Shortcomings in the estimation of clay fraction by hydrometer

Published ArticleThe estimation of clay fraction is important for predicting the engineering properties of a soil. SANS 3001 GR3 (SANS 2011) specifies a procedure for clay fraction determination using a hydrometer. It has long been suspected that there may be flaws in this approach. Some of the possible sources of error have been suggested, but little or no change has been made in the standard procedures for assessment of clay fraction in well over half a century. This paper deals with a microscopic examination of some typical South African clayey soils to assess the adequacy of dispersion and possible consequences for clay fraction determination in currently specified hydrometer procedures. Clays are examined both with and without dispersant, and with and without labelling of clay minerals using an exchangeable cation dye