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/).

To optimize the use of chlorine saline soils commonly found in many coastal areas, ground granulated blast furnace slag (GGBS) and calcium carbide residue (CCR) were used in this study to stabilize/solidify these soils. This study aims at evaluating the suitability of GGBS-CCR as industrial by-products in improving the mechanical behaviors of chlorine saline soil in comparison with the use of Portland cement (PC) as a traditional binder. The optimal proportion of the binder was determined by the unconfined compressive strength, conductivity, and leaching characteristics. Moreover, the water stability coefficients, collapse coefficients and microscopic characteristics of the solidified soil were evaluated. The results reveal that when the ratio of GGBS to CCR in the binder is 4:1, the 28-day unconfined compressive strength reaches 4.53 MPa, and the leaching of chloride ions is reduced by 94.1 %. The excellent water stability and reduced collapsibility further indicate that GGBS-CCR is a preferable binder for solidifying saline soil compared to PC. Furthermore, microscopic analysis revealed that chloride ions in the saline soil were involved in the hydration reaction to form Friedel's salt.

Freeze-thaw cycles and compactness are two critical factors that significantly affect the engineering properties and safety of building foundations, especially in seasonally frozen regions. This paper investigated the effects of freeze-thaw cycles on the shear strength of naturally strongly chlorine saline soil with the compactness of 85%, 90% and 95%. Three soil samples with different compactness were made. Size and mass changes were measured and recorded during freeze-thaw cycles. Shear strength under different vertical pressures was determined by direct shear tests, and the cohesion and friction angle were measured and discussed. Microstructure characteristic changes of saline soil samples were observed using scanning electron microscopy under different freeze-thaw cycles. Furthermore, numerical software was used to calculate the subsoil-bearing capacity and settlement of the electric tower foundation in the Qarhan Salt Lake region under different freeze-thaw cycles. Results show that the low-density soil shows thaw settlement deformation, but the high-density soil shows frost-heaving deformation with the increase in freeze-thaw cycles. The shear strength of the soil samples first increases and then decreases with the increase in freeze-thaw cycles. After 30 freeze-thaw cycles, the friction angle of soil samples is 28.3%, 29.2% and 29.6% lower than the soil samples without freeze-thaw cycle, the cohesion of soil samples is 71.4%, 60.1% and 54.4% lower than the samples without freeze-thaw cycle, and the cohesion and friction angle of soil samples with different compactness are close to each other. Microstructural changes indicate that the freeze-thaw cycle leads to the breakage of coarse particles and the aggregation of fine particles. Correspondingly, the structure type of soil changes from a granular stacked structure to a cemented-aggregated system. Besides, the quality loss of soil samples is at about 2% during the freeze-thaw cycles. Results suggest that there may be an optimal compactness between 90 and 95%, on the premise of meeting the design requirements and economic benefits. This study can provide theoretical guidance for foundation engineering constructions in seasonally frozen regions.