Characteristics of Soil Shear Strength, Disintegration, and Pore Size Distribution at Different Freeze-Thaw Conditions in Brown Soil (Hapludalfs) Region of Northeast China

Freeze-thaw cycles (FTCs) influence soil erodibility through alterations in soil structure and mechanical properties. Despite existing studies, the quantitative relationship between soil erodibility and the number of freeze-thaw cycles (FTCs) at different initial soil water contents (ISWC) is still not fully understood. This study employed direct shear tests, soil disintegration tests, and pore size distribution (PSD) tests to quantify soil erodibility indices and soil structure characteristics for brown soil in Northeast China. Five FTCs (zero, one, five, ten, and fifteen) and five ISWC (10, 15, 20, 25, and 35%) were considered. The results revealed that as ISWC increased, soil cohesion generally declined, particularly with the rise in FTCs, making the difference in cohesion between high and low ISWC more pronounced. In contrast, the differences in internal friction angles between different ISWC gradually decreased as the FTCs increased. The soil disintegration rate was significantly affected by ISWC, showing an initial increase followed by a decrease as ISWC rose (p < 0.05). Notably, the range of 10–15% ISWC was the most easily disintegrated ISWC. FTCs could decrease the soil pore volume of lower ISWC and increase the soil pore volume of higher ISWC. 0.4 μm radius pores were a critical threshold for pore changes in soil under FTCs and five FTCs might be the critical value for influencing macropores. FTCs could affect 0.1–4 μm radius pores indirectly influencing the internal friction angle, and affect 4–25 μm radius pores indirectly influencing the cohesion and disintegration rate. This paper introduces novel insights into the erosion characteristics and micro-mechanisms of brown soil under freeze-thaw conditions.