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the northwest wall (Figure 13) indicate a possibility for planar failure along the foliation <br />plane F-1. Planar failure can occur because the dip direction of the F-1 plane can daylight <br />into the slope face and the trend of the line of intersection formed by F-1 and P-2 can act <br />as a releasing surface. In addition, based on the stereonet plots, intersections between <br />discontinuities F-1 and P-2 and F-1 and P-3 potentially exist, and the plunge line of each <br />of the intersections will fall very close to or within the daylighting envelope and at an <br />inclination approximately equal to the angle of friction. <br />For the 2004 analysis (L&A, 2004), the long-term impact of foliation F-1 on the stability of <br />both the northeast and northwest walls was further investigated using discontinuity data <br />that includes elevation information for the foliation orientation. These data were plotted on <br />a chart showing dip angle in degrees versus elevation in feet (for chart see L&A, 2004). <br />The results resulted in an increase in the number of daylighting foliation planes that start <br />at an elevation of 6,325 feet and continue with increased frequency to below the planned <br />final pit elevation. Due to the possibility of a lower friction angle along the foliation plane <br />(as low as 5°, average 28°) as determined from the 2003 Geotechnical Study (L&A, 2003), <br />the potential exists for future instabilities to occur on the north walls as the pit is excavated <br />to its final depth. These shallow dipping discontinuities along the foliation can have a <br />significant impact on the stability of the north wall and should be monitored closely as <br />excavation progresses. In addition, as discussed in the previous section, an east-west <br />striking unnamed fault zone is mapped in the northwest corner of the main pit. With the <br />combination of faulting and discontinuities on the north walls, the potential impact on the <br />slope stability should be closely monitored as mining continues. <br />5.2.3 West Walls <br />No major stability issues currently exist for the west walls of the main pit of the quarry. <br />Previous analysis indicated that the west highwalls have approximately the optimum <br />orientations for stability. Two west wall orientations were analyzed in this study. The first <br />had a dip direction of 091 °, and using this orientation the stereonet plot (Figure 14) shows <br />one pair of planes having a line of intersection that plunges less than the angle of the <br />-16- <br />tachel & Associates <br />SC14NApf1 C-nG1HfirPING CQ i:VANY