The Design of Bentonite-Sand Mixtures

One-dimensional and isotropic swelling tests, hydraulic conductivity tests and triaxial compression tests have been performed at applied stresses up to 450kPa on sodium bentonite powder, sand and compacted sodium bentonite-sand mixtures (5, 10, and 20% bentonite by dry weight). This was done to investigate the use of bentonite improved soils for waste containment, and study the fundamental geotechnical properties of bentonite-sand mixtures using a classical soil mechanics approach. It was found that air dried bentonite powder swells to reach a state described by a single straight line on a plot of void ratio against the logarithm of vertical effective stress, regardless of preparation technique. The gradient of this line was intermediate between a normal consolidation and rebound line for the same material indicating a different sample fabric when allowed to reach equilibrium from an initially dry state rather than the conventional fully saturated state. Swelling of bentonite-sand mixtures expressed in terms of the clay void ratio show a deviation from bentonite behaviour above a threshold stress which depends on the bentonite content. From this behaviour, a modified principle of effective stress has been proposed. Similar swelling relationships were found for samples under an isotropic confining stress. Hydraulic conductivity data for bentonite and mixtures indicate an approximately linear relationship between the logarithm of hydraulic conductivity and the logarithm of void ratio. Observed differences in hydraulic conductivity between bentonite and mixtures, when represented in terms of the clay void ratio, are attributed to the sand porosity and tortuosity. From a stress-dilatancy analysis of triaxial data, the peak strength of mixtures has been shown to depend on the sand relative density. This parameter indicates how the material will behave during shear. A threshold sand relative density has been postulated, which is dependent on the axial strain. Below the threshold value, it is likely that the stress-strain behaviour will be characteristic of the bentonite alone. A design model based on the clay void ratio, sand porosity and tortuosity, and sand relative density is presented, enabling the hydraulic conductivity or strength of a mixture to be estimated.