The effect of fly ash addition and bottom ash substitution on the gradation and shear strength of clay soil

Abstract Soil is a material that is always associated with civil construction technology, serving as the base support for structures and the location of foundations. One problematic soil type when used as a construction base is clay, which requires improvement efforts to enhance its bearing capacity. This study aims to determine the effect of adding fly ash and substituting bottom ash as stabilizing agents for clay soil on its gradation and shear strength values. The fly ash content was fixed at 10%, while the bottom ash substitution was varied at 10%, 15%, and 20%. The clay soil utilizied in this study was classified using the Unified Soil Classification System (USCS), with testing conducted according to Indonesian National Standards (SNI). The results show that the percentage passing the No. 200 sieve for the TL+10%FA+10%BA variation was 44.346%, TL+10%FA+15%BA was 43.013%, and TL+10%FA+20%BA was 41.070%. The continuous decrease in the percentage passing indicates a shift from fine particles to coarser particles, due to the coarse gradation of the bottom ash. Shear strength values showed a continuous increase with each variation. The highest shear strength value was found in the TL+10%FA+20%BA variation, reaching 55.456 kN/m 2 . This increase is attributed to the bottom ash contributing to a more varied gradation and the fly ash filling the voids between soil particles, thereby enhancing inter-particle bonding.