The response of soil shear strength to heavy rainfall under different land use types on Loess Tableland

Since the implementation of the Green for Grain Project in the Loess Plateau, the vegetation coverage has increased significantly. Vegetation restoration, which not only plays a positive role in improving slope stability and soil erosion resistance but also promotes the efficient development of soil and water conservation, and enhances the shear strength in surface soil through root systems in the Loess Plateau. The purpose of this paper is to explore the difference of soil shear strength under different vegetation types in the Loess Plateau, and to clarify the sensitivity of shear strength of different vegetation types to water. The soil shear strength before and after rainfall was measured by micro-cross plate shear apparatus on four land use types of forestland, grassland, farmland and orchard (with 9 repeated measurements per soil depth), and the soil samples before and after rainfall were collected for soil moisture content determination (with 3 repeated measurements per soil depth). The results showed that: (1) The response of soil moisture content to heavy rainfall was different under different land use types. The response of farmland and orchard to rainfall was more intense, and the increase of surface soil moisture content after rainfall could reach 7.57 and 6.06 times, respectively. In contrast, the response of forestland and grassland to rainfall was smaller, with an increase of 3.15 and 1.65 times, respectively. (2) Before the rainy season, the soil shear strength was relatively high in forestland and orchard, with grassland following, and farmland having the weakest shear strength. After heavy rainfall, the shear strength of grassland and forestland remained relatively high, with orchard next, while farmland had the lowest shear strength. (3) Among various land use types, grassland exhibited the smallest increase in moisture content ratio and the lowest proportion of shear strength reduction. In contrast, orchard and farmland showed a larger decrease in shear strength with the increase in moisture content. The moisture content in grassland increased by 1.57 times, with a corresponding decrease in shear strength of 8.76%. In contrast, the moisture content in Orchard increased by 3.32 times, leading to a 40.46% decrease in shear strength. It was recommended that grassland cover be retained in orchard to enhance soil resistance to erosion. The soil shear strength in farmland was relatively low before and after rainfall, indicating a need for attention to soil and water conservation measures. (4) Before the rainy season, soil bulk density and porosity played a key role in soil shear strength. After the heavy rainfall, the soil moisture content increased, and the influence of soil properties on shear strength decreased. At that time, shear strength was mainly controlled by soil moisture content. By measuring and analyzing the shear strength of soils under different land use types in the Loess Plateau, the study reveals the impact patterns of soil moisture content changes on the shear strength of soils across various land use types. This provides a scientific basis for the high-quality development of ecological construction in the Loess Plateau and aids in formulating more effective soil and water conservation measures.