Stabilisation of Black Cotton Soil Using Fly Ash and Red Mud: Experimental and SPI Based Assessment

Expansive soils, such as Black Cotton Soil (BCS), present significant challenges to infrastructure development due to their high swell-shrink potential and poor geotechnical properties. This study explores the effectiveness of industrial by-products, Fly Ash (FA) and Red Mud (RM), as sustainable stabilising agents for BCS. A series of laboratory experiments were conducted to assess the effects of varying percentages (0–50%) of FA and RM on Atterberg limits, compaction characteristics, free swell index (FSI), and California Bearing Ratio (CBR). The results indicate that FA significantly reduces the liquid limit and plasticity index, but has limited influence on CBR, whereas RM optimally improves strength, compaction, and shrink-swell characteristics at 30% dosage. The study further introduces a stabilisation performance index (SPI) as a composite metric to assess overall effectiveness by integrating CBR, PI, MDD, and OMC. SPI analysis reveals that 30% RM without FA yields the highest performance (SPI = 0.83), while FA alone or in high proportions results in suboptimal or negative stabilisation impact. These findings highlight the potential of RM, both alone and in moderate combination with FA, as a viable alternative to traditional stabilisers for expansive soils. The study contributes to sustainable geotechnical practices by promoting industrial waste reuse for subgrade improvement.