Macro-micro study on mechanical properties of sulfate saline soil under multi-factor conditions

The mechanical properties of sulfate saline soil are governed by the coupled effects of salt, water, and temperature. This study integrates macroscopic direct shear tests with microscopic nuclear magnetic resonance (NMR) analysis to systematically examine how salt content and water content jointly influence the soil’s mechanical behaviour and pore structure. The role of freezing temperature during freeze-thaw cycles is also investigated. Results show that higher salt content mitigates the damaging effect of high water content on pore expansion and mechanical strength, while high water content diminishes the influence of salt on pore structure. During early freeze-thaw cycles (1–3 cycles), soil strength increased by 42%, whereas after more cycles (6–8 cycles), it decreased by 69% due to ice-induced pore expansion. The most pronounced pore structure changes occur during the initial freezing stage, with further cooling having limited effect. Soils with moderate salt and water content exhibit the most stable pore structure under freezing conditions. These findings reveal the macro–micro mechanism governing saline soil behaviour under multi-factor coupling, offering a theoretical basis for saline soil engineering design and maintenance.