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• RULE 6.5 GEOTECHNICAL STABILITY EXHIBIT (ANY REQUIRED SECTIONS) ~_ ~. SURFACE MINE SLOPE STABILITY Introduction The purpose of these calculations is to examine stability of the proposed surface mine walls during mining. The pit could typically have a total depth of about 46 fr as shown in Figure 1. While intermediafe benches will be utilized during mining stages, there may be times when the mine wall is vertical, without benches. This represents the worst-case scenario for the mine slope stability. The calculations utilize existing geotechnical data from the Red Rock Site as a preliminary 2-dimensional limit-equilibrium stability analysis of the proposed mine wall is performed. Upon initial excavation of the mine, overburden materials will be utilized, where possible, for plant and facilities construction, thereby, avoiding a lazge waste rock pile. Once construction materials have been obtained, overburden will be utilized in reclamation of inactive portions of the mine,. further avoiding accumulation of a lazge waste rock pile and associated large unsupported earth slopes. . Assumptions The assumptions for the calculation are listed in Figure 1. Conservative values are selected for the preliminary analysis where possible. Even though some of the Codell sandstone will be quarried, the failure mode is a sliding block controlled by shales underneath the limestone ore body. These shales dip to the north-northeast at about 2 to 3 degrees, however, 5 degrees was assumed for the calculation. It is assumed the bedding is "daylighting" into the surface mine, as shown, a condition that could occur on north facing walls. It is also assumed that there is no lateral strength in the bedrock due to the possibility of interconnecting vertical to subvertical joints, along which tension cracks could form. The shear strength of the shale interbed along the basal sliding surface is critical in terms of evaluating the stability. Direct sheaz tests, performed for the proposed bridge at the Red Rock Site (MWW&AI, 1999), were run on shale interbeds in the Fort Hayes member and are presented on Figure 1. The angle of internal friction, F, ranged from IS to 28 degrees. The tests were performed on remolded specimens so they should be conservative with respect to peak strength. For the following calculation, F, was assumed to be 15 degrees. The slope was assumed to be dry as the surface mine will be continuously dewaiered, as necessary. Ground water is not anticipated as a stability problem at the surface mine site based on previous site studies (Refs. 2 and 3). I • AppExhibitsA-T-APP(] 180)RevB/Ianuary U, 2002 1