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• 4.0 RESULTS AND CONCLUSIONS <br />The stability of the proposed mining geometry is evaluated for <br />overburden depths from 200 ft to 900 ft. The minimum width of <br />the buffer zone is specified for the same overburden range. <br />4.1 Pillar Sizes in Floor Coal Mining Zone <br />Figure 2 summarize the safety factor for the proposed mining <br />geometry for overburden depths ranging from 200 ft to 900 ft. <br />The pillar design model indicates that the proposed mining <br />geometry provides for a factor of safety greater than 1.5 <br />throughout the range of anticipated overburden depths. <br />4.2 Suffer Zone <br />A buffer zone must be left between the caved area and the area of <br />floor coal mining. The minimum required width of the buffer zone <br />is two pillar lines. Figure 3 illustrates the minimum <br />requirement. The width of the buffer zone is independent of <br />pillar length and width for pillar widths in the 50-ft to 55-ft <br />range. Mining height in the buffer zone area must be restricted <br />to no more than 12 ft. <br />• <br />u <br />Cyprus Orchard Valley Mine February b, 1992 <br />Panel B Pi liar 1lrolys is Pepe 6 <br />