A bridge is a structure built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle. Designs of vary depending on the function of the bridge and the nature of the terrain where the bridge is constructed.

History Types of bridges

There are six main types of bridges: bridges, bridges, arch bridges, suspension bridges, cable-stayed bridges and bridges.

Beam bridges

A is basically a rigid horizontal structure that is resting on two piers, one at each end. The weight of the bridge and any traffic on it is directly supported by the piers. The weight is traveling directly downward. The force of compression manifests itself on the top side of the beam bridge's deck (or roadway). This causes the upper portion of the deck to shorten. The result of the compression on the upper portion of the deck causes tension in the lower portion of the deck. This tension causes the lower portion of the beam to lengthen.

Many beam bridges that are found on highway overpasses use concrete or steel beams to handle the load. The size of the beam, and in particular the height of the beam, controls the distance that the beam can span. By increasing the height of the beam, the beam has more material to dissipate the tension. To create very tall beams, bridge designers add supporting lattice work, or a truss, to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force is dissipated through the truss. Despite the ingenious addition of a truss, the beam bridge is still limited in the distance it can span. As the distance increases, the size of the truss must also increase, until it reaches a point where the bridge's own weight is so large that the truss cannot support it.

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Factsheet :Cantilever Bridge Cantilever bridges Ancestor Beam bridge, A cantilever bridge is a bridge built using , structures that project horizontally into space, Descendant supported on only one end. For smallfootbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic Carries Pedestrians, automobiles, use built from structural steel, or box trucks,light rail, heavy rail girders built from . The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can Span range Medium span distances of over 1,500 feet (460 m), and can be more easily constructed at difficult crossings by virtue Material Iron, structural steel, of using little or no falsework. prestressed concrete Function of a Cantilever Bridge Movable No A simple cantilever span is formed by two cantilever arms extending from opposite sides of the obstacle to Design effort Medium be crossed, meeting at the center. In a common variant, Falsework required the suspended span, the cantilever arms do not meet No in the center; instead, they support a central truss Longest Span Quebec Bridge (Quebec, bridge which rests on the ends of the cantilever arms. Canada) (549 m) The suspended span may be built off-site and lifted into place, or constructed in place using special traveling

supports.

A common way to construct steel truss and prestressed concrete cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction, forming a balanced cantilever; when they attach to a solid foundation, the counterbalancing arms are called anchor arms. Thus, in a bridge built on two foundation piers, there are four cantilever arms: two which span the obstacle, and two anchor arms which extend away from the obstacle. Because of the need for more strength at the balanced cantilever's supports, the bridge superstructure often takes the form of towers above the foundation piers. The Commodore Barry Bridge is an example of

2 this type of cantilever bridge. Steel truss cantilevers support loads by tension of the upper members and compression of the lower ones. Commonly, the structure distributes the tension via the anchor arms to the outermost supports, while the compression is carried to the foundations beneath the central towers. Many truss cantilever bridges use pinned joints and are therefore statically determinatewith no members carrying mixed loads. Prestressed concrete balanced cantilever bridges are often built using segmental construction.

Construction methods

Some steel arch bridges (such as the Navajo Bridge) are built using pure cantilever spans from each side, with neither falsework below nor temporary supporting towers and cables above. These are then joined with a pin, usually after forcing the union point apart, and when jacks are removed and the bridge decking is added the bridge becomes a truss . Such unsupported construction is only possible where appropriate rock is available to support the tension in the upper chord of the span during construction, usually limiting this method to the spanning of narrow canyons. Arch Bridges

An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. A (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.

Simple compression arch bridges

Stone, brick and other such materials are strong in compression and somewhat so in shear, but cannot resist much force in tension. As a result, masonry arch bridges are designed to be constantly under compression, so far as is possible. Each arch is constructed over a temporary falsework frame, known as a centring. In the first compression arch bridges, a keystone in the middle of the bridge bore the weight of the rest of the bridge. The more weight that was put onto the bridge, the stronger its structure became. Masonry arch bridges use a quantity of fill material (typically compacted rubble) above the arch in order to increase this dead-weight on the bridge and prevent tension from occurring in the arch ring as loads move across the bridge. Other materials that were used to build this type of bridge were brick and unreinforced concrete. When masonry (cut stone) is used the angles of the faces are cut to minimize shear forces. Where random masonry (uncut and

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Factsheet:

unprepared stones) is used they are mortared together Ancestor and the mortar is allowed to set before the falsework is removed.

Descendant Self-anchored suspension bridge Traditional masonry arches are generally durable, and somewhat resistant to settlement or undermining. However, relative to modern alternatives, such Carries Pedestrians, bicycles, bridges are very heavy, requiring extensive foundations. They are also expensive to livestock,automobiles, build wherever labor costs are high trucks, light rail i i An Arch Br dge n Japan

Construction sequence Span range Medium to long

 Where the arches are founded in a stream bed the water is diverted and the gravels excavated to a Material Steel rope, multiple steel wire good footing. From this the foundation piers are strand cables or forged raised to the base of the arches, a point known as the springing. or cast chain links  Falsework centering is fabricated, typically from timbers and boards. Since each arch of a multi- arch bridge will impose a thrust upon its Movable No neighbors, it is necessary either that all arches of the bridge be raised at the same time, or that very wide piers are used. The thrust from the end Design effort Medium arches is taken into the earth by footings at the canyon walls, or by large inclined planes forming No Falseworkrequired ramps to the bridge, which may also be formed of

arches. Longest Span Akashi-Kaikyo Bridge  The several arches are constructed over the (Japan), 1991 m centering. Once the basic arch barrel is constructed, the arches are stabilized with infill masonry between the arches, which may be laid in horizontal running bond courses. These may form two walls, known as the spandrels, which are then infilled with loose material and rubble.  The road is paved and parapet walls protectively confine traffic to the bridge. Suspension Bridges

A suspension bridge is one where cables (or ropes or chains) are strung across the river (or whatever the obstacle happens to be) and the deck is suspended from these cables. Modern suspension bridges have two tall towers through which the cables are strung. Thus, the towers are supporting the majority of the roadway's weight.

