Depth variations in hydraulic conductivity within a single lift of compacted clay

Abstract

Compacted clay liners are commonly used as components of the lower portion of composite double liner systems for hazardous waste containment. Because the overlying leachate collection and removal systems and the FMLs are not perfect leachate still comes into contact with the lower liner and thus makes it critical that the clay liner component be constructed to achieve the lowest possible hydraulic conductivity. This research was conducted to evaluate the effects of clod size on the hydraulic conductivity of compacted soils and the uniformity of conductivity with depth within a lift of compacted soil. Two subsoils, one from the Beaumont series (smectitic) and one from the Kosse series (kaolinitic), were evaluated in the laboratory and then compacted in large fixed wall permeameters using maximum clod sizes of <2.5, <5.0, and <7.5 cm to a compacted lift thickness of 23 cm. Measurements were made of the hydraulic conductivity of the entire lift, the lower two thirds of the lift, and the lower one third of the lift. The results show that the conductivity of the lower one third of the lift can be as great as 8.7 times that measured for the entire lift and indicates that liners need to be constructed using thin lifts to achieve more uniform low conductivity throughout the liner. The data also indicated that under the carefully controlled conditions of this study and with the clod sizes used, the clod size did not have a significant effect on the hydraulic conductivity of the soils tested. Soil bulk density was poorly correlated with hydraulic conductivity and indicates that measuring the bulk density of a compacted soil is an inadequate method for assuring low hydraulic conductivity. Measurements of the time to the first appearance of leachate indicated that 8 to 17 d are required for water to penetrate a 23 cm thick compacted liner with an average conductivity of 1 × 10−7 cm s−1.