Abstract: The macroscopic properties of soil are mainly affected by its microstructure and pore characteristics. To figure out the microscopic evolution of soil under the influence of external conditions such as freeze-thaw is significant for geotechnical study. As seasonally frozen and special soil, turfy soil has high compressibility and low strength due to its high humus and plant fiber content. Thus, this study took turfy soil to investigate its microstructures, pore characteristics, and effect of freeze-thaw by methods including geotechnical tests, nuclear magnetic resonance (NMR), X-ray Computed tomography (CT), and scanning electron microscope (SEM). Based on geotechnical and NMR theory, micro-image segmentation was completed on CT slices and SEM images to identify air and water-storage pores. Combined with microscopic images and composition analysis, the microstructure of turfy soil reveals that the organic matter component is the soil matrix able to contain and conduct water. The pore size distribution of turfy soil after freeze-thaw showed an increase in the proportion of mesopores and a significant increase in the number of pores. Consequently, the quantitative characterization of microscopic parameters indicates that the pore connectivity of turfy soil before and after freeze-thaw is enhanced, and the complexity of the pore shape is reduced, enhancing permeability. Finally, verifying the theoretical calculation of unsaturated soil shows that the NMR method can effectively characterize the change in permeability of freezing and thawing soil. This work can serve as the basis of studies on soil with high organic matter and high fiber content and can be applied as a parameter basis for engineering construction in turfy soil distributed regions.

Key words: Turfy soil, Freeze-thaw, Microstructures, Pore characteristic, NMR, CT