Analisi dei fenomeni di dissesto a Civita di Bagnoregio ed interventi di stabilizzazione

Civita di Bagnoregio is a typical Mediaeval town, built on top of a volcanic cliff and evolved today into a characteristic morphological peak, as a consequence of extreme erosional phenomena. Landslides involving the cliff-slope system produced a progressive retreat of the cliff edges, causing the destruction of several buildings, so that Civita is commonly known as the “Dying Town”. In the last decade Civita has become a sort of geomorphological field laboratory where Italian scientists and engineers develop new approaches for stabilisation and restoration through low environmental impact reinforcement works. The worst instability phenomena affect the northern cliff of Civita, where rock falls and debris slides have occurred since the last centuries, probably triggered by deforestation in the 17th century and by the climatic deterioration of the Little Ice Age. This paper describes the landslide events in the Northern cliff occurred in the last decade. A geomechanical characterisation of the materials and the back-analysis of the two main landslide events (the rock fall of 1993 and the debris slide of 1996) are described in detail. The back-analyses allow us to understand the instability mechanisms and to propose remedial measures aimed at mitigating the effective causes of instability, also taking into account the landscape and environmental constraints of the site. In particular the interpretation and analysis of the 1993 rock fall have provided indications for designing of stabilisation works at the cliff edge, based on anchored hollow wells, underpinned on micropiles. A possible stabilisation solution based on filling of the Cavon Grande depression, has been analysed adopting an influence line approach and the results of the back-analysis of the debris slide. Key words: landslides, back-analisys, stabilisation works.