Investigation of the mechanical behavior of saline loess using unsaturated true triaxial tests and a constitutive framework

• A modified unsaturated true triaxial test to obtain detailed mechanical behavior of saline loess in the Xining Basin, located in the northeastern region of the Qinghai-Tibet Plateau in China. • The mechanisms of the effects of pore water salinity on the mechanical behavior of unsaturated saline loess are proposed. • An empirical constitutive framework (BBM-π) that considers osmotic suction is proposed, establishing three-dimensional loading-collapse (MOLC) surface and three-dimensional shear yield surface models. Unsaturated saline loess, characterized by significant dissolution collapsibility, is prevalent in arid and semi-arid regions (e.g., Northwest China) due to plateau and inland arid climates. The uneven distribution of soluble salts in these regions poses severe threats to pedestrian safety and infrastructure integrity, causing phenomena such as road heave and subgrade corrosion. We aim at exploring the impact exerted by pore fluid salinity (osmotic suction) and matric suction on the mechanical properties of unsaturated compacted saline loess under three-dimensional stress conditions. Saline loess from the Xining Basin in the northeastern Tibetan Plateau served as the subject of this research. An advanced suction-controlled true triaxial apparatus with rigid-flexible boundaries was utilized to perform isotropic compression and consolidated drained shear tests on loess samples with varying salinities under suction-controlled conditions. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were employed to investigate the pore morphology and pore size distribution (PSD), analyzing the relationship between the change of the loess microstructure induced by salinity (osmotic suction) and its macroscopic mechanical properties. The findings indicate that increased salinity leads to the contraction of micro-pores and expansion of macro-pores in loess, thereby enhancing the compressibility and reducing the yield net mean stress. Post isotropic consolidation, macro-pores induced by elevated salinity are prone to collapse, which promotes particle compaction. Compared to non-saline loess, the effective contact area between particles in saline loess increases, enhancing its shear strength. Drawing from the Barcelona Basic Model (BBM), a semiempirical constitutive framework (BBM-π) that considers osmotic suction is proposed, establishing three-dimensional loading-collapse (MOLC) surface and three-dimensional shear yield surface models. As osmotic suction increases, the elliptical yield surface contracts longitudinally (along the long axis) and expands laterally (along the short axis).