HYDRAULIC AND CONTAMINANT TRANSPORT PERFORMANCE OF COMPACTED LATERITIC SOIL" BENTONITE MIXTURES PROPOSED AS WASTE CONTAINMENT BARRIER

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Department of Engineering

ABSTRACT
Contamination of the geoenvironment due to unsafe waste disposal practices and leakage from waste repositories is a well-known and widespread problem. Emphasis has therefore been put on the hydraulic properties and contaminant transport capability of materials proposed for use as barriers in such facilities. Consistent with the current approaches for evaluating clayey soil liners, data on geotechnical, hydraulic and contaminant transport performance in addition to other relevant engineering properties of lateritic soil associated with bentonite treatment were developed and analyzed. Evaluation of lateritic soil - bentonite mixtures with bentonite content up to 10% has been carried out with respect to hydraulic conductivity, unconfined compression strength, volumetric shrinkage, unsaturated flow behaviour, compatibility with the municipal solid waste MSW leachate at the various compaction states (dry, wet and optimum water content) and compactive efforts (reduced British Standard Light, British Standard Light, West African Standard or ‘intermediate’ and British Standard Heavy). The diffusion properties of inorganic chemical species in MSW leachate through lateritic soil – bentonite mixtures has been characterized in terms of effective diffusion coefficients. Results indicate that unconfined compressive strength together with hydraulic conductivity of compacted samples of the mixtures decreased with increase in the amount of bentonite while volumetric shrinkage strain increased with increase in bentonite content for all the efforts. Acceptable ranges of compaction water content with the corresponding dry unit weight that will provide best performance with respect to the design parameters were delineated in an envelope on the moisture – density plane for the various soil - bentonite mixtures. At the various dosage levels of bentonite, the unsaturated hydraulic conductivity computed using the close form solutions of van Genuchten – Mualem (VGM) and Brooks and Corey - Burdine (BCB) models was essentially the same for specimens prepared either dry or wet of optimum water content. Hydraulic conductivity tests carried out to evaluate compatibility of specimens with MSW leachate showed increase in vii values by a factor of 1.2 to 1.7 at the end of test duration. These increases from a practical standpoint did not jeopardize the use of the mixtures since hydraulic conductivity at the end of the test was still below 1.0 x 10-9 m/s for mixtures that hitherto met this threshold value. Adsorption of metals followed the expected trend of increased metal adsorption characteristics with higher bentonite content. For the range of conditions examined, K cations were found to diffuse faster than the other two cations. The effective diffusion coefficients of the cations were found to be in the order of K+ > Na+ >Pb2+ and diffusion was seen to decrease as the bentonite content increased for all the cations. Evaluation of the influence of model parameters on the reliability associated with a laboratory based hydraulic conductivity model (a dominant failure mode for clay liner materials) for lateritic soil bentonite mixtures showed that reliability index decreased with increasing coefficient of variation of initial saturation, plasticity index and clay content. Both normal and lognormal probability distribution functions of hydraulic conductivity yielded approximately similar trends in reliability – coefficient of variation relationships. The change in reliability levels for variability in these parameters suggests that effective quality control measures must be put in place during construction in order to achieve the specified values for these variables. The result of this study highlights the capability of compacted lateritic soil – bentonite mixtures to function effectively both as hydraulic and contaminant barrier when used alone or in a composite construction in waste containment applications. It is expected to serve as a valuable tool for designers, researchers and regulators as information provided in this work may be helpful for a better understanding of soil materials of similar physical and engineering characteristics to be used as hydraulic barriers.