Assessment of Sherard Chemical Method for Evaluation of Dispersion Potential of Soils

Soil dispersion is a phenomenon through which the soil particles due to their contact with water are suspended and carried away by the flow. This phenomenon causes noticeable damages and destructions in the founded structures on these soils. Regarding the widespread existence of this kind of soil in various areas of Iran including Khuzestan, Ardebil, Fars, Isfahan, and considering the government developmental plans in civil projects such as the modern irrigation and drainage networks, identifying this phenomenon by choosing suitable test method to prevent the probable destructions is necessary and consequently inevitable. Different experimental methods including dual-Hydrometery, Pinhole, and Sherard chemical test are used to study the potentialities of soil dispersivity. These methods have their own merits and demerits and it is mentionable that the results of these methods do not match. This study intends to evaluate, recognize and finally modify the delimitations of Sherard chemical method. To do so, several and various samples were collected out of different regions in the country, especially from the areas where the existing hydraulic structures had been damaged. Furthermore, several artificial soil samples were made by adding the four chemicals including sodium chloride, sodium carbonate, sodium solphate, and poly phosphate sodium with different percentages to clay. Firstly, different identification experiment including, grain size distribution, Aterberg limitations, chemical analysis and classification of the samples were done. Then, the dispersivity potential of the natural and artificial samples through three different methods: Pinhole, dual-Hydrometery, and sherard chemical method. The findings of the investigations and experimentations indicated that soil dispersivity is a physicochemical phenomenon that appears due to the special combinations of type and rate of anions and cations The Sherard chemical method which is based on type and rate of cations in the soil can not recognize dispersivity potentials properly. It was also found that from among the existing Anions in the soil, the chlorine ions on the contrary to sodium cations act as a factor in disperse of the soil. In other words, out of all existing sodium ions in the soil only the extra amount over chlorine rate in it plays the dispersivity role. Therefore changing the vertical axis of the sherard chart to the difference of sodium and chloride percent instead of sodium percent, the efficiency of this method will increase considerably.