Controlling factors of the swelling of various bentonites and their correlations with the hydraulic conductivity of soil-bentonite mixtures

The controlling factors of the swelling of bentonites and its effects on the hydraulic conductivity of soil-bentonite mixtures were investigated using fifteen types of bentonites with a wide range of swelling capacity. The free swell of the bentonites increased linearly with increasing exchangeable sodium percentage below 30%, and almost remained constant beyond 30%. The free swell of the bentonite was correlated with the clay fraction, whereas, no correlation was observed with the methylene blue index and cation exchange capacity. Multiple regression analysis indicated that the variation in the free swell of the bentonites can be explained in terms of their specific surface area and exchangeable sodium percentage. A positive correlation was found between the free swell of the bentonites and the liquid limit of the bentonites and soil-bentonite mixtures. The hydraulic conductivity of the soil-bentonite mixtures at the void ratio corresponding to the maximum dry density decreased from 5.5×10^-8 to 5.3×10^-9cm/s with increase in the free swell from 6 to 20mL/2g, but remained almost constant beyond 20mL/2g. The hydraulic conductivity of the mixtures was found to be a function of the liquid limit of both bentonites and the soil-bentonite mixtures. It was also observed that the hydraulic conductivity of the mixtures containing Japanese bentonites meets the Japanese standards for engineered barriers.