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

Vegetation coverage has increased significantly since the implementation of the Green for Grain Project in the Loess Plateau. Vegetation restoration can also play a positive role in the slope stability and soil erosion resistance. The soil and water conservation can then enhance the shear strength in the surface soil via root systems. The purpose of this study is to explore the soil shear strength under different vegetation types in the Loess Plateau. Vegetation types were then clarified in the sensitivity of shear strength to water conservation. The soil shear strength before and after rainfall was measured by micro-cross plate shear apparatus. Four types of land use were selected as the forestland, grassland, farmland and orchard (with 9 repeated measurements per soil depth). The samples were collected to measure the content of soil moisture before and after rainfall (with 3 repeated measurements per soil depth). The results showed that: 1) The response of soil moisture content to the heavy rainfall was varied in the different types of land use. Among them, there was the more intense response of farmland and orchard to the rainfall. The surface soil moisture content after rainfall was 7.57 and 6.06 times that of before rainfall,respectively.In contrast, there was the small response of forestland and grassland to the rainfall, and the soil moisture content after rainfall was 3.15 and 1.65 times that of before rainfall, respectively. 2) The soil shear strength was relatively high in the forestland and orchard before the rainy season, followed by the grassland, and the farmland was the lowest. After heavy rainfall, the shear strength of grassland and forestland remained relatively high, with the orchard next, while the farmland also shared the lowest. 3) Among various types of land use, the grassland exhibited the smallest increase in the moisture content ratio and the lowest proportion of shear strength reduction. While the orchard and farmland showed the large decrease in the shear strength with the increase in the moisture content. The average moisture content of grassland after rainfall was 1.57 times that of before rainfall,with a corresponding decrease in the shear strength of 8.76%.The average moisture content of orchard after rainfall was 3.32 times that of before rainfall, leading to a 40.46% decrease in the shear strength. Therefore, it was recommended that the grassland cover was retained in orchard to enhance the soil resistance to erosion. The soil shear strength in farmland was relatively low before and after rainfall, indicating the urgent needs of soil and water conservation. 4) Before the rainy season, the soil bulk density and porosity played a key role in the soil shear strength. After the heavy rainfall, the soil moisture content increased to avoid the influence of soil properties on the shear strength. At the same time, the shear strength was controlled by the soil moisture content. The shear strength of soil was measured under different types of land use. There were the impact patterns of soil moisture content on the shear strength of soils among various types of land use. This finding can also provide a scientific basis for the high-quality development of ecological construction in the Loess Plateau.