Multiple Site-Specific Analyses of the Stability of Quarried Slopes in Lebanon Based on Unmanned Aerial Vehicle Photogrammetry

Abstract

Landslide hazards associated with the quarrying sector in Lebanon are a source of public concern due to the adverse environmental and social impacts imposed by illegal quarrying activities. Such activities have been leading to direct threats to human lives and property damage. Regional landslide hazard and risk maps have been developed based on a multi-model analysis performed through the framework of a Geographic Information System (GIS) software using a 15-meter digital elevation model (DEM) by Grant et al. (2016) and Pollock et al. (2019) and their accuracy was validated for natural terrain. The use of these maps for stability assessment of quarry locations was examined by Kaafarani (2020) and was found to be limited because of the steepness of these man-made slopes and the inability of the 15-meter DEM to capture the real topography. The aim of this work is to determine, from multiple site-specific analyses, the most probable mode(s) of failure of such steep rock slopes and to assess the extent to which a failure may impact the surrounding urbanized zones. Remote sensing using Unmanned Aerial Vehicle (UAV) photogrammetry was used to map four limestone quarries located in the Anti-Lebanon Mountain Range. The stability of the studied quarries controlled by geometry and strength of the discontinuities was first evaluated kinematically and then analysed using limit equilibrium methods under dry, rainfall, and seismic conditions. The overall stability of the quarries was also assessed based on the geological strength index (GSI) classification and the rock mass geomechanical properties were based on the generalized Hoek- Brown constants. Risk of block failure along discontinuities was observed and wedge sliding, in particular, had the highest probability of occurrence. The maximum runout distance obtained was less than 9 meters in all sites. The results did not show risk of general failure in any of the four quarries. The outcome of this study can be used to update the regional-scale maps by refining the analysis in quarry locations, to develop strict regulations for future quarrying operations, and to implement a mitigation plan that protected further urbanization projects in areas near dormant and abandoned quarry sites.