2010-02-05_REVISION - C1996083 (8)

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the variability in the overburden realistically. The analyzed areas including both the fine and coarse mesh are shown in Figure lb, rotated for ease of mesh generation. Pore pressure effects are excluded in these boundary element analyses but were included in the finite difference analyses of main pillars during the 2004 detailed analyses. The boundary-element stress analyses results are described in Section 2 followed by conclusions and recommendations in Section 3. 2.0 BOUNDARY ELEMENT STRESS ANALYSES Pillar stability was evaluated using an estimate of pillar strength and stress. Pillar strength was estimated using a method developed by NIOSH (Mark-Bieniaswki formula, Mark and Chase 1997). Pillar stress was calculated using a numerical model. The modeling resulted in vertical stress distributions on the Upper B seam, including variations in topography, and mining geometry. Pillar factor of safety was calculated during the development work beneath the stream valley by dividing pillar strength to stress. Because of the close proximity of the storage panel to the longwall district, longwall retreat was simulated as well. 2.1 Modeling methods and scope The pseudo-three-dimensional boundary-element code MULSIMTI was used for calculating stress distributions over the area of interest. This program incorporates elastic, strain-softening material and is suitable for multiple-seam excavations in dipping seams and variable topographies (Maleki and others 2003). Model input is shown in Table 1 for these elastic analyses. Table I -Model input for boundarv-element analyses Property Value Assumed material behavior Roof and floor, Young's modulus, 106 psi 0.69 Elastic Roof and floor, Poisson's ratio 0.25 Elastic Coal, Young's modulus, 106 psi 0.13 Elastic • Premining vertical stress, psi 1.1-psi stress gradient _ Maleki Technologies, Inc. Page 3