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Message: [QUOTE="sutomcat, post: 2110899, member: 27"] Interesting. Were the original tunnel estimates made with the expectation that boring machines would be used (as opposed to the cut and cover method/top down method it appears to me that you are talking about)? [URL="https://en.wikipedia.org/wiki/Tunnel"]Tunnel - Wikipedia[/URL] [SIZE=4][B]Cut-and-cover[[URL='https://en.wikipedia.org/w/index.php?title=Tunnel&action=edit&section=7']edit[/URL]][/B][/SIZE] [URL='https://en.wikipedia.org/wiki/File:Paris_Metro_construction_03300288-3.jpg'][IMG]https://upload.wikimedia.org/wikipedia/commons/thumb//f7/Paris_Metro_construction_03300288-3.jpg/220px-Paris_Metro_construction_03300288-3.jpg[/IMG][/URL] Cut-and-cover construction of the [URL='https://en.wikipedia.org/wiki/Paris_M%C3%A9tro']Paris Métro[/URL] in [URL='https://en.wikipedia.org/wiki/France']France[/URL] [B]Cut-and-cover[/B] is a simple method of construction for shallow tunnels where a [URL='https://en.wikipedia.org/wiki/Trench']trench[/URL] is excavated and [URL='https://en.wikipedia.org/wiki/Roof']roofed[/URL] over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. Two basic forms of cut-and-cover tunnelling are available: [LIST] [*][I]Bottom-up method[/I]: A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ 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. [*][I]Top-down method[/I]: Side support walls and capping beams are constructed from ground level by such methods as [URL='https://en.wikipedia.org/wiki/Slurry_wall']slurry walling[/URL] 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. [/LIST] 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 [URL='https://en.wikipedia.org/wiki/Tunnelling_shield']tunnelling shield[/URL]. For intermediate levels, both methods are possible. Large cut-and-cover boxes are often used for underground [URL='https://en.wikipedia.org/wiki/Rapid_transit']metro[/URL] stations, such as [URL='https://en.wikipedia.org/wiki/Canary_Wharf_tube_station']Canary Wharf tube station[/URL] 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 [URL='https://en.wikipedia.org/wiki/London_Underground']London Underground[/URL], 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. This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century. [SIZE=4][B]Boring machines[[URL='https://en.wikipedia.org/w/index.php?title=Tunnel&action=edit&section=8']edit[/URL]][/B][/SIZE] Main article: [URL='https://en.wikipedia.org/wiki/Tunnel_boring_machine']Tunnel boring machine[/URL] [URL='https://en.wikipedia.org/wiki/File:TBM_S-210_Alptransit_Faido_East.jpg'][IMG]https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/TBM_S-210_Alptransit_Faido_East.jpg/220px-TBM_S-210_Alptransit_Faido_East.jpg[/IMG][/URL] A workman is dwarfed by the [URL='https://en.wikipedia.org/wiki/Tunnel_boring_machine']tunnel boring machine[/URL] used to excavate the [URL='https://en.wikipedia.org/wiki/Gotthard_Base_Tunnel']Gotthard Base Tunnel[/URL] ([URL='https://en.wikipedia.org/wiki/Switzerland']Switzerland[/URL]), the world's longest. Tunnel boring machines (TBMs) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as quick and cost effective alternative to laying surface rails and roads. Expensive [URL='https://en.wikipedia.org/wiki/Eminent_domain']compulsory purchase[/URL] of buildings and land, with potentially lengthy planning inquiries, is eliminated. Disadvantages of TBMs arise from their usually large size - the difficulty of transporting the large TBM to the site of tunnel construction, or (alternatively) the high cost of assembling the TBM on-site, often within the confines of the tunnel being constructed. There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, the bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, allowing them to be used in difficult conditions below the [URL='https://en.wikipedia.org/wiki/Water_table']water table[/URL]. This pressurizes the ground ahead of the TBM cutter head 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 free from water. Despite these difficulties, TBMs are now preferred over the older method of tunnelling 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 [URL='https://en.wikipedia.org/wiki/Underwater_diving']deep-sea divers[/URL]. In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of the [URL='https://en.wikipedia.org/wiki/Linth%E2%80%93Limmern_Power_Stations']Linth–Limmern Power Stations[/URL] located south of [URL='https://en.wikipedia.org/wiki/Linthal,_Glarus']Linthal[/URL] in the [URL='https://en.wikipedia.org/wiki/Canton_of_Glarus']canton of Glarus[/URL]. The borehole has a diameter of 8.03 metres (26.3 ft).[URL='https://en.wikipedia.org/wiki/Tunnel#cite_note-9'][9][/URL] The four TBMs used for excavating the 57-kilometre (35 mi) [URL='https://en.wikipedia.org/wiki/Gotthard_Base_Tunnel']Gotthard Base Tunnel[/URL], in [URL='https://en.wikipedia.org/wiki/Switzerland']Switzerland[/URL], had a diameter of about 9 metres (30 ft). A larger TBM was built to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the [URL='https://en.wikipedia.org/wiki/HSL-Zuid']HSL-Zuid[/URL] in the Netherlands, with a diameter of 14.87 metres (48.8 ft).[URL='https://en.wikipedia.org/wiki/Tunnel#cite_note-10'][10][/URL] This in turn was superseded by the [URL='https://en.wikipedia.org/wiki/Autopista_de_Circunvalaci%C3%B3n_M-30']Madrid M30 ringroad[/URL], [URL='https://en.wikipedia.org/wiki/Spain']Spain[/URL], and the [URL='https://en.wikipedia.org/wiki/Shanghai_Yangtze_River_Tunnel_and_Bridge']Chong Ming[/URL] tunnels in [URL='https://en.wikipedia.org/wiki/Shanghai']Shanghai[/URL], [URL='https://en.wikipedia.org/wiki/China']China[/URL]. All of these machines were built at least partly by [URL='https://en.wikipedia.org/wiki/Herrenknecht']Herrenknecht[/URL]. As of August 2013, the world's largest TBM is "[URL='https://en.wikipedia.org/wiki/Bertha_(tunnel_boring_machine)']Big Bertha[/URL]", a 57.5-foot (17.5 m) diameter machine built by [URL='https://en.wikipedia.org/wiki/Hitachi_Zosen_Corporation']Hitachi Zosen Corporation[/URL], which is digging the [URL='https://en.wikipedia.org/wiki/Alaskan_Way_Viaduct_replacement_tunnel']Alaskan Way Viaduct replacement tunnel[/URL] in [URL='https://en.wikipedia.org/wiki/Seattle,_Washington']Seattle, Washington[/URL] (US).[URL='https://en.wikipedia.org/wiki/Tunnel#cite_note-NYT_Dec_2012-11'][11][/URL] ... [/QUOTE]

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