Mining induced seismicity in Sweden
Sammanfattning: The virgin stress state in the rock mass is disturbed by mining, which leads to locally increased or decreased stresses. The rock mass in Sweden is generally composed of high strength brittle rock types, so the risk of seismicity and rockbursts (violent failures) increase with increasing depth of mining due to increasing stress levels. Seismicity is the rock mass response to deformation and failure. A seismic event is the sudden release of potential or stored energy in the rock. The released energy is then radiated as seismic waves. A rockburst is defined as a mining-induced seismic event that causes damage to openings in the rock. The most important factors influencing the occurrence and intensity of seismicity are virgin stress state, rock properties, and the influence of the mining method on the stress field. The Swedish cut-and-fill mines are comparable to the studied Canadian mines regarding stress state, mining method, and rock properties, so the same seismicity problems should be expected as mining depth increases. The Swedish sublevel caving mines are not comparable to the studied open stoping mines, regarding stress state and the influence of mining on the stress field. The sublevel caving mining method influences the virgin stress state over a larger area than open stoping methods, which means that the principal stresses around footwall drifts are of the same order or higher at the same depth. This means that seismic events of certain magnitude that are experienced at a depth of 2000 m in Canadian mines, can be expected to occur at less depth in the sublevel caving mines. At present the damage caused by seismicity is limited and can be controlled with the standard stiff reinforcement. When the events become larger, the reinforcement must be complemented with more yielding and energy absorbing components. Practices regarding energy absorbing reinforcement and destressing of drifts should be studied and evaluated for application in the Swedish mines. The application of the correct reinforcement in seismic areas requires that these areas can be identified before seismicity start to occur. This identification can be accomplished by combining a geomechanical model of the mine with 3D stress and energy modeling of the proposed mining sequence. The geomechanical model should include geological structures, properties and locations of different rock types, failure mappings, and rock mass classification. The purpose of the model would be to increase the understanding of the behavior of the rock mass, and to identify areas of high seismic hazard. In the mines where a model already exists, the model should be developed further, to connect geology with seismic events, and to elastic stress analysis on both small and large scale. A seismic monitoring system is an investment worth considering for mines experiencing seismicity, both for localization and estimation of magnitude of seismic events, but also to monitor the behavior of the rock mass during mining. This can provide valuable input for production planning, sequencing etc.
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