Effects of Airflow Induced by Rainfall Infiltration on Unsaturated Soil Slope Stability

The aim of this study was to investigate the characteristics of airflow induced by rainfall infiltration and their effects on unsaturated soil slope stability using numerical methods. The TOUGH2/EOS3 simulator was used to analyze the features of water–air two-phase flow in a soil slope under rain conditions. The results show that when the rain infiltrates the soil, the air is pushed into the soil by the advancing wetting front, the pore-air pressure in the unsaturated zone increases firstly, then the airflow is reversed, and the pore-air pressure decreases until it is close to atmospheric pressure. The features of water single-phase flow (i.e., the gas phase is neglected) were also simulated using the TOUGH2/EOS9 simulator. When the infiltration of rainwater into the soil slope in the water single-phase flow model is set as same as that in the water–air two-phase flow model, the water saturation and the capillary pressure distributions in the unsaturated zone are probably the same. Slope stability studies were then performed based on the simulated water–air two-phase and water single-phase seepage conditions, respectively. The slope stability analysis on a given slip surface based on the simulated water–air two-phase seepage condition shows that the safety factor decreases during rainfall, reaches the lowest value when rain stops, generates immediately a sudden rise, and then increases slowly to a steady value. The capillary pressure is beneficial to slope stability, while the pore-air pressure is unfavorable to slope stability. The variation tendency of the safety factor is similar to that of the negative total air pressure acting on the slip surface. The slope stability analysis on the given slip surface based on the water single-phase seepage condition shows that the safety factor is approximate to FS1 (is the safety factor for the case that does not take into account the air pressure based on the water–air two-phase seepage condition). Therefore, difference between the safety factor and FS1 based on the water–air two-phase seepage condition can approximately represent the effects of airflow induced by rainfall on unsaturated soil slope stability under the condition that the infiltration of rainwater into the soil slope remains unchanged.