Innovative analysis and advanced modeling of UCS and CBR in fly ash-treated soils: evaluating the impact of hydraulic index, chemical alteration, lime modulus, and geochemical indices

Unconfined compressive strength (UCS) and California bearing ratio (CBR) are key indicators of soil strength, particularly in fine-grained soils that often fail to meet project standards for roads and embankments. This study investigates the effects of fly ash on UCS and CBR, demonstrating an increase in both, though not symmetrically, due to varying percentages of chemical oxides in the soil-fly ash matrix. The relationships between UCS, curing time, chemical oxides (silica, alumina, calcium, magnesia, ferric), maximum dry density, and optimum moisture content (OMC) were analyzed. Three mathematical models, pure quadratic (PQ), interaction (IA), and full quadratic (FQ), were used to model UCS for 111 fly ash-treated and 49 untreated soils. While FQ and IA offered excellent predictions, their complexity led to applying geochemical indices like the hydraulic index (HI) and lime modulus (LM) to simplify the equations, with FQ remaining the most accurate. Sensitivity analysis showed that curing time was the most influential factor on UCS, followed by calcium oxide (CaO). When geochemical indices were applied, the hydraulic index (HI) emerged as the most significant factor. These findings underscore the importance of grouped chemical oxides, particularly SiO2, Al2O3, and Fe2O3, in enhancing soil properties, providing valuable insights for geotechnical engineering applications.