Geotechnical characterization and slope stability evaluation of the gypsum quarry of Sogerela, Óbidos, Portugal

A geotechnical characterization of the gypsum quarry of Sogerela and its slope stability assessment, were carried out. The quarry is located at Avarela, in the municipality of Óbidos (western Portugal). For the evaluation of rock mass quality, the characteristics of the discontinuities were defined, which correspond to an important factor in the stability and behavior of the fractured gypsum rock mass affected by diapiric deformations. Laboratory characterization tests were performed, determining several parameters such as: apparent density, open porosity, slake durability index, Schmidt hammer rebound hardness, point load strength index and mineral identification with X-Ray Diffraction (XRD). Results obtained from field survey and the laboratory tests allowed to define the values of the geotechnical parameters and to perform the characterization and classification of the rock mass, using the Rock Mass Rating (RMR) and Geotechnical Strength Index (GSI).

The data obtained was used in the geomechanical simulation for slope stability analysis using SLIDE2 V9.0 and RocFall V8.0 software programs. The quarry slope stability was assessed with the definition of the possibility of slope failure considering the raise of the water table and in the eventual occurrence of an earthquake. The results allowed to verify that that the actual conditions seem to be stable, concerning global failure of the excavation slopes, despite the frequent fall of gypsum fragments adjacent to the quarry walls during and after rainy periods.