Materiały konferencyjne SEP 1992

378 Underground Explołtation School * 92 classical rarefraction burst wave with explosive collapsing of micro- porous system "coal-gas" transient pneumatic shifting of smali coal pieces (blocks) by gradually expandlng gas 4. Monitoring sind prevention Routlne measurements of gas pressure lndex or desorption ratio in 3 m boreholes do not give results describing properly the situa- tion. Usually testing boreholes do not achieve actlve zones (the second or even the third) and so measured values are not repre- sentative. On the other hand drilling of long boreholes łn very thin seauns in highly stressed conditions Is rather a very hard task and often impossible to realize. The preventlon has simi- lair problems resulting from the exlstence of wide precrushed zones of coa1 ahead the face. The use of 1 ong 1 arge diameter boreholes seems to be quite an effective measure here. But again smali dimensions of working space in thin seams are a very seri- ous ł imitation. It makes the method impossible to use at all then. The analysis of the naturę of this type of phenomena led to designing of novel way of its prevention. The new method is based on blasting of explosives in long boreholes in the roof sandstone bed. The boreholes are dri1led parałlely with coal face front within short distance (several meters) aiiead of it. The di p of boreho1es depends on a combi nat i on of sandst one thickness, dip of the beds, the length of coal face and possible length of the borehole. Such a blasting forms a set of micro- cracks in sandstone bed very near to the edge of coal face. As the exploitation is going on the microcracks become active cau- sing a failure of massive bed. It changes stress conditions ahead the face rapidly. Namely the pick of abutment stresses is decreasing remarkably. The experience from several coal faces where this method was used showed that it was successful. In case of roadways the measures based on long large diameter bore- holes seems to be still the most effective ones.