Predicting Hydraulic Conductivity of Clay Liners

Data are presented from four projects in which rates of leakage from ponds lined with clay significantly exceeded the rates that would have been predicted on the basis of laboratory permeability tests. The actual hydraulic conductivities of the clay liners were generally found to be 10 to 1,000 times larger than values obtained from laboratory tests on either undisturbed or recompacted samples of the clay liner. The source of difficulty with laboratory permeability tests is the problem of obtaining a representative sample of soil for testing. Neither recompacted samples nor small, undisturbed samples are likely to contain a representative distribution of desiccation cracks, fissures, slickensides, or other hydraulic defects that may be present in the liner. Field permeability tests were performed for three of the four case histories and yielded results that compared well with field performance. Field permeability tests seem to produce much better results than laboratory tests. The four projects had certain features in common: (1) In all cases involving comparatively high rates of leakage, the liners were relatively thin (less than 24 in. or 0.6 m thick); (2) with only one exception, all of the liners were subjected to some desiccation; and (3) in essentially all cases, the construction inspection was not as extensive as it might have been.