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• -27- <br />results of the analysis indicates a factor of safety of 1.6 for massive <br />failure which is slightly higher than for a continuous 2:1 slope. The <br />factor of safety of a deep - seated failure of individual 1.5:1 slopes is <br />also greater than 1.5. In our opinion, the risk of shallow failures is <br />greater at this configuration but failures should be limited to individual <br />slopes and involve a quantity of material which could readily be cleaned up <br />with mining equipment. <br />None of the above analyses considered the presence of a significant <br />layer of fire clay between the bottom of the spoil and the old pit floor. <br />As discussed during our evaluation of the existing spoil configurations, if <br />the fire clay existed in sufficient quantities to control mass stability, <br />failures would be evident within the pit. No failures, even minor, were <br />• evident during our investigation. If the fire clay is relatively intact at <br />this time and experiences weathering and strength loss in the future, evidence <br />would become apparent in the form of slow slope creep, and not a catastrophic <br />type failure. Because it is impossible to confidently know of the presence <br />or extent of the fire clay, we recommend that the spoil crest and slope be <br />monitored to detect slope creep. <br />We present the following recommendations for short -term and long -term <br />slope configurations, based upon our analyses. <br />Short -Term Slopes <br />1. Our analysis indicates a factor of safety of 1.2 for mass failure <br />of slopes at a ratio of 1.5 horizontal to 1 vertical. Shallow <br />failures could occur at this ratio. We recommend that slopes be <br />graded no steeper than 1.5:1 provided some shallow failures can be <br />tolerated and adequate protection around entries are provided. <br />