Experimental infiltration of unsaturated loess under air action and simulations of a water-air two-phase flow model on slopes

Loess, which is widely distributed in China, forms unique geographical areas prone to frequent geological disasters. The thick vadose zone in these regions is characterized by the coexistence of solid, liquid, and gas phases, significantly complicating water infiltration processes. The influence of soil air pressure must be considered when modeling water infiltration in such areas, rather than relying solely on single-phase flow models. In this study, water and air migration mechanisms are investigated under open and closed conditions through laboratory experiments and numerical simulations. After simulating scenarios such as precipitation and irrigation, a comparative analysis revealed the effects of air–water two-phase flow versus single-phase flow. The results demonstrate that under closed conditions, the pore air pressure increases substantially, retarding water infiltration. Air pressure fluctuations cause variations in the infiltration velocity of the wetting front. Moreover, the presence of gas connects the water above and below the wetting front, causing the water flux beneath the wetting front to fluctuate in response to air pressure changes. Minor air pressure changes due to precipitation have negligible impacts, whereas irrigation-induced air pressure variations significantly affect water infiltration and slope stability.