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The force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are firmly entrenched.

The supporting cables, running between the two anchorages, are the lucky recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated.

Almost all suspension bridges have, in addition to the cables, a supporting truss system beneath the bridge deck (a deck truss). This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple.

Construction Sequence

Typical suspension bridges are constructed using a sequence generally described as follows. Depending on length and size, construction may take anywhere between a year and a half (construction on the original Tacoma Narrows Bridge took only 19 months) to as many as a decade (the Akashi-Kaikyō Bridge's construction began in May 1986 and was opened in May, 1998 - a total of twelve years). 1. Where the towers are founded on underwater piers, caissons are sunk and any soft bottom is excavated for a foundation. If the bedrock is too deep to be exposed by excavation or the sinking of a caisson, pilings are driven to the bedrock or into overlying hard soil, or a large concrete pad to distribute the weight over less resistant soil may be constructed, first preparing the surface with a bed of compacted gravel. (Such a pad footing can also accommodate the movements of an active fault, and this has been implemented on the foundations of the cable- stayed Rio-Antirio bridge. The piers are then extended above water level, where they're capped with pedestal bases for the towers. 2. Where the towers are founded on dry land, deep foundation excavation or pilings are used. 3. From the tower foundation, towers of single or multiple columns are erected using high- strength reinforced concrete, stonework, or steel. Concrete is used most frequently in modern suspension bridge construction due to the high cost of steel. Large devices called saddles, which will carry the main suspension cables, are positioned atop the towers. Typically of cast steel, they can also be manufactured using riveted forms, and are equipped with rollers to allow the main cables to shift under construction and normal loads. 4. Anchorages are constructed, usually in tandem with the towers, to resist the tension of the cables and form as the main anchor system for the entire structure. These are usually anchored in good quality rock, but may consist of massive reinforced concrete deadweights within an excavation. The anchorage structure will have multiple protruding

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open eyebolts enclosed within a secure space.

5. Temporary suspended walkways, called catwalks, are then erected using a set of guide wires hoisted into place via winches positioned atop the towers. These catwalks follow the curve set by bridge designers for the main cables, in a path mathematically described as acatenary arc. Typical catwalks are usually between eight and ten feet wide, and are constructed using wire grate and wood slats. 6. Gantries are placed upon the catwalks, which will support the main cable spinning reels. Then, cables attached to winches are installed, and in turn, the main cable spinning devices are installed. 7. High strength wire (typically 4 or 6 gauge galvanized steel wire), is pulled in a loop by pulleys on the traveler, with one end affixed at an anchorage. When the traveler reaches the opposite anchorage the loop is placed over an open anchor eyebar. Along the catwalk, workers also pull the cable wires to their desired tension. This continues until a bundle, called a "cable strand" is completed, and temporarily bundled using stainless steel wire. This process is repeated until the final cable strand is completed. Workers then remove the individual wraps on the cable strands (during the spinning process, the shape of the main cable closely resembles a hexagon), and then the entire cable is then compressed by a traveling hydraulic press into a closely packed cylinder and tightly wrapped with additional wire to form the final circular cross section. The wire used in suspension bridge construction is a galvanized steel wire that has been coated with corrosion inhibitors.

8. At specific points along the main cable (each being the exact distance horizontally in relation to the next) devices called "cable bands" are installed to carry steel wire ropes called Suspender cables. Each suspender cable is engineered and cut to precise lengths, and are looped over the cable bands. In some bridges, where the towers are close to or on the shore, the suspender cables may be applied only to the central span. Early suspender cables were fitted with zinc jewels and a set of steel washers, which formed the support for the deck. Modern suspender cables carry a shackle-type fitting. 9. Special lifting hoists attached to the suspenders or from the main cables are used to lift prefabricated sections of bridge deck to the proper level, provided that the local conditions allow the sections to be carried below the bridge by barge or other means. Otherwise, a travelingcantilever derrick may be used to extend the deck one section at a time starting from the towers and working outward. If the addition of the deck structure extends from the towers the finished portions of the deck will pitch upward rather sharply, as there is no downward force in the center of the span. Upon completion of the deck the added load will pull the main cables into an arc mathematically described as aparabola, while the arc of the deck will be as the designer intended ² usually a gentle

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upward arc for added clearance if over a shipping channel, or flat in other cases such as a span over a canyon. Arched suspension spans also give the structure more rigidity and strength. 10. With completion of the primary structure various details such as lighting, handrails, finish painting and paving are installed or completed.

Cable-stayed bridges

A cable-stayed bridge is a bridge that consists of one or more columns (normally referred to as towers or pylons), with cables supporting the bridge deck. There are two major classes of cable-stayed bridges: In a harp design, the cables are made nearly parallel by attaching cables to various points on the tower(s) so that the height of attachment of each cable on the tower is similar to the distance from the tower along the roadway to its lower attachment. In a fan design, the cables all connect to or pass over the top of the tower(s). Compared to other bridge types, the cable-stayed is optimal for spans longer than typically seen in cantilever bridges and shorter than those typically requiring a suspension bridge. This is the range in which cantilever spans would rapidly grow heavier if they were lengthened, and in which suspension cabling does not get more economical were the span to be shortened. Truss bridges

A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by nineteenth and early twentieth century engineers. A truss bridge is economical to construct owing to its efficient use of materials.

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Different Tutes of Truss Bridge

Additional Bridge Forces

There are dozens of forces that must be taken into consideration when designing a bridge. These forces are usually specific to a particular location or bridge design.

