Pirone, Marianna (2009) Analysis of slope failure mechanism in unsaturated pyroclastic soils, based on testing site monitoring. [Tesi di dottorato] (Inedito)

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Abstract

In the Campania region the pyroclastic soils resting on the limestone massif are often affected by mudflows. These landslides involve more superficial deposits, products of the volcanic eruption of the districts: Campi Flegrei and Somma Vesuvius. The triggering mechanism in these soils are induced by rainfalls that can produce a significant increase in the degree of saturation and, consequently, significant reductions in suction and shear strength. In order to analyse the failure mechanism, the experimental research project on mudflows was developed at University of Naples and it is going on from the winter 2005. Based on geological and geomorphologic considerations, the testing site of Monteforte Irpino (AV) was selected to collect experimental data through laboratory testing on undisturbed samples recovered in site and by monitoring climatic conditions, matric suction and water content in the field. The soils recognized in site are constituted by the products of a series of eruptions of Somma Vesuvius, hence they represent well the large area of Campania affected by the mudflows. Mechanical and hydraulic characterization of pyroclastic soils sampled in the test site are available, thanks to a previous PhD thesis (Papa, PhD thesis 2007). The instrumentation installed in site consists of 94 tensiometers, 40 TDR probes, 6 Casagrande piezometers and a weather station. To analyse the factors predisposing the failure, the “hydraulic regime” in the subsoil and the influence of rainfall on the distribution of pore water pressure have to be investigated. Hence, in this thesis the data collected in situ: suction, water content and climatic conditions are shown. The elaboration of measurements are presented too, in particular the intensity and direction of the water flows in the subsoil. Moreover it was possible to get some conclusion about the water balance in the subsoil and to identify the rule of the pumices interposed between the pyroclastic layers. The central part of thesis concerns the numerical analyses of the triggering mechanism by using the monitoring data and the results of the test lab already available (Papa, 2007). The numerical code used was ICFEP (Imperial College Finite Element Program). The model was validated simulating the observed conditions over one hydrologic year. Then the slope stability analyses were carried out to understand how the failure mechanism changes varying the rainfall history applied on the upper boundary, the saturated permeability, the critical state angle, the initial conditions. By processing the numerical results, the influence on the failure mechanism of each factor is identified and the critical threshold in terms of daily rain intensity can be developed.

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