Stability analysis of unsaturated soil slopes considering water-air flow caused by rainfall infiltration
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 pressure response at depth are critical for understanding the transient conditions 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 introduced. The most widely used infiltration analysis method is to solve the Richards (1931), which considers single-phase flow of water by ignoring 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 unsaturated region, the flow of air also occurs, or the air is compressed by the interaction 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 mechanical 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 atmospheric pressure based on the assumption that the air flow is free relative 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 simulate 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 characteristics 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 surface using the limit equilibrium method were then performed based on the simulated water-air tp seepage conditions, taking into consideration the contribution from pore air pressure. The results by Zhang et al. (2009) showed that pore air pressure generated in the unsaturated zone reduces the safety factor, and as the distance between the slip surface and the underground water level increased, the influence of pore air pressure on soil slope stability also increased. Sun et al. (2015) showed that capillary pressure is beneficial, while pore air pressure is unfavorable to slope stability for a given slip surface.
Hu et al. (2011) adopted a linear elastic model for simplicity to describe the mechanical behavior of the soils. Therefore, the effects of nonlinear deformation and the shear strength of unsaturated soils on the process of rain infiltration and the evolution of slope stability were not considered. Zhang et al. (2009) and Sun et al. (2015) used a fixed given slip surface to analyze the slope stability. In other words, they did not reflect the continuous transition of the critical failure surface with the progression of the wetting front.
In this study, water-air tp flow analyses were conducted to investigate the effect of air flow due to rainfall infiltration on the stability of unsaturated soil slopes. In order to study the infiltration behavior with respect to soil type, flow analysis was performed for two types of soil under similar conditions. The result obtained from the tp infiltration analysis was then used as input for the stability analysis that accounted for mechanical equilibrium of the system, taking into consideration the contribution from pore air pressure. For the purpose of comparison, infiltration and stability analyses based on a water sp. (single-phase) flow model were also carried out, which helped clarify the effects of air flow induced by rainfall infiltration on unsaturated soil slopes.
Section snippets
Two-phase flow analysis
In this study, two-dimensional finite difference code FLAC Ver. 7.0 (Itasca, 2011) was used to analyze rainfall infiltration of unsaturated soil slopes. The tp flow option in FLAC allows numerical modeling of the flow of two immiscible fluids through porous media.
Slope stability analysis by the strength reduction method
The slope safety factor can be determined by the strength reduction method (SRM) at any time step during the rainfall infiltration analysis. SRM is typically used to calculate the safety factor by progressively reducing or increasing the shear strength of a material to bring the slope to a state of limiting equilibrium (Zienkiewicz et al., 1975). The definition of the safety factor is identical to the one adopted by the conventional limit equilibrium method.
Zienkiewicz et al. (1975) first used
Numerical study
The air flow generated in the unsaturated region by the infiltration of rainfall affects the flow of water. This interaction between water and air, in turn, will affect slope stability. In this study, water-air tp flow analysis was carried out to evaluate the effect of water-air flow on the stability of unsaturated soil slopes. By comparing this analysis with water sp. flow, the effect of air flow induced by rainfall infiltration on the stability of unsaturated soil slopes was investigated.
Conclusions
In this study, water-air two-phase flow analyses were conducted to investigate the effect of air flow caused by rainfall infiltration on the stability of unsaturated soil slopes. In order to study the infiltration behavior with respect to soil type, flow analysis was performed for two types of soil under similar settings. The result obtained from the tp infiltration analysis was then used as input to the stability analysis, which accounted for mechanical equilibrium of the system, taking into
Acknowledgements
This research was supported by the National Research Foundation of Korea (NRF) with funding from the Ministry of Science, ICT & Future Planning (2012M3A2A1050981) and by the Korea Agency for Infrastructure Technology Advancement (KAIA) with funding from the Ministry of Land, Infrastructure and Transport of the Korean government (16SCIP-B065985-04).
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