Torsion, which is a rotational or twisting force, is one which has been effectively eliminated in all but the largest suspension bridges. The natural shape of the arch and the additional truss structure of the beam bridge have eliminated the destructive effects of torsion on these bridges. Suspension bridges, however, because of the very fact that they are suspended (hanging from a pair of cables), are somewhat more susceptible to torsion, especially in high winds.

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All suspension bridges have deck-stiffening trusses which, as in the case of beam bridges, effectively eliminate the effects of torsion; but in suspension bridges of extreme length, the deck truss alone is not enough. Wind-tunnel tests are generally conducted on models to determine the bridge's resistance to torsional movements. Aerodynamic truss structures, diagonal suspender cables, and an exaggerated ratio between the depth of the stiffening truss to the length of the span are some of the methods employed to mitigate the effects of torsion.

Efficiency

A bridge's structural efficiency may be considered to be the ratio of load carried to bridge mass, given a specific set of material types. In one common challenge students are divided into groups and given a quantity of wood sticks, a distance to span, and glue, and then asked to construct a bridge that will be tested to destruction by the progressive addition of load at the center of the span. The bridge taking the greatest load is by this test the most structurally efficient. A more refined measure for this exercise is to weigh the completed bridge rather than measure against a fixed quantity of materials provided and determine the multiple of this weight that the bridge can carry, a test that emphasizes economy of materials and efficient glue joints.

A bridge's economic efficiency will be site and traffic dependent, the ratio of savings by having a bridge (instead of, for example, a ferry, or a longer road route) compared to its cost. The lifetime cost is composed of materials, labor, machinery, engineering, cost of money, insurance, maintenance, refurbishment, and ultimately, demolition and associated disposal, recycling, and replacement, less the value of scrap and reuse of components. Bridges employing only compression are relatively inefficient structurally, but may be highly cost efficient where suitable materials are available near the site and the cost of labor is low. For medium spans, trusses or box beams are usually most economical, while in some cases, the appearance of the bridge may be more important than its cost efficiency. The longest spans usually require suspension bridges.

Double-decker bridge

Double-decker bridges have two levels, such as the San Francisco ʹ Oakland Bay Bridge, with two road levels. Tsing Ma Bridge and Kap Shui Mun Bridge in Hong Kong have six lanes on their upper decks, and on their lower decks there are two lanes and a pair of tracks for MTR metro trains.

Tower Bridge is different example of a double-decker bridge, with the central section consisting of a low level bascule span and a high levelfootbridge.

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Movable bridge

Some bridges are not fixed crossings, but can move out of the way of boats or other kinds of traffic which, ideally, moves under them, but is sometimes too tall to fit. These are generally electrically powered.

Resonance

Resonance (a vibration in something caused by an external force that is in harmony with the natural vibration of the original thing) is a force which, unchecked, can be fatal to a bridge. Resonant vibrations will travel through a bridge in the form of waves. A very famous example of resonance waves destroying a bridge is the Tacoma Narrows bridge, which fell apart in 1940 in a 40-mph (64-kph) wind. Close examination of the situation suggested that the bridge's deck-stiffening truss was insufficient for the span, but that alone was not the cause of the bridge's demise. The wind that day was at just the right speed, and hitting the bridge at just the right angle, to start it vibrating. Continued winds increased the vibrations until the waves grew so large and violent that they broke the bridge apart. When an army marches across a bridge, the soldiers are often told to "break step." This is to avoid the possibility that their rhythmic marching will start resonating throughout the bridge. An army that is large enough and marching at the right cadence could start a bridge swaying and undulating until it broke apart. In order to mitigate the resonance effect in a bridge, it is important to build dampeners into the bridge design in order to interrupt the resonant waves. Interrupting them is an effective way to prevent the growth of the waves regardless of the duration or source of the vibrations. Dampening techniques generally involve inertia. If a bridge has, for example, a solid roadway, then a resonant wave can easily travel the length of the bridge. If a bridge roadway is made up of different sections that have overlapping plates, then the movement of one section is transferred to another via the plates, which, since they are overlapping, create a certain amount of friction. The trick is to create enough friction to change the frequency of the resonant wave. Changing the frequency prevents the wave from building. Changing the wave effectively creates two different waves, neither of which can build off the other into a destructive force.

Some Special Bridges

 Some bridges carry special installations such as the tower of Nový Most bridge in Bratislava which carries a restaurant. Other suspension bridge towers carry transmission antennas.  A bridge can carry overhead power lines as does the Storstrøm Bridge.  Costs and cost overruns in bridge construction have been studied by Flyvbjerg et al. (2003). The average cost overrun in building a bridge was found to be 34%.

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Tunnel

A tunnel is an underground passageway. The definition of what constitutes a tunnel is not universally agreed upon. Tunnels in general, however, are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, save for the openings at each end. Some civic planners define a tunnel as 0.1 miles (0.16 km) in length or longer, while anything shorter than this should be called an underpass or a chute. A tunnel may be for pedestrians or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are aqueducts, constructed purely for carrying water ² for consumption, for hydroelectric purposes or as sewers ² while others carry other services such as telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species. Some secret tunnels have also been made as a method of entrance or escape from an area, such as the Cu Chi Tunnels or the tunnels connecting the Gaza Strip to Egypt. Some tunnels are not for transport at all but rather, are fortifications, for example Mittelwerk and Cheyenne Mountain. In the United Kingdom, a pedestrian tunnel or other underpass beneath a road is called a subway. This term was used in the past in the United States, but now refers to underground rapid transit systems. The central part of a rapid transit network is usually built in tunnels. To allow non-level crossings, some lines run in deeper tunnels than others. Rail stations with much traffic usually provide pedestrian tunnels from one platform to another, though others use bridges.

