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• <br />• <br />This section describes the pillar design methods utilized in this <br />study, the model calibration methods, and the determination of <br />the minimum required factor of safety for pillars. <br />2.0 METHODOLOGY <br />2.1 Pillar Design Methods <br />evaluate pillar stability. Wilson (1972) originally published <br />the method. Wilson subsequently expanded the method to include <br />the effects of yield in the roof and floor strata adjacent to a <br />pillar. Wilson (1977, 1981, 1982a, and 1982b) describes this <br />modified method. <br />The confined core pillar design method of A. H. Wilson is used to <br />Wilson's method differentiates between a rigid rock mass model <br />and a yield rock mass model. The yield model is characterized by <br />yield in the roof, pillar, and floor. The rigid model restricts <br />yield to the pillar while the roof and floor remain rigid. <br />The yield model was selected as the most representative rock mass <br />model for the proposed mining in the Steven's Gulch area of <br />Panel C at the Orchard Valley Mine. Mining in this area will <br />occur within the coal seam, leaving roof and floor coal. The <br />immediate roof and floor rocks are classified as moderately weak <br />silty shale. <br />Wilson's method requires physical rock property data for the coal <br />seam and overburden density. The Table 2 summarizes the required <br />data and the design values used in the pillar stability analyses. <br />These data were developed from existing test data from the East <br />Mine area. Figure 1 summarizes the results of the test data <br />analysis. <br />Table 2 <br />Physical Properties Data <br />Panel C Pillar Stability Analyses <br /> <br />Parameter Design Value <br />Maximum overburden depth 800 ft <br />Overburden density 160 lb/ft^3 <br />Laboratory compression strength 5681 psi <br />Insitu strength reduction factor 5 <br />Internal angle of friction 40.2 deg <br />Cyprus Orchard Valley Mine <br />Panel C Piltar Analysis <br />Narch 12, 1992 <br />page 2 <br />