum air pressure at which workers can safely work.
The procedure was first developed by an American engineer, W.J. Wilgus, for the construction (1906–10) of the Detroit River twin-tube railroad tunnel between Detroit, Mich., and Windsor, Ont., where it was successfully used for the 2,665-foot river-crossing portion. A structural assembly of steel tubes was prefabricated in 262-foot-long sections with both ends temporarily bulkheaded or closed. Each section was then towed out and sunk in 60 to 80 feet of water, onto a grillage of I-beams in sand at the bottom of a trench previously dredged in the river-bottom clay. After being connected to the previous section by locking pins driven by a diver, the section was weighted down by surrounding it with concrete. Next, after removal of the temporary bulkheads at the just-completed connection, the newly placed section was pumped out, permitting completion of an interior concrete lining in free air. With subsequent refinements these basic principles still form the basis of the immersed-tube method.
After use on a four-tube New York City subway crossing under the Harlem River in 1912–14, the method was tried for a vehicular tunnel in the 1925–28 construction of the 3,545-foot-long, 37-foot-diameter Posey tunnel at Oakland in California. Because these and other experiences have indicated that the problems encountered in building large vehicular tunnels could be better handled by the immersed-tube method, it has been preferred for subaqueous vehicular tunnels since about 1940. While shield tunneling continued in a transition period (1940–50), subsequently nearly all of the world’s large vehicular tunnels have been constructed by the immersed-tube method, including such notable examples as the Bankhead tunnel at Mobile, Ala.; two Chesapeake Bay tunnels; the Fraser River tunnel at Vancouver, B.C.; the Maas River tunnel in the Netherlands; Denmark’s Limfjord tunnel; Sweden’s Tingstad tunnel; and the Hong Kong Cross Harbor tunnel.
Modern practice
The world’s longest and deepest application to date is the twin-tube subway crossing of San Francisco Bay, constructed between 1966 and 1971 with a length of 3.6 miles in a maximum water depth of 135 feet. The 330-foot-long, 48-foot-wide sections were constructed of steel plate and launched by shipbuilding procedures. Each section also had temporary end bulkheads and upper pockets for gravel ballast placed during sinking. After placement of the interior concrete lining at a fitting-out dock, each section was towed to the site and sunk in a trench previously dredged in the mud in the bottom of the bay. With diver guidance, the initial connection was accomplished by hydraulic-jack-powered couplers, similar to those that automatically join railroad cars. By relieving the water pressure within the short compartment between bulkheads at the new joint, the water pressure acting on the forward end of the new section provided a huge force that pushed it into intimate contact with the previously laid tube, compressing the rubber gaskets to provide a watertight seal. Following this, the temporary bulkheads were removed on each side of the new joint and interior concrete placed across the connection.
Most applications of the immersed-tube procedure outside the United States have been by a Danish engineer-constructor firm, Christiani and Nielsen, starting in 1938 with a three-tube highway crossing of the Maas River in Rotterdam. While following American technique in essence, European engineers have developed a number of innovations, including prestressed concrete in lieu of a steel structure (often consisting of a number of short sections tied together with prestressed tendons to form a single section 300 feet in length); the use of butyl rubber as the waterproofing membrane; and initial support on temporary piles while a sand fill is jetted beneath. An alternate to the last approach has been used in a Swedish experiment on the Tingstad tunnel, in which the precast sections were supported on water-filled nylon sacks and the water later replaced by grout injected into the sacks to form the permanent support. Also, the cross section has been greatly enlarged—the 1969 Schelde River tunnel in Antwerp, Belg., used precast sections 328 feet long by 33 feet high by 157 feet wide. This unusually large width accommodates two highway tubes of three lanes each, one two-track railroad tube, and one bicycle tube. Particularly unusual was a 1963 use of the immersed-tube technique in subway construction in Rotterdam. Trenches were dug or, in some cases, made out of abandoned canals and filled with water. The tube sections were then floated into position. This technique had been first tried in 1952 for a land approach to the immersed-tube Elizabeth tunnel in Norfolk, Va.; in low-elevation ground with the water table near the surface, it permits a considerable saving in bracing of the trench because keeping the trench filled eliminates the need for resisting external water pressure.
Thus, the immersed-tube method has become a frequent choice for subaqueous crossings, although some locations pose problems of interference with intensive navigation traffic or the possibility of displacement by severe storms (one tube section of the Chesapeake Bay tunnel was moved out of its trench by a severe storm during construction). The method is being actively considered for many of the world’s most difficult underwater crossings, including the long-discussed English Channel Project.
