In this study, a pioneer work on characterizing rainfall-induced shallow landslides in layered soils using unsaturated flow equation is proposed. Several models using different mathematical approximations for describing the nonlinear physical relationships of the soil-water characteristic curve were introduced. The Gardner exponential model was adopted to derive analytical solutions of unsaturated flow problems. To model the rainfall-induced shallow landslides, the slope stability analysis coupled with the hydrological model with the consideration of the fluctuation of pore water pressure for soil water characteristic curve was developed. The results obtained from this study demonstrate that the slope stability of landslides is strongly dependent on the hydraulic conductivity. It is found that the variation of pore water pressure in unsaturated layered influences the stability of a slope. Besides, the pressure head at the interface increases quickly and the lowest safety factor may occur at the interface between two consecutive soil layers during a rainfall event.
Due to the effects of climate change and global warming, severe weather conditions such as the large amount of precipitation are becoming much more frequent around the world. The extreme rainfall events affect the stability of slopes and trigger extensive landslides. It is therefore necessary to characterize how the variability of severe climate variables, such as the rainfall, affects the vulnerability to landslide hazards. Shallow landslides often occurred in the unsaturated zone. The assessment of rainfall-induced shallow landslides has drawn much attention in branches of engineering and science such as geotechnical engineering, civil engineering and engineering geology (Liu et al, 2017). In the past, numerical models have been developed to analyze the slope stability, assuming soil was nearly saturated to further solve the numerical solution of the Richards equation in the simplified form of a one-dimensional linear diffusion equation. Since the appearance of layered heterogeneous porous media is much more common than homogeneous soils in engineering problems, the hydrological process in layered unsaturated soils has been studied (Ku et al, 2017). However, the modeling of rainfall-induced shallow landslides in layered unsaturated soils using the variably saturated flow equation has hardly been reported.