3D Geotechnical Stability Analysis of the Open Pit Mine at Styrian Erzberg

Current slope stability assessments at the Styrian Erzberg are primarily based on predefined two-dimensional cross-sections evaluated using limit equilibrium methods. Within the scope of this study, a three-dimensional geotechnical model based on available data and previous reports was developed and implemented within an appropriate numerical simulation environment. Subsequently, numerical analyses were carried out and compared with earlier studies. Finally, areas of the model with potential need for further investigation were identified. The geotechnical model is fundamentally composed of the geological units, structures, rock mass properties and the hydrogeological conditions. This model is supported by the geometric model, which provides the relevant information of mining stages. Following the parameter study and geotechnical model setup, the three-dimensional numerical model was implemented in the Software FLAC3D (Itasca, 2025) The results of the numerical simulations indicate agreement with previous assessments in domains with limited three-dimensional stress influence. Whereas significant changes within the potential failure mode occur in areas with significant three-dimensional influence. Evaluation of the plastified zones, developing during mining progression, shows concentration of yielding within the transition zone between the Porphyroid and Carbonate Formation. With advancing excavation, both the extent and connection of these plastified regions increase, extending across the eastern main slope towards the crusher slope. The assessment demonstrates that the introduced transition zone with reduced rock mass strength has a significant influence on the overall stability of the Styrian Erzberg. In summary, the transition from two- to three-dimensional slope stability assessment by means of numerical simulation is inherently more time and computationally intensive. However, it provides essential insight into the governing geotechnical mechanisms of the Styrian Erzberg. At the same time, the analysis highlights the necessity for further investigations, particularly regarding geotechnical parameterisation, in order to fully exploit the advantages of numerical modelling.