Seismic activity induced by exploitation in vicinity of mined-out space, case study from deep copper ore mine in Poland

Abstract

Rock bursts are one of the most difficult to recognize and assess natural hazards in deep copper ore mines in Poland. The occurrence of rock bursts is conditioned by a number of complex geological and mining factors. The impact of each of the factors on the state of seismic hazard is sometimes impossible to be determined. The correct recognition of the rock mass condition and the causes of rock burst threat gives a chance for advance preparation of adequate prevention, and thus for eliminating or reducing the threat to the level of tolerated risk. In Poland’s copper ore mines, continuous observation of seismic activity is carried out, which is expressed in the number of tremors and their energy over a specified period of time. Moreover, the effectiveness of prevention measures and, in particular, of the active ones, which are aimed at provoking seismic energy emissions through properly executed winning blasting works, is calculated and analysed. Reducing the amount of energy accumulated in the rock mass reduces the risk of rock bursts. Exploitation is carried out in a way that avoids parallel approach of the operational front towards the goafs, drifts, and faults with drops greater than the height of the mined deposit. However, over forty years of mining of the copper ore deposit, which has created the large areas of mined-out space mean that mining works are increasingly carried out under restrained conditions. There are unfavourable geological and mining situations in the exploitation fields, disrupting rhythmic development and an even front line. The specific structure of the copper ore deposit in Poland makes it possible to conduct multidirectional cutting and to introduce local changes in the adopted directions of exploitation. Cases of operational fronts approaching goafs occur often in the vicinity of old mining infrastructure and when there is the need to mine remnant parts of the deposit not previously intended for exploitation or not mined yet. Generally, exploitation in the direction of goafs or yielding zones is limited to the cutting and yielding works. The purpose of the study was to analyse the seismic and rock burst hazard in an exploitation field located at a depth of 850 m in a copper ore mine, over a period of 5 years, when mining was carried out towards goafs. Seismic activity and the effectiveness of rock burst prevention measures were assessed and analysed. The active prevention i.e. the group winning blasting works was an inseparable element of the excavation technology. It was found that when the operational front progressed towards adjacent goafs, there were symptoms of an increase in rock mass pressure causing the destruction of roof rocks and, as a consequence, deterioration of roof stability, squeezing of floor, relaxation of the side walls and increase of seismic activity, which posed the potential threat of rock bursts.