A Review of the Geotechnical Characterisation of Indian Coal Ash

ABSTRACT Coal is a reliable source of energy, and its combustion generates two primary by-products: fly ash and bottom ash. A significant portion of fly ash is utilised by the cement industry; however, the remaining fly ash, along with bottom ash, is often mixed with water and stored in ash ponds, which is later known as pond ash. To effectively use coal ash as a construction material, it is essential to understand its engineering properties, including volume change behaviour, stress-strain response, particle dilation, effect of particle crushing, and the appropriate material model to capture coal ash’s engineering behaviour. This paper provides a comprehensive review of the geotechnical properties of coal ash. Coal ash possesses lower specific gravity, which results in a lower unit weight. The coal ash is non-plastic, and its grains are sand to silt size. Its permeability is comparable to that of silt or fine sand-silt mixtures. The collapse potential of bottom ash is lower than that of fly ash at the same vertical stress level. The characteristics of the Critical State Line (CSL) and Normal Compression Line (NCL) in the e-ln p’ space closely resemble those of granular soils, with some exceptions. Coal ash exhibits both strain hardening and softening stress-strain behaviour, along with contractive volume change behaviour. Significant amounts of coal ash particles are crushed during shearing, which leads to a stress-dependent internal friction angle. The mechanical behaviour of coal ash resembles that of calcareous sand more than that of natural soil.