The swelling behaviour and hydraulic conductivity of Na-bentonite powder an
d 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 s
eparate the sand particles and reach a clay void ratio similar to that achi
eved by bentonite alone. At high stresses, or in strong solutions, the bent
onite in a mixture has insufficient swelling capacity to force the sand par
ticles apart and swelling is limited by the sand pore volume. The hydraulic
conductivity of a mixture depends on the bentonite void ratio, and the por
osity and tortuosity of the sand matrix. A design model is proposed to pred
ict the engineering properties of a mixture over a range of confining stres
ses from the properties of its constituents and the permeant.