## 5 Conclusions  
In this study, a water-air two-phase flow analysis was conducted in order to study the effect of pore gas pressure caused by rainfall infiltration on the stability of unsaturated soil slopes. During this process, the variation of the load as a result of the pore gas pressure was obtained. Together with the unsaturated soil shear strength theory and the residual thrust method, pore gas pressure was incorporated into the slope stability analysis method. In order to compare the infiltration behavior with respect to initial saturation, we investigated the movement of pore gas pressure and pore water pressure for silty sand. The following conclusions could be drawn from the analysis:  
(1) When the ground surface of the soil approached the saturated condition, the pore gas was squeezed by infiltrated water, and the support for the gas opposite to the gravity gradient occurred. Therefore, the magnitude of infiltration decreased and tended to be stable because of the support of gas, and the stable infiltration magnitude was mainly affected by matric suction and the water relative permeability coefficient. When there was  
Figure 17: Evolution of safety factor with saturation degree. Figure 18: Diagram for distribution of residual thrust for different saturation degrees. low saturation, the stable magnitude of infiltration was mainly affected by matric suction and decreased as the saturation increased. When the saturation approached 100%, the magnitude of infiltration was mainly affected by the water relative permeability coefficient, and it increased with the increase in the degree of saturation. (2) The maximum value of stable infiltration magnitude occurred at the residual saturation degree (saturation = 0%) or at saturation (saturation = 100%). This value was affected by the soil-water characteristic curve as well as the water and gas relative permeability coefficients. In this research, the maximum value of the stable magnitude of infiltration for silty sand occurred at the degree of residual saturation, while the minimum value for it occurred at saturation levels between 80% and 90%. When the saturation was more than 85%, the degree of declination for the safety factor became slower compared with a saturation degree of less than 85%. This was because the magnitude of infiltration increased when the saturation degree was more than 85%. (3) During the process of infiltration, it was difficult for the ground surface soil to reach full saturation. The greater the initial saturation was, the greater the pore gas pressure and the stronger the influence of gas on the soil saturation. The saturation by the soil surface was much smaller. In addition, affected by the water relative permeability coefficient, water was infiltrated by the matric suction gradient at the higher gas content and water was infiltrated by the pore channel at the lower gas content. (4) As the coefficient of viscosity for air is much smaller than for water, the flow of air is much better than the flow of water. As a result, pore water pressure in the saturated zone can be transferred to the toe of the slope by pore gas pressure. This could also result in a pore gas pressure gradient oriented towards the slope's toe, increasing the thrust for the slipping mass. This mechanism of force transfer has a negative impact on the safety factor. (5) This research adopted the results of the calculation of stress and seepage using the finite element method. The pore gas pressure in the unsaturated zone and the pore water pressure in the saturated zone were as a result of the gradient. In addition, the impact of the saturation degree on the slope stability was realized by modifying the shear strength parameters, cohesion, and internal friction angle. When analyzing slope stability, this method is much safer than other traditional methods. Conceptualization, W.T. and H.P. ; methodology, W.T. ; software, S.X. ; validation, W.T., H.P. and B.M. ; formal analysis, Z.C. ; investigation, W.T. ; resources, Z.C. ; data curation, H.P.