Three-dimensional Landslide Susceptibility Analysis at the reservoir scale by Limit Equilibrium Models

Most of the reservoirs located in mountainous areas are exposed to landslides as well as bank slope erosion phenomena, which induces hazard conditions and undermines the integrity and operativity of the reservoir. It is therefore imperative to develop reliable quantitative approaches aimed at assessing landslide susceptibility of the slopes delimiting reservoirs and other slopes within the reservoir basin, so that appropriate preventive and mitigation measures can be explored and implemented accordingly. The main purpose of this study is to extend the application of three-dimensional (3D) limit equilibrium technique for slope stability analysis to the entire reservoir scale in order to conduct landslide susceptibility assessment for both shallow and deep-seated instability processes affecting artificial impoundments, under both different groundwater conditions and other relevant landslide conditioning factors. Based on the available information on the geological settings as well as the soil physical and mechanical data, the approach has been applied to the reservoir basins of the San Pietro Dam and the Occhito Dam, which are both located in Southern Italy. A schematized 3D geotechnical model was created for each of the reservoir basins upon which 3D limit equilibrium analysis of slope stability was carried out, from which safety factor maps were obtained at the entire reservoir basin scale. Different scenarios were run considering both peak and residual geotechnical strength parameters as well as different groundwater depths. In general, the obtained results enabled the identification of slopes highly susceptible to failure within the reservoir basins based on the obtained low safety factor (SF) values. The derived SF maps were validated by comparison with the available landslide inventory maps for the two reservoir basins. This showed that there is good agreement between landslides in the basins and the areas identified as more susceptible to landsliding based on the obtained low SF values confirming that the proposed approach can serve as a valuable tool for basin scale landslide susceptibility assessments. As a quantitative-based approach, the methodology has several advantages for the sake of dam safety, since it provides a clear overview of the slope stability conditions of the entire basin and, hence, can be highly useful in risk management activities.