engleski

Importance of geotechnical cross-section unsaturated zone for landslide occurrence in flysch deposits

Factors primary controlling rainfall-induced slope failures are both rainfall characteristics and soil properties. Rainfall causes a transient infiltration process through the unsaturated part of the slope which affects sloping material by increasing the moisture content and pore-water pressure, changing the effective stress and hydro-mechanical properties of the soil. To study the physical process of rainfall infiltration and how it affects the slope stability state, the dependency between water retention properties, hydraulic permeability, shear strength properties and the water content or negative pore-water pressures existing above the phreatic surface have to be known. Unlike for the shallow-type landslides, very few studies were performed for the case of deep-seated landslides built of material with relatively low hydraulic conductivity. This kind of studies were never performed for landslides occurring in flysch deposits. This study presents the results of different field investigations, laboratory tests, and numerical activities that were performed to build up a numerical model capable to determine the influence of rainfall infiltration process on slope stability state through time. Undisturbed samples of the residual soil from a flysch rock mass were used to perform different laboratory tests in saturated and unsaturated conditions, aimed to provide an advanced hydro-mechanical characterization of the soil behavior under negative pore-water pressure. For the first time, the water retention curves, hydraulic conductivity functions, and unsaturated shear strength characteristics of the investigated soil were determined. Simulation results showed that rainfall duration, instead of intensity, is the crucial factor causing the landslide instability. Results suggest that only about 8 mm of rainfall can infiltrate into the slope during a day. The shear strength component associated with the negative pore-water pressure was found to have a negligible influence on the slope stability at the time of failure since positive pore-water pressure exists along the entire sliding surface. However, it is the unsaturated zone with specific hydraulic features that delay the landslide triggering event. Reduced hydraulic permeability and retention capacity of the soil between the phreatic line and slope surface are factors that postpone build-up of positive pore-water pressure along the sliding surface and enable the slope to remain stable during a long-lasting heavy precipitation.

flysch, unsaturated residual soil, rainfall-induced landslides, water retention curve, hydraulic conductivity function, unsaturated shear strength, numerical analysis

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