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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 />10 psi 0.69 Elastic <br />Roof and floor, Poisson's ratio 0.25 Elastic <br />Coal, Young's modulus, 10 psi 0.13 Elastic <br />Premining vertical stress, psi 1.1 -psi stress jzradient <br />C <br />Maleki Technologies, Inc. Page 29 <br />