53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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ABSTRACT: Damaging seismic events in mines can have a significant impact on safety and production, therefore it is important to understand these events to help prevent or mitigate the effect in future. Seismicity is caused by changing stress conditions in the local volume of rock. Studying the temporal evolution of seismicity allows us to study the evolving stress state in the volume of interest. In this case study, we investigate these trends before and after a damaging event in a pillar at Carola Mine, Chile. The temporal changes in the stress state are examined through the use of time histories of seismic parameters such and Energy Index (EI) and through the evolution of the moment tensors of the events. The time history of EI, a proxy for seismic stress, shows the stress in a volume around the pillar shortly before the large event is high. EI then decreases rapidly, specifically in the pillar, before stabilizing after the large event. Moment tensor analysis of the events in two pillars show how the P-axes, a proxy for sigma 1, point through different sections of the pillars as the seismicity in the pillars progress. This suggests partial pillar failure which is also supported by the analysis of EI in time and space. By studying the seismicity in these pillars, rich information about the changing stress conditions was obtained. This gives useful information about the support capability of the pillars which can be used for future planning and support design.
1. MINE SETTING
Carola mine is located in the well-known Punta del Cobre formation. It exploits chalcopyrite from which Copper is extracted, with traces of gold and silver. The chalcopyrite is found in the form of veins, veinlets, stockwork and scattered in Andesite rock (a rock with red tones, locally known as ‘Red Sediment’, represents the top of the mineralization with a variable thickness of 5 to 13 meters. On the red sediment, there is tuff, and on the tuff there are limestones). The geological bodies that are exploited are a few kilometres long, tens of meters wide and hundreds of meters high, trending some 10° to the south and with variable orientations from 40° to 90° towards West. The main faults of the mine have a general subvertical mantle towards the West, some of the strata become more horizontal with depth (40°). The latest references classify the deposit type as iron oxide copper gold deposit (IOCG) (Marschik and Fontboté, 1996 and 2001). Figure 1 shows the geology of the region.
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