Stability analysis of unsaturated soil slopes considering water-air flow caused by rainfall infiltration Sung Eun Cho Department of Civil, Safety and Environmental Engineering, Hankyong National University, 327 Chungang-Ro, Anseong-Si, Gyeonggi-Do 456-749, South Korea abstract article info Article history: Received 29 February 2016 Received in revised form 18 July 2016 Accepted 20 July 2016 Available online 22 July 2016 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 rain- water 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. © 2016 Published by Elsevier B.V. Keywords: Slope stability Two-phase flow Rainfall infiltration Strength reduction method 1. Introduction Slope failures due to rainfall are very frequent worldwide, and the damage caused by such failures is substantial. According to previous studies, rainfall can cause the development of a perched water table, a rise in the main groundwater level, surface erosion, and an increase in unit weight of soil due to a rise in moisture content (Ng and Shi, 1998; Cho and Lee, 2001). The role of water infiltration in soil and the subsequent pore pres- sure response at depth are critical for understanding the transient con- ditions that lead to slope failure (Lu and Godt, 2013). Since soil is a mixture of solids with voids that are filled by fluids such as air and water, in order to exactly interpret the infiltration of rainfall through the slope surface, a fully coupled formulation of the water and air flow and the stress-strain behavior of soil should be considered. However, assumptions for the sake of simplicity have been intro- duced. The most widely used infiltration analysis method is to solve the Richards (1931), which considers single-phase flow of water by ig- noring the stress-strain behavior and air flow in soil (e.g., Ng and Shi, 1998; Rahardjo et al., 2001). When rainfall infiltration occurs through pore space in an unsaturat- ed region, the flow of air also occurs, or the air is compressed by the in- teraction at the water-air interface. It is well known, based on experiments and analyses, that the flow of air through the pore space in unsaturated soil affects the infiltration of water (Touma and Vauclin, 1986; Sun et al., 2015). However, because of the difficulties of measuring pore-air pressure and analyzing air flow, the air flow induced by rainfall is ignored in general slope stability analyses by setting the pore air pressure to zero (Sun et al., 2015). In order to study the me- chanical behavior due to rainfall infiltration of the slope, coupled hydro-mechanical analyses have been conducted. However, most of the solutions considered the air pressure in soil to be equal to the atmo- spheric pressure based on the assumption that the air flow is free rela- tive to the flow of water (e.g., Alonso et al., 1995; Cho and Lee, 2001; Smith, 2003; Borja and White, 2010; Borja et al., 2012; Hamdhan and Schweiger, 2011; Wang et al., 2015). Only a few studies have been conducted regarding the effect of air flow due to rainfall infiltration on the stability of soil slopes. Hu et al. (2011) applied a coupled three phase (solid-water-air) model to simu- late the coupled deformation, water flow, and gas transport processes in a homogeneous soil slope and to assess the evolution of the safety factor of the slope using the Morgenstern-Price Method under a long, heavy rainfall. They showed that air transport in a homogeneous soil slope that was subjected to a heavy rainfall has a significant effect on slowing the propagation of the wetting front and decreasing slope stability. Zhang et al. (2009) and Sun et al. (2015) investigated the character- istics of airflow response to rainfall in a soil slope using a water-air tp (two-phase) flow model. Slope stability analyses on the given slip sur- face using the limit equilibrium method were then performed based Engineering Geology 211 (2016) 184–197 E-mail address: drsecho@hanmail.net. http://dx.doi.org/10.1016/j.enggeo.2016.07.008 0013-7952/© 2016 Published by Elsevier B.V. Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo