Geomechanical safety assessment for transversely isotropic rock mass subjected to deep mining operations

Abstract

The largest risk for mining operations conducted within the Legnica–Głogów Copper Basin is created mostly by high-energy tremors, the hypocenters of which are located within the main roof strata composed of sedimentary-type rocks — mainly dolomite and anhydrite — about 40–200 m above the excavated copper ore body. These categories of rock clearly exhibit anisotropic strength–deformation characteristics that may significantly affect the safety level value represented by the appropriate safety margin (or safety factor) based on an adequate strength hypothesis. As the focal mechanism most often encountered in such tremors is a slipping-type mechanism with a rupture plane, typically the Mohr–Coulomb theory of strength is applied for a safety level assessment in Polish copper mines. It has been assumed, however, that strength theories based on anisotropic failure criteria should serve as better indicators of correlation between observed and well-characterized sedimentary rock strata failure mechanisms and the location of concentrated areas of the negative values of margin of safety within the rock mass. As changing levels of stress in the rock mass during the mining process may be tracked effectively using solutions offered by appropriate three-dimensional geomechanical models (e.g., finite element method), the assessment of these changes due to mining-face progress is also possible in the location where the seismic tremor occurred. This assessment is characterized by its focal mechanism using the appropriate geophysical methods that permit finding such geomechanical conditions, engaging also the rock mass’ strain–stress states and the material anisotropic characteristics. On the basis of the long-term path of rock mass loading — due to mining predicted by numerical modeling — this could indicate the necessary conditions that should be fulfilled if the anticipated methods of the geophysics failure mechanism could be developed. This is particularly important for anisotropic rock structures. The proposed approach is illustrated using an example of a strong seismic energy event of 0.22 GJ that occurred in 2005 in an area of the Rudna mine.