Predicting the properties of bentonite-sand mixtures

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One-dimensional swelling tests and hydraulic conductivity tests have been performed at vertical effective stresses up to 450 kPa on Na-bentonite powder and compacted sand/Na-bentonite mixtures (5, 10 and 20% bentonite by weight) to investigate the use of bentonite-improved soils for waste containment. It was found that bentonite powder swells to reach a final state described by a single straight line on a plot of void ratio against the logarithm of vertical effective stress, regardless of preparation technique. Swelling of sand/bentonite mixtures expressed in terms of the clay void ratio show a deviation from bentonite behaviour above a stress which depends on the bentonite content. Hydraulic conductivity data for bentonite and sand/bentonite mixtures indicate an approximately linear relationship between logarithm of hydraulic conductivity and logarithm of void ratio. A design model based on the clay void ratio, and the sand porosity and tortuosity is presented enabling the hydraulic conductivity of a mixture to be estimated.