Currently, nearly 170 million tonnes of coal ash is generated annually in India from 85 thermal power plants, and because of the limited availability of land, vertical expansions of ash dykes are unavoidable. The present paper compares the variability in engineering behaviour of coal ash from two different thermal power plants (TPP) A and B and highlights the influence of the variability in coarse particle content on design of ash dykes. Using the geotechnical properties of compacted ash as well as hydraulically deposited ash, ash dyke sections have been designed and a series of stability runs are carried out to map the factor of safety at various stages of ash dyke raising. The results indicate that slopes of ash dykes have to be made flatter as bottom ash content decreases in pond ash due to increased utilization or separate storage of bottom ash. © Springer Nature Singapore Pte Ltd 2019.