Slope stability analysis of a small intermittent failure

During significant precipitation events water that does not directly run off infiltrates the soil. If the soils are thin or overlie an impermeable layer, the water will percolate to the impermeable layer and build-up forming a perched water table. If the build-up of infiltrating water generates sufficient pore fluid pressure, the shear strength of the material will be reduced, resulting in slope failure. The objective of this study was to determine the conditions resulting in periodic slope instability on gently sloped terrain. It was hypothesized that that a perched water table, generated by excessive or intense precipitation events, affects the slope stability in the area and results in intermittent movement of the landslide. Data-gathering activities included monitoring precipitation, measuring the height and extent of the perched water table using a piezometer array, interpreting the location and morphology of impermeable boundaries with ground penetrating radar and assessing relative slide movements. Field measurements suggest that there was a correlation between the amount of rainfall received and the change in height of the rise of water above the impermeable surface; a slope-stability analysis was performed to quantify probable conditions resulting in stability. Conditions that lead to slope instability on gentle slopes in this study were identified as precipitation falling on soils with high antecedent moisture conditions, causing the formation of a perched water table. According to the factor of safety analysis, the height of the perched water table need for movement is met seasonally. Because of the gentle slopes, movement occurred slowly.