The marble deposit of Gummern is located in the northwest of Villach. In the course of this review, a long-term rock slope stability analysis of the permitted Gummern open pit high wall was conducted with numerical methods. A few other versions of open pit high walls were also conducted. Furthermore, the effects of an underground mine on the stability of the rock slopes were investigated. In the reason of the enforcement of a numerical analysis, some geotechnical inquiries must be done, because it is necessary to identify geotechnical areas of homogeneity. On the basis of these areas, material properties, which are necessary for a numerical simulation, can be identified. The geotechnical inquiries are divided into a qualitative geotechnical rock mass classification and a quantitative rock mass classification. For the qualitative rock mass classification, the Rock Mass Rating System of Bieniawski in the version of 1989, Bartons Q-System in the Version of 2002 and the GSI (Geological Strength Index) in the Version of 2002 were applied. These three systems were compared. After the comparison, it was decided to base the quantitative rock mass classification on the GSI. After a comparison, the numerical analysis was conducted with the program PHASE2D from RocScience. The simulation based on the Mohr-Coulomb failure criteria. After a comparison with the Hoek & Brown failure criteria it was decided to work with Mohr-Coulomb, because the Mohr-Coulomb criteria is easier to handle than Hoek & Brown, and the calculation time for the simulation is faster. It has to be concerned that safety factors, gained by Mohr-Coulomb, are a little bit higher in areas near the surface, than safety factors gained by Hoek & Brown for these areas.