Lean clay soils are considered among the most problematic soil types in civil engineering due to their low shear strength and high compressibility, which necessitate remediation to limit post-construction settlement and ensure adequate bearing capacity. Chemical stabilization is widely adopted for improving such weak ground conditions, with cement and lime traditionally used as stabilizing agents. However, the high economic and environmental costs associated with these materials have encouraged researchers to explore more sustainable alternatives. The effect of supplementary stabilizing materials such as Fly ash (FA), Ground Granulated Blast Furnace Slag (GGBS) and Silica Fume (SF) on the unconfined compressive strength of soft soil are studied in this research work. The research aims to determine the strength characteristics (UCS) of varied eight mix proportions of (Fly Ash-GGBS-Silica Fume) are experimentally studied in the research. The unconfined compressive strength (UCS) test was conducted at curing periods of 3, 7, and 28 days to evaluate the performance of the treated soil. The results showed an increase in soil strength with the addition of these materials compared to natural soil. Among all mixtures, the R3 sample (20% fly ash, 9% GGBS, and 5% silica fume) exhibited the highest compressive strength, reaching 955 kPa at 28 days, indicating that this mix represents the best-performing tested mixture within the investigated dosage range for soil stabilization. The California Bearing Ratio (CBR) results showed significant improvement in the bearing capacity of the treated soil compared with untreated soil. This enhancement was mainly attributed to pozzolanic and cementitious reactions that produced C–S–H gel as reported in previous studies, improving particle bonding and reducing voids. Within the investigated dosage ranges, the mixture containing 20% Fly Ash, 9% GGBS, and 5% Silica Fume showed the best overall engineering performance in terms of UCS and CBR. Fly Ash improved soil structure, while GGBS increased soil stiffness and strength. Silica Fume enhanced soil densification and penetration resistance due to its ultrafine particles. However, excessive additive contents caused a slight reduction in CBR values because of increased fines content and water demand.
