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Theoretical and practical applications of electro-osmosis have been utilized for many years. Successful applications have included slope stability, dewatering, pile driving, and consolidation. In addition, electro-osmosis has been used in electrochemical injection applications, including the use of aluminum anodes and additives such as potassium chloride and aluminum acetate. Although there is a wide range of applications of electro-osmosis, there are many unknowns with respect to ion flow through soil and the resulting changes in soil properties.
The principal mechanism behind electro-osmosis is migrating ions. Oxidation and reduction take place at the electrodes as electrons are transferred in and out of the system. These reactions are associated with changes in soil pH which result in changes in the thickness of the double layer. In addition, the structure of alumino-silicates allows for the availability of certain ions for chemical reaction when subjected to an electrical gradient. These reactions may be dependent on pH or facilitated by a reduction in pH. Changes in thickness of the double layer as well as chemical reactions in the soil produce changes in soil properties, including plasticity and volume change characteristics.
Electro-osmosis was applied to a moderately swelling soil in the laboratory. The mass of soil treated was 12 by 12 by 6 in. Solution tanks on both ends of the soil contained the electrodes. A direct current power supply applied a constant voltage of approximately 16 V across the electrodes. Current, voltage, and pH were monitored throughout treatment. Electro-osmosis continued for a length of time sufficient to effect changes in soil pH near the anode and cathode.
Changes in soil properties were determined by performing Atterberg limits and swell tests on soil samples taken before and after electro-osmosis was applied. Samples were taken at different locations between the anode and cathode to determine variations in soil properties as a function of pH. Near the anode, where the soil pH decreased, significant changes in soil properties were noted. The plasticity index and swell were both found to decrease while the shrinkage limit increased. The changes, which are attributed to changes in the thickness of the double layer and to electrochemical hardening, indicate that soils considered questionable for use in engineering projects may be improved by electro-osmosis.
Ionic flow, changes in soil pH, and chemical reactions combine to produce changes in soil properties. As more is learned about the nature of these mechanisms, applications of electro-osmosis may be developed which fully utilize the electrokinetic phenomena.
swelling soils, soil stabilization, electro-osmosis, shrinkage limit, clay mineralogy
Senior engineer I, GAI Consultants-NC, Inc., Raleigh, NC
Associate professor of civil engineering, University of Missouri-Rolla, Rolla, MO