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workers are on the face. If the pressure zone is an adequate <br /> distance ahead of the face, then the face can be mined with- <br /> out danger to the miners . <br /> These stresses occur regardless of the mining <br /> method, whether longwall or room-and-pillar. Abutment <br /> pressures from previous workings superimpose on areas of <br /> current mining. Thus , the effects of mining in the lower <br /> seam are manifested in pressures on the upper seam. As an <br /> example, mining in No. 2 mine must contend with abutment <br /> pressures created by previous workings in the No. 3 mine. <br /> Further, the corner of a longwall is under very <br /> heavy abutment pressures. Ours is the classic situation for <br /> every deep longwall in the world where successive panels are <br /> being worked that the abutment pressure from the face being <br /> worked is superimposed on the abutment pressures from the <br /> lower flank of the previously worked longwall panel. <br /> Coal and rock have the characteristic of failing <br /> suddenly when the compressive strength is exceeded, rather <br /> than of yielding by flexure, such as a piece of metal or <br /> steel. <br /> Manifestations of that failure are, as mentioned, <br /> outbursts, rockbursts, bumps and bouncing. All are very <br /> dangerous. Bumps and bounces, which are characterized by <br /> a sudden upheaval of the floor, can toss men, and equipment <br /> weighing several tons, and can result in the possible failure <br /> of roof and ribs . Outbursts and rockbursts also occur as <br /> sudden failing of the coal. All have great potential for <br /> damage to air courses , escapeways and water lines. In <br /> addition, in outbursting, the liberation of gas causes <br /> the coal to flow horizontally because it behaves in a fluidized <br /> manner by the rapid release of the methane. <br /> Safety in our mines is dependant on detection and <br /> release of stress in the coal and surrounding rock strata <br /> before the compressive strength fails , and pressure is thus <br /> relieved in the form of outbursting, rockbursting and <br /> bouncing. <br /> While our instrumentation and experience afford <br /> us a substantial accuracy in identifying excessive abutment <br /> pressures and stressed conditions, we cannot predict when <br /> or precisely where such conditions will develop and must <br /> rely on frequent inspections , monitoring and prompt action <br /> where needed. <br /> 2 - <br />