Abstract: Solute ions migrate, collide with and associate with both free water and bound water, indicating a virtual semi-permeable membrane effect in pore water. Electrostatic attraction draws some mobile ions into bound water, converting them into immobile ions. Spatially nonuniform ion distribution creates a chemical potential in pore water, causing a discrepancy between the actual pore-water pressure and its measured value in unsaturated saline soil. Therefore, the effective osmotic suction is used to characterize the chemical factors to describe the mechanical behavior of unsaturated saline soil. By clarifying the types and characteristics of pore water and ions in soil, considering the non-movable ions in liquid water, the calculation method of effective osmotic suction is determined by a representative elementary volume (REV). The contribution of chemical potential in pore water to Gibbs free energy of gas-liquid interface is analyzed. Based on soil-skeleton stress-balance analysis and the fact that the surface tension of the gas–liquid interface satisfies the Young–Laplace equation, we propose a generalized effective-stress equation for unsaturated saline soil that accounts for capillary and physicochemical effects. For loess with varying salinity and matric suction, isotropic compression and shear tests under controlled suction were conducted using an unsaturated true-triaxial apparatus. The relationship between the effective stress and isotropic deformation, the shear strength under different intermediate principal stress parameters of the saline loess is obtained, and the accuracy of the generalized effective stress equation is verified.

Key words: unsaturated saline soil, physicochemical effect, effective stress, effective osmotic suction, unsaturated true triaxial test