Rockburst represents a very dangerous phenomenon in the Upper Silesian Coal Basin (USCB). The rockburst problem relates to the natural and mining conditions in the rock mass. It is very difficult to decide which of the factors prevails, but we generally assume that the existence of competent adjacent rocks in the roof of the seams and incomplete seam extraction in the overburden, cause irregular stress distribution in the rock mass and stress overflow in those regions with the highest stress concentration. Destress blasting in overlying competent rocks is a very important rockburst control technique.The case study describes the control of rockburst risk by destress blasting in an example of the longwall of the Czech part of the USCB. Destress blasting, in the competent overburden of the exploited seam, was applied as the main rockburst prevention method. Groups of boreholes were fired ahead of the longwall face with explosive charges of up to 4500 kg during the longwall mining of the thick coal seam. An analysis of the effectiveness of stress release following destress blasting and generally, during the course of mining was realized. Despite its rarity, rockburst in the longwall face was induced by the applied destress blasting during the course of longwall mining. The authors evaluate the causes and effects of the induced rockburst from in situ stress and deformation measurements. The case study presented confirms that destress blasting is a very important mine safety tool that contributes to the reduction of rockburst hazard.

1 Introduction

Ostrava-Karvina coalfield (hereafter OKR) is the only hard coal deposit in the Czech Republic. It is a part of the Upper Silesian Coal Basin (USCB), which mostly belongs to Poland (see Fig. 1). Owing to the geological and geomechanical characteristics of the rock massif and because of long-term mining activity, irregular stress distribution occurs in the rock mass. Irregular distribution of stress and of its concentration in certain parts of the rock mass, when exceeding the rock strength limit, can lead to a sudden release of elastic deformation energy and to the occurrence of a rockburst. Considerable attention has been dedicated to this problem in the OKR for about one hundred years, as the first reports of rockburst are dated from the very beginning of the last century (Holecko et al. 1999). In the OKR, a multiseam sedimentary hard coal deposit is exploited. The Czech part comprises an area of about 1550 km2. The total thickness of the Carboniferous strata extends up to 2000 m. The geological structure is the result of several geological genesis cycles. It is essentially changing in a west-east direction and based on geological differentiation, the Czech part of the deposit has been structured into a western part (Ostrava part of basin), and into an eastern part (Karvina part of basin). It should be mentioned here that at the present time only the eastern Karvina part of the OKR is exploited, whereas mining in the western Ostrava part of the OKR has been halted. In the Karviná sub basin, the paralic series were covered by sedimentation of continental molasse beds. In contrast to the lithological nature of the western part of the OKR, the sedimentary cycles are conspicuously longer and moreover, very competent sandy rocks prevail (Dopita et al. 1997).

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