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-i i I ,~ ,.~ .. :~ The buttress is stable with a ].8 static: factor of safety. The buttress will be constructed of interburden waste rock material end dumped from the pit benches. The buttress will be constructed in two lifts. As shown on Figure E.3.1-2, the lower lift will be end dumped and graded to a 3H:1V outslope. The upper lift will be left at the angle of repose. The lower slope will be topsoiled with 18 inches of topsoil material and revegetated with the approved seed mi;cture. Trees will be planted at a minimum rate of 100 trees per acre. Stability of the buttress was analyzed using .a simplified Janbu method. The buttress is stable with a 1.8 static factor of safety. The buttress is estimated to be 600 ft long. Figure F-2 shows the approximate extent of the buttress. -,i Final design for the buttress will be determined when the exact characteristics of the geology and seepage in the south wall are known. The design will be finalized in the first two years of mining along the south wall and subm~~tted to the Division for approval. ''2 4 -~ :.a If seepage management operations indicate that there is a potential for large volumes of seepage which may affect the long-term stability of'the rock buttress, a low permeability barrier such as a, slurry wall or cutoff trench will be installed through the alluvium and keyed into the surrounding Santa Fe Conglomerate. The typical design is shown on Figure E.3.1-2. Figure E.3.1-3 shows the estimated extent of the slurry wall. figures E.3.1-4 and E.3.1-5 show a 'ongitudinal and transverse cross-section, respectively.The need for the low permeability barrier will be evaluated during the first two years of mining in the south wall area. Final riesigns will be submitted to the Division for approval. E-12 (Revised 4/20/89)