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described in Stark (2003). An average 2D -to -3D factor of 1.30, based on the height and width of the <br />mobilized slope, was used for estimating the factor of safety as a function of refuse elevation. It <br />should be noted that this average factor is consistent with previously reported 2D and 3D FLAC <br />simulations in which an average factor of 1.34 was obtained. These factors of safety are consistent <br />and in the same range of values obtained when using the limit equilibrium method previously <br />implemented during the initial design. Table 4 and Figures 2-9 and 2-10 show the 2D and 3D factors <br />of safety for the RPEE as function of elevation for static (ESSA) and seismic (ESSA), respectively. <br />The accompanying FLAC outputs are shown in Figures 2-11 through 2-14. <br />The minimum long-term static factor of safety of 1.50 is met up to its ultimate capacity top of pile <br />elevation (El. 7,440 ft). It should be noted, however, that the factor of safety decreases with increased <br />refuse height and the lowest 3D factor of safety computed for the static ESSA case is 1.54. This <br />value is very close to the minimum accepted value such that fairly minor changes in waste rock pile <br />geometry can lead to inadequate factors of safety. <br />It can be seen in Table 4 that the computed 3D factors of safety are approximately 0.4 higher than the <br />2D factors of safety. This is likely the result of the effect of the shear resistance along the flanks of <br />the potential failure surface, and the slope geometry is more accurately represented in this manner. <br />The 3D results indicate that the static ESSA factors of safety are greater than 1.5. The seismic factors <br />of safety are also considered adequate based on the results of the modeling exercise because they are <br />greater than 1.0. <br />2.3.2 Assessment of Stability — Permit Envelope Slope <br />To establish an acceptable permit envelope slope, a generalized slope that conforms to the overall <br />design slope was established. The steepness of the slope was then increased until the long-term <br />factor of safety was just above 1.5. Figure 2-15 shows the results of the 2D FLAC model for a slope <br />that is 0.5 degrees steeper than the design slope. The same adjustment used to adjust the design slope <br />evaluation results from 2D to 3D are applicable to these results, so the 3D long-term factor of safety <br />for this model is 1.51. The factor of safety for seismic conditions is above 1.0. <br />The permit envelope profile line and the footprint projected from the permit envelope profile are <br />shown on the permit drawings in Appendix D. <br />12 <br />P:\Mpls\06 CO\26\06261003 MCC Refuse Pile Site Review & Permit\WorkFiles\Permit Application\2014 Revision\Exhibit 82 2014-05- <br />29.docx <br />