Information AboutTunnels |
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, Belgium ]] A tunnel is an underground passage. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels 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. A tunnel may be for Pedestrian s or Cyclist s, for general road Traffic , for Motor Vehicle s only, for Rail traffic, or for a Canal . Some are Aqueduct s, constructed purely for Carrying Water — for consumption, for Hydroelectric purposes or as Sewers — while others carry other services such as Telecommunications cables. There are even tunnels designed as Wildlife Crossing s for European Badger s 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 . In the United Kingdom a pedestrian tunnel or other underpass beneath a road is called a Subway . This term was also used in the past in the United States , but is now used to refer to underground Rapid Transit systems. The longest canal tunnel is the Standedge Tunnel in the United Kingdom, which stretches over three miles. In the Czech republic, the verb to 'tunnel' is a synonym for to embezzle. For example: the manager 'tunnelled' the company and now lives on the Bahamas; or, many banks collapsed because they were 'tunnelled'. Tunneling (fraud) airport.]] in Singapore is a fully-underground rail line.]] 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. At metro stations there are usually pedestrian tunnels from one Platform to another. Often, ground-level Railway Station s also have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without having to walk across the tracks. However, in the United Kingdom bridges are an equally popular method of pedestrian access between two or more different Railway Station platforms. CONSTRUCTION under construction]] Tunnels are dug in various types of materials, from soft clays to hard rocks, and the method of excavation heavily depends on the ground conditions. Cut-and-cover Cut-and-cover is a simple method of construction for shallow tunnels where a Trench is excavated and Roof ed over. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing. Two basic forms of cut-and-cover tunnelling are available:
Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a Tunnelling Shield . For intermediate levels, both methods are possible. Boring machines , Nevada ]] Tunnel Boring Machine s (TBMs) and associated back-up systems can be used to highly automate the entire tunneling process. There are a variety of TBMs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types, (bentonite slurry and earth-pressure balance machines), have pressurised compartments at the front, allowing them to be used in difficult conditions below the Water Table . This pressurizes the ground ahead of the TBM cutter head in order to balance the water pressure. The operators work in normal air pressure behind the pressurised compartment, but may occasionally have to enter that compartment to renew or repair the cutters; this requires special precautions, such as local ground treatment or halting the TBM at a position locally free from water. Despite these difficulties, TBMs are now preferred to the older method of tunneling in compressed air, with an air lock/decompression chamber some way back from the TBM, which required operators to work in high pressure and go through decompression procedures at the end of their shifts, much like divers. Until recently the biggest TBM built was used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. Its diameter is 14.87 m. {Link without Title} Nowadays 4 even larger machines exist: 2 for the M30 ringroad in Madrid, Spain, 2 for the Chong Ming tunnels in Shanghai, China. These machines are 15,2 m and 15,4m in diameter respectively. The two machines for Spain were built by Mitsubishi/Dura Fuelgo and Herrenknecht {Link without Title} . The TBMs for China were built by Herrenknecht. NATM The New Austrian Tunneling Method (NATM) was developed in the 1960s. The main idea of this method is to use the geological Stress of the surrounding Rock Mass to stabilize the tunnel itself. Based on Geotechnical measurements, an optimal Cross Section is computed. The excavation is immediately protected by thin Shotcrete , just behind the excavation. This creates a natural load-bearing ring, which minimizes the rock's Deformation . By special Monitoring the NATM method is very flexible, even at surprising changes of the Geomechanical rock consistency during the tunneling work. The measured rock properties lead to appropriate Tool s for tunnel Strengthening . In the last decades also Soft Ground excavations up to 10 km became usual. Pipe jacking Pipe Jacking, also known as '''pipejacking''' or '''pipe-jacking''', is a method of tunnel construction where hydraulic jacks are used to push specially made pipes through the ground behind a tunnel boring machine or shield. This technique is commonly used to create tunnels under existing structures, such as roads or railways. Underwater tunnels There are also several approaches to underwater tunnels, for instance an Immersed Tube as in the Sydney Harbour , and the Posey And Webster Street Tubes which connect the cities of Oakland and Alameda, California , running beneath the Alameda-Oakland Estuary . Other Other tunneling methods include:
CHOICE OF TUNNELS VS. BRIDGES For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or ), 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 . SHORT TUNNELS A short tunnel can be built as an alternative to an Overpass . One example of a short tunnel is the Croom Tunnel on the South Coast Railway Line . ARTIFICIAL TUNNELS Overbridges can sometimes be built by covering a road or river or railway with brick or still arches, and then levelling the surface with earth. In railway parlance, a surface-level track which has been built or covered over is normally called a covered way. Snow Shed s are a kind of artificial tunnel built to protect a railway from avalanches of snow. Similarly the Stanwell Park , New South Wales steel tunnel, on the South Coast Railway Line , which protects the line from rockfalls. Common Utility Duct s are man-made tunnels created to carry two or more utility lines underground. Through co-location of different utilities in one tunnel, governments and companies are able to reduce the costs of building and maintaining utilities. EXAMPLES OF TUNNELS In history , London , mid 19th century]]
See also the History of Rapid Transit . Longest See Also: List of tunnels by length
Notable
Other uses Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with Military Use During Armed Conflict , or civilian responses to threat of attack. MEDIA NATURAL TUNNEL Snow tunnels are created by voles, chipmunks and other rodents for protection and access to food sources. Larger versions are created by humans, usually for fun. For more information regarding tunnels built by animals, see Burrow SEE ALSO
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