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rock drain to convey seepage water past the base of the buttress. In-place colluvium in the waste rock <br />pile footprint has not been and will not be removed prior to placement of refuse. Colluvium was <br />removed below the rock and colluvium buttress and within a zone of about 200 feet up the gulch <br />south of the south toe of the buttress. The modeled 2D section is placed approximately in the thalweg <br />of the draw because this has been the deepest zone of refuse and is, therefore, the critical design <br />section. <br />The Rules require a long-term static factor of safety of 1.5 or higher and a seismic factor of safety of <br />1.0 or higher. It should be noted that the required seismic factor of safety remains 1.0 for long-term <br />(ESSA) conditions. <br />Initial slope stability analyses provided early guidance on the RPEE waste rock pile geometry, but <br />updated stability analyses have been performed to verify that the RPEE waste rock pile design meets <br />the long-term factor of safety of 1.5 required under the CDRMS rules for coal mine waste banks. The <br />slope stability evaluation used colluvium shear strength data based on Fall 2013 drilling, sampling, <br />and laboratory testing. It also utilized updated monitoring information from in-place instrumentation, <br />consistent with the Observational Approach. The evaluation was performed because of deformation <br />measured by inclinometers in the colluvium buttress and in the waste rock pile. The initial design <br />was based only on the limit equilibrium method using GeoStudio 2007. The analyses performed in <br />2013 compared deformation model results to deformation observed at the RPEE based on <br />inclinometer data. The 2013 evaluation used new laboratory values for the colluvium shear strength, <br />and the colluvium stiffness values were back -calculated such that the measurements from the field <br />inclinometer were matched by the FLAC model. <br />In this re -analysis, the three-dimensional (31)) effects were again taken into account by following the <br />procedure suggested by Stark (2003). Consideration of the 3D effects is important especially for the <br />geometry features present in the RPEE (e.g., long and narrow footprint). This revised assessment <br />provides a better understanding of the stability of the RPEE waste rock pile by incorporating current <br />geometry of both buttress and waste rock pile and by using deformation and shear strength properties <br />that account for in-situ conditions. The analysis includes the following sections: <br />1. Fall 2013 drilling and subsequent laboratory shear strength testing on the sampled colluvium <br />PAMpls\06 CO\26\06261003 MCC Refuse Pile Site Review & Permit\WorkFiles\Permit Application\2014 Revision\Exhibit 82 2014-05- <br />29.docx <br />