2006-10-26_PERMIT FILE - C1996083

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cobbles and boulders were encountered during drilling. Although no in -situ permeability testing was conducted to our knowledge, the reported gradation of the foundation soils, the performance of foundation soils at the existing gob pile, and the anticipated rate of gob placement suggest that the foundation soils will not develop excess pore pressures (i.e., will be drained). Therefore, effective stress parameters may be more appropriate for the foundation soils. Effective stress parameters, represented by Mohr- Coulomb criteria, were estimated for the foundation soils. In- situ permeability testing of the foundation soils may be warranted to determine if the foundation soils will behave in a drained manner. Based on Lambert (2001), we assumed the groundwater table to be deep enough to not affect the stability analyses. Table 2. Strength Parameters Used in the Stability Evaluations Notes: 1. d = depth in feet 6.3 Stability Evaluation Results Factors of safety from critical failure surfaces are summarized in Tables 3 and 4. Table 3 presents analyses considering the foundation soils to be undrained while Table 4 presents analyses considering the foundation soils to be drained. Output from these critical stability analyses is presented in Appendix A. As indicated in Table 3, the slope factor of safety ranged from approximately 1.7 to 2.7 for the sections analyzed. Analyses considering the foundation soils to be drained (Table 4), determined the critical factor of safety to range from approximately 1.7 to 3.2. The critical failure surfaces for both drained and undrained foundations were generally observed to be deep surfaces through the gob and extending partially into the foundation soils for those models considering constant undrained shear strength with depth for the gob. As expected, the critical failure surfaces for models considering drained gob shear strengths and undrained gob shear strengths increasing with depth were shallow. For these analyses, the failure surface was forced a minimum of 5 feet into the gob to avoid extremely shallow critical failure surfaces at or near the surface of the gob. Bowie No 2 South Gob Pile preliminary stability evaluation Project #05- 126- GEO -08 Page 7 of 10 Foundation Soils Coal Waste Undrained 1200 - 1650 psf Shear and Total Stress Strength, Su Su = 53d Analyses (psf) 24 -- � (deg) 0 -- c s Effective �� (deg) 30 30 Stress 0 0 Analyses C' S Notes: 1. d = depth in feet 6.3 Stability Evaluation Results Factors of safety from critical failure surfaces are summarized in Tables 3 and 4. Table 3 presents analyses considering the foundation soils to be undrained while Table 4 presents analyses considering the foundation soils to be drained. Output from these critical stability analyses is presented in Appendix A. As indicated in Table 3, the slope factor of safety ranged from approximately 1.7 to 2.7 for the sections analyzed. Analyses considering the foundation soils to be drained (Table 4), determined the critical factor of safety to range from approximately 1.7 to 3.2. The critical failure surfaces for both drained and undrained foundations were generally observed to be deep surfaces through the gob and extending partially into the foundation soils for those models considering constant undrained shear strength with depth for the gob. As expected, the critical failure surfaces for models considering drained gob shear strengths and undrained gob shear strengths increasing with depth were shallow. For these analyses, the failure surface was forced a minimum of 5 feet into the gob to avoid extremely shallow critical failure surfaces at or near the surface of the gob. Bowie No 2 South Gob Pile preliminary stability evaluation Project #05- 126- GEO -08 Page 7 of 10