A further examination of the nature of dispersive soils in Natal, South Africa

Abstract

Dispersive soils are found in the drier areas of Natal, in regions where the annual rainfall is less than 850 mm. They are derived from a variety of rock types. Dispersive soils deflocculate in the presence of relatively pure water to form colloidal suspensions and are therefore highly susceptible to erosion and piping. Dispersive erosion depends on the chemistry of the clay on the one hand, and the dissolved salts in the pore and eroding water on the other. The presence of exchangeable sodium on clay mineral surfaces is one of the main chemical factors contributing towards dispersive behaviour. Susceptibility to dispersion also is influenced by the total content of dissolved salts in the pore water. The fabric of the soils was analysed by means of a scanning electron microscope. Turbostratic and flocculated structures were found in the clay material associated with low and high TDS contents respectively. The mineralogy, particle size distribution and consistency of these soils had little or no influence on dispersivity. A series of physical and chemical tests was carried out in an attempt to assess dispersivity. Then an evaluation of the reliability of the results of these tests was undertaken by means of discriminant analysis. This indicated that the results of the crumb test, the pinhole test, the SAR value and the assessments provided by the CEC vs ESP and TDS vs % Na charts represented the best parameters to use in a rating system. Such a system is proposed to aid the recognition of dispersive soils.