ABSTRACT
Compacted clay liners are an integral component of the lining systems for covering or capping new or old landfills or waste impoundments. As the use of laterites that is widespread in the tropics, for waste contaminant has not been quite documented, this study was aimed at assessing the influence of fines content on water and chemical flows through lateritic liners. Seven laterites (samples A-G) from Zaria (Lat. 11o 511N and Long. 7o 451E) environs were subjected to various laboratory tests such as index properties, differential thermal analysis, compaction, hydraulic conductivity, volumetric shrinkage, unconfined compressive strength, suction test for soil water characteristics curves (SWCC), batch equilibrium, and diffusion tests using three compactive efforts (i.e. British Standard Heavy (BSH), West African Standard (WAS), and British Standard Light (BSL)). From their index properties, Laterite A with the highest clay content, activity, and cation exchange capacity of 40%, 0.75, and 19.2meg/100g, respectively, is the most suitable liner material based on the specifications (Das, 1998) for municipal solid wastes liners. Some compositional variables (i.e. initial saturation, plasticity index, and clay content) obtained were further subjected to First Order Reliability analysis using a FORTRAN- based FORM programme as well as analysis of variance, while SWCC results were subjected to multiple regression analysis and analysis of variance. Results obtained indicate that the hydraulic conductivity is sensitive to compaction conditions, initial saturation, plasticity index, liquid limit and fines content. Furthermore, the shape of the SWCC depends on compaction conditions, and plasticity product of the soil. Soils with higher plasticity index have greater air-entry suction. Changes in SWCC were also reflected in the mineralogical composition of the soil. Based on a minimum hydraulic conductivity k ≤ 1x10-7 cm/s, minimum shear strength of 200 KN/m2, and a maximum volumetric shrinkage strain of 4%, acceptable zones for the seven laterites were delineated. vi The results of the diffusion tests were analyzed using an analytical solution to Fick's second law. The transport parameters obtained, which included the effective diffusion coefficient, D* (ranges from 0.003x10-8m2/s to 0.880x10-8m2/s for Na+; from 0.010x10-m2/s to 1.24x10-8m2/s for K+; from 1.120x10-8m2/s to 2.010x10-8m2/s for Ca2+), retardation factor (ranges from 2.481 to 3.653 for Na+; from 3.589 to 3.998 for K+; and from 5.821 to 7.011 for Ca2+), and tortuosity factor for Cl- (ranges from 0.241 to 0.328) are within the ranges of values found in literature; and are also sensitive to the plasticity product of the soils. The lateritic samples are compatible with the leachate. The breakthrough curves obtained show that the retardation of Na+ and K+ is a function of the soil compositional properties. Generally the fines content has an effect on the water and chemical flows through the compacted lateritic soils in the following way; as the fines content increases from 53% (Laterite G) to 70% (Laterite A), the hydraulic conductivity decreases from 2.42E-06 to 3.14E-08 for BSL specimens, 1.56E-06 to 2.91E-08 for WAS specimens, and 1.10E-06 to 2.76E-08 for BSH specimens. This shows that lower hydraulic conductivities are associated with soils compacted with the highest effort, BSH, followed by WAS specimens, while BSL specimens have the highest conductivities. Also, from the results of the reliability analysis, the fines content as well as the initial saturation, and compactive effort have a great influence in controlling the hydraulic conductivity of the compacted lateritic soils. Generally, as the fines content decreases, the hydraulic conductivity increases. Furthermore, the results show that Laterite G, with the least fines content, has the least retardation factor for Na+, K+, and Ca2+, and as the fines content increases to the highest in Laterite A, the retardation factor for the three cations increases, while their effective diffusion coefficient decreases from Laterite G to the lowest values in Laterite A.
