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substantial barrier pillar which is too wide to fail, such as an outcrop barrier pillar or mains <br />barrier pillars. Alternatively, when adjacent panels are extracted, flexure of the overburden <br />beds as they pass over the barrier pillar between the panels will result in tensile strains <br />immediately above the pillar if the barrier pillar does not yield. <br />Field measurements of subsidence cracks in the Mesaverde Formation by Dunrud <br />(1976) provide and alternative basis for estimating the overburden depths to which <br />subsidence cracks will occur. He found subsidence cracks above a 40 foot wide barrier <br />pillar gradually narrowed and disappeared at about the 450 foot overburden thickness <br />level. Subsidence cracks and pits in soil and colluvium occurred through 500 to 600 feet <br />of overburden above wide pillars. <br />Mining of the 1st West and 2nd West panels at the Bowie #1 Mine has resulted in <br />substantial subsidence cracks propagating to the surface through approximately 520 to 840 <br />feet of overburden. There is some indication that geologic conditions were unusual in this <br />area. The coal at the southern end of the 1st West panel in the vicinity of the outcrop <br />barrier pillar was unusually highly oxidized and was relatively incompetent. The roof strata <br />were also incompetent, limonite stained, and difficult to support. Areas exhibiting similar <br />characteristics are no longer mined for safety reasons. <br />Panels 1st through 7th East in the Bowie #1 Mine have been developed and <br />extracted below overburden depths of less than 800 feet, and no surface cracks have <br />developed. Geologic conditions in this area are considered to be more representative of <br />the Mesaverde Formation than in the area of the 1st and 2nd West panels in that mine. <br />Based on this data, it appears that subsidence cracks may develop through <br />overburden thicknesses of up to 800 feet under unfavorable conditions. While unfavorable <br />conditions can not be defined exactly, they may include zones of weathered coal and <br />overburden. Under normal conditions, subsidence cracks do not appear likely to propagate <br />through more than 500 feet of overburden. Overburden thicknesses of 800 feet or less <br />have been classified as having a high risk of surface fracturing developing. This is a <br />conservative upper limit under normal conditions. <br /> <br />- 11 - 10/00 <br />P,2-o y <br />