| Abstract: | Compacted soil liners are essential components of municipal and hazardous waste landfills. These liners should maintain hydraulic conductivity (K) of less than 1x10^-7 cm/s throughout the intended life of the landfill. Discrepancies between the hydraulic conductivity determined from field leakage rates and those obtained in the laboratory for the same soil have been attributed, in part, to the presence of preferential flow paths caused by desiccation shrinkage and cracking. A two-phased program was designed to evaluate factors affecting soil shrinkage and cracking and means to minimize shrinkage. In the laboratory phase, effects of moisture content, compaction effort, and compaction method on the shrinkage of three selected soils were determined. Soils were then stabilized with four different agents, and compared for shrinkage and hydraulic conductivity with unamended soils. Soil samples were then compacted into large permeameters (I.D. = 57.5 cm) to determine the effects of cracking on hydraulic conductivity in the field. For each soil, two sets of samples were used. The first set was prevented from drying prior to testing while the second set was allowed to dry. Then the K values of the samples were determined. In all three soils studied, both shrinkage and hydraulic conductivity correlated with the type and amount of clay in the soil. Shrinkage increased linearly with moisture content, and all additives with the exception of polypropylene fiber were effective in reducing the soil shrinkage. In addition, the additives influenced the hydraulic conductivity of the soils. Lime added to all three soils decreased shrinkage and increased the hydraulic conductivity. In general, cement decreased both the volumetric shrinkage and hydraulic conductivity. Addition of sand to soils decreased the amount of shrinkage while it increased the hydraulic conductivity. A study of desiccated samples showed that shrinkage can cause cracks which influence the K and are related to the amount of shrinkage. Results indicate that soil with volumetric shrinkage of less than 10% will resist increased K values. This amount of shrinkage corresponds to soils with plasticity index (PI) of less than 25. Thus, soils to be used for liner construction should either have volumetric shrinkage and PI of less than 10% and 25%, respectively, or will require amendments to bring these values into this range. |
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