Shear strength enhancement and optimum dosage prediction of fly ash-stabilized cohesive and cohesionless soils

Abstract

Fly ash, generated in large quantities as a by-product of coal-fired power plants, presents both an environmental disposal challenge and a potential resource for sustainable geotechnical applications. Soil stabilisation using industrial by-products has gained increasing attention as a low-cost, environmentally friendly alternative to conventional binders such as cement and lime, which carry significant carbon emissions and cost implications. In Bangladesh, the Barapukuria Coal-Fired Thermal Power Plant alone generates approximately 52,000 metric tons of fly ash annually, the majority of which is disposed of in ash ponds, creating both land-use and environmental concerns. The main objective of this study is to investigate soil stabilization using fly ash, with a particular focus on assessing its influence on the shear strength of both cohesive (clay) and cohesionless (sand) soils. To comprehensively evaluate performance, fly ash (FA) was incorporated into the natural soil at proportions of 8.5%, 12%, and 15%. The effectiveness of these additions was examined through direct shear and unconfined compression tests. Notably, the approach included determining the baseline properties of both sand and clay samples, then systematically introducing fly ash and retesting to identify changes in mechanical behavior. The observed pattern in the results demonstrates a non-linear behaviour with the increase of fly ash content. At 12% fly ash content, the sandy soil and clayey soil exhibited improvements of 27% and 34%, respectively. Beyond this content, strength declined due to excess fines and reduced particle bonding efficiency. The observed improvements result from different mechanisms: improved particle interlocking and densification in sand, and pozzolanic cementation in clay. A quadratic regression model is proposed to predict the optimum fly ash dosage and corresponding strength gain. In summary, these results confirm that adding 12% fly ash is highly effective for maximizing mechanical benefit and provides a practical framework for the sustainable stabilization of subgrades, embankments, and other geotechnical applications.