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• disappeared at about the 950 foot overburden thickness <br />level. Subsidence cracks and pits in soil and colluvium <br />occurred through 500 to 600 feet of overburden above wide <br />pillars. <br />Mining of the 1st West and 2nd West panels at the <br />Orchard Valley Mine has resulted in substantial subsidence <br />cracks propagating to the surface through approximately 520 <br />to 890 feet of overburden. There is some indication that <br />geologic conditions were unusual in this area. The coal at <br />the southern end of the 1st West panel in the vicinity of <br />the outcrop barrier pillar was unusually highly oxidized and <br />was relatively incompetent. The roof strata were also incom- <br />petent, limonite stained, and difficult to support. Areas <br />exhibiting similar characteristics are no longer mined for <br />safety reasons. <br />Panels 1st through 7th East have been developed and <br />extracted below overburden depths of less than 800 feet, and <br />no surface cracks have developed. Geologic conditions in <br />this area are considered to be more representative than in <br />the area of 1st and 2nd West panels. <br />Based on these data, it appears that subsidence cracks <br />may develop through overburden thicknesses of 800 feet or. <br />• more under unfavorable conditions. While unfavorable <br />conditions can not be defined exactly, they may include <br />zones of weathered coal and overburden. Under normal <br />conditions, subsidence cracks do not appear likely to <br />propagate through more than 500 to 600 feet of overburden. <br />Overburden thicknesses of 800 feet or less have been <br />classified as having a high risk of surface fracturing <br />developing. This is probably a conservative upper limit <br />under normal conditions. <br />5.7 ZONES ALONG OUTCROP BARRIER PILLAR <br />The proposed lease area and the future mine are limited <br />by the D-seam outcrop on the south side. An outcrop barrier <br />pillar will be left between the future mine and the coal <br />outcrop. The width of such a pillar as well as its <br />properties are important for the assessment of the stability <br />of the slope above it and for the assessment of the <br />subsidence potential close to this pillar. The limited <br />overburden height, weathered rocks close to the surface and <br />the potential of the coal burn line extending into or close <br />to the workings may contribute to the subsidence potential <br />in this area. <br />• - 3 9 - <br />