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تونل-Tunnel

Tunnel

 An underground space of substantial length, usually having a tubular shape. Tunnels can be either constructed or natural and are used as passageways, storage areas, carriageways, and utility ducts. They may also be used for mining, water supply, sewerage, flood prevention, and civil defense.

Construction

Tunnels are constructed in numerous ways. Shallow tunnels are usually constructed by burying sections of tunnel structures in trenches dug from the surface. This is a preferred method of tunneling as long as space is available and the operation will not cause disturbance to surface activities. Otherwise, tunnels can be constructed by boring underground. Short tunnels are usually bored manually or by using light machines (such as a roadheader or backhoe). If the ground is too hard to bore, a drill-and-blast method is frequently used. For long tunnels, it is more economical and much faster to use tunneling boring machines which work on the full face (complete diameter of the opening) at the same time. In uniform massive rock formations without fissures or joints, tunnels can be bored without any temporary supports to hold up the tunnel crowns. However, temporary supports are usually required because of the presence of destabilizing fissures and joints in the rock mass (Fig. 1). A layer of shotcrete serves as the primary lining to protect the newly exposed surface and to support the tunnel crowns as well. The shotcrete is frequently reinforced by steel meshes and, if necessary, braced by steel lattices.  See also: Drilling, geotechnical

 

 

Fig. 1  Temporary supports for a four-lane road tunnel. (RESA Engineering Corporation)

 

 

In soft ground, it has become popular to use shield machines for boring and reinforced concrete segments for lining. The largest shield machine ever, 14 m (46 ft) in diameter, was used in constructing the Tokyo Trans-Bay Highway (Tokyo Wan Aqua-Line) between Kawasaki City and Kisarazu City of Japan (completed in 1997). There are various types of shield machines available to serve different purposes. The multiface shield machine (Fig. 2) was first used in constructing the Osaka Business Park subway station in Japan (completed in 1995). The machine is 17 m (56 ft) in width and 7.5 m (25 ft) in height and has three cutters which operate independently. Theoretically, the two side cutters can be detached from the center cutter upon the completion of station excavation, leaving the center cutter to continue boring toward the next station. However, this technique was not applied in this case, and the machine was used for the Osaka Business Park station only.

 

 

Fig. 2  Multiface shield tunneling machine. (Hitachi Zosen Corporation)

 

 

 

 

 

Immersed tunnels

 For tunnels to be constructed across bodies of water, an alternative to boring is to lay tunnel boxes directly on the prepared seabed. These boxes, made of either steel or reinforced concrete, are prepared in dry docks and sealed at their ends by the use of bulkheads. They float as the docks are flooded, and are towed to the site by tugboats. The boxes are then flooded to allow them to sink to the seabed after they are properly positioned. Immersed tunnels are usually buried in shallow trenches dug for this purpose and covered by ballast so they will not be affected by the movement of the water. The joints between tunnel sections are made watertight by using rubber gaskets, and water is pumped out of the tunnel to make it ready for service. Among the numerous immersed tunnels the Øresund Link, completed in 1999 [3.5 km (2.2 mi)], between Denmark and Sweden is second in length to the cross-bay tunnel [5.8 km (3.6 mi)] for the Bay Area Rapid Transit system of San Francisco, California.  See also: Concrete; Steel

 

Microtunnels

 Small tunnels, such as sewer lines and water mains, are usually installed by jacking steel or concrete pipes into the ground (Fig. 3). The soil core inside the tubes can be removed manually or by using “moles,” which are essentially small shield tunneling machines. The alignment of the pipes is continuously monitored and adjusted. With the moles guided by a computerized navigation system, it is possible to align pipes to a precision within 100 mm (4 in.) regardless of length. This technique has been used for jacking pipes as large as 2 m (7 ft) or so in diameter for distances more than 100 m (330 ft). Pipes with smaller sizes can be jacked to distances of more than 300 m (990 ft).

 

 

Fig. 3  Microtunneling and pipe jacking technique.

 

 

 

 Special tunneling techniques

 Auxiliary measures are frequently required to ensure the safety of tunnels during boring in soft ground. Compressed air was used in the past, but is it is seldom used anymore because improper decompression may cause aeroembolism (diver's disease) in workers. Instead, grouting and ground freezing are now preferred. In the Central Artery Project (scheduled to be completed in 2004) in Boston, Massachusetts, ground freezing is carried out to permit three tunnels to be bored under the railway tracks leading to the South Station Railway Terminal. Roughly 1600 freezing pipes are installed to depths varying from 13.7 to 16.8 m (45 to 55 ft), and the volume of frozen soil is more than 60,000 m3 (2,100,000 ft3). This could be the largest undertaking of this nature.

An underpass, scheduled to be completed in 2001, is being constructed beneath the Taipei International Airport by using the Endless-Self-Advancing method. To minimize ground settlements and ensure the safety of air traffic, interlocked steel pipes are first jacked into the ground to form a protective shelter. The soil core inside the shelter is excavated at the rate of 400 mm (16 in.) per lift. Concrete segments are moved one by one into the space created, by jacking the segments behind. Each time only the last segment is jacked forward, and the jacking force is taken by the frictional resistance acting on all the rest of the segments. Cables are anchored to the first and last segments so the force acting on the last segment can be transmitted to the segments in front of it. The movements of these segments resemble the movements of centipedes. In theory, there is no limit on the length of tunnels installed by this method.

 

Longest tunnels

 The longest tunnel of any kind is the New York City/West Delaware water supply tunnel (completed in 1944). It runs for 169 km (105 mi) from the Rondout Reservoir into the Hillview Reservoir in Yonkers, New York. The longest rail tunnel is Seikan Tunnel (53.6 km; 33.3 mi) in Japan (completed in 1985). The longest road tunnel was Saint Gotthard Tunnel (16.9 km; 10.5 mi) in Switzerland (completed in 1980). In mid2001 the title was taken over by the Laerdal Tunnel (24.5 km; 15.2 mi).

The longest undersea tunnel is the Channel Tunnel [49.4 km (30.7 mi), of which 38 km (23.6 mi) is undersea] across the English Strait. It runs from Folkestone in Britain to Calais in France. There are two running tunnels plus one service tunnel in the center, 7.6 and 4.8 m (25 and 16 ft) in internal diameter, respectively. The tunnel was officially inaugurated on May 6, 1994, when the Queen of England and President Mitterrand of France became the first official passengers to pass by train between the two countries. Eurotunnel has a concession from the British and French governments to run the tunnel until 2052. Shuttle trains, carrying up to 180 cars, and freight shuttles, carrying 28 lorries, will take 35 minutes to cross the strait.

Bibliography

  •  C. J. Kirkland (ed.), Eurotunnel: Engineering the Channel Tunnel, 1995
  • Alifazeli = egeology.blogfa.com
  • J. O. Bickel, T. R. Kuesel, and E. H. King (eds.), Tunnel Engineering Handbook, 1995
  • Alifazeli = egeology.blogfa.com
  • Z. C. Moh et al., Underpass beneath Taipei International Airport, Proceedings of the Conference on New Frontiers and Challenges, Bangkok, Thailand, November 8–12, 1999
  • Alifazeli = egeology.blogfa.com
  • A. M. M. Wood and A. M. Wood, Tunnelling: Management by Design, 2000
  • Alifazeli = egeology.blogfa.com

 

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