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The objective of this research project was to study, through a geological-technical modeling, the possible trigger parameters of rapid kinematic landslide phenomena involving the pyroclastic deposits of some geological contexts of Campania area. In particular, the study area, located on the eastern edge of the Piana Campana, includes the slopes of the municipal territory of Palma Campania, part of the wider carbonate ridge of the Monti di Sarno. During the geological survey, relatively recent landslides with initial movement of the translational flow type were mapped (Cruden & Varnes, 1996). Particularly, the detailed field analysis allowed to hypothesize that the initial rupture surface could be located at the transition between pumice and scoriae levels of different granulometry where an interface characterized by a significant variation in porosity and permeability was developed. In the case study, the deposits are constituted by primary fallout layers, which can be attributed to the Vesuvian eruption of the so-called Pumici di Base dated about 22 ka (Santacroce et al., 2008). In order to characterize in detail the possible triggering mechanisms of the observed landslide phenomena, representative samples were taken at different stratigraphic heights in the pyroclastic deposit. The samples were analyzed both with conventional methods for grain size, specific gravity and density, and with innovative methods, such as RX microtomography (micro-ct), for porosity and hydraulic conductivity. Field observation, conventional and innovative micro-ct analyses showed a variability of the granulometry, specific gravity and density of the materials along the examined pyroclastic sequence and allowed the identification and quantification of the total and connected porosity and hydraulic conductivity both for reconstituted samples and for individual clasts. The results obtained have made it possible to hypothesize that at the transition between grey pumice and black scoriae (corresponding to the rupture surfaces of landslides observed in the field), it is possible the formation of so called "capillary barriers" (as defined by Shackelford et al. 1994) during the infiltration phenomena within unsaturated pyroclastic layers of different granulometry. This hypothesis, which for the adopted methodologies contains aspects that are certainly original and never applied to the microscale, has been verified, on the basis of the stratigraphic succession type, through numerical simulations allowing the reconstruction of the process of infiltration of meteoric water throughout the different stratigraphic levels 7 as the textural characteristics change (grain size, porosity, hydraulic conductivity obtained from laboratory analysis on natural samples at macro and microscopic scale). For numerical simulations, two models with different scales were considered: - meso-scale (2D), using the VS2DTI software open source to finite differences developed by the U.S.G.S. for the simulation of infiltration processes in unsaturated soils; - micro-scale (3D) on segmented tomographic images, using the open source software OpenFOAM, (Open Field Operation And Manipulation) for fluid dynamics analysis. The results obtained from the simulations support the interpretative model proposed in this doctoral thesis, able to explain instability phenomena in partially saturated pyroclastic coulters, through the detailed analysis of the infiltration process thanks to a more precise characterization of the local stratigraphy, from the field scale to the micro-scale. |
