The San Leo Cliff in the Northern Apennines, Italy: slope instability and risk mitigation measures after the 2014 landslide

Abstract

The medieval town of San Leo and its fortress have been classifi ed as a “settlement to be consolidated”, following the Italian law. They rise on a rocky, steep cliff that is prone to failure, as well as the entire area surrounding the cliff is subject to slope instability phenomena. Over the past seventy years, a series of consolidation works have been conducted on the rock walls. These interventions focused mainly on the south face, where the town and its access road are located, and on the east face, particularly in the area where the fortress is situated. Since the 1980s, efforts to reduce the risk have also focused on the base of the cliff with the aim to prevent its erosion and undermining. The latter is considered the cause of major rock failures, such as the landslide that occurred in 2014. Despite past interventions, however, landslide risk in the San Leo cliff still remains high. The slope failure of 2014 marks a dividing line: after that a comprehensive multidisciplinary study was started. The study involved different investigation and monitoring techniques in order to understand, prevent, and reduce landslide risk. The in-depth knowledge gained through this study was crucial in managing the emergency after the collapse, providing the essential elements for defi ning risk scenarios and for planning and designing interventions to reduce risk after the emergency phase. These interventions were carried out on the north face of the cliff, where the main landslide occurred in 2014, but also the east and south faces.