Change characteristics of soil and water parameters of loess landfill slopes during rainfall scouring

In recent years, with China's "One Belt, One Road" "Western Development" and other national strategies vigorously promoted, the Loess Plateau's urbanization process accelerated, the demand for urban land increased, but the Loess area is dominated by gullies and hills, the terrain is fragmented, and the ditches are longitudinal and transverse, which seriously restricts the local urban construction and economic development, local urban construction and economic development. In order to expand the urban space, many cities have carried out the project of leveling hills to create land, but during the implementation of the project, loess dumps are usually piled up along the gullies, forming a larger number and area of loess landfill slopes. Therefore, in order to investigate the variation rules of soil and water parameters in different slopes of loess landfill slopes during extreme rainfall, this study, based on the field investigation, took the typical loess landfill-Jianshui gully watershed in Lanzhou urban area (an important node city of Loess Plateau) as an example to carry out the simulation test of indoor rainfall, with the slope gradient of the side slopes set at 25°, 30°, 35°, 40°, 45°, and the water content and substrate suction rate of the side slopes were recorded at different parts of the side slopes in the process of rainfall. During the rainfall process, the water content and matrix suction change data were recorded at different parts of the slope. In order to analyze the damage mechanism of cut-gully slumping of landfill loess slopes, the theory of unsaturated soil shear strength was coupled with the gravity erosion model of loess gully slopes, and the influence of slope gradient on the erosion and sediment yield of slopes was further analyzed. The results showed that: 1) the development process of rainfall erosion on the loess slope of landfill was divided into three stages (spattering and sheet erosion stage; rill erosion stage; and gully-cutting erosion and collapse damage stage), in which the rill erosion stage lasted for the longest time, and the degree of erosion of the slope surface was intensified with the increase of the slope gradient, and the first damage site of rainfall erosion was gradually moved from the bottom of the slope to the top of the slope, and at the same time the more common phenomenon of the gravitational erosion was at the stage of the gully-cutting. 2) With the increase of the slope gradient, the matric suction strength was increased, the sediment yield was increased. slope increased, the response rate of matrix suction sensor first increased and then decreased, and the fastest responded near 40°, indicating that with the increase of slope, the erosion rate of the slope first increased and then decreased. 3) Through theoretical derivation, we obtained the critical rupture angle for the slope of the fill loess, which was inversely proportional to the slope of the gully bed, i.e., the bigger the slope of the gully bed, the more likely the gully slope reached the critical rupture angle, and the more likely to be destabilized and damaged. The larger the slope was, the more likely to occurred gravity erosion in the gully-cutting erosion stage. However, the critical slope was around 40°, i.e., the runoff shear force was the largest in this slope range, the slope surface was seriously eroded by gully, and the response rate of matrix suction on the slope was accelerated. This may also be the reason for the accelerated response rate of matrix suction near 40°. The research results can provide theoretical support for the prevention and control of geologic hazards and the comprehensive management of soil erosion of engineering accumulations in loess areas.