Numerical modelling and back analysis of a rock slope failure occurred in 2005 at Scascoli (Bologna, Italy)

The rock slope failure object of this study occurred on the 12th of March, 2005, within the Scascoli Gorges in the Savena Valley, 25 km south of Bologna, in the Northern Apennines, Italy. The failure involved a volume of rock of nearly 30000 m3 that detached from an 80 m high cliff and fell on the river bed and onto the adjacent road, denoted as “Strada Provinciale – Fondovalle Savena”, damming the first and destroying the latter. The conformation of the cliff, known as “Mammellone 1”, was rather convex, overhanging and undercut at the base where in contact with the river bed. The event is the last of a series of mass movements which occurred in a 15-year span in the area. With integration of past analyses and surveys, possible causes and mechanism of failure have been investigated by means of two and three-dimensional kinematic analysis (using the software DIPS and SWEDGE by Rocscience, 2016), photogrammetry and terrestrial laser scanning comparison (Cloud Compare, Daniel Girardeau-Montaut, 2016; Autocad, Autodesk, 2016) and two-dimensional finite element numerical modelling (RS2, Rocscience, 2016). The use of a finite element method to model a predominantly blocky structure has shown to be effective and to produce good results if data integration, boundary conditions and geometry of the site are well correlated between each other to best fit the resulting scenario. The design of the numerical model considered the relative position of crown and scarp to the discontinuity families and to the geometry of the cross section, to better costrain the failure surface. Furthermore, the process of formation of the valley was taken into account in order to consider also stress-strain conditions prior to the road construction and river erosion. This was carried out by multi-staging the modelling process considering the natural erosion and the advancement of the landslide on the hydrogeological left side of the Savena steam before the last rockfall event.