A Critical Evaluation of the Suction and Swelling Characteristics of Sand–Bentonite–Coal Ash Mixes

In recent times, compacted bentonite–sand/coal ash mixtures have been suggested as backfilling materials for waste disposal systems. These soil mixes containing mineral montmorillonite from the varying percentage of bentonite may exhibit swell/shrink behaviour with fluctuating moisture conditions. This paper presents the results of matric suction using pressure plate apparatus and swelling characteristics in terms of swelling pressure, free swell and swell potential on compacted sand–bentonite–coal ash mixtures. Bentonite content was varied as 5, 10, 15, 20, 30, 40% in case of sand–bentonite mixes; where as it was kept as 5, 10, 15, 20% for coal ash–bentonite mixes. Coal ash percentage was varied from 10, 20, 30, 50% for sand–bentonite–coal ash mixes. Data was fitted using two models—Van Genuchten, Fredlund and Xing to obtain the soil water characteristic curve (SWCC) and to estimate the SWCC variables for different soil mixes. Test results depict that soils with high bentonite content exhibited a prominent swelling behaviour, while addition of coal ash had a stabilizing effect on the mix by reducing the swelling characteristics. The air entry value (AEV) and residual suction (Ψr) values decreased as the effective grain size (D₁₀) of soil increased. A distinction in the values of the AEV and Ψr was observed using the Van Genuchten, Fredlund and Xing Models. Also, keeping in mind the statutory requirement of low hydraulic conductivity for use as a liner material, hydraulic conductivity of soil mixes was ascertained in both saturated and unsaturated state. Addition of 10% bentonite to sand and coal ash both is recommended for use as liner and cover materials in case of engineered landfills considering the hydraulic conductivity requirement and swelling characteristics.