Some structures may require excavation similar to tunnel excavation, but are not actually tunnels. Shafts, for example, are often hand-dug or dug with boring equipment. But unlike tunnels, shafts are vertical and shorter. Often, shafts are built either as part of a tunnel project to analyze the rock or soil, or in tunnel construction to provide headings, or locations, from which a tunnel can be excavated. The diagram below shows the relationship between these underground structures in a typical mountain tunnel. The opening of the tunnel is a portal. The "roof" of the tunnel, or the top half of the tube, is thecrown. The bottom half is the invert. The basic geometry of the tunnel is a continuous arch. Because tunnels must withstand tremendous pressure from all sides, the arch is an ideal shape. In the case of a tunnel, the arch simply goes all the way around.

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Tunnel engineers, like bridge engineers, must be concerned known as statics. Statics describes how the following forces interact to produce equilibrium on structures such as tunnels and bridges: y Tension, which expands, or pulls on, material y Compression, which shortens, or squeezes material y Shearing, which causes parts of a material to slide past one another in opposite directions y Torsion, which twists a material The tunnel must oppose these forces with strong materials, such as masonry, steel, iron and concrete.

In order to remain static, tunnels must be able to withstand the loads placed on them. Dead load refers to the weight of the structure itself, while live load refers to the weight of the vehicles and people that move through the tunnel. Types of Tunnels

There are three broad categories of tunnels: mining, public works and transportation.. Mine tunnels are used during ore extraction, enabling laborers or equipment to access mineral and metal deposits deep inside the earth. These tunnels are made using similar techniques as other types of tunnels, but they cost less to build. Mine tunnels are not as safe as tunnels designed for permanent occupation, however.

Public works tunnels carry water, sewage or gas lines across great distances. The earliest tunnels were used to transport water to, and sewage away from, heavily populated regions. Roman engineers used an extensive network of tunnels to help carry water from mountain springs to cities and villages. These tunnels were part of aqueduct systems, which also comprised underground chambers and sloping bridge-like structures supported by a series of arches. By A.D. 97, nine aqueducts carried approximately 85 million gallons of water a day from mountain springs to the city of Rome. Before there were trains and cars, there were transportation tunnels such as canals -- artificial waterways used for travel, shipping or irrigation. Just like railways and roadways today, canals usually ran above ground, but many required tunnels to pass efficiently through an obstacle, such as a mountain. Canal construction inspired some of the world's earliest tunnels. The Underground Canal, located in Lancashire County and Manchester, England, was constructed from the mid- to late- 1700s and includes miles of tunnels to house the underground

12 canals. One of America's first tunnels was the Paw Paw Tunnel, built in West Virginia between 1836 and 1850 as part of the Chesapeake and Ohio Canal. Although the canal no longer runs through the Paw Paw, at 3,118 feet long it is still one of the longest canal tunnels in the United States. By the 20th century, trains and cars had replaced canals as the primary form of transportation, leading to the construction of bigger, longer tunnels. The Holland Tunnel, completed in 1927, was one of the first roadway tunnels and is still one of the world's greatest engineering projects. Named for the engineer who oversaw construction, the tunnel ushers nearly 100,000 vehicles daily between and New Jersey. Construction

Tunnel Planning Almost every tunnel is a solution to a specific challenge or problem. In many cases, that challenge is an obstacle that a roadway or railway must bypass. They might be bodies of water, mountains or other transportation routes. Even cities, with little open space available for new construction, can be an obstacle that engineers must tunnel beneath to avoid. In the case of the Holland Tunnel, the challenge was an obsolete ferry system that strained to transport more than 20,000 vehicles a day across the Hudson River. For New York City officials, the solution was clear: Build an automobile tunnel under the river and let commuters drive themselves from New Jersey into the city. The tunnel made an immediate impact. On the opening day alone, 51,694 vehicles made the crossing, with an average trip time of just 8 minutes. C S Sometimes, tunnels offer a safer solution than other onstruction of the eikan Tunnel structures. The Seikan Tunnel in Japan was built involved a 24-year struggle to because ferries crossing the Tsugaru Strait often overcome challenges posed by soft encountered dangerous waters and weather conditions. rock under the sea. After a typhoon sank five ferryboats in 1954, the Japanese government considered a variety of solutions. They decided that any bridge safe enough to withstand the severe conditions would be too difficult to build. Finally, they proposed a railway tunnel running almost 800 feet below the sea surface. Ten years later, construction began, and in 1988, the Seikan Tunnel officially opened. How a tunnel is built depends heavily on the material through which it must pass. Tunneling through soft ground, for instance, requires very different techniques than tunneling through hard rock or soft rock, such as shale, chalk or sandstone. Tunneling underwater, the most challenging of all environments, demands a unique approach that would be impossible or impractical to implement above ground.

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That's why planning is so important to a successful tunnel project. Engineers conduct a thorough geologic analysis to determine the type of material they will be tunneling through and assess the relative risks of different locations. They consider many factors, but some of the most important include:

y Soil and rock types y Weak beds and zones, including faults and shear zones y Groundwater, including flow pattern and pressure y Special hazards, such as heat, gas and fault lines Often, a single tunnel will pass through more than one type of material or encounter multiple hazards. Good planning allows engineers to plan for these variations right from the beginning, decreasing the likelihood of an unexpected delay in the middle of the project. Once engineers have analyzed the material that the tunnel will pass through and have developed an overall excavation plan, construction can begin. The tunnel engineers' term for building a tunnel is driving, and advancing the passageway can be a long, tedious process that requires blasting, boring and digging by hand. Tunnel Construction Workers generally use two basic techniques to advance a tunnel. In the full-face method, they excavate the entire diameter of the tunnel at the same time. This is most suitable for tunnels passing through strong ground or for building smaller tunnels. The second technique, shown in the diagram below, is thetop-heading-and-bench method. In this technique, workers dig a smaller tunnel known as a heading. Once the top heading has advanced some distance into the rock, workers begin excavating immediately below the floor of the top heading; this is a bench. One advantage of the top-heading-and-bench method is that engineers can use the heading tunnel to gauge the stability of the rock before moving forward with the project.

