## 1 Introduction  
Slope failures induced by rainfall infiltration are very common all over the world, and the damage raised by such failures can be extremely extensive [1, 2]. Based on past research, rainfall infiltration could lead to a high groundwater level, an increase in the unit weight of the sliding soil mass because of the increase in moisture content as well as a decrease in soil shear strength [3, 4]. Among all these influences, the rainwater infiltrating the soil and the resulting increment of pore pressure are significant aspects to understand slope failures [5, 6]. Two types of pore pressure exist in the soil slope, including pore gas pressure that is present in the unsaturated zone and pore water pressure that is present in the saturated zone. As we all know, soil is a three-phase material that consists of solid particles which make up the soil skeleton and voids that are filled with air and water. However, when researching infiltration behavior and its impact on slope stability, many scholars choose the one-phase fluid model by analyzing the increase of pore water pressure caused by infiltrated water. In addition, most of them make the assumption that the pore air pressure is equal to atmospheric pressure [7; 8]. Recently, there has been a growing interest in the research on the influence of pore gas pressure on slope stability [9]. In order to research the influence of rainfall infiltration on a soil slope, a fully coupled model of water-air two-phase flow should be considered, especially the pore air pressure in the unsaturated zone. It is difficult to simulate and monitor pore gas pressure in an unsaturated zone. Some studies have been conducted to consider the influence of pore gas pressure as a result of rainfall infiltration acting on soil slopes. [PERSON] et al. [10] proposed a theoretical model to simulate the transfer processes of water and gas in a homogeneous soil slope and to evaluate safety factor variations after long periods of heavy rainfall. [PERSON] et al. [11] also developed a water-air two-phase flow model to simulate the seepage process and analyzed the variation of pore pressure in the soil. [PERSON] et al. [12] analyzed the characteristics of subsurface airflow triggered by rainfall infiltration and estimated the important effect of airflow on unsaturated soil slope stability. These researchers simulated the infiltration process of water and gas during the infiltration period and verified the important impact of gas pressure. However, most of these previous studies did not consider pore gas pressure in the slope stability analysis method. There are several methods to evaluate slope stability, including the limit equilibrium condition, the finite element method as well as numerical methods [13]. The limit equilibrium method is considered the most common method adopted by many scholars because of its simplicity and practicability. The residual thrust method is also a commonly used method for a limit equilibrium analysis considering a strip division, which could be suitable for geotechnical slopes where the sliding surface has an arbitrary shape. This method is also recommended by many scholars [14]. In this research, a new slope stability analysis method that considers pore gas pressure was developed and the effect of airflow on slope stability was examined using a numerical analysis. First of all, a water-air two-phase flow analysis was conducted to investigate the distribution of pore air pressure, pore water pressure as well as water saturation induced by rainfall infiltration on unsaturated soil slopes. Then, pore gas pressure was incorporated into the slope stability analysis method on the basis of the unsaturated soil shear strength theory and the residual thrust method for the simulation of the influence of the pore air pressure on the slope stability. After establishing the new method to evaluate the slope stability, a practical engineering slope was adopted to conduct a simulation to research the important impact of pore gas pressure.