Water has long been a major source of instability and deterioration in the strength of gypseous soil. This study examines the effect of adding a water-repellent agent on the collapsibility and shear strength of gypseous soil. This silane agent (also known as KH-570) is a low-viscosity organic silane solution that imparts water-repellent properties to surfaces and materials. Samples of disturbed soil containing 45% gypsum were collected from Salah Al-Din Governorate in Iraq at a depth of 1.5 meters below the natural ground surface. The additive concentration was selected by spraying and mixing with soil samples at 2%, 4%, and 6% by weight of the soil used, Consolidation and direct shear tests are conducted for both treated and untreated soil samples. Results show significant improvement in collapse potential values for treated soil in comparison with untreated ones. It was found that the optimal weight ratio for the additive is 2%, and this value is used when calculating shear resistance coefficients. In the case of semi-dry soil, the cohesion strength and the angle of internal friction increased by 89% and 9.3%, respectively., and for the soaked case were 119% and 250% respectively compared to the untreated soaked sample. Although KH-570 may have a relatively higher unit cost compared with traditional stabilizers such as lime or cement, the low required dosage (2%) and the significant improvement in waterproofing as well as reduce the long-term maintenance costs make it a cost-effective stabilization technique for gypseous soils.