Soft Ground (Earth) Workers dig soft-ground tunnels through clay, silt, sand, gravel or mud. In this type of tunnel, stand-up time -- how long the ground will safely stand by itself at the point of excavation -- is of paramount importance. Because stand-up time is generally short when tunneling through soft ground, cave-ins are a constant threat. To prevent this from happening, engineers use a special piece of equipment called ashield. A shield is an iron or steel cylinder literally pushed into the soft soil. It carves a perfectly round hole and supports the surrounding earth while workers remove debris and install a permanent lining made of cast iron or precast concrete. When the workers complete a section, jacks push the shield forward and they repeat the process.

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Marc Isambard Brunel, a French engineer, invented the first tunnel shield in 1825 to excavate the Thames Tunnel in London, England. Brunel's shield comprised 12 connected frames, protected on the top and sides by heavy plates called staves. He divided each frame into three workspaces, or cells, where diggers could work safely. A wall of short timbers, or breasting boards, separated each cell from the face of the tunnel. A digger would remove a breasting board, carve out three or four inches of clay and replace the board. When all of the diggers in all of the cells had completed this process on one section, powerful screw jacks pushed the shield forward. In 1874, Peter M. Barlow and James Henry Greathead improved on Brunel's design by constructing a circular shield lined with cast-iron segments. They first used the newly-designed shield to excavate a second tunnel under the Thames for pedestrian traffic. Then, in 1874, the shield was used to help excavate the London Underground, the world's first subway. Greathead further refined the shield design by adding compressed air pressure inside the tunnel. When air pressure inside the tunnel exceeded water pressure outside, the water stayed out. Soon, engineers in New York, Boston, Budapest and Paris had adopted the Greathead shield to build their own subways. Hard Rock Tunneling through hard rock almost always involves blasting. Workers use a scaffold, called a jumbo, to place explosives quickly and safely. The jumbo moves to the face of the tunnel, and drills mounted to the jumbo make several holes in the rock. The depth of the holes can vary depending on the type of rock, but a typical hole is about 10 feet deep and only a few inches in diameter. Next, workers pack explosives into the holes, evacuate the tunnel and detonate the charges. After vacuuming out the noxious fumes created during the explosion, workers can enter and begin carrying out the debris, known as muck, using carts. Then they repeat the process, which advances the tunnel slowly through the rock. Fire-setting is an alternative to blasting. In this technique, the tunnel wall is heated with fire, and then cooled with water. The rapid expansion and contraction caused by the sudden temperature change causes large chunks of rock to break off. The Cloaca Maxima, one of Rome's oldest sewer tunnels, was built using this technique. The stand-up time for solid, very hard rock may measure in centuries. In this environment, extra support for the tunnel roof and walls may not be required. However, most tunnels pass through rock that contains breaks or pockets of fractured rock, so engineers must add additional support in the form of bolts, sprayed concrete or rings of steel beams. In most cases, they add a permanent concrete lining.

Soft Rock

Tunneling through soft rock and tunneling underground require different approaches. Blasting in soft, firm rock such as shale or limestone is difficult to control. Instead, engineers use tunnel- boring machines (TBMs), or moles, to create the tunnel. TBMs are enormous, multimillion- dollar pieces of equipment with a circular plate on one end. The circular plate is covered with disk cutters -- chisel-shaped cutting teeth, steel disks or a combination of the two. As the

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circular plate slowly rotates, the disk cutters slice into the rock, which falls through spaces in the cutting head onto a conveyor system. The conveyor system carries the muck to the rear of the machine. Hydraulic cylinders attached to the spine of the TBM propel it forward a few feet at a time. TBMs don't just bore the tunnels -- they also provide support. As the machine excavates, two drills just behind the cutters bore into the rock. Then workers pump grout into the holes and attach bolts to hold everything in place until the permanent lining can be installed. The TBM accomplishes this with a massive erector arm that raises segments of the tunnel lining into place.

A TBM used in the construction of Yucca Mountain Repository, a U.S. Department of Energy terminal storage facility

Underwater Tunnels built across the bottoms of rivers, bays and other bodies of water use the cut-and-cover method, which involves immersing a tube in a trench and covering it with material to keep the tube in place. Construction begins by dredging a trench in the riverbed or ocean floor. Long, prefabricated tube sections, made of steel or concrete and sealed to keep out water, are floated to the site and sunk in the prepared trench. Then divers connect the sections and remove the seals. Any excess water is pumped out, and the entire tunnel is covered with backfill. The tunnel connecting England and France -- known as the Channel Tunnel, the Euro Tunnel or Chunnel -- runs beneath the English Channel through 32 miles of soft, chalky earth. Although it's one of the longest tunnels in the world, it took just three years to excavate, thanks to state-of-the-art TBMs. Eleven of these massive machines chewed through the seabed that lay beneath the Channel. Why so many? Because the Chunnel actually consists of three parallel tubes, two that carry trains and one that acts as a service tunnel. Two TBMs placed on opposite ends of the tunnel dug each of these tubes. In essence, the three British TBMs raced against the three French TBMs to see who would make it to the middle first. The

16 remaining five TBMs worked inland, creating the portion of the tunnel that lay between the portals and their respective coasts. Unless the tunnel is short, control of the environment is essential to provide safe working conditions and to ensure the safety of passengers after the tunnel is operational. One of the most important concerns is ventilation -- a problem magnified by waste gases produced by trains and automobiles. Clifford Holland addressed the problem of ventilation when he designed the tunnel that bears his name. His solution was to add two additional layers above and below the main traffic tunnel. The upper layer clears exhaust fumes, while the lower layer pumps in fresh air. Four large ventilation towers, two on each side of the Hudson River, house the fans that move the air in and out. Eighty-four fans, each 80 feet in diameter, can change the air completely every 90 seconds. Other tunneling methods include: y Drilling and blasting y Slurry-shield machine y Wall-cover construction method.

