A quick combined approach for the characterization of a cliff during a post-rockfall emergency

The research presented in this paper is aimed at testing an innovative surveying protocol based the integration of quick methodologies employed for the characterization of a carbonate cliff affected by a rockfall and showing signs of further instability. During a post-rockfall emergency, one of the most important activities is evaluating if reactivation of the movements is possible, and if it may represent a threat for a series of elements at risk. On 5 January 2019, intense rainfalls triggered the detachment of a significant volume of fractured limestone and dolostone below the main square of a historical village located in southern Italy in the municipality of Messina, giving rise to one of the greatest landslides occurred at one of the most tourist areas of northeastern Sicily. Fallen blocks damaged the two main infrastructures of the area reaching also a bus terminal, which fortunately was vacant at the time of the rockfall. With the aim of proposing a quick surveying protocol during a post-rockfall emergency, terrestrial laser scanner, infrared thermography, and horizontal to vertical spectral ratio surveys were employed for the geostructural characterization of the cliff and for the investigation of the subsoil below the main square. In particular, the survey through terrestrial laser scanner returned an accurate 3D model of the cliff, where some key structures were highlighted; infrared thermography allowed recognizing caves and fractures along the cliff, with specific reference to a hollow area arising from past rockfalls. Such remote data, along with direct rock mass surveys performed by expert climbers allowed ascertaining that the instability of this cliff is driven by wedges formed by the intersection of 2 and 3 discontinuity systems, which are likely related to the main tectonic systems of the area. The horizontal to vertical spectral ratio survey allowed the identification of a peculiar vertical contrast of impedance, which may be related to a mechanical discontinuity located below the main square of the village, well matching with one of the systems responsible of the instability. The integration of these surveys methodologies resulted a useful quick protocol for the achievement of the key information on the stability of a rock cliff in the initial stage of its securing