On the prediction of slope saturated time considering unsaturated soils characteristics under rainfalls

Many studies on slope stability have indicated that the slope stability decreases due to infiltration of rain water into a slope. In Korea, the fully saturated ground condition during rainy season, which is the worst, is used for the slope stability analysis where rainfall intensity, soil type and slope geometry are ignored. However, use of such a condition in rainy season would be conservative because most of the soil slopes is maintained at unsaturated condition. To evaluate slope stability properly, therefore, seepage analyses in an unsaturated soil need to be considered. Although, based on these results, slope stability analyses need to be performed by considering pore water pressure, many difficulties exist due to complicated processes. This paper suggests a simple method to predict saturated time of weathered granite soil slope in unsaturated ground condition on the basis of a result of seepage analyses in various conditions. If predicted saturated time exists within possible range (within 24 hours), we can understand that the fully saturation ground condition is appropriate for the slope stability analyses. To investigate the effects of soil-water characteristics, permeability at saturation, slope geometry, and rainfall intensity on saturated time of soil slope, the finite element analyses of transient water flow in unsaturated slopes are carried out. From the results, the chart for predicting saturation time of soil slope by considering soil-water characteristics, permeability at saturation, and slope height is presented. To verify the proposed chart, the prediction time is compared with laboratory model tests.