Based on the setting, tunnels can be divided into three major types:

Soft-ground tunnels are typically shallow and are often used as subways, water-supply systems, and sewers. Because the ground is soft, a support structure, called a tunnel shield, must be used at the head of the tunnel to prevent it from collapsing.

Rock tunnels require little or no extra support during construction and are often used as railways or roadways through mountains. Years ago, engineers were forced to blast through mountains with dynamite. Today they rely on enormous rock-chewing contraptions called tunnel boring machines.

Underwater tunnels are particularly tricky to construct, as water must be held back while the tunnel is being built. Early engineers used pressurized excavation chambers to prevent water from gushing into tunnels. Today, prefabricated tunnel segments can be floated into position, sunk, and attached to other sections. Choice of tunnels vs. bridges

For water crossings, a tunnel is generally more costly to construct than a bridge. Navigational considerations may limit the use of high bridges or spans intersecting with shipping channels, necessitating a tunnel. Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in tunnels' favor. Boston's Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.

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The 1934 Queensway Road Tunnel under the River Mersey at Liverpool, was chosen over a massively high bridge for defence reasons. It was feared aircraft could destroy a bridge in times of war. Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, and the , the 1934 River Mersey road Queensway Tunnel and the Westerschelde tunnel, Zeeland, Netherlands. Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather and shipping during construction (as in the 51.5-kilometre or 32.0 mi Channel Tunnel), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge). Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in the eastern United States. Hazards

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in Switzerland in 2001. One of the worst railway disasters ever, the Balvano train disaster, was caused by a train stalling in the Armi tunnel in Italy in 1944, killing 426 passengers. Notable Tunnels

 The Fredhälls Tunnel in Stockholm, Sweden, and the New Elbe Tunnel in Hamburg, Germany, both with around 150,000 vehicles a day, two of the most trafficked tunnels in the world.  The Lincoln Tunnel between New Jersey and New York is one of the busiest vehicular tunnels in America, at 120,000 vehicles/day, although the Central Artery Tunnel in Boston probably has around 200,000 vehicles/day..  New York City Water Tunnel No. 3[2], started in 1970, has an expected completion date of 2020.  The Chicago Deep Tunnel Project is a network of 109 mi (175 km) of tunnels designed to reduce flooding in the Chicago area. Started in the mid 1970s, the project is due to be completed in 2019.  The Fenghuoshan tunnel on Qinghai-Tibet railway is the world's highest railway tunnel, about 4,905 m (16,093 ft) above sea level.

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Road

A road is an identifiable thoroughfare, route, way or path between two places which may or may not be available for use by the public; public roads, especially major roads connecting significant destinations are termed highways. Modern roads are normally smoothed, paved, or otherwise prepared to allow easy travel although historically many roads were simply recognizable routes without any formal construction or maintenance.

Historical road construction

A Greek street from the 3rd to 4th century BC in Velia, Italy. The Porta Rosa was the main street of Elea. It is paved with limestone blocks, with a gutter for the drainage of rain water.

The first pathways were the trails made by animals has not been universally accepted, arguing that animals do not follow constant paths. Others believe that some roads originated from following animal trails.TheIcknield Way is given as an example of this type of road origination, where man and animal both selected the same natural line. By about 10,000 BC, rough pathways were used by human travelers.

Construction

Road construction requires the creation of a continuous right-of-way, overcoming geographic obstacles and having grades low enough to permit vehicle or foot travel and may be required to meet standards set by law or official guidelines. The process is often begun with the removal of earth and rock by digging or blasting, construction of embankments, bridges and tunnels, and removal of vegetation (this may involve deforestation) and followed by the laying of pavement material. A variety of road building equipment is employed in road building

After design, approval, planning, legal and environmental considerations have been addressed alignment of the road is set out by a surveyor. The Radii and gradient are designed and staked out to best suit the natural ground levels and minimize the amount of cut and fill. Great care is taken to preserve reference Benchmarks. controls are constructed to prevent detrimental effects. Drainage lines are laid with sealed joints in the road easement with runoff coefficients and characteristics adequate for the land zoning and storm water system. Drainage systems must be capable of carrying the ultimate design flow from

19 the upstream catchment with approval for the outfall from the appropriate authority to a watercourse, creek, river or the sea for drainage discharge.

A Borrow pit (source for obtaining fill, gravel, and rock) and a water source should be located near or in reasonable distance to the road construction site. Approval from local authorities may be required to draw water or for working (crushing and screening) of materials for construction needs. The top soil and vegetation is removed from the borrow pit and stockpiled for subsequent rehabilitation of the extraction area. Side slopes in the excavation area not steeper than one vertical to two horizontal for safety reasons.

Old road surfaces, fences, and buildings may need to be removed before construction can begin. Trees in the road construction area may be marked for retention. These protected trees should not have the topsoil within the area of the tree's drip line removed and the area should be kept clear of construction material and equipment. Compensation or replacement may be required if a protected tree is damaged. Much of the vegetation may be mulched and put aside for use during reinstatement. The topsoil is usually stripped and stockpiled nearby for rehabilitation of newly constructed embankments along the road. Stumps and roots are removed and holes filled as required before the earthwork begins. Final rehabilitation after road construction is completed will include seeding, planting, watering and other activities to reinstate the area to be consistent with the untouched surrounding areas.

Processes during earthwork include excavation, removal of material to spoil, filling, compacting, construction and trimming. If rock or other unsuitable material is discovered it is removed, moisture content is managed and replaced with standard fill compacted to 90% relative compaction. Generally blasting of rock is discouraged in the road bed. When a depression must be filled to come up to the road grade the native bed is compacted after the topsoil has been removed. The fill is made by the "compacted layer method" where a layer of fill is spread then compacted to specifications, the process is repeated until the desired grade is reached.

