Shear Strength Enhancement and Optimum Dosage Prediction of Fly Ash–Stabilized Cohesive and Cohesionless Soils

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 internal friction angle of sandy soil increased from 29° to 37°, and the cohesion of clayey soil increased from 79.6 kN/m² to 106.4 kN/m², representing 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.