Four-field Coupled Equation Correction of Hydrothermal Salt Force for Embankment of Coarse Particle Saline Soil and Its Experimental Verification

This study aimed to elucidate the water-salt transfer mechanism, understand the effect of partition in the subgrade, and establish the relationship between water-salt transfer and salt expansion and thaw collapse in the subgrade. Coupled differential equations of water-heat-salt-stress for embankment of coarse-grained sulfate saline soil were established basing on the theory of seepage and heat conduction in unsaturated soil; considering the influence of water-salt phase change on temperature field, water field, salt field, and stress field; and correcting the established temperature, water, and salt fields that satisfy the engineering characteristics of coarse-grained sulfate saline soil subgrade. Simultaneously, a water-heat-salt-stress four-field coupled numerical model of saline soil subgrade was established by using COMSOL Multi-physics software, and the validity of the established mathematical model was verified by the results of an indoor large-scale coarse particle saline soil freeze-thaw cycle test. The results show that in the freeze-thaw cycle with groundwater recharge, the depth of the low-temperature-sensitive area of gravel sulfate saline soil subgrade is about 45 cm and the location of the geotextile partition should comprehensively consider the influence of regional low temperature on sensitive depth and the strong rise of the capillary water in the subgrade filler. Under the effects of capillarity and temperature, the water and salt in the gravel saline soil subgrade transfer to the cold end, leading to salt accumulation under the geotextile and the strong enrichment of sulfate ion concentration. The geotextile partition in the subgrade can effectively hinder the longitudinal migration path of water and salt, and weaken the transfer of water and salt to the low-temperature-sensitive area, thereby reducing the effect of salt expansion and thaw collapse in the subgrade. In addition, the degree of reduction can reach more than 30%. The obtained calculation model can effectively predict the water-salt transfer mode, thereby providing a theoretical support for the design and construction of coarse-grained sulfate saline soil subgrade.