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Because of their relatively low costs and numerous possibilities for use in soil and rock free of groundwater, open shield machines with mechanical partial excavation enjoy a wide application for jacking lengths up to 1000 m [FI-Herreb]. Water retention measures (open water retention or groundwater lowering) are required when jacking in groundwater (Abschnitt 4.4) (Abschnitt 12.8.2). In the cases in which this is not possible or permissible, a closed … |
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(Image: Open cowl shield with spiral conveyor for spoil removal and hydraulically extendable breast plate for gravel-sand soils without groundwater (external diameter 4280 mm) [FI-Eppin]) Shield machines with special excavation and conveying installations have been developed, and used for special purposes. The features of these excavation tools are that the working face is only partially excavated at some points in the shield, e.g. more or less passively. |
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Shield machines with compressed air support are used [Krets72] :
The limits of use for jacking in compressed air are determined by [Krets72] :
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(Image: Qualitative depiction of compressed air consumption [FI-Baben]) In order to ensure the stability of the working face, a high support pressure as possible is required on the one hand, but, on the other hand, with increasing air pressure, there is the danger that the resulting large pneumatic gradient causes a ground failure at the surface and thus an increasing loss of air in the working chamber (blow out). The increasing loss of air can lead … |
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Compressed air installations for spaces in which work is being carried out by people are subject to special safety demands and require special safety measures in order to protect the personnel from health hazards. Important for these in the Federal Republic of Germany are the:
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(Image: Design and method of operation of the membrane shield method with reference to [FI-Zübli] [Image: S&P GmbH]) In order to expand the area of application of shields with mechanical partial excavation and compressed air supported working face to inhomogeneous soils with a water permeability coefficient k > 10-4 m/s (e.g. sand or gravel with intrusions of cobbles and boulders), shield types have been developed in which the air support is combined … |
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Shield machines with fluid support (SM-T4) … |
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(Image: Closed cowl shield (external diameter 2.0 m) with integrated microtunnelling machine of type AVN 800 B (external diameter 975 mm) [FI-Herreb]) The shield machines with mechanical partial excavation correspond in concept to the design of the hand shield. They feature a mechanised open (SM-T1 or SM-T2) or closed (SM-T3 or SM-T4) shield with a fixed installed excavator for partial excavation of the soil or rock at the working face. The spoil is … |
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Project example - construction of the utility tunnel "Versorgungstunnel Kieler Förde" |
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Shield machines with partial excavation feature an open or closed shield of the models SM-T1, SM-T2, SM-T3 or SM-T4, in which the soil at the working face is partially loosened by means of fluid jets. The spoilfluid mixture is transported hydraulically to the surface. Representatives of these types of shield, which is no longer in use today, are discussed in the following for the sake of completeness. They are:
Flushing … |
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Shield machines with partial excavation feature an open (SM-T1 or SM-T2) or closed shield (SM-T3 or SMT4) and partial loosening of the soil or material at the working face. The spoil is transferred directly or by a conveyor belt, chain conveyor or a spiral conveyor (auger) to the downstream conveying system (belt conveyor, transport cart or conveying bucket). In exceptional cases, the spoil can also be conveyed hydraulically. Shield machines with partial … |
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The cutting wheel is a mostly open designed boring head that does not serve to support the working face mechanically ( Bild 9.2.2.1.1 a and b) (Bild 9.2.2.1.1) (Bild 9.2.2.1.1) (Bild 9.2.2.1.1) (Bild 9.2.2.1.1) (Bild 9.2.2.1.1). Its advantages are in the small cover and thus good accessibility to the working face if desired.
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Scraper cutting heads are nearby, closed, disc-shaped boring heads or cutting wheels with adjustable or fixed supporting plates with soil entry openings that are adapted to suit the size of the maximum particle to be transported Bild 9.2.2.1.2 (Bild 9.2.2.1.2) (Bild 9.2.2.1.2) (Bild 9.2.2.1.2). Besides their excavation function, the mechanical support of the working face becomes important with this boring head variant. Scraper cutting heads have a … |
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Rock cutting heads consist of almost closed, disc shaped basic constructions with usually slot shaped entry openings (removal slots) and are flat with a rounded periphery, slightly coned or almost convex (Bild 9.2.2.1.3) (Bild 9.2.2.1.3) (Bild 9.2.2.1.3) (Bild 9.2.2.1.3) (Bild 9.2.2.1.3) (Bild 9.2.2.1.3). Set into the boring head there are additional carriers for attaching or bearing the roller cutters. So-called removal teeth are situated along the … |
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For shield machines with full-face excavation, special attention must be paid to the ability to change tools without changing the support conditions of the working face. One method of carrying this out in a safe and simple manner underground without withdrawing the machine from the rear of the boring head is the so-called "Backloading system" [Kollm00]. Examples of the fastenings used for the various tools are shown in Bild 9.2.2.1.5, Bild 9.2.2.1.5 |
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Shield machines with full-face excavation are equipped with various boring heads [Arnol93] depending of the subsoil conditions and especially the strength of the soil and rock material. Depending on the design, the boring head, besides excavation, can also undertake an earth pressure balance function. A difference is made between:
The … |
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Shield machines with full-face excavation and natural support of the working face are open, fully mechanised shields (Tabelle 9.2). They are the simplest form of shield machines … |
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When jacking in groundwater with the open shield machines of the types SM-V1 as well as SM-V2, the working face must be continuously, i.e. over the complete distance of the jacking, pressurized with compressed air and a change over must be made to a closed shield insofar as the groundwater cannot or must not be lowered [Tunnel98]. In this case, the corresponding discussions of the Abschnitt 9.2.1.2.3 apply. The main areas of application of this type … |
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With slurry shields (Bild 9.2.2.5.1) (Bild 9.2.2.5.1), the support pressure control is carried out within the excavation chamber by means of the pump-controlled supply and discharge of the support fluid (Abschnitt 8.3). As the support pressure control can react very insensitively to sudden losses of the supporting fluid, for instance when approaching fault zones or inhomogeneous subsoil, leading to an uncontrolled collapse of the working face and … |
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The most important deviating design feature of hydroshields (Bild 9.2.2.5.2) (Bild 9.2.2.5.2) from the slurry shields is that the excavation chamber is divided by a scum board into two regions (Bild 9.2.2.5.2). In the rear chamber (pressure chamber) that is separated from the trailing shield segment by a pressure bulkhead, there is a compressed air cushion through which the support pressure at the fluid balanced working face is controlled by means … |
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All the boring head variants mentioned in Abschnitt 9.2.2.1 (cutting … |
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