Materiały konferencyjne SEP 1992

376 Underground Explołtation School * 92 3. The mechanism of phenomena The mechanism of "typical" outburst could be briefly discri- bed as follows (Rakowski, 1991, Barron 1990): - High brłttłeness SLnd Iow tensile strenght of coal make it prone for rapid changing of mechanical state as the result of sudden changing of boundary conditions. - Removing of the certain volume of coal for new advance of the face leads to sudden forming of new open surface of the rock new boundaj-y. - There was a state of eąuilibrium on this boundaj^y before blas- t ing. The non-equi 1 i brium state instantaneously occures on the boundary formed after blasting or cutting. - Due to high internal energy the conditions for forming the dynamie stress drop on the boundary are fulfilled. The combina- tion with Iow tensile strength and high brittleness enables to generate the bursting of thin layer of coal from newly formed surface. By repeating this situation the so ca11ed burst wave develops and spreads to the coal seam. - Thin coal pieces are explosively disintegrated to grains by the expansion of gas captured and sorbed in the pores. - Fine grains or their parts are pneumatically transported to underground space. With some simplification outburst mechanism could be descri- bed by such a definition: The outburst of coal and gas is a series of instantaneous static events driven by tensile failures generated by rapid drops of internal energy on the piane which spreads to the mass like rEirefraction shock (burst) wave forming the cavity. The mechanism of considered phenomena is different in some parts. In an example of the coal face as it is showed in Fig. 2 and it could be described in this way. With developing face the span between coal pillars is growing gradually. It causes elastic bending of the sandstone bed in the roof and conseąuent increasing of abutment stresses. The pick of abutment stress is greater then and its position is shłfted toward coal pil lar. Yet primarily very friable coal is crushed forming a ąuite wide yield zone. The greater is the span