Compacted soil–bentonite mixtures are finding wide application as buffer material for waste repositories for their favorable self-sealing qualities. The swelling properties of such materials which serve as a measure of their self-sealing capabilities and, thus, the efficiency of the repository in sealing off their contents from the environment are closely related to the chemistry of the leachate that emanate from the wastes. For this reason, the swelling parameters (namely swelling potential and pressure) of compacted lateritic soil–bentonite mixtures under consideration for use as barrier in municipal waste landfill were evaluated. Series of swelling potential and pressure tests were performed using variable content (0–10 %) of bentonite at predetermined optimum moisture content. Soil mixtures were compacted with British Standard Heavy compactive effort and saturated with processed tap water as well as three leachate solutions of varying ionic strength that were generated in active open dump landfills. Experimental results showed that swelling potential based on the free swell together with the maximum swell pressures of compacted soil mixtures measured at equilibrium increased approximately linearly with increase in the amount of bentonite when inundated with processed tap water and the three leachate solutions. On the other hand, these swelling parameters decreased as the ionic strength of the leachate solutions measured by their electrical conductivity increased for the various soil mixtures. These results provide an insight into the swelling behavior and the possible degradation in the efficiency of the proposed lateritic soil–bentonite mixtures in relation to their use as buffer material in waste landfills.

Swelling characteristics of compacted lateritic soil–bentonite mixtures subjected to municipal waste leachate contamination

In current geoenvironmental practice, design engineers usually require that soil liners in waste landfills be compacted within a specified range of water content and dry unit weight. This specification is based primarily on the need to achieve a minimum dry unit weight for factors controlling the performance of compacted soil liners most especially the hydraulic conductivity, k. In this study, lateritic soil treated with up to 10% bentonite, prepared at various compaction states (dry of optimum, optimum and wet of optimum moisture content) was compacted with four compactive efforts (i.e., the reduced British Standard Light, British Standard Light, West African Standard, and British Standard Heavy) to simulate the range of compaction energies expected in the field. Prepared soil mixtures were permeated with water and specimens that yielded the permissible limit of k ≤ 1 × 10−9 m/s were enclosed in an envelope (known as the acceptable zone) on the water content–dry unit weight curve. It was observed that compaction conditions resulting in moisture content slightly wet of optimum led to the lowest values of k and that the shapes and boundaries of the acceptable zones gradually increased in extent, shifting to wet side of optimum moisture content as the bentonite content increased to 10%. This approach provides good control over the quality of compacted soils and has great potential for field application.

Delineation of compaction criteria for acceptable hydraulic conductivity of lateritic soil-bentonite mixtures designed as landfill liners

This study was initiated to evaluate changes in the plasticity characteristics and related geotechnical properties of lateritic soil treated with low percent bentonite (up to 10% by dry weight of soil). The evaluation included some index parameters with very wide universal acceptance such as particle size distribution, Atterberg limits, compaction characteristics together with some engineering properties related to the plasticity characteristics namely swelling and shrinkage behaviour as well as hydraulic conductivity. Soil mixtures for the engineering properties tests were compacted with British Standard Light (BSL) compactive effort at predetermined optimum moisture content. From the results, it was observed that bentonite treatment generally caused an increase in plasticity of the soil. While soil mixtures containing 0 – 5% bentonite classifies as inorganic clay soils of intermediate plasticity (CI), mixtures with 7.5 – 10 bentonite are classified as CH soils in accordance with Unified Soil Classification System.  Correlation between bentonite content and the measured engineering properties of the resulting soil mixtures revealed an increase in optimum moisture content, free swell as well as desiccation induced shrinkage strain but a reduction in dry unit weight and hydraulic conductivity of specimens with higher bentonite content. Keywords : Bentonite, desiccation shrinkage, hydraulic conductivity, lateritic soil, plasticity characteristics,

Variations in plasticity characteristics of lateritic soil associated with bentonite treatment

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Variations in plasticity characteristics of lateritic soil associated with bentonite treatment