In this study, the effect of variation of suction stress due to transient infiltration on the stability of unsaturated soil slope during rainfall was investigated for more understanding on landslide. In order to achieve the aim of the research, a series of soil-water retention tests were performed for three soils (e.g. Jumunjin standard sand, Inje and Dogye granite weathered soils). In the tests, both first drying and wetting
processes were measured by the semi-auto soil-water retention apparatus. Based on the results of tests, the soil-water characteristic curves, hydraulic conductivity characteristic curves and suction stress characteristic curves for drying and wetting processes of each soil were estimated. In addition, using these characteristic curves, transient seepage and stability analyses were carried out by the finite element method and limit equilibrium analysis, respectively.
According to the results of the experiments, the soil-water characteristic curve is dependent on the soil type and subsequently, hydraulic conductivity characteristic curve and suction stress characteristic curve are also influenced. It is observed that there are hysteresis phenomena between drying and wetting curves and these are also ependent on the soil type.
Moreover, according to the results of the numerical analysis, rainfall-induced infiltration into the soil slope changes a magnitude of suction stress, which attributes to the shear strength of unsaturated soil, and consequently the factor of safety of the slope is varied. Especially, due to the continuously infiltration of rainwater, the suction stress is vanished and thus, the safety factor in subsurface soil is reduced below 1 and the failure of slope is occurred. In comparisons between analyses using drying and wetting characteristic curves, it is shown that there is a significant difference in an advance of a wetting front and a development of a wetting band according to the characteristic curves applied in seepage analysis and also in the variation of the factor of safety in stability analysis. Therefore, it is thought that in the analysis of slope stability, appropriate characteristic curves should be used.