R.M. Schmitz1, 3, Ch. Schroeder1, J. Thorez1, 2, R. Charlier1
1 Geomecanics and Engineering Geology - Géomac - Université de Liège
2 Clay Mineral Laboratory - Université de Liège
3 National Scientific Research Fund (F.N.R.S.)
Evolution of mechanical properties of natural clays: predictions on the basis of their clay mineralogy

Clays are geomaterials with are used in various (over 100 applications) in our society. The more common geotechnical applications are: barriers to contain waste, slurry walls etc. But even when clays are not used as construction material the geotechnical engineer encounters them frequently during construction of e.g. foundations and tunnels. As clays are the end product of the weathering chain of geomaterials they are stable as such, but, within this group of clay minerals we can find individual clay species that posses geotechnical properties that can vary enormously. One of these aspects is: chemo plasticity e.g. reflected in the effect the composition of the pore fluid has on the mechanical properties of the clay.

An approach to deal with this chemo plastic effects is to separate the processes depending on their affect on clay fabric, the clay particles or the clay minerals:

  1. Changes of the clay fabric:
  2. Changes of the clay particles:
    Double layer: cations, originating from salts entering the sample by advection or diffusion, decrease the thickness of the double layer and allow clay particles to approach each other, this goes hand in hand with a decrease of the liquid limit, decrease of the coefficient of consolidation etc. [2]
  3. Changes of clay minerals:
    Some ions, like potassium, can enter the pseudo hexagonal voids of Smectite. This causes a to some degree irreversible collapse of the interlayer space [3]. The properties of this collapsed material will correspond to that of an Illitic clay or Illite-Smectite mixed layer, which generally have a lower liquid limit and higher shear strength. These mineral alteration processes can be related to geotechnical index properties using the Equivalent Interlayer Concept [4].

Changes on e.g. level C) followed by consolidation will of course affect changes in geotechnical properties on level B) and A). An analysis of the processes on all these levels as for example during a leaching test: by performing analysis of the clay mineralogy by X-ray diffraction, analysis of the changes in clay fabric in thin sections and relating the lab experiments to appropriate geological analogues [5] the measured changes in hydraulic conductivity can lead to an assessment of the performance of a clay barrier on the long term.

For more detail:
  1. SCHMITZ, R.M., SCHROEDER Ch., CHARLIER, R. (2003) A correlation between clay mineralogy and Atterberg limits: suitability of clays as engineered barrier systems. Industrial Minerals - Resources, Characteristics and Applications. Aardk. Mededel., 2003, 13, 000-000 (submitted)
  2. Schmitz R.M., van Paassen L.A. (2003) The decay of the liquid limit of clays with increasing salt concentration. Ingeokring Newsletter, published by the Dutch association of Engineering geology. Vol 9., N1, 2003, pp.10-14, ISSN 1384-1351. (Journal)
  3. Schmitz R.M., Ourth A-S., Dosquet D., Illing P., Schroeder Ch., Verbrugge J-C., Bolle A., Charlier R. , Thorez J. (2002) The suitability of some Belgian Tertiary clays as construction material for landfill seals: Interaction with domestic landfill leachates. Aardkundige Mededelingen 12. General Editor: F. Gullentops. Leuven University Press. ISBN 9058672468. Leuven 2002. (Journal)
  4. SCHMITZ, R.M., SCHROEDER Ch., CHARLIER, R. A correlation between clay mineralogy and atterberg limits. Proceedings International Workshop of Young Doctors in Geomechanics. Editors V. De Gennaro and Pierre Delage. December 4th-6th 2002. Ecole Nationale des Ponts et Chaussées. pp. 27-30 (Proceedings)
  5. Thorez, J., Dosquet, D., Illing, P., Schroeder, Ch. (2001) Behaviour of two Tertiary clayey materials leached with a complex domestic waste fluid under high pressure: triaxial permeability, and qualitative and semi-quantitative clay mineral changes. th KIWIR International Workshop on Key Issues in Waste Isolation Research, Ecole Nationale des Ponts et Chaussés, 28 - 30 November 2001, Paris France.)(Proceedings)
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