Clay barriers : laboratory study of hydraulic defects and preferential flow paths

A laboratory testing program was carried out to evaluate the importance of preferential flow in fractured clay. The saturated hydraulic conductivity was found to be insufficient in capturing the contribution of preferential flow paths during flow, especially for smaller samples. The reason for this is that these pathways closed as the sample swelled upon saturation. These pathways seem to be much more hydraulically active during the dynamic stage of infiltration before the cracks close. Three series of tests were conducted to evaluate the development of preferential flow paths, show the dynamics of flow through clay fractures, and explore the importance of preferential flow paths during infiltration. Flow tests were performed on desiccated slurry samples to investigate the impact of swelling on fractures. An apparent hydraulic conductivity defined for unsaturated conditions in fractured clay was shown to decrease by as much as three orders of magnitude as the fractures closed. Digital images of the samples before and after the flow tests showed closing of the fractures. Flow interruptions were used to show how the head can influence the rate at which fractures closed. Infiltration tests performed with a red dye showed the influence of preferential flow in compacted kaolinite samples. The dye stains dramatically illustrated the importance of preferential flow paths during infiltration. Fractures and clod size were found to have a strong influence on both the rate and depth of infiltration. These pathways may be especially important in governing contaminant transport in compacted clay barriers.