Sudden temperature drops cause soils in natural environments to freeze unidirectionally, resulting in soil expansion and deformation that can lead to damage to engineering structures. The impact of temperature-induced freezing on deformation and solute migration in saline soils, especially under extended freezing, is not well understood due to the lack of knowledge regarding the microscopic mechanisms involved. This study investigated the expansion, deformation, and water-salt migration in chlorinated saline soils, materials commonly used for canal foundations in cold and arid regions, under different roof temperatures and soil compaction levels through unidirectional freezing experiments. The microscopic structures of saline soils were observed using scanning electron microscopy (SEM) and optical microscopy. A quantitative analysis of the microstructural data was conducted before and after freezing to elucidate the microscopic mechanisms of water-salt migration and deformation. The results indicate that soil swelling is enhanced by elevated roof temperatures approaching the soil's freezing point and soil compaction, which prolongs the duration and accelerates the rate of water-salt migration. The unidirectional freezing altered the microstructure of saline soils due to the continuous temperature gradients, leading to four distinct zones: natural frozen zone, peak frozen zone, gradual frozen zone, and unfrozen zone, each exhibiting significant changes in pore types and fractal dimensions. Vacuum suction at the colder end of the soil structure facilitates the upward migration of salt and water, which subsequently undergoes crystallization. This process expands the internal pore structure and causes swelling. The findings provide a theoretical basis for understanding the evolution of soil microstructure in cold and arid regions and for the management of saline soil engineering. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Coral sand, as a kind of filling material, is widely used in the construction of artificial reefs or roadbeds in coastal areas. With the widespread use of the artificial grounding freezing method in the coastal area and the seepage environment faced, it will exert a significant impact on the macroscopic properties and micro-structure of coral sand. To study the macroscopic and microscopic properties of coral sand subjected to freeze-thaw with and without seepage, the one-dimensional soil column freezing test, scanning electron microscope, direct shear test, and particle sieve test were carried out. The test results showed that there was obvious water migration in the coral sand during the freezing process, and the increase of water content in the frozen zone was large in the absence of seepage, while the difference of water content in the frozen zone, phase-transition zone and unfrozen zone of coral sand was not significant under seepage effect. The scouring effect of seepage resulted in increased friction on the surface of sand particles, and the small particles produced by freeze-thaw damage of large particles increased inter-particle shading, resulting in greater shear strength of coral sands with seepage than without seepage. Regardless of the effect of seepage, all zones of the coral sand increased in the range of particle size from 0.5 mm to 0.25 mm and decreased in the range of particle size from 0.2 mm to 0.1 mm. Freezing increased the number of small pores within the coral sand. This study provides a reference basis for understanding the macro and micro mechanical properties of artificially frozen coral sand under seepage.