HYDROLOGICAL MONITORING AND NUMERICAL MODELLING OF LANDSLIDE-PRONE PYROCLASTIC SOILS COVERING CAMALDOLI HILLSLOPES (NAPLES, ITALY)

On March 4-6, 2005, several rainfall-induced landslides occurred along slopes of the Camaldoli hill (Naples, southern Italy). These landslides were characterized as soil slips as they involved mainly failure of the surficial weathering zone of a volcanoclastic series deposited during the last active period of the Phlegraean Volcanic Field in the Plio-Quaternary. Along these slopes, falls and topples involving both lithoid (tuff and lavas) and incoherent materials (top-soil and pyroclastic deposits) typically occur after long-duration and/or intense rainfall, thus representing a recurrent threat for the population living along the foothill areas.

This study shows the preliminary results of in-situ pressure-head monitoring carried out at 0.3, 0.5, and 2.2 m depths in the surficial pyroclastic horizons. Monitoring started in 2015 along the southwestern slope of the Camaldoli hill and was focused on understanding the hydrological processes leading to the initiation of slope instability. In-situ investigations consisting of borehole excavations, dynamic penetrometric tests, topographic surveys and permeability tests were carried out, as well as geotechnical laboratory tests on soil specimens collected in the field.

Monitoring data showed a constant pressure head regime ranging from -1.0 m to -4.0 m during the winter rainy season due to a combined effect of rainfall, evapotranspiration and unsaturated water flow circulating into volcanoclastic soil horizons. Starting from spring until the late summer, an exponential decrease of pressure head was recorded (down to -20.0 m) due to the combined effect of lower rainfall, and increased air temperature and evapotranspiration rate. This experimental observation was used to set and calibrate a numerical hydrological model. By coupling model results with slope stability analyses, the critical unsaturated/saturated hydrological conditions leading to slope failures under variable rainfall intensities were estimated by a physically-based approach.