# A Theory of Slope Shear Scouring and the Failure Mechanism of PFC3D on a Gangue Slope
## 3 Results  
### Failure Mode  
Under the action of fluid scouring and penetration, weak bonds between the surface particles of the gauge slope are lost, and the slope's mechanical strength characteristics are weakened, forming a gully, which affects the stability of the slope. Therefore, it is important  
Figure 10: PFC3D fluid–solid coupling scouring scheme. Figure 9: PFC3D coal-waste slope model and monitoring arrangement. to study the failure mode of the PFC gangue slope model of scouring under rainfall conditions to determine the gully formation mechanisms of such slopes and methods for slope protection and treatment. We recorded the failure mode of the PFC gangue slope within 80 min of rainfall scouring; see Figure 11. The failure types of the whole process can be divided into four stages: (1) overall splash erosion and local schist erosion at the bottom of the slope; (2) erosion diversion and sheeting in the middle of the slope; (3) tributary-slope crest extension and the etching stage; and (4) integral gully erosion and the landslide stage. Figure 11: Gully formation process. (A-Catchmentarea; B-Rainfallscouring area; C-Catchmentarea). (1) Integral splashing and local etching of the slope bottom. This phase corresponds to the 10-20 min period shown in Figure 11. The early stage of rainfall occurs from 0 to 10 min and no obvious slope erosion occurs. Clump particles do not disengage from the slope. However, a displacement shape similar to that of gully 5 is formed in area B of the slope's surface. At this time, the scouring condition of the slope's surface is still in the early stage of catchment. The effect of hydraulic force on the clump particles is mainly concentrated in the valley. An arc-shaped radial flow channel is formed in the whole area (B), with the displacement of clump particles at 0-0.4 m and the maximum displacement concentrated at the bottom of the slope. As the hydraulic downward action destroys the particle contact model, the interaction between the particles at the bottom of the slope increases and the displacement increases. At 20 min, the clump particles on the surface of the slope begin to be destroyed and they disperse and disengage from the slope. The particles are taken away from the suspension module, and the maximum displacement is 9.79 m. This process is called sheet erosion, where fills appear on the bottom surface of the slope. At the same time, the catchment on the slope's surface gradually transfers from Zone C to Zone A, and the particle displacement can reach 1.5 m due to catchment erosion. (2) Erosion diversion and sheet erosion stages in the middle section of the slope. The rainfall continues to progress, and the scouring time reaches 30 min. At this time, the denuded particles at the bottom of the slope gradually increase in number, forming a muddy shape and accumulating downward. At the same time, denudation develops upward and reaches the middle of the slope, leading to the start of the sheet erosion. It is not difficult to determine that the erosion resistance of the soil particles on the slope is less than the runoff scouring force, and the particles are in a local unstable state. At this time, the shear force between the runoff particles washes away the adjacent particles and forms a small erosion hole in the middle of the slope. Since the inter-particle hydraulic action gradually shifts to Zone A during the catchment stage, the main gully begins to flow in the direction of Zone A in the middle of the slope. It is interesting to note that the particles are eroded transversely in the direction of Zone A and form a Z-shaped scouring ditch with the rill at the bottom of the slope. After the rainfall scouring reaches 40 min, the downward scouring capacity of the main stream is greater than the lateral damage capacity of the catchment tributary. This results in the upward denudation speed of the main gully, which exceeds that of the lateral tributaries. With the progression of the rainfall, the runoff energy continues to increase.