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near the outcrop. Thus, the outcrop barrier pillar will consist of one row of development <br />pillars 100' wide, plus the width of burnt and /or oxidized coal between that row of <br />development pillars and the outcrop. For design purposes, the burnt and /or oxidized coal <br />was assumed not component enough to be considered as a barrier pillar or contributing <br />any strength for the use as a barrier pillar. <br />Chimney Collapse Prediction <br />A form of subsidence called chimney collapse occurs above mine openings and is <br />important because a chimney collapse has the potential to breach the surface, possibly <br />reactivating an old land slide or capturing streams or springs in near outcrop areas if it <br />should occur directly underneath such a surface feature. <br />Piggott and Eynon (1977) presented a mathematical method of predicting the height <br />of chimney development based on the mining height and the percent free swell of the <br />collapsing roof rock. This prediction will assume a maximum possible 9.5 foot mining <br />height. If 9.5 feet of coal is extracted and the overburden is entirely sandstone, 67 percent <br />swell, the maximum height of potential conical, worst -case, chimney collapse would be <br />approximately 42 feet. If twelve feet of coal is extracted and the entire overburden is shale, <br />33 percent swell, the maximum height of conical, worst -case, chimney collapse is <br />approximately 86 feet. <br />PR -14 _15- 03/14 <br />