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Clearly vertical stress concentration is influenced by variations in overburden thickness • <br />and is mostly low to moderate (<1,500 psi) and thus pillar instability is not a concern. <br />Pillar stress levels are low (<1000 psi) directly under the drainage and increase to the <br />west. The minimum average pillar factor-of-safety is approximately 1.64 in an area to <br />the west near modeled face position. The maximum pillar factor-of-safety exceeds 2.5 <br />directly under the drainage. <br />Table 5 .-Model input for boundary-element analyses <br />Property Value Assumed material behavior <br />Roof and floor, Young's modulus, <br />106 psi 0.69 Elastic <br />Roof and floor, Poisson's ratio 0.25 Elastic <br />Coal, Young's modulus, 106 psi 0.13 Elastic <br />Premining vertical stress, psi 1.1-psi stress gradient <br />11 <br />• <br />Maleki Technologies, Inc. Page 29