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with 3 benches and a vertical height of 120 feet. The south wall is much smaller with <br /> heights near 50 feet. The north and east walls of the pit are the highest and, therefore, the <br /> focus of the stability evaluation. <br /> 4.3 ASSESSMENT OF GLOBAL STABILITY <br /> Global stability was evaluated on a comparative basis. The total vertical height of <br /> the north and east pit walls is relatively small, compared to large, open pit operations. <br /> Both the gradient of vertical stress over the depth, and the horizontal stress at the toe of <br /> the slope is small compared to the strength of the rock. The probability of massive <br /> failure is, therefore,relatively small. <br /> Comparative data presented by Hoek and Bray (1977) for open pit mines in Spain <br /> for porphyry clopper deposits provides a relative estimate of global stability at the <br /> Sunnyside Pit. Figure 4.2 shows case history data for both stable and unstable cases as a <br /> function of slope height and slope angle. The dimensions of the Sunnyside Pit are also <br /> plotted and indicate: <br /> • the range of experience that includes the Sunnyside Pit contains only stable <br /> slopes; <br /> • the comparative factor of safety was approximately 2.0; and <br /> • based upon the histograms of stable versus unstable slopes in Figure 4.2, only <br /> 5 percent of the slopes with comparative factors of safety greater than 1.2 <br /> were unstable. <br /> 4.4 EVALUATION OF STRUCTURAL DISCONTINUITIES <br /> Slope instability due to the occurrence of structural discontinuities is the most likely <br /> mechanism for failure of the north and east walls of the Sunnyside Pit. The structural <br /> mapping data presented in Section 3.0 was, therefore, analyzed to indicate the potential <br /> for occurrence of plane failures or wedge failures in the slope wall. Figure 4.3 presents <br /> 13 <br />