## 4 Model Studies
### Results of the Simulated Rainfall Infiltration
#### 4.4.1 Water-Air Coupling Effect on Rainfall Infiltration  
At the onset of rainfall infiltration, the saturation of the ground surface was relatively small, so the matric suction was comparatively high. As a result, the water pressure gradient and the magnitude of infiltration were relatively high. At this time, the gas permeability coefficient for soil was relatively high and the flow of gas was good. The air could flow out of the ground surface. As the pore air pressure was close to atmospheric pressure, the effect of air on infiltration could be ignored at the ground surface in the initial stage. As the ground soil gradually reached a fully saturated state, the gas permeability coefficient decreased rapidly until the value was close to zero and could not flow anymore out of the ground surface. The pore gas increased rapidly, and the resulting pressure was the opposite direction to the water pressure because of the infiltration of water. The variation of gas pressure was much more sensitive at the location between the slope surface and the slip band. As a result, pore water pressure and the intensity of infiltration decreased. When the soil was saturated, the air pressure was equal to the water pressure and gradually stablilized, while the supporting effect of gas on the gravity gradient also tended to be stable. At this time, infiltration was mainly affected by the combined action of the matric suction and the water relative permeability coefficient in the wetting front area. This meant it was a relatively stable infiltration status. Figure 5 shows the time series of pore gas pressure at three monitoring points (H, I, and J) in the slope. When the ground surface was saturated, it was discovered that pore gas pressure increased rapidly. This was affected by the variation of the surface ventilation condition as well as the squeezing and infiltration of water. As the degree of infiltration was relatively high in the early stage, it was too late for the pore gas generated by the extrusion to move in the soil. Therefore, there was a limit on the maximum value for the pore gas pressure (Figure 5). When the magnitude of infiltration decreased later and pore gas moved in the soil, the pore gas pressure dropped slowly and then tended to be stable. At this time, the magnitude of infiltration tended to be stable. Figure 6 presents the development of the pore water pressure at three monitoring points (H, I, J). The evolution of pore water pressure was related to the time series from its initial negative value at the early stage of the rainfall. It was also shown that with the decrease in the distance from the slope surface, the response of pore water pressure became faster. With the infiltration of the arrival rainwater, pore water pressure increased sharply  
Figure 5: Time series of pore gas pressure at three monitoring points. and then finally converged to zero. The variation of the water pressure was mainly affected by the pore air pressure in the unsaturated zone. The evolution of pore air pressure, pore water pressure, and matric suction calculated by this model at three monitoring points H, I, and J are plotted in Figure 7a-c. Matric suction decreased over time as a result of infiltrating water, but it did not converge to zero. This was because the compressed air could not escape freely through the soil surface. It was also noted that this response was different according to the distance from the slope surface. The shorter this distance, the faster the reduction of matric suction. Traditional seepage models assume that the air pressure is atmospheric. However, when air pressure and water pressure are considered, matric suction mainly comprises air pressure as well as water pressure, and it is computed as the difference between these two pressures \((p_{a}-p_{w})\). The gas relative permeability coefficient of the sliding bed was much larger than the sliding body; pore gas decayed along the direction of the sliding bed.