Rock slope stability and runout analysis for geological risk mapping in the Marina Piccola bay (Capri Island, Italy)

Thanks to modern geotechnologies, recent studies reveal that many urbanized areas are sited by rock fall hazard of high degree. Due to their particular location, buildings and houses in such areas are often exposed to high risk caused by the overhead presence of potentially unstable blocks. In this paper, the case study of “Grotta delle Felci” cliff (Solaro Mountain, Naples – Italy) is presented. With the aim of updating the existing geological risk map of the Marina Piccola bay, a study of the rock slope stability was performed using traditional geological surveys, Digital Terrestrial Photogrammetry (DTP), Laser Scanning (LS) and topo-graphic GPS and Total Station measurements. The use of DTP is recommended especially in inaccessible sites such as, quarries, mountain slopes and natural cliffs. From LS points cloud and oriented stereo-images, re-spectively the Digital Dense Surface Model (DDSM) and the cliff orthophoto were produced. The analysis of DDSM and stereopairs allowed to derive fundamental data for studying the slope stability such as the geometrical characteristics of joints (position, attitude, spacing and persistence), the blocks shape and volume and the local morphology dip and dip direction. Then, a runout analysis of unstable blocks and wedges was performed with the goal of highlighting the most dangerous areas at the bottom of the cliff and of defining the kinetic energy of possible blocks along their falling paths. Following a deterministic approach, the geometrical information, combined with data from traditional engineering-geological survey, was used to study the slope stability both by the limit equilibrium conventional method and distinct elements numerical analysis. Final results about cliff stability and runout analysis, combined with the vulnerability of buildings and roads, allowed to propose the updated geological risk map and to advise the most suitable protection works to mitigate the hazard.