Volumetric and hydraulic behaviour of compacted natural clay–sand mixtures during wet–dry cycles

Landfill liners located in the active zone are subjected to hydraulic and volumetric changes with time after their installation due to wet–dry (W–D) cycles. Drying results in the formation of desiccation cracks which increases the hydraulic conductivity, whereas the swelling aids in the closure of cracks. Recently, the clay–sand mixtures have been widely used as landfill liners since the clay–sand mixtures in correct proportions are very effective in imparting low hydraulic conductivity with fewer desiccation cracks and better stability. However, the studies that bring out the effect of W–D cycles on volumetric and hydraulic behaviour of compacted natural clay soil–sand mixtures are not available. In this study, two different natural clay–sand mixtures were prepared with low (32%) and high (68%) sand contents. Cyclic W–D tests and hydraulic conductivity tests were performed on these compacted natural clay–sand specimens. X-ray CT tests were also carried out to examine the change in internal structure and propagation of cracks towards the interior of the specimen during drying. Addition of 32% sand to the expansive soil resulted in the reduction of volumetric deformations by 19% and also the desiccation cracks, whereas the addition of 68% sand led to the reduction of 36% volumetric deformations without any desiccation cracks. Even though higher hydraulic conductivities were observed for both the compacted natural clay–sand specimens, the specimen with 32% sand content satisfied the hydraulic conductivity criterion at higher confining pressures, making it the most suitable liner material among the various natural clay–sand combinations considered.