HYDROLOGICAL AND STABILITY MODELLING OF INITIAL LANDSLIDES TRIGGERING DEBRIS FLOWS IN ASH-FALL DEPOSITS COVERING HILLSLOPES SURROUNDING SOMMA-VESUVIUS (SOUTHERN ITALY).

Rainfall-induced debris flows involving ash-fall pyroclastic deposits covering steep mountain slopes that surround the Somma-Vesuvius volcano, are natural events representing the main cause of risk for urban settlements located at footslopes. The presented research was based on the review of the wide scientific literature and was aimed to the improvement of some crucial aspects regarding the initiation of debris flows by means of field and laboratory experimental methods and modelling applied in representative sample areas of the Sarno Mountain Range, where deadly flow-like landslides initiated on May 5th - 6th 1998. Detailed stratigraphic and topographic surveys carried out in three representative initiation areas led to recognise that, depending on the slope angle, ash-fall pyroclastic deposits are discontinuously distributed along slopes, showing a total thickness that varies from a maximum value recognisable in the slope angle range lower than 30° up to be negligible for slope angle values greater than 50°, thus being strongly related to bedrock morphology itself. This distribution influences stratigraphical setting of ash-fall pyroclastic mantle leading to a downward thinning up to pinch out of pyroclastic horizons. Three fundamental quantitative engineering geological models were identified, in which the most part of the initial landslides occurred in May 1998 can be classified: i) knickpoints, characterised by a downward progressive thinning of pyroclastic mantle; ii) rocky scarps, identified as causing an abrupt interruption of pyroclastic mantle; iii) road cuts in pyroclastic mantle, whereas they occur in a critical slope angle range. Coupled geotechnical and saturated/unsaturated hydraulic characterisations of pyroclastic soils, led to the hydro-mechanical modelling of slope stability in the initiation areas. Results demonstrated that initial instabilities of pyroclastic mantle can occur without a hydraulic contribution from the carbonate bedrock, therefore critical increase of pore pressure derives from the infiltration and throughflow processes. Finally, the hydro-mechanical modelling of slope stability permitted the deterministic definition of intensity/duration hydrological thresholds.