Geomorphological and geotechnical analysis of the December 3, 2013, Montescaglioso landslide (southern Italy)

The In the second half of 2013, the area between Apulia and Basilicata (southern Italy) was struck by severe rainfall events that produced many effects on the environment, including slope movements, sinkholes, and floods. Beside the severe economic losses, four fatalities had to be recorded. The town of Montescaglioso, located at the top of a prominent hill in a highly-prone to landslides setting, was in particular interested on its south-western slope by a large landslide on December 3, 2013. The phenomenon, that covered a total area of some 3.0 × 105 m2 as a whole, started rapidly, accordingly to eyewitness accounts, and in a time span of 15-20 minutes destroyed more than 500 m of the main road connecting the town of Montescaglioso to the Province Road SP175, and involved a few warehouses, a supermarket, and private homes. Soon afterward the landslide activation, the main consequences of the slope movements were evaluated through detailed field surveys, which allowed, at the same time, to compile a map of the surface deformations in the affected area, aimed at identifying zones within the landslide body that showed different kinematics. The field surveys were aided by the visual analysis of post-event terrestrial photographs and photographs taken during helicopter flights. Many different geomorphological features were identified and mapped, including, but not limited to, single fractures, sets of fractures, tension cracks, trenches up to 6 m in depth or width, and pressure ridges. This first surveys, and the deriving map, were of crucial importance, since many of the geomorphological features mapped immediately after the landslide event were few days later destroyed, due to the first remediation works. Based upon the geomorphological data thus collected, and the outcomes of a campaign of boreholes and geophysical surveys, a geotechnical model of the area affected by the large landslide was produced. In particular, a transient seepage finite element analysis, coupled with limit equilibrium analysis, was developed to explore the role of the rainfall history as the main triggering factor of the instability process. Also, a three-dimensional finite element analysis was carried out in order to investigate the overall failure mechanism and the directivity of the landslide movement. The modelling results highlights the role of a pre-existing landslide surface, so that the landslide process recently occurred can be identified as a slope reactivation. Moreover, the numerical results are consistent with the landslide displacement mapping derived from both the field observations and the application of interferometry techniques.