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Table 5. Strength Parameters Used in the Stability Evaluations Foundation Soils Coal Waste Undrained Shear Strength, -- 1578 psf Total Slress Su Analyses ~ (deg) 38 -- c (psf) 100 - Ettective ~' (deg) 38 30 Stress Analyses c' (osfl 100 0 5.3 Findings of Stability Evaluations Factors of safety for total and effective stress analyses are summarized in Table 6. Output from these critical stability analyses is presented in Appendix E. Total stress analyses model undrained conditions for the gob using undrained shear strengths. Foundation soils were modeled using drained strength parameters (Mohr-Coulomb ~' and c') Based on pore pressure monitoring at Gob Pile No. 2, gob is likely to develop excess pore pressures during construction and behave undrained. Excess pore pressures will dissipate once construction is completed. The time required for complete pore pressure dissipation is a function of the hydraulic conductivity of the gob and length of drainage path. Circulaz and block type failures were considered by the • total stress analyses. Critical failures surfaces for the undrained analyses were found to be mostly deep failures through the gob. Effective stress analyses model drained conditions (i.e., no excess pore pressures) for the gob and represent long-term conditions. Effective stress parameters were estimated based on CU triaxial shear tests. As expected, the critical failure surfaces for the drained models were founda to be shallow. For these analyses, the failure surface was forced a minimum of 5 feet into the gob to avoid prediction of extremely shallow critical failure surfaces at or near the surface of the gob. i• Possible excavation slopes were first evaluated. These analyses consider a completed excavation prior to gob placement. It is more likely that excavation will proceed during gob placement, however, these analyses provide an indication of the stability of the overall excavation. As indicated in Table 6, the factor of safety for excavations ranges from 1.34 fora 1.SH:1 V excavation to 1.65 for an excavation of 2.SH:1 V. We recommend a minimum factor of safety for the excavation of 1.50 due to the uncertainty of the strength pazameters. It should be noted that the excavation is temporary, and will be buttressed relatively quickly by gob. The gob buttress will increase the factor of safety of the excavation and native soils. An overall excavation slope of 2H:1 V results in a factor of safety of 1.50. Based on the results, a maximum excavation slope of 2H:1 V is recommended for Gob Pile No. 4. The excavation may be made in a benched malmer as long as the overall slope is 2H:1 V or gentler. Following evaluation of the excavations, possible gob geometries were evaluated for excavation slopes of 2H:1 V and 2.SH:1 V. As indicated in Table 6, the critical factors of safety were greater than 1.50 for an excavation subgrade of 2H:1 V and gob slopes up to 3H:1 V. Gob slopes of Bowie Gob Pile No. 4 stability report Project #OS-126-GEO Page I2 of IS