Standardized Hydraulic Conductivity Testing of Compacted Sand-Bentonite Mixtures

Compacted sand-bentonite mixtures can be used as engineered barriers (liners) for waste containment applications. A primary consideration for such applications is the ability of the mixtures to achieve a suitably low hydraulic conductivity, k, typically <= 1.0 x 10(-9) m/s, based on permeation with water in the laboratory. The purpose of this study was to evaluate in a systematic manner the applicability of ASTM D5084-10 for the measurement of k of compacted sand-bentonite mixtures. Final degrees of saturation, S-f, for 4 of the 19 test specimens were outside of the required range 95 % <= S-f <= 105 %, even though the measured k values for these 4 specimens were consistent with those tested under the same conditions with values of Sf that met the standard. Thus, S-f apparently was an unreliable measure of true saturation for the materials tested in this study. Otherwise, the results indicated that conservatively high values of k were measured for the largest specimen size, the highest applied hydraulic gradient, and the highest ionic strength of the permeant water. A difference in the method of hydraulic gradient application had essentially no effect on the magnitude of k. Finally, as expected, specimens containing the highest amount of bentonite (i.e., 15 % by dry weight) resulted in the lowest values of k, although calculation of some values of k for one specimen were negative (k < 0), which was attributed to swelling of the specimen during permeation and the manner in which k was calculated based on ASTM D5084-10.