Performance Evaluation of Bentonite Embedded Pond Ash as Landfill Liner

Abstract

Rapid urbanization and growth in population are key reasons for a massive increase in generation of solid wastes. This is becoming a major environmental concern. Landfilling is an extensively accepted practice for disposal of solid wastes. Conventionally, clay is used as a landfill liner due to its cost-effectiveness, high stability, and self-healing ability. The lack of suitable clay at a site often triggers the use of bentonite based materials as a substitute to since it is highly colloidal in nature with expanding lattice structure having high adsorption capacity and low hydraulic conductivity. However, compacted clays with higher bentonitic contents undergo extensive changes in properties when exposed to freeze-thaw or shrink-swell cycling. The problems associated with higher volume change and developments of cracks are found to be minimized by the addition of a substantial amount of non-swelling coarser fraction. Typically, soil-bentonite, sand-bentonite, zeolite-bentonite, etc. are used as an alternative liner material for waste disposal facilities. However, the scarcity of natural soil encourages the use of alternate materials as landfill liners.
In this context, the present research work focuses on assessing the suitability of pond ash-bentonite (PAB) mixture as an alternative to sand-bentonite (SB) mixture for liner material. As the coarse fraction of pond ash resembles natural sand in terms of gradation with higher interlocking and frictional properties, there is a potential for utilization of the coarser fraction of pond ash as a substitute to sand. The performance of the PAB mixtures is accessed by examining their physicochemical and hydro-mechanical properties. The physicochemical properties considered comprised of gradation, particle shape parameters, consistency limits, free swell index and cation exchange capacity, and the hydro-mechanical properties considered include unconfined compressive strength, shear strength parameters, hydraulic conductivity, compressibility and volumetric shrinkage characteristics. A comparative assessment is made between SB and PAB mixtures for a range of bentonite content varying from 0 to 30% by weight at an interval of 5% to ensure an effective substitution of sand with pond ash. The optimized bentonite contents are determined by examining the relevant properties of compacted PAB and SB mixtures as per the USEPA (1988) regulatory for liner material. Influential parameters such as clay content, molding water content, fibre content, and chemical environment on strength, durability, hydraulic and microstructural characteristics of PAB and SB mixtures have also been investigated.
The experimental results reveal that the addition of bentonite to sand or pond ash significantly influences the plasticity, strength, compressibility, permeability, and volumetric shrinkage properties. At comparable conditions, compacted PAB mixtures exhibit higher unconfined viii compressive strength (UCS), cohesion, frictional angle, hydraulic conductivity, and lower volumetric shrinkage than SB mixtures. Both PAB and SB mixtures met the liner requirements as per USEPA (1988) regulatory when compacted with modified Proctor energy at a minimum bentonite content of 20% and 15% respectively. Compression index (Cc) of the mixtures is found to maintain a linear relationship with liquid limit. Empirical equations have been developed to estimate the Cc and hydraulic conductivity of bentonite-based liner material from the basic parameters of the mixtures which have been compared and validated with existing corelations and experimental datasets. The compressive strength, failure strain, unit cohesion, and frictional angle of the compacted specimens are found to increase whereas the volumetric shrinkage strains are decreased with an increase in fibre content irrespective of the molding water content. With an addition of 1% Recron-3S polypropylene fibre, the UCS values are found to increase by 2 to 3 times as compared to those of unreinforced specimen. Both unreinforced and reinforced specimens exhibit the maximum UCS value at relative water content of 90% and 80% when compacted to standard and modified Proctor density respectively. Reinforced specimens compacted at dry of optimum did not show any significant variation in hydraulic conductivity whereas specimens compacted at wet of optimum exhibit an increased value of hydraulic conductivity with fibre content. The increase in volumetric shrinkage strain with relative water content is reduced as the fibre content increases. Furthermore, alternate freeze-thaw cycles are found to have a significant influence on volume, moisture content, unit weight, UCS, and hydraulic conductivity of the compacted mixtures. The changes in these properties are found to be stabilized after 10 freeze-thaw cycles. The rate of increment in hydraulic conductivity and reduction in compressive strength of compacted SB mixes are higher than those of compacted PAB mixes. Specimens compacted at dry of optimum exhibit lower reduction in compressive strength than those compacted at wet of optimum. In contrast, specimens compacted at dry of optimum show lower increment in hydraulic conductivity than those compacted at wet of optimum. The study on the effects of chemical environment on the engineering properties of PAB and SB mixtures reveals that the consistency limits, free swell indices, and volumetric shrinkage strain are reduced whereas the hydraulic conductivity increased as the concentration of salt solutions and their permeation time is increased. The UCS values are found to increase with the permeation of lower concentration of salt solutions and the same reduces substantially with further increase of concentration. Salt solutions of polyvalent cations are found to have more impact than those of monovalent cations. PAB specimens are less vulnerable to these changes as compared to those of SB specimens. The sorption concentrations of permeating cations are found to increase substantially in the liner materials with an increase of salt concentration and their permeation time, whereas the concentrations of inherently adsorbed elements decreased. The observed variations in the said ix properties associated with the influential parameters are correlated to the shape of coarse fraction particle and the corresponding microstructural arrangements of the compacted specimens.
Although, PAB mixtures experience marginally higher hydraulic conductivity than those of SB mixtures, the mixtures possess higher CEC, improved strength and lower volumetric shrinkage strain at a particular bentonite content and compactive effort. Further the engineering properties of PAB mixtures are less susceptible to adverse environmental conditions than SB mixtures. The use of pond ash as a coarse fractioned element in bentonite-based liner material will provide a greater avenue for its utilization as well as the preservation of the natural resources