Effects of calcium concentration and differential settlement on permeability characteristics of bentonite-sand mixtures

Liner systems consisting of clay or bentonite-sand mixtures can be installed on the bottom of landfills to prevent contaminants from seeping into the surroundings. Differential settlement of the foundation and presence of heavy metal or alkali metal ions in the leachate can vastly affect permeability characteristics of the liner system. In this study laboratory model tests were performed to investigate the influence of differential settlement and Ca2 + on permeability characteristics of bentonite-sand mixtures. The results showed that when the thickness of bentonite-sand mixtures was 100 and 200 mm, cracks were formed within the bentonite-sand mixtures with respective deflections of 2.5 and 7.0 mm. When the height of developed cracks exceeded 50% and 70% of the thickness, the permeability characteristics of bentonite-sand mixtures was > 10− 7 cm/s. Ca2 + concentration has great impact on the permeability characteristics of bentonite-sand mixtures. Na-bentonite will be converted to Ca-bentonite when the concentration of Ca2 + is two times greater than that of Na+ in the solution, and the anti-seepage performance of bentonite-sand mixtures will consequently deteriorate.