Investigation of Hydraulic Conductivity and Matric Suction in Sand–Bentonite–Coal Ash Mixes

Abstract

Compacted bentonite–sand/coal ash mixtures have been used nowadays as backfilling materials for nuclear waste disposal systems and as a landfill liner as they possess low permeability and good adsorption capacity, which proves to be beneficial in order to encapsulate waste from the surrounding environment. This paper deals with the evaluation of matric suction using pressure plate apparatus and filter paper technique on compacted sand–bentonite–coal ash mixtures. While the bentonite content was varied from 5, 10, 15, 20, 30, 40% in case of sand–bentonite mixes 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 at fixed bentonite contents of 10, 15 and 20%. 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. The air entry value (AEV) and residual suction (Ψr) values decreased as the effective grain size (D 10) of soil increased. A distinction in the values of the AEV and Ψr was observed using the Van Genuchten, Fredlund and Xing Models. The effect of varying bentonite content for sand–bentonite, coal ash–bentonite mixes and varying coal ash content in case of sand–bentonite–coal ash mixes was ascertained upon the SWCC modelling parameters viz. a, n, m. It was concluded that the parameter ‘a’ is directly related to the air entry value while ‘n’ and ‘m’ follow a reverse relationship as the ‘m’ parameter decreases with an increase in the bentonite content. It was concluded from the study that suction pressure is an important factor related to the performance evaluation of the bentonite enriched soils to serve as a barrier material as the soil tends to be in the unsaturated condition throughout the year. As per the chosen materials in this study, i.e. bentonite and coal ash, it can be witnessed that the addition of high percentages of bentonite and coal ash to the soil lead to higher suction pressures, which may prove to be detrimental to the stability of the landfills. Also, keeping in mind the statutory requirement of low hydraulic conductivity for use as a liner material, hydraulic conductivity of soil mixes was determined in both saturated and unsaturated state.