General fill material should be free of organics, meet minimum California bearing ratio (CBR) results and have a low plasticity index. The lower fill generally comprises sand or a sand-rich mixture with fine gravel, which acts as an inhibitor to the growth of plants or other vegetable matter. The compacted fill also serves as lower-stratum drainage. Select second fill (sieved) should be composed of gravel, decomposed rock or broken rock below a specified Particle size and be free of large lumps of clay. Sand clay fill may also be used. The road bed must be "proof rolled" after each layer of fill is compacted. If a roller passes over an area without creating visible deformation or spring the section is deemed to comply.

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The completed road way is finished by paving or left with a gravel or other natural surface. The type of road surface is dependent on economic factors and expected usage. Safety improvements like Traffic signs, Crash barriers, Raised pavement markers, and other forms of Road surface marking are installed.

When a single carriageway road is converted into dual carriageway by building a second separate carriageway alongside the first, it is usually referred to as duplication, twinning or doubling. The original carriageway is changed from two-way to become one-way, while the new carriageway is one-way in the opposite direction. In the same way as converting railway lines from single track to double track, the new carriageway is not always constructed directly alongside the existing carriageway.

Maintenance

Like all structures, roads deteriorate over time. Deterioration is primarily due to accumulated damage from vehicles, however environmental effects such as frost heaves, thermal cracking and oxidation often contribute. According to a series of experiments carried out in the late 1950s, called the AASHO Road Test, it was empirically determined that the effective damage done to the road is roughly proportional to the 4th power of axle weight. A typical tractor-trailer weighing 80,000 pounds (36.287 t) with 8,000 pounds (3.6287 t) on the steer axle and 36,000 pounds (16.329 t) on both of the tandem axle groups is expected to do 7,800 times more damage than a passenger vehicle with 2,000 pounds (0.907 t) on each axle. Potholes on roads are caused by rain damage and vehicle braking or related construction works.

Pavements are designed for an expected service life or design life. In some UK countries the standard design life is 40 years for new bitumen and concrete pavement. Maintenance is considered in the whole life cost of the road with service at 10, 20 and 30 year milestones. Roads can be and are designed for a variety of lives (8-, 15-, 30-, and 60-year designs). When pavement lasts longer than its intended life, it may have been overbuilt, and the original costs may have been too high. When a pavement fails before its intended design life, the owner may have excessive repair and rehabilitation costs. Many concrete pavements built since the 1950s have significantly outlived their intended design lives. Some roads like Chicago, Illinois's "Wacker Drive", a major two-level viaduct in downtown area are being rebuilt with a designed service life of 100 years.

Virtually all roads require some form of maintenance before they come to the end of their service life. Pro-active agencies continually monitor road conditions and apply preventive maintenance treatments as needed to prolong the lifespan of their roads. Technically advanced agencies monitior the road network surface condition with sophisticated equipment such as laser/inertial Profilometers. These measurements include road curvature, cross slope, unevenness, roughness, rutting and texture (roads). This data is fed into a pavement management system, which

21 recommends the best maintenance or construction treatment to correct the damage that has occurred.

Maintenance treatments for asphalt concrete generally include crack sealing, surface rejuvenating, fog sealing, micro-milling and surface treatments. Thin surfacing preserves, protects and improves the functional condition of the road while reducing the need for routing maintenance, leading to extended service life without increasing structural capacity

Slab Stabilization

Distress and serviceability loss on concrete roads can be caused by loss of support due to voids beneath the concrete pavement slabs. The voids usually occur near cracks or joints due to surface water infiltration. The most common causes of voids are pumping, consolidation, subgrade failure and bridge approach failure. Slab stabilization is a non-destructive method of solving this problem and is usually employed with other Concrete Pavement Restoration (CPR) methods including patching and diamond grinding. The technique restores support to concrete slabs by filing small voids that develop underneath the concrete slab at joints, cracks or the pavement edge. The process consists of pumping a cementitous grout or polyurethane mixture through holes drilled through the slab. The grout can fill small voids beneath the slab and/or sub-base. The grout also displaces free water and helps keep water from saturating and weakening support under the joints and slab edge after stabilization is complete. The three steps for this method after finding the voids are locating and drilling holes, grout injection and post-testing the stabilized slabs.

Common stabilization materials are pozzolan-cement grout and polyurethane. The requirements for slab stabilization are strength and the ability to flow into or expand to fill small voids. Colloidal mixing equipment is necessary to use the pozzolan-cement grouts. The contractor should place the grout using a positive-displacement injection pump or a non-pulsing progressive cavity pump. A drill is also necessary but it must produce a clean hole with no surface spalling or breakouts. The injection devices must include a grout packer that is capable of sealing a hole. The injection device must also have a return hose or a fast-control reverse switch in case workers detect slab movement on the uplift gauge. The uplift beam helps to monitor the slab deflection and has to have sensitive dial gauges.

Road safety

For major roads risk can be reduced by providing limited access from properties and local roads, grade separated junctions and Median dividers between opposite-direction traffic to reduce likelihood of head-on collisions. The placement of energy attenuation devices (e.g. guardrails, wide grassy areas, sand barrels). Some road fixtures such as road signs and fire hydrants are designed to collapse on impact. Light poles are designed to break at the base rather than violently stop a car that hits them.

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Airport

An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport. An airport consists of at least one surface such as a runway for a plane to take off and land, a helipad, or water for take offs andlandings, and often includes buildings such as control towers, hangars and terminal buildings. Larger airports may have fixed base operator services, seaplane docks and ramps, air traffic control, passenger facilities such as restaurants and lounges, and emergency services. A military airport is known as an airbase or air station. The terms aerodrome, airdrome, airfield, andairstrip may also be used to refer to airports, and the terms heliport, seaplane base, and STOLport refer to airports dedicated exclusively to helicopters, seaplanes, or short take-off and landing aircraft.

