Adhesion of clay during tunnel drives with slurry-type shield machines

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The stickiness of clay, due to adhesion, is the cause of considerable difficulties during tunnel drives with slurry-type shield machines. The scope of this article is to propose a definition of sticky soils and to improve the understanding of the mechanisms that cause stickiness. This includes the evaluation of past experiences with sticky clays, and the introduction of laboratory tests of the adhesion between various moistened clays and a steel surface. The results pertain to the composition and consistency of the soil samples, as well as the type of fluid used for moistening. A correlation analysis shows the relationship between mineralogical composition and adhesion. A simplified model is introduced, explaining the basic mechanism causing adhesion after previous moistening. Finally, some recommendations are given regarding future soil investigations and the need for further development.

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The application field, in which fluid-supported and earth-pressure-balanced shields are used, for both types of machines has been extended to almost the full spectrum of soils including a partial use in solid rock over the past few years. In this respect mixed-face situations with an interchanging occurrence of rock and soil belong to the most challenging applications of fluid-supported and EPB-shields. Such a geological setting is very demanding for both, the design of the tunnelling equipment as well as the tunnelling crews on site. In this paper, the special problems for the excavation, the face support and for the wear and tear of cutting tools and other parts of the machine in mixed-face situations are identified and described. Possible countermeasures in designing and operating a shield machine regarding such ground conditions are given. Finally, the advantages and disadvantages of fluid-supported and EPB-shields in relation to a tunnel drive in a changing or mixed face with rock and soil are discussed.
Shield drives in fine-grained clay soils are often associated with clogging. The tendency of soils to clog depends on numerous parameters. The separation of fines out of the cohesive soil structure also often leads to negative effects. Whether a certain clay soil can become critical depends not only on the soil properties but also the hydrogeological and operating conditions. According to the quantity of water available (groundwater, support fluid), the soil can be altered to a critical consistency range. An evaluation diagram, which takes into account possible changes of water content during the tunnel drive, can be used to evaluate soils under changing conditions (change of available water quantity). Schildvortriebe in feinkörnigen, tonhaltigen Böden sind häufig mit Verklebungen verbunden. Die Verklebungsneigung von Lockergesteinen hängt von einer Vielzahl an Parametern ab. Auch das Auflösen von Feinkorn aus dem kohäsiven Bodengefüge führt vielfach zu negativen Auswirkungen. Ob ein bestimmtes tonhaltiges Lockergestein kritisch werden kann, hängt neben den Bodeneigenschaften auch von den hydrogeologischen und baubetrieblichen Randbedingungen ab. Je nach Angebot an freiem Wasser (Bergwasser, Stützflüssigkeit) können die Böden in kritische Konsistenzbereiche überführt werden. Mit einem Bewertungs-Diagramm, das mögliche Wassergehaltsänderungen während des Vortriebs berücksichtigt, können Böden auch unter wechselnden Randbedingungen (unterschiedliches Wasserangebot) beurteilt werden.
  • In Dortmund
in Dortmund. Gütersloh: Bertelsmann (erscheint Dezember 2003).