Macro-microscopic deformation evolution of thawing frozen fine-grained soil used in heavy-haul railway subgrades in cold regions

During the thawing process of a railway subgrade, bidirectional thawing complicates water-heat transfer, leading to serious thaw settlement issues under train loads. Focusing on the severely frozen section of the Shuozhou-Huanghua port heavy-haul railway, this study conducted indoor soil-column laterally-limited compression tests on thawing fine-grained soil specimens to analyze the cumulative deformation during thawing. The deformation evolution was examined from both macroscopic and microscopic perspectives. The test results revealed a significant increase in the water content at the frozen interlayer during thawing, with minimal thaw settlement under no-load conditions. However, under dynamic loads, the thawing soil exhibited rapid settlement during the initial stages of the process. Increasing the dynamic load amplitude did not result in significant additional thaw settlement compression. Particle image velocimetry revealed substantial thaw settlement and compression at the top of thawing soil. Microscopically, the porosity at the top of the specimens significantly decreased, whereas the porosity in the frozen interlayer remained largely unchanged. Under dynamic loading, the specimens exhibited a concentrated distribution of large pores with scattered smaller pores. The phase change from ice to water, combined with dynamic loading, induced particle movement and expanded the inter-particle pore space, leading to macroscopic thaw settlement and soil compression. The findings can provide a theoretical foundation for maintaining and ensuring the safety of railway subgrades in cold regions.