Laserfiche WebLink
is slightly reduced to 2.5 directly under the Creek and moderately reduced to the east under <br />higher cover. Pore pressure effects are excluded from these boundary element analyses but <br />accounted for during the 2004 analyses of the mains using detailed finite-difference analyses. <br />The stresses are higher and the factor of safety is lower for the rock storage area using 60--ft wide <br />square pillars. Figure 2 shows minor load transfer toward the south eastern portion of this panel <br />resulting from simulated longwall extraction. The minimum pillar factor of safety exceeds 1.4 <br />near the rock storage area for the worst assumed coal strength and load transfer conditions. <br />Because this panel is only needed for the short term, and will soon be backfilled to the top by <br />waste rock, subsidence is not expected for assumed continuous (nonfaulted; Brinton 2009) strata <br />conditions. To increase pillar stability factors, BRL should consider increasing pillar dimensions, <br />reducing the mining height, controlling panel width and implementing properly designed water <br />drainage and handling programs. <br />3.0 CONCLUSIONS AND RECOMMENDATIONS <br />• Based on review of available geotechnical data and the experience in the neighboring longwall <br />areas, the model predicts more than adequate factor of safety for mains pillars directly under the <br />Creek using Mark-Bieniawski method and boundary-element modeling results. The minimum <br />pillar factor of safety is reached within the rock storage area and influenced by smaller pillar <br />dimensions and minor load transfer from the longwall area. By limiting the mining height to 9-ft <br />(as planned), increasing pillar dimensions (such as increasing pillar lengths by 20-40 feet), and <br />by following careful excavation and support practice, short-term pillar stability can be enhanced <br />for these temporary rock storage areas. <br />It is recommended to study depositional and structural conditions within the study area by <br />projecting data from the longwall and other neighboring areas, supplementing the corehole data <br />collected in 2003. Design of prudent excavation and support techniques, including cut depth, <br />entry width, support designs, timing of support installation, and particularly groundwater control <br />should be completed. Pillar behavior and ground movements should be monitored during the <br />construction project for verification of site specific pillar strength. <br />• <br />Maleki Technologies, Inc. Page 5