From UAV acquisition to risk assessment: An integrated approach for improved geotechnical modeling of rock slopes

The use of Unmanned Aerial Vehicles (UAVs) for geological surveys has become a widespread and transformative technique in modern engineering geology (Wstoby et al., 2012). The availability of highquality UAV-mounted digital cameras, combined with advanced photogrammetry software, has significantly enhanced the accuracy and resolution of engineering and structural geological data collection. By generating photogrammetric 3D models, it is now possible to perform virtual geomechanical surveys and acquire detailed structural data from areas that are otherwise inaccessible using traditional field methods. Rock slope stability is governed by the orientation, persistence, and spacing of joint sets, which can substantially decrease the overall strength of the rock mass. Understanding these structural patterns is therefore a crucial aspect of engineering geological investigations. The present study introduces an integrated methodology that combines UAV-based photogrammetry with advanced analytical techniques to perform comprehensive 3D geotechnical modelling. The proposed workflow comprises several key steps: UAV data acquisition, 3D model generation, virtual geomechanical survey, cluster analysis, extraction of principal joint sets, 2D spatial analysis, and final 3D geotechnical modelling. To test the proposed methodology, the Monte Conero (Italy) case study was selected, focusing on the Piangrande area. This site consists of a highly fractured rocky slope developed within the Scaglia Rossa Formation, which represents part of an eroded, tectonically deformed anticline (Gigli & Casagli, 2011). The slope analyses have been undertaken using UAV based LiDAR and photogrammetric models and DEM (Itasca Consulting Group, Inc. 2023. 3DEC—Three-Dimensional Distinct Element Code (Version 9.6), Itasca Consulting Group, Inc., Minneapolis, MN, USA), and FEM-DEM (Geomechanica Inc. Irazu: 2D/3D Finite-Discrete Element Software for Geomechanical Analysis, Geomechanica Inc, Toronto, ON, Canada) numerical codes, so to verify the advantages and limitations of the different methodologies. A preliminary runout analysis is then computed to evaluate the risk for the buildings located in proximity of the slope.