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Tunnel

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Transporation tunnels such as the Boston Interstate 1-93 are the most common type of tunnel constructed today.
Transporation tunnels such as the Boston Interstate 1-93 are the most common type of tunnel constructed today.
A tunnel can be best described as a hollow underground passageway carved out through soil or stone. Tunnels, or more appropriately, man-made tunnels, are excavated using a variety of means that include manual labor, explosives, rapid heating and cooling, tunneling machinery, or any combination thereof.

Man-made tunnels were first constructed during the Roman era to carry water. In the 17th century, tunnels were incorporated in the structural design of complex canal systems throughout Europe. These canals were used as the primary means of transporting and moving goods.

With the invention of steam and eventually steam locomotives, extensive railway systems were being built, replacing canals as the new form of transportation. Later on, the invention of the automobile also precipitated a need for new and better roadway infrastructures including new highways, bridges and tunnels. The Holland Tunnel, constructed in 1927, was one of the world’s first roadway tunnels.[1]

Tunneling methods have come a long way since then. A tunnel can be directly constructed under the bustling city and high-rise buildings of Boston, as was the case in the Central Artery Tunnel Project. Tunnels have also been constructed deep under the ocean’s surface, through treacherous bodies of water in place of bridges, such as the infamous Channel Tunnel connecting France to the United Kingdom, and currently the world’s longest undersea tunnel, the Seikan Tunnel that  connects the islands of Honshu and Hoikkaido.

Tunnels today are constructed primarily for use in mining, public works, and transportation. Mining tunnels are built deep inside the earth for the purpose of extracting iron ore and enabling workers and equipment access to the ore. Mining tunnels and mine shafts are not always permanent and pose a higher safety risk than tunnels constructed for permanent occupation. Public works tunnels carry sewage or water and gas lines across long distances. The tunnels built by the Romans were the earliest use of tunnels for this purpose. The most common type of a tunnel is a transportation tunnel.

Contents

[edit] History

Many ancient civilizations hand dug tunnels using rudimentary digging tools such as copper rock saws. Fire was used to heat up rock obstructions. Dousing the rock after it was heated would cause it to crack, making it easier to move. Another tunneling technique still used widely today called the cut-and-cover method was used in Babylon more than 4,000 years ago.[2]

The ancient Romans were among the first to become skilled tunnel builders. This involved the creation of extensive networks of tunnels. Sloping arched structures called aqueducts, in conjunction with carved out underground chambers were used to facilitate the transport of water and the removal of waste products away from city centers.[3]

Some of these underground passages extended for kilometers. For instance, a 3.5-mile (5.6-km) long tunnel was constructed for draining Fucino Lake just east of Rome. The project was undertaken to impede the flooding of the area and require the physical labor of 30,000 slaves for the duration of 10 years.[4]

[edit] Canal Tunnels

By the 17th century, tunnels were being built to support canal infrastructure.[5] Canals were the primary means by which materials were moved from the country to the city and therefore, existed as the most viable way to transport goods over long distances. Sometimes the canals, constructed above ground, would have to pass through a mountain or large hill. The only way to overcome this barricade was to build a tunnel. As a result, canal construction inspired some of the world’s earliest tunnels.[6]Gunpowder was used to blast a long canal tunnel in France in 1681 that was 515 feet (157 m) long.[7] The use of gunpowder to blast away debris was one of the first advances made in tunneling that went beyond basic hand digging.

[edit] Railway Tunnels

The use of steam-powered locatives led to the need to dig railway tunnels. Earlier knowledge in constructing channel or canal tunnels was applied to the process. Railway tunnel construction was done manually and the excavated debris was carted away by horses. In the construction of longer tunnels, wells were often routinely built along the tunnel route in several different places. This reduced the time spent carrying out tunneling projects.[8]

Two substantial developments in tunneling occurred in the 1850s that revolutionized the construction of railway tunnels in the latter part of the century. The first development was that nitroglycerine replaced gunpowder as a blasting mechanism. Secondly, in combination with nitroglycerine, steam and compressed air were used to power drills. The drills bore holes into rock that would then be filled with explosive charges and detonated.[9]

[edit] Tunneling Underwater

One of the first under river roadway tunnels, the Rotherhithe Tunnel, was constructed beneath the Thames River in London.[10] The tunnel, started in 1825, was initially constructed to serve horse-drawn traffic. In its time, it was the longest underwater tunnel in existence. It was not built without complication—an overflow of water and mud into the tunnel delayed its completion until 1843.[11]

