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• GEO-HYDRO Consulting, Inc. (1983) provides guidelines for a necessary width of <br />an outcrop barrier pillar in different geologic and hydrologic conditions. According to their <br />evaluation, a pillar width of some 50 feet should be sufficient in most conditions of the <br />Bowie #1 Mine to prevent crushing or sliding of the outcrop barrier pillar. <br />A barrier pillar of any width, assumed or otherwise, composed primarily of burnt <br />and/or oxidized coal may not be sufficient to minimize the effects of subsidence near the <br />outcrop. The Operator plans to leave one row of developed pillars (competent coal) <br />adjacent to the burnline (the point that poor quality coal is intercepted), and that this pillar <br />(100' width of competent coal) should prevent the potential crushing, sliding, orother failure <br />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 />Conservative application of Piggott and Eynon's (1977) worst-case conical chimney <br />height prediction method indicates that even aworst-case conical chimney collapse should <br />not breach more than 110 foot of overburden under normal geologic conditions. <br /> <br />- 13 - 10/00 <br />P~-o y <br />