Hazard Assessment of Rocky Slopes: An Integrated Photogrammetry–GIS Approach Including Fracture Density and Probability of Failure Data

Abstract

Natural rock slopes require accurate engineering–geological characterization to determine their stability conditions. Given that a natural rock mass is often characterized by a non-uniform fracture distribution, the correct, detailed, and accurate characterization of the discontinuity pattern of the rock mass is essential. This is crucial, for example, for identifying the possibility and the probability of kinematic releases. In addition, complete stability analyses of possible rockfall events should be performed and used to create hazard maps capable of identifying the most dangerous parts of a rock mass. This paper shows a working approach that combines traditional geological surveys and remote sensing techniques for engineering–geological investigations in a natural rock slope in Northern Italy. Discontinuities were identified and mapped in a deterministic way by using semi-automatic procedures that were based on detailed 3D Unmanned Aerial Vehicle photogrammetric-based point cloud data and provided georeferenced representations of thousands of fractures. In this way, detailed documentation of the geo-mechanical and geo-structural characteristics of discontinuities were obtained and subsequently used to create fracture density maps. Then, traditional kinematic analyses and probabilistic stability analyses were performed using limit equilibrium methods. The results were then managed in a GIS environment to create a final hazard map that classifies different portions of the rock slope based on three factors: kinematic predisposition to rockfall (planar sliding, wedge sliding, toppling), fracture density, and probability of failure. The integration of the three hazard factors allowed the identification of the most hazardous areas through a deterministic and accurate procedure, with a high level of reliability. The adopted approach can therefore be very useful to determine the areas in which to prioritize remediation measures with the aim of reducing the level of risk.

Rockfall Susceptibility Assessment and Landscape Evolution of San Nicola Island (Tremiti Islands, Southern Adriatic Sea, Italy)

“…Concerning outcropping lithologies, they exert a significant influence on rockfall susceptibility and slope instability, according to their nature, degree of cementation, and behavior towards degradation processes, as also supported by literature data [68,69]. The structural discontinuities density, here meant as the number of faults, joints, open fissures, and trenches per unit of surface, is directly linked to the degree of rock fracturing, which in turn affects slope firmness, with higher values associated with a higher instability [70]; it was computed by means of the "Line Density" tool implemented in QGIS 3.22 Białowie ża, setting a search radius of 10 m, after the digitization of elements derived from literature data (especially the work by [52]) and geomechanical investigations. Moreover, the spacing and persistence of the discontinuity sets were analyzed together with their relationship with the bedding attitude and slope orientation to preliminarily define the size of blocks susceptible to fall, as well as their fall mechanism [71].…”

Contribution of High-Resolution Digital Twins for the Definition of Rockfall Activity and Associated Hazard Modelling

“…For this purpose, information about geometric parameters such as discontinuity sets and joint spacing have been computed by integrating 2D and 3D UAV-derived remote sensing data. We stress that in the case of complex and heterogeneous lithologies, semiautomatic software need to be integrated with expert-controlled tools to obtain accurate results [57]. Based on the 3D point cloud, five different joint sets and their spacing values were identified along the slope.…”

Assessment of geological hazard susceptibility based on remote sensing images

“…In processing the photogrammetric results, Vanneschi et al us ed AgisoftTM Metashape software to process the photogrammetric results and generate a geo-referenced dense 3D point cloud of the entire slope. They then manually categorized the digital terrain model by removing the vegetation to obtain the final digital terrain model (DTM) [24]. Kyriou et al processed the photographic data t hrough Agisoft PhotoScan software after measuring the data using a UAV, thus generating an orthophoto map and DSM [5].…”