Characteristics of coal ash mixtures as replacement materials in ground improvement works

Abstract

Fly ash (FA) and bottom ash (BA) are two of the coal ashes by-products produced from coal-fired power plants. They are usually disposed off together as a waste in utility disposal sites with a typical disposal rate of 80% FA and 20% BA. The use of coal ash in construction projects that require large volume materials, such as in soil improvement works, is highly promising in solving the disposal problem. The aim of this research is to determine the suitability of FA-BA mixtures as replacement materials for soft soil foundation. Representative samples of coal ash were collected from Tanjung Bin Power Plant, Pontian, Johor. Six mixtures of FA and BA with different mix ratios of 0%, 30%, 50%, 70%, 90% and 100% FA content by weight had been used in this study. The coal ash mixtures were compacted at 95% of maximum dry density, sealed and cured for 0, 14, and 28 days before being analysed for material characterization, mechanical properties and chemical analysis. The performance of FA-BA mixtures as replacement materials had been determined through laboratory physical model tests. In model test, 40 cm height (H) of soft kaolin clay (about 20 kPa undrained strength) was replaced fully and partially by FA-BA mixtures (Hm) at replacement ratio, Hm/H = 0, 0.125, 0.375 and 1.0. In general, the results of the laboratory tests indicate good performance of FA-BA mixtures. Morphological analysis shows that the number of irregular shaped particles increased confirming change in material type with curing period. The results also show that mixtures with higher fly ash composition have less drainage characteristics but can be improved by prolonging the curing period. The shear strength of coal ash mixtures varied depending on the FA content. The maximum shear strength was obtained at the mixture of 50%FA with the friction angle values ranged from 270 to 370 that increased with curing period. The California Bearing Ratio (CBR) values increased while compressibility decreased with curing periods due to pozzolanic reaction. CBR values and compressibility of the mixtures also generally decreased with the increased of FA content. Results suggest that ash mixtures are non-corrosive while the heavy metals concentration is below the limit set by respective authority. Since the mixture of 50%FA-50%BA has the highest strength and considerably low compressibility, it can be concluded that this mixture is the most suitable mixture for replacement of soft soil. The result of physical model tests concluded the suitability of FA-BA mixtures as full or partial replacement materials of soft clay that gives promising effect in terms of decreasing the settlement of the footing placed on top of the soil. Based on this, preliminary design charts had been developed for the usage of FA-BA mixtures in geotechnical engineering works. This could help the engineers not only in designing the depth of soil to be replaced in soil improvement works but also in other purposes that rely on the strength of the eco-friendly ash mixtures.