Geomechanical Characterization of a Rock Cliff Hosting a Cultural Heritage through Ground and UAV Rock Mass Surveys for Its Sustainable Fruition

Abstract

In mountainous areas around the world, dealing with rockfalls means facing some technical survey difficulties due to the low accessibility of areas and the height of slopes. If a cultural heritage is also threatened by such mass movement, the need of specific survey solutions, even in a combined asset, is required. This paper deals with the integration of ground and UAV rock mass surveys aimed at defining the rockfall attitude of an unstable rock cliff sector hosting an example of cultural heritage in tourist area of southern Italy, whose fruition has already been threatened by the occurrence of rockfalls. As an example of the defensive architecture of XII and XIII centuries, the Saracen Castle in Taormina is reached by hundreds of visitors each year, but its access path and the surrounding area are threatened by the unstable condition of the cliff, hosting unstable rock volumes on kinematically critical planes. In order to achieve a reliable geostructural setting of the cliff, aiming at its possible securing through proper mitigation works, ground rock mass surveys could not provide enough information due to the bad accessibility of the rock faces. Therefore, a survey by an Unmanned Aerial Vehicle, with a reliable verified accuracy, was carried out to map the discontinuity planes especially occurring at the highest portions of the cliff, achieving geostructural data of different fronts of the cliff. Ground and aerial data were combined and statistically analyzed to define the main kinematic failure patterns. In this perspective, a critical comparison between the two employed surveying methodologies is proposed herein, highlighting that both approaches are affected by potential and limitations and that the integration of the mutual dataset represents a suitable solution for a complete rock mass characterization in this type of areas. Furthermore, rockfall simulations allowed ascertaining that potential falling blocks would cross the access path to the castle, thus representing a natural threat to the fruition of cultural heritage, thus proving the need of mitigation measures to ensure the safe fruition of the cultural heritage.