In cold-region high-speed railway (HSR) subgrade engineering, coarse-grained soils are commonly used as frost heave prevention fillers. However, coupled water-heat migration during freeze-thaw cycles still induces frost heave. This study innovatively employs a nuclear magnetic resonance (NMR) system to elucidate the hydro-thermal transport mechanisms in coarse-grained soils during freezing. The results reveal that under identical temperature and freezing duration, high-water-content soils release substantial latent heat from pore water freezing, resulting in higher freezing zone temperatures than low-water-content soils. During freezing, unfrozen water content decreases as a power function with freezing time at different depths of soil samples, with the frozen zone experiencing the fastest water reduction, followed by the freezing front and then the unfrozen zone. Both free and bound water progressively decrease in frozen and unfrozen zones. After freeze-thaw, the change in soil pore structure leads to a decrease in bound water and an increase in free water in frozen zones, while both decrease in unfrozen zones. Furthermore, higher initial water content results in more pronounced reductions of bound water and increases of free water in frozen zones. These findings advance the understanding of hydro-thermal coupling mechanisms and provide theoretical foundations for frost damage mitigation in high-speed railway subgrades.

The Sanjiangyuan region, known as the Chinese Water Tower, serves as a crucial ecological zone that is highly sensitive to climate change. In recent years, rising temperatures and increased precipitation have led to permafrost melt and frequent occurrences of thermokarst landslides, exacerbating soil erosion issues. Although studies have explored the impact of freeze-thaw action (FTA) on soil properties, research on this phenomenon within the unique geomorphological unit of thermokarst landslides, formed from degrading permafrost, remains sparse. This study, set against the backdrop of temperature-induced soil landslides, combines field investigations and controlled laboratory experiments on typical thermokarst landslide bodies within the permafrost region of Sanjiangyuan to systematically investigate the effects of FTA on the properties of soils within thermokarst landslides. Furthermore, this study employs the EPIC model to establish an empirical formula for the soil erodibility (SE) factor before and after freeze-thaw cycles (FTCs). The results indicate that: (1) FTCs significantly alter soil particle composition, reducing the content of clay particles in the surface soil while increasing the content of sand particles and the median particle size, thus compromising soil structure and enhancing erodibility. (2) FTA initially significantly increases soil organic matter content (OMC); however, as the number of FTCs increases, the magnitude of these changes diminishes. The initial moisture content of the soil significantly influences the effects of FTA, with more pronounced changes in particle composition and OMC in soils with higher moisture content. (3) With an increasing number of FTCs, the SE K-value first significantly increases and then tends to stabilize, showing significant differences across the cycles (1 to 15) (p < 0.05). This study reveals that FTCs, by altering the physicochemical properties of the soil, significantly increase SE, providing a scientific basis for soil erosion control and ecological environmental protection in the Sanjiangyuan area.

To solve the problem that the mechanical behavior of undisturbed loess in seasonally frozen soil area is affected by freeze-thaw action, triaxial shear tests of undisturbed loess under freeze-thaw condition were carried out. The results show that the mechanical properties of undisturbed loess are greatly affected by factors including freeze-thaw process, water content, natural density and confining pressure. Freeze-thaw action has a certain impact on the failure surface shape and stress-strain curve. Before and after freeze-thaw, the shape of the shear failure surface is complex, including single oblique failure surface, double oblique failure surface, vertical failure surface, X-shaped failure surface, bulging failure, etc. And under the conditions of low water content, low confining pressure and high dry density, the stress-strain curve tends to be softened. Conversely, the curve tends to harden. Freeze-thaw action can make the stress-strain curve transition from softening to hardening. In addition, the freeze-thaw action significantly weakens the failure strength, shear strength, cohesion, initial tangent modulus and failure ratio of undisturbed soil, but does not change the internal friction angle obviously. Also, the heterogeneity of natural soil is also an important factor affecting the mechanical parameters, failure surface shape and stress-strain curve of undisturbed loess.

Landslides induced by freeze-thaw processes on grasslands are one of the major geohazards, and their scale and frequency are increasing as the global warms. Freeze-thaw induced landslides degrade surface vegetation and soil properties, reduce biodiversity, intensify landscape fragmentation, and lead to losses in economy, human and animal lives. Despite substantial progress in research on landslides, there has been little study focused on how ground freeze-thaw events affect landslides. By critically analyzing previous studies, this paper proposes a conceptual framework for the forms and types, development, dominant factors, monitoring techniques, and impact mechanisms of freeze-thaw induced landslides. Landslides are controlled by soil characteristics and topographic slope, which are major intrinsic determinants. Increased rainfall, rising temperatures, and thickening active layer due to climate change are all direct drivers of freeze-thaw induced landslides. Vegetation conditions, animal behavior interference, and wind erosion all affect the occurrence and development process of landslides by modifying vegetation cover, soil physical and chemical properties, and structure. Currently, landslide monitoring techniques have evolved rapidly with improved efficiency and accuracy, but with only few applications for freeze-thaw induced landslides. There are a variety of prediction models for landslides, but few consider freeze-thaw effects and lack field validation. The new perspective on the occurring types and dominant factors enhances theoretical understanding of the formation mechanisms, which helps further monitor and analysis of freeze-thaw induced landslides. Future studies should concentrate on the coupling mechanism of multiple factors and the development of an accurate prediction system, which will greatly benefit the understanding and early detection of freeze-thaw induced landslides.