Sustainable decommissioning: analysis of hydrogeological behavior in rock massifs and its influence on geotechnical stability in open pit mine slopeand acid mine drainage generation

Abstract

Mining projects aim at the extraction and processing of mineral supplies, which have an extraction area (open or underground mine), processing area and ore concentration, tailings release area and dams. The quality of rocks and degree of fracture of rocky massifs are determinant in the formation of areas of geotechnical instability in open pit mines. Another factor of particular relevance is the hydrogeological behavior within rocky massifs, especially in the case of fractured crystalline rocks, where there is not necessarily an established groundwater level, but rather a flow restricted to certain fracture orientations. Geotechnical instrumentation systems are alternatives of consecrated use in the direct monitoring of rocky slope. However, particularly in fractured aquifer systems, piezometers can often be positioned in the wrong way, do not cross fractures with water and induce evaluation errors, with consequent risk and economic losses. In many cases, the occurrence of water accumulation zones or preferential flow within unknown or improperly understood rock massifs can result in unforeseen and disastrous mass movements. The mining of ores of hydrothermal origin often implies the exposure of rocks with high concentrations of sulfides in rocky slope during or after the end of activities. The hydration of mineral sulfides under surface conditions causes oxidation, release of cations and large amount of sulfur and dissolution in surface and groundwater, which configures an effluent called acid mine drainage, characterized by acid pH and high amount of metals. This effluent has a reactive character and interacts with cement in pores and fractures, besides accelerating internal weathering in rocky massifs, a phenomenon that can cause geotechnical instability in slopes sometimes silently and difficult to predict. Given the complexity of factors related to geotechnical stability of slope in open pit mines, with emphasis on sulfite deposits, this project aims to evaluate the benefits and limitations of the combination of geophysical methods (Ground Penetration Radar, DC resistivity, Induced Polarization) combined with the use of geotechnical reconnaissance surveys, with the objective of relating the hydrogeological behavior in a fractured aquifer system inside the rocky massif, with areas of geotechnical instability recognized in slope. Another issue to be evaluated is the weathering and accelerated alteration of mineralized rocks in sulfides within the rocky massif, by hydration in groundwater flow systems, and its reflections on the geotechnical stability of open pit mine slope. In the end, it will also be possible to evaluate the relevance of the geophysical diagnosis prior to the planning of geotechnical monitoring systems, specifically for the correct leasing of piezometers in places with greater flow representativeness in fractured rock massifs, with consequent increase in the quality and representativeness of direct data. (AU)