A New Technique for Diffusion Testing of Unsaturated Soil

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Courtesy of ASTM International

A technique is proposed for determining the coefficients of diffusion and adsorption for ions in unsaturated soils. The method integrates the single reservoir diffusion testing of saturated soil with the axis-translation technique for the control of matric suction. The procedure allows for the application of a net total stress and matric suction to the soil during the diffusion test. The use of this new technique to determine the coefficients of diffusion and adsorption of ions from a single test is demonstrated. The effects of the degree of saturation on the coefficients of diffusion and adsorption were evaluated on a sandy soil with water contents ranging from saturation to near the residual degree of saturation. Potassium and chloride ions were used as the primary tracers.

The test results showed that both the coefficients of diffusion and adsorption can be determined from a single test. The results illustrate a decrease in both the effective diffusion and adsorption coefficients with a decrease in the degree of saturation. The extent of the decrease in the effective diffusion coefficient from saturation to near the residual degree of saturation was approximately 80%. The decrease in the adsorption coefficient was gradual and small from saturation to a degree of saturation of approximately 10%; however, it then decreased rapidly as the water content approached the residual degree of saturation.

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