<![CDATA[Clay soil is frequently encountered in road and building construction, yet its low bearing capacity, high plasticity, and sensitivity to moisture variations can limit its suitability as a subgrade material. This study aims to characterize clay soil from the Kaligawe area of Semarang and evaluate the effects of fly ash, hydrated lime, and their combination on California Bearing Ratio (CBR) and Unconfined Compression Strength (UCS). An experimental laboratory approach was employed using untreated soil and stabilized specimens with additive contents of 2%, 4%, 6%, 8%, 10%, 12%, 15%, and 20% based on dry soil weight. The testing program comprised Atterberg limits, Standard Proctor compaction, soaked and unsoaked CBR, and UCS tests. The untreated soil exhibited a Liquid Limit of 65.375%, Plastic Limit of 27.611%, Plasticity Index of 37.764%, unsoaked CBR of 4.80%, soaked CBR of 3.10%, and UCS of 0.867 kg/cm². Fly ash achieved optimum CBR values at 15%, reaching 12.20% unsoaked and 8.80% soaked, whereas hydrated lime achieved 19.60% unsoaked and 17.00% soaked at 10%. The combined treatment produced the highest unsoaked CBR of 21.20% and UCS of 2.284 kg/cm² at 10% fly ash + 10% hydrated lime. UCS values for fly ash and hydrated lime individually reached 1.725 and 2.198 kg/cm², respectively. The findings demonstrate that combined stabilization effectively enhances clay performance, with 10% + 10% identified as the optimum composition.]]>
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