Effects of freezing and thawing on soil shear strength in the western farming pastoral ecotone of northern China.

Freezing and thawing have important effects on soil erosion, especially in the western part of the agricultural and pastoral intertwined belt in northern China where seasonal freezing and thawing are significant. In this study, the effects of water content and the number of freeze-thaw cycles on the shear strength characteristics of soil were investigated by freeze-thaw cycle test and straight shear test using loessial soil, sierozem, and chernozem in this region. The results indicate that: (1) The soil shear strength decreases first and then stabilizes with the increase of freeze-thaw cycles and decreases with the increase of water content. After 10 freeze-thaw cycles, the shear strength of loessial soil, sierozem, and chernozem with 10% water content decreases by an average of 7.92%, 8.23%, and 12.24%, respectively. (2) Freeze-thaw cycles cause the soil's cohesion to decrease initially and then stabilize, while the increase in water content weakens this trend. After 10 freeze-thaw cycles, the cohesion of loessial soil, sierozem, and chernozem with 10% water content decreases by an average of 33.33%, 31.25%, and 16.39%, respectively. (3) Freeze-thaw cycles cause varying changes in the internal friction angle of the three soils, while an increase in water content reduces the internal friction angle. When the water content increases from 10 to 30%, the internal friction angle of loessial soil, sierozem, and chernozem decreases by an average of 7.14%, 8.79%, and 10.99%, respectively. These findings are of significant importance for deepening the understanding of the mechanisms underlying soil erosion and formulating targeted soil conservation measures.