Soft clay soils generally exhibit low bearing capacity, high plasticity, and limited shear resistance, making stabilization necessary for construction applications. This study investigates the effect of foamed mortar addition at 0%, 15%, 20%, and 25% on the physical and mechanical properties of soft clay obtained from Semarang City, Indonesia. The research employed a quantitative experimental approach through laboratory testing, including grain-size analysis, Atterberg limits, direct shear testing, Standard Proctor compaction, and laboratory California Bearing Ratio (CBR). The untreated soil was dominated by clay-sized particles at 89.88% and exhibited a specific gravity of 2.487, liquid limit of 53.21%, plastic limit of 29.82%, and plasticity index of 23.39%. The untreated soil also showed an internal friction angle of 11.06°, optimum moisture content of 34.00%, maximum dry density of 1.27 g/cm³, and unsoaked CBR of 5.10%. Increasing the foamed mortar content substantially modified the grain-size composition and improved the mechanical performance of the soil. At 25% foamed mortar content, the internal friction angle increased to approximately 34.50°, while the maximum unsoaked CBR increased to 25.70%, approximately 5.04 times the untreated value. The findings demonstrate that foamed mortar is effective in improving the shear resistance and bearing capacity of soft clay, with the 25% mixture providing the highest documented mechanical performance.
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