Swell-shrink and hydraulic behaviour of compacted red soil-bentonite mixture

Clay liners are provided in waste landfills to prevent the leachate from percolating into underlying soil and ground water and polluting it. Hence, soils used as landfill liners must possess low hydraulic conductivity (< 10 −7 cm/s). In the initial as-compacted state, liners satisfy this design criterion. However, in the field, liners are subjected to alternate wet-dry cycles and the initial microstructure of liners change, thereby affecting their hydraulic properties. The present study was conducted on a locally available red soil mixed with bentonite so that it satisfies the requirement of hydraulic conductivity in the as-compacted state. To achieve this objective, identical specimens compacted at optimum moisture content to its maximum dry density were inundated with distilled water under a surcharge load of 12.5 kPa. Upon attainment of maximum swelling, hydraulic conductivity tests were conducted at swollen state under a hydraulic gradient of 20. The soil specimens were then dried at a temperature of 45±2 °C. The weight, height and diameter of specimens were measured continuously during drying to study the shrinkage behaviour of the soil. The dried specimens were subjected to subsequent wet-dry cycles until the specimens reached an equilibrium state and the hydraulic conductivity was determined after each wetting cycle. SEM images were also captured to analyse the changes in the soil structure in the as-compacted state and at the end of different wetting cycles.