Stability analysis of unsaturated soil slopes considering water-air flow caused by rainfall infiltration

Abstract

Even though soil is a mixture of solids with voids that are filled by air and water, most previous studies on rainfall infiltration and its influence on slope stability were based on a single-phase water flow model by assuming that the pore air pressure was atmospheric. The purpose of this study is to examine the effect of interactions between air and water flow due to heavy rainfall on the mechanical stability of an unsaturated soil slope. Water-air two-phase flow analyses were conducted to investigate the contribution of pore-air pressure on infiltration by rainfall. In order to study the infiltration behavior with respect to soil type, flow analyses were performed with two types of soil under similar settings. Results obtained from the two-phase infiltration analysis were then used as input to the stability analysis by the strength reduction method. Infiltration and stability analyses based on a single-phase water flow model were also carried out, which helped clarify the effects of air flow induced by rainfall infiltration on an unsaturated soil slope. The results showed an increase in pore air pressure during infiltration because rainwater displaced the air in the unsaturated zone; hence, remarkable delaying effects on water flow were induced. Such water-air interactions in the pore space of soil significantly affected the stability and behavior of the soil slope.