ABSTRACT
After a period of intense rains, on 3 December 2013, already from the day before preceded by several warning signals, a landslide of about 900 m in length, 850 m wide, maximum depth of 40 m, with a surface area of the first detachment body of about 420,000 square meters (42 ha) and a volume of about 8 million cubic meters was mobilized from the southern slopes of Montescaglioso (Basilicata, Italy). The body moved towards the south of about 20 m, stopping against the opposite bank of a deep ditch. The distension caused by this first event triggered the movement of additional plates in the upper part of the slope, extending the total surface interested by the instability phenomenon. Despite the extensive damage to houses and commercial buildings, no casualties occurred. The landslide of 3 December 2013 triggered after 56 hours of continuous rains, from 30 November to 2 December. This severe meteoric event caused in the three consecutive days, respectively, 11, 125 and 21 mm/day of rainfall. The studies and monitoring of sensible parameters, carried out after the landslide movement, revealed numerous specificities prodromal to the landslide phenomenon: 1) a stratigraphic context, even if simple, but disrupted by late-Pleistocene tectonics and by the eustatic deepening of the base level of the hydrography; 2) a widespread aquifer over the entire surface of the landslide body inside the sandy cover laying over blue clays basement; 3) the groundwater flow which revealed the same direction of the landslide displacement; 4) a river network strongly deformed from its natural configuration, which caused a reduction of the outflow capacity and an increase of the infiltration process. Among the points of weakness, whose coalescence enveloped the large surface of the landslide, the following processes were also taken into account: loss of cementation by sandy and conglomeratic soils, partial loss of the fine soil matrix operated by groundwater flow in the stretch near the clayey bedrock and interaction of the stiff blue clays with low salinity fluids at the foot of the landslide and elsewhere. The result was a rapid movement of a rigid body, which allowed to recognize a process of progressive failure. The mean shear strength mobilized shortly after the failure has been derived by modeling an energy balance. The particularly low values of the computed shear strengths are the result of the processes of chemical destructuring of sands and clays in contact with low salinity flushing fluid, rather than the resizing of soils at residual shear strength under previous instability phenomena.
