Using Rock Mechanics Classification to Reduce Ground Instability in Amethyst Underground Mining

Brazilian amethyst production is mostly obtained from underground mines in the southernmost part of the country, involving approximately 6000 miners and generating an income of US$ 20 million per year. Amethyst-bearing geodes are hosted by Cretaceous continental flood basalts of the Serra Geral Formation. The economic zone occurs in the massive (colonnade) intermediary section of the Sao Gabriel Basalto. The tunnels and stopes follow the occurrence of geodes leading to an erratic and complex layout. A ground control survey revealed excessive roof convergence. Topographic results show excessive roof deformation and some roof falls have already been reported. To understand the ground conditions, prevent roof falls and better define the pillar dimensions detailed geomechanical mapping was carried out using the scanline method. Uniaxial compressive strength tests provided complementary information for rock mass classification. Results of the geomechanical classification were used to modify the mining layout taking into account the roof and pillar geomechanical characteristics. Retreat pillar mining minimised ground instability.