Effect of clod size on hydraulic conductivity of compacted clay

A study has been conducted to identify the causes for field-measured values of hydraulic conductivity of clay liners being typically two orders of magnitude greater than laboratory-measured values. It has been found through laboratory testing on large specimens that the size and gradation of clods and clumps of clay, prior to compaction, can significantly affect the measured hydraulic conductivity. The results of this study are compared to existing studies on the effect of clods on hydraulic conductivity, and general agreement is found. It is concluded that, for clod sizes and gradations likely to occur in field compaction of clay liners, that differences in clod gradation and size between lab and field conditions can essentially account for the observed roughly two orders of magnitude discrepancy between lab and field-measured conductivities. Revised laboratory testing procedures for both the before-liner construction and after-liner construction cases are recommended. The revised laboratory testing procedures are designed to correct the deficiencies in conventional lab testing and to produce unbiased estimates of field hydraulic conductivity values.