This paper considers the engineering behaviour of bentonite-enhanced sand (BES) mixtures in relation to their performance as environmental barriers. Data on the swelling and hydraulic conductivity are presented. At low effective stresses the bentonite within BES mixtures swells sufficiently to separate the sand particles. In such states two factors affect the void ratio reached by the bentonite after swelling, the ionic concentration of the pore solution and the bentonite fabric after compaction. Bentonite swelling is very sensitive to the pore solution concentration because increasing concentration suppresses the diffuse double layer component of swelling. Remoulding during compaction can result in a slight reduction in bentonite swelling, probably because of disruption to the cluster-based fabric of bentonite. At high effective stresses the bentonite has insufficient swelling capacity to force the sand particles apart, and the sand pore volume thus limits swelling.A model to predict the swelling and hydraulic conductivity of BES in distilled water and various salt solutions is described. This model requires the swelling behaviour and hydraulic conductivity of the bentonite in the relevant solution, and the compressibility and porosity of the sand component as input parameters. Soil tortuosity is used as a fitting parameter, and is estimated from Archie's equation. Application of this model to the swelling of compacted mixtures is shown to produce a good fit with the experimental data.
The swelling behaviour and hydraulic conductivity of Na-bentonite powder and bentonite-sand mixtures (10 and 20% of bentonite by dry weight) have been measured with distilled water and various salt solutions (0.01, 0.1 and 1 mol/l concentrations), It was found that in dilute solutions, the bentonite in mixtures subjected to small confining stresses swells sufficiently to separate the sand particles and reach a clay void ratio similar to that achieved by bentonite alone. At high stresses, or in strong solutions, the bentonite in a mixture has insufficient swelling capacity to force the sand particles apart and swelling is limited by the sand pore volume. The hydraulic conductivity of a mixture depends on the bentonite void ratio, and the porosity and tortuosity of the sand matrix. A design model is proposed to predict the engineering properties of a mixture over a range of confining stresses from the properties of its constituents and the permeant.
This paper considers the engineering properties of bentonite-enhanced sand (BES) mixtures in relation to their performance as landfill liners. Data on the swelling, hydraulic conductivity, strength and desiccation of such mixtures are presented. The hydraulic conductivity depends on the proportions of sand and clay, the confining stress and the permeating fluid. The drained strength of mixtures typically used as landfill liners depends primarily on the relative density and critical state friction angle of the sand component. The susceptibility to desiccation cracking is a function of the moisture content and the proportion of bentonite used in the mixture. A model for BES mixtures is proposed that can be used to predict their swelling and hydraulic conductivity in water and salt solutions. The model requires the swelling behaviour and hydraulic conductivity of the bentonite in the relevant solution, and the compressibility, porosity and tortuosity of the sand component as input parameters. Sand tortuosity is estimated. The strength can be predicted using the void ratio of the sand component calculated from the proposed model.
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