Improvement of Barrier Soil Properties with Fly Ash to Minimize Desiccation Shrinkage

Barrier systems built with fine grained soils frequently loose their hydraulic integrity due to desiccation cracking either during construction or shortly thereafter. Moreover, typical specifications for the construction of compacted soil liners and covers require that the soil be compacted wet of optimum water contents to achieve the lowest possible hydraulic conductivity, a condition that results in high desiccation shrinkage values. However, such soils can be treated with fly ash to maintain low and tolerable desiccation shrinkage strains. In this study, volumetric shrinkage strains of representative fine grained soil containing 0 - 20% fly ash by dry weight of soil compacted with the British Standard Light (BSL), West African Standard ( WAS) and British Standard Heavy, (BSH) compaction efforts at moisture contents ranging from 10 - 20% were evaluated. Measurements indicate that volumetric shrinkage strain decreased with higher fly ash content and that fly ash effectively reduced the shrinkage of untreated soil prepared wet of optimum from 4.4 - 7.7% to values well below the 4% threshold. The measured shrinkage strains were related to water content and dry unit weight on the dry unit weight - moulding water content curve in what is referred to as acceptable zone. Data points within the acceptable zone represent test results with shrinkage strain <= 4% which ensures compaction efficiency. This study therefore established that fly ash application and appropriate regulation of the moulding water content are feasible means of reducing the risk of barrier soil damage by shrinkage cracks while still realizing very low hydraulic conductivity and adequate strength.