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Rainfall infiltration and stability analysis of an unsaturated slope in residual soil from flysch rock mass

Weathering process in the flysch rock mass results with a geotechnical profile consisting of residual soil, weathered flysch rock mass and fresh flysch bedrock. Back-analysis of landslides occurred in flysch slopes often indicated high values of pore water pressures that should exist on the sliding surface decreasing the soil strength and causing a slide. Associated high groundwater levels (often close to the terrain surface) have an important role in landslide occurrence but the upper unsaturated part of the geotechnical cross-section and developed negative pore pressures caused by suction may have a critical contribution in keeping a slope stable. Due to the very low hydraulic conductivity of residual soils from flysch rock mass, rainfall infiltration is very slow and necessary conditions for saturation of the unsaturated zone are long-term rainfall events. The Valići Landslide, Croatia, occurred on flysch slope in February 2014 after few months of heavy, mostly uninterrupted rainfall. To identify the impact of negative pore pressures in unsaturated zone on slope stability and landslide activation, comprehensive soil laboratory testing and numerical analysis were conducted. Undisturbed samples of residual soil were collected from the superficial part of the landslide body and advanced laboratory measurements were performed to obtain hydro-mechanical properties of soil in unsaturated conditions. Obtained soil parameters were used to assess the influence of negative pore water pressures existing in the unsaturated zone of residual soil on rainfall infiltration process and slope stability in time. In this paper, the results of carried out laboratory tests and numerical analysis will be presented. Based on these results the behavior and landslide activation in slopes built in residual soil from flysch rock mass will be explained.

Unsaturated residual soil ; Flysch ; Rainfall-induced landslides ; Infiltration ; Stability analysis

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