The influence of hydraulic gradient and rate of erosion on hydraulic conductivity of sand-bentonite mixtures

The use of sand-bentonite mixtures as liner materials for waste disposal is very common. In the laboratory, this study investigated hydraulic conductivities of such mixtures at different hydraulic pressure (hydraulic gradient), dry unit weights, and bentonite contents. The bentonite content and the dry unit weight of the samples were both important factors, significantly affecting the hydraulic conductivity of the liner material. A bentonite content of 5% was found to be sufficient in reaching a hydraulic conductivity under 10(-9) m/s, when the liner material was compacted under near optimum moisture content. Nevertheless, hydraulic conductivity was found to increase with hydraulic pressures, especiallyfor the 5% bentonite mixtures subjected to pressure above 40 kPa, suggesting some degree of internal erosion (washing out of particles). Therefore, this paper discuses the influence of internal erosion of the mixtures under a given hydraulic gradient, on the final value of k. The internal erosion of the tested mixtures was found to be influenced mainly by porosity, which can be reduced by properly selecting the sand particle size distribution and the bentonite percentage. Furthermore, this study proposed an empirical expression to predict the risk of internal erosion in the sand-bentonite mixtures, and therefore of k being higher than planned. This expression can be used for designing bentonite content and compaction to achieve very low permeability.