Planning

Any major airport has lots of customers, most of them passengers. Atlanta's Hartsfield International Airport, for example, handles 2,400 flights every day (one flight every 40 seconds, 24 hours a day!) carrying hundreds of thousands of people. That adds up to 72-million domestic and 78-million international passengers passing through Hartsfield each year. That's a lot of people, and most of those 150-million are going to want to grab a bite, use the restroom, maybe buy a magazine. To meet passengers' needs, an airport must:

y be accessible by roadways and public transportation, plus have plenty of parking y have areas for ticketing, check-in and baggage handling y keep the passengers safe y offer food and other services y maintain areas for the customs service Airports have other customers to take care of, too.

y Airlines need space for airplanes, facilities for routine maintenance, jet fuel and places for passengers and flight crews while on the ground. y Air-freight companies need space for loading and unloading cargo airplanes.

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y Pilots and other crew members need runways, aircraft fuel, air-traffic information, facilities for aircraft storage and maintenance and places to relax while on the ground.

Ground Transportation An airport can't exist in isolation. It depends on a massive surface-transportation system so that people can get to and from the airport, park and get from place to place within the airport structure itself. While your first thought about an airport is air travel, ground transportation is pretty crucial to an aiport's operation. The busiest airport in the world is Atlanta's Hartsfield International Airport. Here are some ways ground transportation is critical to that airport:

y Roads allow access to and from the airport: In Atlanta, four interstate highways move traffic to and from Hartsfield. There's also a station for MARTA trains to connect into the city's rapid transit system. y Parking allows short- and long-term storage of automobiles. Parking can be on or off airport grounds, and some parking systems are run by private vendors under airport regulation. Hartsfield has 30,000 public parking spaces. y Passenger drop-off and pick-up areas make it easier for passengers to get into the terminals, although they are often plagued by traffic congestion because so many people are trying to get in and out. y Rental car companies serve airports. Hartsfield has eight rental car companies on airport grounds and another three off airport grounds. y Shuttle services provide passengers with transportation to local hotels and off-site parking facilities. Hartsfield is served by 18 hotel/motel shuttle buses. y Private transportation is available in the form of limousines, vans and taxis. y Public transportation (such as municipal buses and subways) may have stations at an airport. Besides the MARTA station at Hartsfield, 12 bus lines (public and private) serve the airport. y Internal subway trains and trams may be available to help passengers get to the terminal gates from the concourse. Hartsfield's People Mover is a 3.5-mile (5.6-km) loop track that has 13 stations serving six concourses with nine four-car trains; the trip is two minutes between stations. Terminals While the terms are often used interchangeably, concourses are defined as the long halls and large, open areas where you'll find shops, restaurants and lounges, and terminals as long halls lined by the gates where you board and disembark airplanes. Atlanta's Hartsfield airport has 5.7-million square feet (529,547 square meters) of concourses and terminals -- that's 130 acres!

Most of the time, and in most airports, concourse areas are accessible to the general public (passengers and non- passengers). The gate areas may be restricted by airport security to ticket-holding passengers only, especially during alerts (for instance, during the Gulf War, non-

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passengers could not pass security points). Generally, airport security and/or customs lie between the concourse and the gates.

Airline freight and private air-freight services such as Fed Ex and DHL may have their own terminals at the airport. Gates The gates are where the airplanes park for passenger boarding and deplaning. Passengers wait in the immediate area of each gate to board the plane. Gates are rented by each airline from the airport authority, and some airlines may rent a whole terminal building in their "hub" airport, in which case the rental fee alone can run into the millions of dollars. Routine airplane maintenance, such as washing, de-icing and refueling, is done by airline personnel while the plane is parked at the gate. In some cases, other maintenance tasks might be performed at the gate, possibly with passengers onboard the plane -- it is not uncommon to sit on a plane at the gate while maintenance personnel replace something like a hydraulic brake line on an aircraft.

Runways

Runways are amazing -- a typical one is about 2 miles long, as wide as a 16-lane highway and about 3 feet thick! Runways have to be specially constructed to take the strain of a huge aircraft like Boeing 747 or 767 without cracking or, worse, buckling. As they're designing runways, engineers have to consider the number of wheels an airplane has, how far apart those wheels are and the size of the tires. As planes get bigger and bigger, runways have to be re-built to accommodate the increased stresses. When the Denver International Airport was built, it took 2.5-million cubic yards of concrete to create five 12,000- foot runways, plus taxiways and aprons. First, 6 feet of compacted soil was put down; then, a foot-deep layer of soil was spread, topped by an 8-inch-thick cement-treated base; that was followed by 17 inches of concrete paving. Main runways are usually oriented to line up with the prevailing wind patterns so that airplanes can take-off into the wind and land with it. Local and ground air traffic controllers determine which runways are used for take-off and which for landing, taking into account weather, wind and air-traffic conditions. In some airports, main runways cross each other, so the controllers have to pay even closer attention. Planes use taxi runways to get from the gate to a main runway for take-off and from a main runway to the gate after landing. Ground controllers direct ground traffic from the airport's tower.

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Conclusion

Transportation is the key for dynamic society. Bridges connects two lands across obstacles like rivers. Tunnels cut through Mountains and running under sea provides faster mode of transport. Transportation Engineering projects make better society. Bridges, tunnels, airports not only provide efficient way of communication but serves aesthetic purposes also.

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Reference

1. Google 2. Wikipedia.org 3. www.About.com 4. www.howstaffswork.com 5. www.eHOW.com 6. en.structurae.de/ 7. whatiscivilengineering.csce.ca/ 8. http://www.ite.org/ , Institute of Transportation Engineers 9. Brown, David J. Bridges: Three Thousand Years of Defying Nature. Richmond Hill, Ont: Firefly Books, 2005. ISBN 1-55407-099-6. 10. Aakaar, society of Civil Engineers, IIT Bombay 11. Railway Tunnels in Queensland by Brian Webber, 1997

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