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r? <br />U <br />• <br />• <br />Clearly vertical stress concentrations are influenced by variations in overburden thickness <br />and are mostly low to moderate (<1,500 psi), and thus pillar stability is expected to be <br />good. Table 6 presents the average pillar factor of safety for a row of pillars located near <br />the northern boundary of the planned mains (shown in figure 6). The minimum average <br />pillar factor of safety is 2.2 in this area. MTI considers such high levels of factor of <br />safety very beneficial to long-term stability of the mains. <br />Table 5-Model input for boundary-element analyses <br />Property Value Assumed material behavior <br />Roof and floor, Young's modulus, 106 psi 2.57 Elastic <br />Roof and floor, Poisson's ratio 0.25 Elastic <br />Coal, Young's modulus, 106 psi 0.28 Elastic <br />Premining vertical stress, psi 1.1-psi stress gradient <br />Table 6.-Pillar stress and factor-of-safety for one row of pillars shown in figure 6 <br />Pillar Stress, psi Factor of safety` <br />1 1,395 2.42 <br />2 1,450 2.32 <br />3 1,530 2.20 <br />4 1,530 2.20 <br />` Using Mark-Bieniawski pillar strength of 3,372 psi <br />Maleki Technologies, Inc. <br />Page 17