2012-02-03_REVISION - M1978315 (6)

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Slope Stability Analyses: Peabody Sage Creek Mining, has provided us with topographic and cross - section information for the existing conditions, as well as the proposed reclaimed slope configurations. Based on our review of the provided cross - sections, we have determined, that the final reclaim cut slopes are proposed to be constructed at a 1 (horizontal) to 1 (vertical) configuration. Stability analyses were performed using the Slope /W 2007 computer program developed by GEO -SLOPE International, Ltd. Due to limited subsurface soil and bedrock information, conservative soil/bedrock material properties were used in our analysis. The Slope/W 2007 program uses a Limit Equilibrium Analysis for accessing the stability of the proposed slopes. The Morgenstem -Price method was used to calculate the factors of safety against slope failure. The slope stability model analyzes the driving forces (i.e., forces causing the slope to move) versus the resisting forces (i.e., the shear strength of the soils tending to resist movement). The ratio of these forces yields the factor of safety of the slope against movement (failure). A factor of safety of 1.0 indicates failure of the slope and subsequent movement. A minimum factor of safety of 1.5 under static conditions is recommended for the proposed construction. Stability analyses were conducted for the anticipated cut slope and natural slope configurations, as well as with varying soil parameters to determine the factor of safety against failure for the proposed slopes. Based the laboratory and field test results, an internal friction angle of 5 degrees, a cohesion value of 12,500 psf and a unit weight of 140 pcf were used for the weathered shale - claystone and an internal friction angle of 35 degrees, a cohesion value of 20,000 psf and a unit weight of 150 pcf were used for the undisturbed sandstone bedrock materials in the stability analyses. Using the slope configurations and soil/bedrock parameters outlined above, we have calculated that the theoretical factor of safety against a global slope failure in the overburden weathered shale - claystone after the cut slope is constructed is 1.7. It should also be noted that the stability of the overburden weathered shale - claystone materials were not affected significantly by the removal of the sandstone materials from the toe of the slope. We have also calculated that the factor of safety of a global slope failure of the overburden materials through the proposed sandstone cut will be greater than 2. The factor of safety for a global failure through the sandstone cut was determined to be greater than 5. As noted above, conservative strength parameters for the sandstone bedrock materials and the weathered shale - claystone materials were used in the analyses. Slope Stability Slip Surface Diagrams are shown in Figures #1 through #3 along with the corresponding data. Conclusions: Based on our observations, NWCC does not believe there are hazardous geologic conditions that would render the proposed quarry activities unfeasible as currently planned. Minor intermittent rockfall and erosion of the exposed bedrock materials associated with the proposed cut slopes should be anticipated. Rockfall hazards should be assessed periodically during mining operations. The greatest risk of rockfalls will likely occur in the spring and fall when moisture flows into the fractures and freezing occurs causing a rock fragment to break and fall down the slope. In addition, we recommend that the slopes be observed by NWCC during construction to identify any potential slope stability problems with the natural and constructed cut slopes. Job Number. 11-9090 NWCC, Inc Page 3