A method for engineering management of induced seismicity in deep-level hard rock mining

Abstract

Induced seismicity in mining is possibly the least managed risk in current rock mechanics practice. In some Australian mines, an increase in the occurrence of damage and injury related to seismicity has resulted in severe restrictions on previous operating practices. In some cases, the viability of mines has been challenged.A method is proposed in this work for managing seismic hazard in mines by comparing proposed excavation options in terms of a quantitative assessment of seismic hazard. This quantitative assessment is produced from three-dimensional, elastic, boundary element modelling of a continuum. Parameters from the modelling can be compared to historical observations of seismicity to generate probabilistic relations between seismic event occurrence and event strength.The parameters used in the event spatial occurrence relations and event strength estimates are the Factor of Safety against seismic failure for seismic types inferred from back analysis and the Modeled Ground Work (MGW). The MGW is a new concept in the field of seismicity.The modeled estimates of these parameters, are analagous to the controlling quantities for a seismic event. Namely, an unstable accumulation of energy - energy in excess of that which can be released non-violently - and a sufficient stress condition to allow failure of rock.At Brunswick Mining and Smelting, Number 12 Mine and at Kalgoorlie Consolidated Gold Mines, Mount Charlotte Mine, the method produced useable relations in case studies. In both mines, a prediction of seismicity for selected mining steps was demonstrated that would have allowed adaptation of the mining plan to achieve a better seismic outcome.The method requires considerable analytical effort, comprising detailed analysis of mining steps with well measured seismic histories, before it can be applied in a predictive manner. The steps that must be achieved to develop and apply the method in a mine are:i. identification of the mechanics of seismic source types from numerical back analysis of historic seismicity. The types can usually be defined in terms of analytical measures of rock strength - typically an oriented Coulomb slip criterion for deviatoric behaviour or a simple relation between major and minor principal stresses for other events;ii. generation of volumetric grids or test blocks around a volume exceeding that of the seismic influence of existing excavations, related to induction of events in the seismic history;iii. modelling of the Factor of Safety for different seismic source types and the MGW, associated with generation of the possible seismic source types in each test block, by substitution of alternative materials or introduction of an appropriately oriented displacement discontinuity;iv. comparison of the relative proportions of test blocks at given paired values of MGW and Factor of Safety, containing and not containing inferred events (the event/non-event ratio);v. Generation of an event spatial occurrence relation, on the basis of the event/non-event ratio, at given paired values of FOS and MGW;vi. Generation of event strength relation to MGW, from historical data and back analysis;vii. Generation of proposed mining geometries and test blocks in the likely volume of seismic influence;viii. Modelling of the FOS and MGW of all test blocks, subject to all possible seismic source types, to generate event probabilities and most likely event strength estimates. This uses the observed event spatial occurrence relation and the observed MGW/event strength relations as predictors;ix. Generation of predicted seismic populations from the predicted event probability and the strength relations.