Abstract: The macroscopic properties of soil are primarily influenced by its microstructure and pore characteristics. Understanding the microscopic evolution of soil under external conditions like freeze-thaw is crucial for geotechnical studies. Turfy soil, a seasonally frozen and unique type of soil, exhibits high compressibility and low strength due to its high humus and plant fiber content. Therefore, this study focused on turfy soil to investigate its microstructures, pore characteristics and the effects of freeze-thaw using methods such as geotechnical tests, nuclear magnetic resonance (NMR), X-ray computed tomography (CT) and scanning electron microscopy (SEM). Utilizing geotechnical and NMR theories, micro-image segmentation was performed on CT slices and SEM images to identify air and water-storage pores. Combined with microscopic images and compositional analysis, the microstructure of turfy soil reveals that the organic matter component forms a matrix capable of containing and conducting water. The pore size distribution of turfy soil after freeze-thaw shows an increased proportion of mesopores and a significant increase in the number of pores. Consequently, quantitative characterization of microscopic parameters indicates enhanced pore connectivity and reduced pore shape complexity in turfy soil before and after freeze-thaw, thereby enhancing permeability. Verification of theoretical calculations for unsaturated soil shows that the NMR method effectively measures the permeability of freezing and thawing soil. The research findings can serve as a basis for studies on soil with high organic matter and fiber content and can be applied as a parameter basis for engineering construction in regions with turfy soil.

Key words: turfy soil, freeze-thaw, microstructures, pore characteristic, nuclear magnetic resonance (NMR), computed tompography (CT)