Stress concentration and rockburst risk in coal mine are associated with the occurrence of competent strata in overburdened coal seams. Benches of sandstone and conglomerate are difficult unbreakable and a high level of elastic energy is stored there. It can be released as a rockburst. Destress blasting is a rockburst protective measure in hardcoal longwall mining. For optimal use of this method it is useful to know the stress changes induced by blasting. This paper describes the results of a project's in situ stress and its changes measurement ahead of an advancing longwall face in rock mass affected by destress blasting. The project was carried out in a longwall panel with a high rockburst risk. The newly developed modified overcoring method of stress testing was used. Although the explored longwall is still in progress the first results of stress development in the overburden of a mined seam influenced by blasting are interpreted.
Rockbursts in the Upper Silesian Coal Basin (USCB)have represented a very dangerous phenomenon for a long time (e.g. Konicek et al. 2011). The rockburst problem relates primarily to the existence of competent rocks in rock mass and to the non-uniformstress fields induced by long-term excavation. The occurrence of a critical stress field leads to the brittle failure of coal and also of adjacent rocks. Such rockbursts are usually the result of long-term formation of unfavourable stress conditions in a specific geological structure (Holecko et al. 1999).
The impact of destress blasting on the original high stress concentration ahead of advanced longwall face is main goal of our stress research. There is a general consensus that destress blasting softens the rock and reduces its effective elastic deformation modulus. There are conflicting views over the importance of the destress blasting for reducing the stress and the stored strain energy from the rock (e.g. Sedlak 1997, Konicek et al. 2013). Destress rock blasting has been used in underground coal mining in the Czech part of the USCB since 1990 to prevent rockbursts and more than 2000 destress rock blastings occurred in this region between 1990 and 2010 to rockbursts control (Konicek et al. 2011).
This paper presents the first results for in situ stress and measured changes. The measurement was carried out in a selected longwall panel in the Czech part of the USCB.