The excavation of the tunnel was reliant on the first tunneling shield ever used in the construction of a tunnel through soft ground. The shield, invented by a French engineer by the name of March Isambard Brunel, was comprised of 12 connected frames protecting on the top and sided by heavy plates called staves. Each of these frames was divided into a separate workspace or cell permitting workers to excavate under safer conditions. Breasting boards, a wall of short timbers, separated each cell from the tunnel face. A digger would remove the breasting board, excavate a few inches of clay and then replace the board. When all the diggers in each cell completed this process in one section, screw jacks were used to push the shield forward.[12]
The world's first subway tunnels were built in the UK starting in the early 1870s.
The world's first subway tunnels were built in the UK starting in the early 1870s.

In 1874 two engineers by the names of Peter M. Barlow and James Henry Greathead improved on Brunel’s existing design by producing a circular shield lined with pre-cast iron segments. This shield was used in carving out a second pedestrian tunnel under the Thames River. Also in 1874 the shield was instrumental in excavating the London Underground, the world’s very first subway system. The addition of compressed air pressure refined the shield’s capability to generate air pressure that exceeded the pressure of water outside the tunnel, thereby minimizing water leakage. The shield, eventually referred to as the Greathead shield, was quickly adopted by engineers in the construction of Subway tunnels for systems in New York, Boston, Paris, and Budapest.[13] 

[edit] A Tunneling Breakthrough: TBMs

Perhaps the the most substantial advancement made in tunneling technology, in addition to the the shield, was the invention of tunnel boring machines (TBMs). Tunnel boring machines were first used in the latter part of the 19th century. The first tunneling machine to bore through rock was invented by an American engineer by the name of Charles Wilson in 1851. Early TBMs were used in the late 1870s, when the ambitious construction of an underwater tunnel, the Channel Tunnel, began under the English Channel in an effort to link France to the United Kingdom. Tunnel boring machines were used to excavate and bore the first holes, replacing manual labor. Politics and financing ultimately lead to simultaneous setbacks in the tunnel’s final completion until 1994.[14] It wasn't until the early 1950s, however, that James S. Robbins redesigned and manufactured a TBM measuring 25.5 feet (7.8 m) in diameter and 149 kW that the successful application of using TBMs to bore through hard rock became more common and progress in TBM technology was made.

[edit] Process

Each tunnel constructed exists as an answer to a particular obstacle that needs to be by-passed such as a mountain, body of water, or even a city. In some situations, a tunnel is chosen over another structure such as a bridge because it stands as a safer alternative. For example, a tunnel was the obvious choice for Japanese officials when they sought a means to safely cross the Tsugaru Strait, a body of treacherous water often hit by unpredictable weather conditions. In 1954, five ferries sank when a typhoon swept through the strait. The Japanese government first proposed a number of solutions but finally decided on an underwater tunnel deeming a bridge to dangerous to construct. The result was the Seikan Tunnel that opened in 1988.[15]

[edit] The Parts of a Tunnel

A tunnel is basically a continuous arch. This shape is the most ideal form for a tunnel to assume because it enables a tunnel to withstand intense external pressure from all sides. A tunnel usually consists of an opening called the portal or face,a roof part referred to as the crown,and a bottom, called the invert.Tunnels can be excavated through the portal or face of the tunnel or through a shaft,a shorter, vertical passageway in the roof of the tunnel. Shafts are built in the construction of tunnels to provide an access point to conduct geological testing of the soil or rock bed or for excavation.[16]

[edit] Engineering Expertise

The construction of a tunnel is no easy feat. It requires the expertise of engineers with knowledge in the field of statics,a specialized area of physics that deals with how forces interact with each other to produce equilibrium on structures such as tunnels and bridges. These forces include tension, compression, shearing and torsion. Tension refers to that which expands or pulls on the material, compression, that which shortens or squeezes material, shearing, that which causes materials to slide past one another in opposite directions and torsion,that which causes material to twist. This is vital information to know when constructing a tunnel because the structure must be able to withstand the weight of the load that is placed on it. The term load encompasses dead load, the actual weight of the tunnel, and live load, the weight of people and vehicles moving through the tunnel.[17] 

[edit] Tunnel Planning

The first stage in constructing a tunnel is to determine what kind of ground conditions the tunnel is going to be excavated through. This involves taking and analyzing soil and rock samples to determine the earth’s composition. Preliminary probe drilling of test holes is conducted to accomplish this phase and assess the relative risks. Sometimes a smaller tunnel may actually be excavated along the same route as the main tunnel. This way rock layers can be investigated more thoroughly. Some of the more important factors engineers will look at include but are not limited to:

