IDENTIFICATION OF EXPANSIVE CLAYS

Clays contract when they lose moisture and increase in volume when they become wet. Particles of clay can be considered to be formed essentially of a nucleus of colloidal anions, surrounded by absorption cations at the surface. These cations may be replaced and, owing to their electropositive charge, they may absorb dipolar molecules, such as water molecules. The surrounding liquid will vary in thickness, depending on the degree of absorption. However, instead of speaking of the particles of clay, it would be more accurate to speak of minute sheets or flakes, and in the minerals of the clays, two kinds of sheets should be considered: silica sheets and alumina sheets (gibbsite). Taking these sheets into account, clays are usually divided into three large groups: kaolinites, illites or hydromicas, and montmorillonites; two other groups of clays (the vermiculites and attapulgites) are of much less importance in highway engineering. The kaolinitic minerals have a tend to store water in the interreticular spaces; this "storage" property leads, in turn, to an ability to withstand very large increases in volume. In highway engineering, it is therefore very important to identify the expansive clays. the best known methods for doing this are the use of an electronic microscope; an x-ray diffraction system; thermodifferential analysis; cationic-change capacity, and, in addition, densimetric, optical, and chemical methods; thermal analysis and ionic change produce very good results in highway engineering. Movements of water in clay masses are not in general gravitational, but are usually produced by capillary action, thermo-osmosis, and the attraction of vegetation; the phenomenon of thermo-osmosis explains much of the serious damage that occurs to roads constructred on expansive roadbeds. In addition to the methods of identification listed in point 2, when the Atterberg limits point to the presence of expansive clays, use may be made of free expansive tests and hydrometric tests (the latter to ascertain the amount of colloidal clay). If the clay is expansive, it is very important to determine its expansion capacity, which can be done by means of consolidometers or the normal AASHO test: T 174-60. To demonstrate the means of establishing the identification of clays suspected of having a dangerous expansive capacity, several examples are given of Colombian soils and a comparative analysis is made of their geotechnic characteristics in connection with highway engineering. In roadbuilding, expansive clays are the cause of very serious problems owing to considerable increase in volume and the high expansion pressure. To solve these problems, several of the measures recommended are: wide protected shoulders; thermo-osmotic control by laying coats of non-expansive material; stabilizing the surface of the roadbed; and other systems. Emphasis is placed on the importance of drainage and maintenance of roads having expansive roadbeds. Finally, a method is proposed that applies the usual grading tests, Atterberg limits, and classification of roadbeds, and reveals the clays suspected of having expansive properties, which can then be subjected to other tests in order to identify the expansive roadbeds (for highway engineering purposes) and to ascertain their expansion capacity (expressed in kilograms per square inch, for example). /Author/

  • Supplemental Notes:
    • Ninth Pan American Highway congress.
  • Corporate Authors:

    Organization of American States

    17th and Constitution Avenue, NW
    Washington, DC  USA  20006
  • Authors:
    • Lara, G C
  • Conference:
  • Publication Date: 1963-5

Media Info

  • Features: Appendices; Tables;
  • Pagination: p. 105-117
  • Serial:
    • Volume: 2
    • Issue Number: 194

Subject/Index Terms

Filing Info

  • Accession Number: 00263864
  • Record Type: Publication
  • Report/Paper Numbers: Conf. Proc
  • Files: TRIS
  • Created Date: Jan 29 1975 12:00AM