2010-02-05_REVISION - C1996083 (8)

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• Figure lb presents the planned mining geometries and the location of detailed models. Mining geometry includes the set of mains being driven under the drainage, the storage panel and three longwall panels in both the Upper D and the Upper B Seams. For clarity the Upper D Seam workings are excluded from the Figures. Table 2 presents the scope of the modeling. Isolated development conditions are analyzed in step 1. By simulating the longwall retreat in the neighboring district, load transfer toward the corridor is conservatively estimated in step 2. Table 2-Scone of boundary-element modelinLy Analyses Seam Comment 1- Development B and D Two seam development 2- Retreat B and D Load transfer from longwalls 2.2 Results • Modeling results are presented as vertical stress and factor of safety levels (Figures 2 and 3). In Figure 2, square elements are 18- by 18- by 9.5-ft coal elements, the color of which depends on the stresses acting on them. Figure 3 presents the factor of safety for each element. The factor of safety is based on the pillar strength which is calculated using Mark-Bieniaswki formula (Mark and Chase 1997). This equation depends on the pillar dimensions and the excavation height. For clarity, only the results from the northern end of the model are presented. Clearly vertical stress concentration is influenced by variations in overburden thickness and is mostly low to moderate (<1,500 psi) directly beneath Terror Creek, even after considering simulated longwall load transfer. The minimum average pillar factor-of-safety for the mains exceeds 3.0 directly under the creek for simulated development mining using the 900 psi coal strength suggested by NIOSH. Using a more conservative coal strength value of 600 psi, commonly used by BRL for very week geologic conditions encountered in portions of neighboring longwalls (Stewart 2009), the minimum average pillar factor-of-safety for the mains • Maleki Technologies, Inc. Page 4