Influence of water repellency on granite residual soil

Purpose This study explores the use of octadecylamine (ODA) as a surface modifier to treat granite residual soil, aiming to enhance its water repellency and address the threat to engineering safety posed by the soil’s water sensitivity. Design/methodology/approach In this study, the surface wettability of granite residual soil with ODA addition was investigated through a series of tests, including contact angle measurement, water drop penetration time (WDPT) test, soil–water characteristic curve (SWCC) test, water loss test, scanning electron microscopy analysis, disintegration test and gray correlation analysis. Findings The water repellency of granite residual soil modified with ODA is effectively improved. Moreover, the surface contact angle increases with the rise in ODA content and influences the soil’s surface wettability in three aspects: It exhibits stable wettability under low moisture content conditions and the failure of the hydrophobic barrier under high moisture content conditions; the increase in contact angle reduces the soil’s matric suction and inhibits capillary action, leading to the attenuation of the SWCC hysteresis effect; The increase in contact angle accelerates the water loss rate of granite residual soil. In addition, the incorporation of ODA significantly enhances the anti-disintegration performance of granite residual soil, and the effective suppression of structural degradation under multiple wet–dry cycles further verifies the durability of ODA modification. Originality/value Existing improvement technologies mainly focus on chemical solidification methods such as cement and lime or traditional structural modification approaches like microbial-induced calcium carbonate precipitation (MICP). Chemical reagents including cement and lime enhance the cementation between soil particles through complex physical and chemical reactions with clay minerals, but they are associated with high carbon emissions and pose potential hazards to the environment. The relatively environmentally friendly MICP technology improves soil stability by depositing calcium carbonate crystals between soil particles to fill pores; however, it is sensitive to soil pH and difficult to regulate. In this study, ODA is used as a surface modifier, which can regulate the hydraulic migration of granite residual soil by altering its surface properties without changing the soil structure. This provides a new technology and insight for the prevention and control of geological hazards in granite residual soil areas.