  • soil and rock types
  • weak beds and fault and shear zones
  • flow pattern and pressure of ground water
  • potential hazards including heat, gas, and fault lines

Effective tunnel planning will account for any of these variations from the start. After analysis of the material in which the tunnel will pass through is conducted and an excavation plan determined, construction of the tunnel can resume.[18]

[edit] Excavating the Tunnel

The term typically used for building a tunnel is referred to as driving or advancing a tunnel. There are two common techniques in advancing a tunnel. In the full-face method, the entire diameter of tunnel is excavated at the same time. The technique is more common in the construction of tunnels passing through hard ground and for building smaller tunnels. The second technique, known as the top-heading-and-bench method, entails workers digging a smaller tunnel called a heading.The heading tunnel is advanced first to a certain distance. A second tunnel, below the floor of the heading, is then excavated. This secondary tunnel is called the bench. The advantage in advancing a tunnel using this technique is that the heading tunnel can be used to gauge the rock or soil’s stability.

Depending on whether the ground type is soft earth, hard rock, soft rock, or under the water, excavation and construction methods and techniques will vary.[19] However, the construction of a stable tunnel will take into account the following three critical steps: excavation, support, and lining.[20]

[edit] Soft Ground Tunnel Construction

A tunnel excavated in soft ground such as clay, silt, gravel or mud always poses the threat of caving in because the ground is unstable. The amount of time the ground is able to remain stable at the point of excavation is therefore known as stand-up time. To prevent cave-ins, the shield method is used during the tunnel’s construction. Shields today are iron or steel cylinders with cutters that are pushed into the soft soil.The shield’s function is two-fold; to carve out a perfect round hole and provide critical support while excavation occurs. Once a section of the tunnel is excavated, hydraulic jacks push or jack the shield forward and the process is repeated. This is known as tunnel jacking. Eventually, a more permanent lining of pre-cast concrete or cast iron is used in lieu of the shield to provide more permanent support. Today, subways constructed deeper under the ground employ the shield method. Shields permit fast, relatively safe construction work to be carried out without interfering with traffic above ground or compromising nearby structures underground. A shield may also be used in combination with a slurry wall that provides additional support.[21]

[edit] Hard Ground Tunnel Construction

A tunnel excavated from hard rock requires blasting the debris away with explosives or by removing it using tunnel boring machines (TBMs) that chew away at the rock. The one advantage in the construction of tunnels passing through hard rock such as a mountain is that they require very little if no additional support, as the stand-up time for hard rock tunnels can last for years.[22] Sometimes tunnels passing through hard rock can contain pockets, breaks, or fractures that can compromise stability. In this situation, engineers will either reinforce the tunnel walls with bolts, shotcrete or sprayed concrete, rings, steel beams or a permanent concrete lining.[23]

If a tunnel boring machine is not used, workers may use a scaffold known as a Jumbo to disperse the explosives. The jumbo moves to the tunnel’s face and drills mounted to the jumbo penetrate several holes into the rock bed simultaneously. The depth of the holes varies depending on the rock type. The typical depth of holes is on average 10 feet (3 m) and only a few inches in diameter.[24] The explosives are then packed into the holes, the tunnel evacuated and the charges to the explosives detonated. The fumes generated from the explosion are vacuumed out of the tunnel and the debris or muck is carted away.[25]

An alternative to blasting called fire-setting is also sometimes used in the excavation of hard rock tunnels.In this technique, the tunnel’s wall is heated with fire and cooled down with water. This sudden change in temperature causes the rock to expand and contract resulting in the breaking off of large chunks of rock.[26]

[edit] Underwater Tunnel Construction

Constructing a tunnel underwater presents many unique challenges, the main one being holding back the water while the tunnel is being excavated and constructed. The approach to building a tunnel underwater may vary. Blasting is not a very viable option and is often difficult to control as the ground may be soft, firm rock such as shale or limestone as was the case in the building of the Channel Tunnel. For this very reason, engineers may opt to use tunnel boring machines or moles to carve out the tunnel.[27] Most tunnel boring machines are specifically designed and customized to operate in certain geological conditions with the objective of meeting specific project requirements.[28]

