Coal seams in the Upper Silesian Coal Basin are currently extracted under more and more disadvantageous geological and mining conditions. Mining depth, geological dislocations and mining remnants are factors which affect the rockburst hazard during underground mining to the greatest extent. This hazard can be minimized by employment of active rockburst prevention, where long-hole destress blasts in roof rocks (torpedo blasts) have an important role. The main goal of these blastings is to either destress local stress concentrations in rock mass and to fracture the thick layers of strong roof rocks to prevent or minimize the impact of high energy tremors on the excavations. Sometimes, these blastings are performed to make the roof rocks caving behind the longwall face easier. The efficiency of blasting is typically evaluated from the seismic effect, which is calculated based on seismic monitoring data (seismic energy) and the weight of the charged explosive. This method, as used previously in the Czech Republic, was adopted in a selected Polish hard coal mine in the Upper Silesian Coal Basin. This method enables rapid and easy estimation of destress blasting effectiveness, adjusted to conditions occurring in the designed colliery. Destress blasts effectiveness may be evaluated via the seismic source parameters analysis as well, as was carried out in the selected colliery in the Polish part of the Upper Silesian Coal Basin. These parameters provide information, for example, on its size, state of stress and occurrence of slip mechanism in the source of provoked tremors. Long-hole destress blasting effectiveness in selected colliery has been evaluated using the seismic effect method and seismic source parameters analysis. The results were compared with each other and conditions were observed in situ.

1. Introduction

Rockburst is a dangerous phenomenon occurring during deep underground hard coal mining in the Upper Silesian Coal Basin (USCB). To minimize this hazard, special prevention techniques are applied. One of them are destress blasts in the rocks surrounding the coal seam, especially in roof rocks. The main purpose of such blasts is to reduce stress concentrations in these rocks, and to reach a new advantageous energy equilibrium state by the rock mass due to stress drop. Fracturing the thick layers of strong roof rocks to prevent or minimize the impact of high energy tremors on the excavations is important too. Sometimes blasts are performed to facilitate roof rocks caving and goaf formation. Hanging-up of strong roof rocks behind longwall face may be responsible for high-energy tremors occurrence in close distance from the longwall face, which is dangerous for the working crew.

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