Although the application of destress blasting as an active rockburst mitigation measure is not yet commonplace in hard coal longwall mining, the method is assuming heightened importance due to increases in mining depth and high horizontal stresses in the rock mass. The main goals of destress blasting are the softening of competent rock layers, the reduction of strain energy storage and rock mass stress release, which together contribute to minimizing rockburst occurrence and risk. One such region in which destress blasting in competent rock is applied is the Upper Silesian Coal Basin, mainly in its Czech part.
Here a case study of destress blasting is presented and evaluated in terms of rockburst prevention, focusing on a thick coal seam (about 5 m) subject to longwall mining under very unfavourable geomechanical conditions (great depth – 1200 m, competent rigid rocks between coal seams, unfavourable stress field due to long-term mining). Destress blasting stages were carried out in a group of boreholes (4–8; diameter 95 mm) with the total explosive charge ranging from 2100 kg to 3750 kg, detonated regularly at a distance of 43 m to 114 m ahead of the advancing longwall face. Evaluation of destress blasting based on seismic monitoring revealed that the longwall blastings were very successful in terms of rock mass stress release and decreasing the rockburst risk, with only one rockburst occurring on a roadway 210 m from the longwall face. The following longwall section was mined safely without rockburst occurrence.
In the hard coal reserve of the Upper Silesian Coal Basin (USCB), which is shared by the Czech Republic and Poland, longwall mining is the dominant underground mining method. The Czech part of the USCB, known as the Ostrava–Karvina Coalfield (OKR), lies in the northeastern part of the country (Figure 1). The exhaustion of the upper seams due to the continuation of coal mining activities for around 200 years has shifted activity to a greater depth (> 800 m). Under existing mining and geological conditions in the Karvina sub-basin of the USCB, underground extraction of coal is typically accompanied by rockbursts, the first of which occurred in 1912 (Pelnar 1938). A number of attempts have been made to address rockburst activity in both the Czech (e.g. Straube et al., 1972; Holecko et al., 1999; Takla et al., 2005; Holub, Rušajová, and Holecko, 2011) and Polish parts of the USCB (e.g. Dubinski and Konopko, 2000; Drzewiecki and Kabiesz, 2008).
