Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube station in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral, owing to the sheer size of the excavation. This contrasts with many traditional stations on London Underground, where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the london Underground network, the metropolitan and District railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam. A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction.
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Two basic forms of cut-and-cover tunnelling are available: Bottom-up method : A trench is excavated, with ground support as necessary, and the tunnel is constructed. The tunnel may be of in situ positioning concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated. Top-down method : Side support walls and capping beams are constructed from ground level by such methods as slurry walling or contiguous bored piling. Then a shallow excavation allows making the tunnel roof of precast beams or in situ concrete. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed. 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.
7 Increased taxes to finance a large project may cause opposition. 8 Construction edit main article: Tunnel construction Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length presentation and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel. There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally a tube can be sunk into a body of water, which is called an immersed tunnel. Cut-and-cover edit cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.
Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of bonds. The costs and benefits for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the us house of Representatives approved a 100 million federal grant to build a tunnel under New York harbor. However, the port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project.
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Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were write considered higher than for a tunnel. Similar conclusions were reached for the 1971 Kingsway tunnel under the mersey. In Hampton roads, virginia, tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges would prevent us navy vessels from leaving naval Station Norfolk. Water-crossing tunnels built instead of bridges include the holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York city ; the queens-Midtown Tunnel between Manhattan and the borough of queens on Long Island ; the detroit-Windsor Tunnel between Michigan and Ontario ;. Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the.5-kilometre.0-mile Channel Tunnel aesthetic reasons (preserving the above-ground view, landscape, and scenery and also for weight capacity reasons (it may. Some water crossings are a mixture of bridges and tunnels, such as the denmark to Sweden link and the Chesapeake bay bridge-tunnel in Virginia.
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. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage. Project planning and cost estimates edit government funds are often required for the creation of tunnels. 6 When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use project management techniques for developing a major structure.
The most common way to control groundwater is to install dewatering pipes into the ground and to simply pump the water out. 4 a very effective but expensive technology is ground freezing, using pipes which are inserted into the ground surrounding the excavation, which are then cooled with special refrigerant fluids. This freezes the ground around each pipe until the whole space is surrounded with frozen soil, keeping water out until a permanent structure can be built. Tunnel cross-sectional shape is also very important in determining stand-up time. If a tunnel excavation is wider than it is high, it will have a harder time supporting itself, decreasing its stand-up time. A square or rectangular excavation is more difficult to make self-supporting, because of a concentration of stress at the corners.
5 Choice of tunnels versus bridges edit The harbor Tunnel in Baltimore, which carries I-895, serves as an example of a water-crossing tunnel built instead of a bridge. 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 drawbridge 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 favor of a tunnel. 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. The 1934 queensway tunnel under the river Mersey at liverpool was chosen over a massively high bridge for defense reasons; it was feared that aircraft could destroy a bridge in times of war.
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To give more information, a pilot tunnel (or "drift tunnel may be driven ahead of the barbing main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face. Other key geotechnical factors: "Stand-up time" is the amount of time a newly excavated cavity can support itself without any added structures. Knowing this parameter allows the engineers to determine how far an excavation can proceed before support is needed, which in turn affects the speed, efficiency, and cost of construction. Generally, certain configurations of rock and clay will have the greatest stand-up time, while sand and fine soils will have a much lower stand-up time. 3 Groundwater control is very important in tunnel construction. Water leaking into a tunnel or vertical shaft will greatly decrease stand-up time, causing the excavation to become unstable and risking collapse.
The etymology of the terms "mining" (for mineral extraction or for siege attacks "military engineering and " civil engineering " reveals these deep historic connections. Antique and early middle ages edit Predecessors of modern tunnels were adits to transport water for irrigation or drinking, and sewerage. The first Qanats are known from before 2000. Geotechnical investigation and design edit main article: geotechnical investigation A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from boreholes and by other geophysical techniques. An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is thesis common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction. Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels.
it is passing underneath, it may be called an underpass, though the official term when passing under a railway is an underbridge. A longer underpass containing a road, canal or railway is normally called a "tunnel whether or not it passes under another item of infrastructure. An underpass of any length under a river is also usually called a "tunnel whatever mode of transport it is for. In the us, the term "subway" means an underground rapid transit system, and the term pedestrian underpass is used for a passage beneath a barrier. Rail station platforms may be connected by pedestrian tunnels or footbridges. History edit This section needs expansion. You can help by adding. (March 2013) Much of the early technology of tunneling evolved from mining and military engineering.
Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers. Utility tunnels are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment. Secret tunnels are built for military purposes, or by civilians for smuggling of weapons, contraband, or people. Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely. Tunnels can be connected together in tunnel networks. Contents, terminology edit, a tunnel is relatively long and narrow; the length is often much greater than twice the diameter, although similar shorter excavations can be constructed, such as cross passages between tunnels. The write definition of what constitutes a tunnel can vary widely from source to source. For example, the definition of a road tunnel in the United Kingdom is defined as "a subsurface highway structure enclosed for a length of 150 metres (490 ft) or more." 1 In the United States, the nfpa definition of a tunnel is "An underground structure with.
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This article is about underground passages. For other uses, see. For the john Foxx song, see. For a tunnel for pedestrians, see. A tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods. A tunnel may be for foot or vehicular road traffic, margaret for rail traffic, or for a canal. The central portions of a rapid transit network are usually in tunnel.