Rainfall infiltration into unsaturated soils and its influence on slope stability

To understand the relationship between rainfall infiltration and slope stability, this study conducted a series of column test using weathered soils from gneisses, granites, and mudstones. In this study, volumetric water content and pore water pressure were measured using TDR sensors and tensiometers at a regular time interval. Unit weight conditions of soils are determined based on the in-situ conditions. For the column test, three steps of unit weights are set as in-situ condition, looser condition and denser condition. Rainfall intensities are selected as 20mm/h and 50mm/h. As the results of this test, the higher rainfall intensity and the lower unit weight of soil, the faster average infiltration velocity. Granite weathered soils of rainfall infiltration velocities were faster than the other weathered soils. These results are related with grain size distribution and mineral composition such as clays. This study also analyzed changes of safety factor of an infinite slope based on the result of rainfall infiltration velocity by the column test. The safety factor was calculated by an equation based on the concept of quasi-dynamic wetness index considering saturation depth ratio dependent on rainfall duration. The equation can calculate the safety factor of a slope on each time due to changes of saturation depth ratio with rainfall in soil. The analyses results show that safety factors were changed rapidly with lower unit weight of soil and higher rainfall intensity. Therefore, it can consider the effect of rainfall on the slope stability.