Rodrigues Afonso Dias, Ana Sofia (2018) The effect of vegetation on slope stability of shallow pyroclastic soil covers. [Tesi di dottorato]

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The effect of the local vegetation, composed of cultivated Castanea sativa, on slope stability was investigated on a test site in Mount Faito (Campania, Southern Italy). In Campania, shallow pyroclastic soil covers are susceptible to landslides triggered by rainfall. Prolonged rainfall periods followed by heavy short-term rainfall events trigger fast moving and highly destructive landslides in road cuts and pyroclastic scarps on rocky cliffs in the areas surrounding the Vesuvius volcano. Undisturbed pyroclastic soil samples containing roots of mature C. sativa were used for hydraulic characterization through an extensive set of laboratory experiments. Saturated permeability, evaporation and imbibition response, water content for high suction ranges, and the root dry biomass were determined. The presence of roots increased the hydraulic permeability by one order of magnitude in the most surficial soil (10-7 to 10-6 m s-1) and decreased the air-entry value of the water retention curves (6 to 4 kPa). The variability of soil permeability among soil layers was identified as conditioning of the groundwater flow with regard to the speed of the wetting front movement and generation of positive pore-water pressures within the soil profile. The calibration of hysteretic model to characterize natural pyroclastic soil provided a more approximate manner of modelling in situ hydraulic responses. A good agreement between the model and the field observations was obtained. Field monitoring was performed with the intent of showing that the distribution of roots of C. sativa is associated to the groundwater regime. The spatial and vertical distribution of root density and traits were quantified for C. sativa roots collected from several boreholes performed in Mount Faito. Minimum suction, minimum water content and minimum gradient (indicative of downward water flow), were monitored throughout the year and related to root distribution and spatial distribution of trees. An increasing root density was found to be associated to lower values of suction and higher gradients of infiltration, which can potentially have a negative influence of the slope stability. A modelling investigation on the mechanical reinforcement of soil by tree roots allowed us to understand the importance of hydraulic and mechanic components on the stability of a slope. Roots increase greatly the shear strength of soil (up to 25.8 kPa in Mount Faito) through mechanical reinforcement and consequently, the safety factor of the slope increased significantly. Considering the root reinforcement in the estimation of potential failure surfaces safety factor showed that the weakest failure surface was found at 2.2 m, where the root reinforcement was 1.3 kPa, instead of 0.9 m without the root reinforcement of 13.8 kPa. The weakest failure surface found was in agreement with the failure surfaces observed from previous landslides near the test site. The test site did not present the characteristics of a landslide triggering area. The slope angle of the landslide triggering areas (35° to 45°) can easily exceed the soil friction angle (36.5° to 38.5°) and the hydraulic effect (the contribution of suction to the shear strength) would not be enough to guarantee the stability of the slope during the wet season (0 to 10 kPa). However, the root reinforcement was estimated to be able to sustain the slopes until an angle of 42°. Therefore, the presence of tree roots was found to affect hydraulically and mechanically stability of pyroclastic soil covers. Such conclusions may be extended to the areas of Campania where C. sativa plantations are present. The hydraulic effect of vegetation was greatly compensated by the mechanical reinforcement of roots.

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