These mammoth multi-million dollar pieces of equipment use a circular plate with disc cutters  (chisel-shaped cutting teeth), steel discs, or a combination of the two, to slice away at the rock. The slices of rock move through spaces in the cutting head onto a conveyer belt that carries the debris to the machine’s rear. Hydraulic cylinders fastened to the spine of the machine propel it forward through the tunnel a few feet at a time. The machines also provide critical support. As the machine cuts away into the rock two drills directly located behind the cutters bore into the rock. Workers pump grout out into the holes and attach bolts to everything in place while a permanent lining is eventually installed in the tunnel. This task is done with a massive erector type arm on the TBM that is essentially used for lifting segments of lining into place.[29]

Tunnels constructed under bodies of water such as rivers and bays may rely on the cut-and-cover tunnel method or  be built as an immersed tube tunnel. A trench in the surface is excavated and a tube is immersed inside of it then covered with material to keep the tube in place. A trench is dredged into the riverbed or ocean floor and long pre-fabricated sections of steel or concrete are sealed to prevent water from leaking inside the tube. Sections of tube are then floated to the site and sunk into the prepared trench. Divers then connect the sections and remove the seals. Excess water is pumped out and the tunnel, filled in with backfill.[30]

[edit] Other Tunneling Methods

Roader header method excavation is a mechanical machine that has a large rotating cutting head mounted onto a movable boom. The cutting head moves over the periphery of the tunnel face creating a profile of a desired shape or size. Debris is loaded onto a conveyer within the machine and eventually discharged onto a truck for removal.[31]

Rock hammer excavation is when excavators fitted with rock hammers are used to excavate shafts. Similar to a jackhammer, rock hammers are resourceful tools for excavating through hard rock.[32]

[edit] New Austrian Tunneling Method (NATM)

NATM is described as a “modern tunneling method,”[33] developed between 1957 to 1965 in Austria by Ladislaus von Rabecwicz, Leopold Muller, and Franz Pacher. The method exists more as a philosophical approach to tunneling and has become the topic of much debate within the tunneling industry. Rather than an explicit set of excavation and support techniques the NATM is “based on a concept whereby the ground surrounding an underground opening becomes a load bearing structural component through the activation of a ring like body supporting ground.”[34] Another more generic definition that has been presented is a “method of producing underground space by using all available means to develop the maximum self-supporting capacity of the rock or soil itself to provide stability of the underground opening.”[35]

[edit] Advancements in Tunneling

Recent developments in imaging technology have now given tunnel planners the ability to scan right inside the earth to compute how sound waves may travel through the ground. This technology is extremely beneficial because it provides quite an accurate picture of what a tunnel's possible environment may contain, including surrounding rock and soil types and and geological anomalies such as faults or fissures.

Other advancements are being made in excavation methods and in providing critical ground support. Tunnel boring machines are able to cut 1,600 tons of muck in an hour. Other rock cutting methods that utilize high-pressure water jets, lasers, and ultrasonics are being explored. New types of concrete with resins and other polymers that harden faster than cement are also being worked on.[36]

[edit] Equipment Used

[edit] References

  1. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  2. ↑ Technology. Answers.com. 2008-09-09.
  3. ↑ Tunnel. PBS. 2008-09-09.
  4. ↑ How to Build a Tunnel. Softpedia. 2008-09-09.
  5. ↑ Tunnel. PBS. 2008-09-09.
  6. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  7. ↑ Technology. Answers.com. 2008-09-09.
  8. ↑ How to Build a Tunnel. Softpedia. 2008-09-09.
  9. ↑ Technology. Answers.com. 2008-09-09.
  10. ↑ Henry Jephson, The Making of Modern London: progress and reaction: twenty-one years of the London County council, p. 62. (The London Liberal Federation, 1910)
  11. ↑ How to Build a Tunnel. Softpedia. 2008-09-09.
  12. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  13. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  14. ↑ How to Build a Tunnel. Softpedia. 2008-09-09.
  15. ↑ Tunnel. Howstuffworks.com 2008-09-09.
  16. ↑ Tunnel. Howstuffworks.com 2008-09-09.
  17. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  18. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  19. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  20. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  21. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  22. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  23. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  24. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  25. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  26. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  27. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  28. ↑ Tunnel Construction. Cross Harbour Study. 2008-09-09.
  29. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  30. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  31. ↑ Article. Tidc. 2008-09-09.
  32. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
  33. ↑ Tunnel Method. INONU. 2008-09-09.
  34. ↑ Tunnel Method. INONU. 2008-09-09.
  35. ↑ Tunnel Method. INONU. 2008-09-09.
  36. ↑ Tunnel. Howstuffworks.com. 2008-09-09.
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