Soil stabilization is one of several ground improvement methods used around the world. Researchers in geotechnical engineering have investigated the use of waste materials as a cost-effective and environmentally friendly substitute for physical or chemical methods of soil stabilization, which have become increasingly expensive. The aim of this investigation was to analyze the impact of adding different proportions of quarry dust and water hyacinth ash on the properties of expansive soils, and to assess the resulting changes. It also makes a connection between the theory of unsaturated soil and the relevant engineering problem by analyzing the improvement effect of expansive soil utilizing the soil-water characteristic curve. This will then be utilized as a tool for the proper geotechnical engineering designs that rest on unsaturated soils. For laboratory analysis, soil samples were prepared with different quantities of quarry dust (5 %, 10 %, 15 %, and 20 % by weight) and water hyacinth stem ash (2 %, 4 %, 10 %, and 15 % by weight). The tests performed on these samples included particle size analysis, free swell tests, Atterberg limit tests, compaction tests, and CBR tests. The inclusion of quarry dust-water hyacinth stem ash (15 % QD + 10 % WHSA) resulted in notable enhancements in the geotechnical properties of the expansive soil, according to the test findings. The plasticity of the soil decreased by 68.3 %, while the CBR values increased from 1.21 % to 8.56 % after 7 days of curing and from 1.21 % to 8.8 % after 14 days of curing, as more quarry dust and water hyacinth stem ash were added to the mixture. The expansive soil's CBR swell value decreased from 10.85 % to 1.88% and 1.76 % after 7 days and 14 days of curing. Therefore, it was concluded that using a combination of quarry dust and water hyacinth stem ash significantly enhances the engineering properties of expansive clay soil. The expansive soil's maximum dry density (MDD) increased from 1.22 gm/cm3 to 1.321 gm/cm3 and 1.336 gm/cm3 after 7 and 14 days of curing, respectively. This satisfies the subgrade requirement. The plastic index also decreased from 48.9 % to 15.5 % and 11.6 % after 7 and 14 days of curing, respectively, meeting the standards set by ERA. Therefore, quarry dust and water hyacinth stem ash can be utilized as a substitute stabilizer. When the stabilization dosage percentage goes up, the air-entry value decreases in response. The findings of this study indicate that the SWCC fitting parameters 'a' and 'm' decrease, while parameter 'n' increases, when stabilizers cause the accumulation of clay particles through cation exchange with the clay soil particles. This results in the formation of a more open, hard, and aggregated soil structure, which leads to an improvement in the expansive soils. Keywords: Expansive soil, unsaturated soil, pressure plate, SWCC