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mechanical calculation of FLAC 7.0, or it may be done in parallel with the mechanical modeling to <br />capture the effects of fluid/soil interaction. <br />A 2D slope stability analysis calculates the factor of safety against failure assuming a plane -strain <br />condition. Therefore, it is implicitly assumed that the slip surface is infinitely wide, and thus three- <br />dimensional (3D) effects are negligible because of the infinite width of the slide mass. This <br />assumption is reasonable in cases where the width to height ratio is greater than 5 (Stark, 2003). In <br />the case of the RPEE, the approximate width to height ratio is about 2 to 3, and thus the impact of the <br />3D effect is significant. As a result of this determination, a deformation analysis with FLAC31) was <br />performed to assess the proposed slope conditions and evaluate the 3D effects which more accurately <br />assess the stability of a slope with well-defined 3D geometry. In other words, the actual geometry <br />was modeled and the effect of the shear resistance along the flanks of the potential failure surface <br />was determined. Three-dimensional analyses are significantly more complex and consequently are <br />not often performed, but they more accurately represent field conditions. <br />2.2.2 Model Parameters <br />The two most important materials relating to the stability of the RPEE waste rock pile are the coal <br />waste rock (refuse) itself and the underlying colluvium. The bedrock underlying the colluvium is <br />generally sandstone and siltstone. While zones of shale may be present within the stratigraphy in the <br />area of the waste rock pile, they are anticipated to be unsaturated. The critical materials (refuse and <br />colluvium) are addressed in the sections below. The parameters used in modeling for these and the <br />other materials are found in Table 1. Colluvium properties were later calibrated in the deformation <br />model using field inclinometer data. <br />2.2.2.1 Waste Rock (Refuse) <br />As stated above, four samples of refuse generated by the CPP were collected from the existing RPE <br />area. Laboratory tests performed on this material included moisture content, Atterberg limits, dry <br />density, organic content, standard Proctor, fully -softened direct shear, and hydraulic conductivity. <br />The 3 -inch -minus material was classified in the laboratory as a silty gravel with sand and coal (SM) <br />based on the gradations which included sieve and hydrometer. The four samples were very well <br />graded, and generally compared well with each other. The percent passing the #4 sieve ranged from <br />35.1 to 52.4 percent and the percent passing the #200 sieve ranged from 12.3 to 18.4 percent. In <br />addition to the gradations carried out on the 3 -inch -minus material as received from the field, some <br />r <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 />