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-26- <br />of safety are probably between the values calculated for 0 =34 and 0 =370. <br />In our opinion, the spoil will be adequately stable for short -term life <br />at a slope ratio of 1.5 horizontal to 1 vertical provided some shallow failures <br />can be tolerated. Even though failures may be shallow with repsect to the <br />overall thickness of the spoil bank, the failure could involve the full <br />height of the slope face. If it is decided to use a slope ratio of 1.5:1, <br />we recommend that some type of barrier be provided near mine entries to prevent <br />spoil sliding into mine entries. In addition, the slope should be monitored <br />for movement. <br />Most regulatory agencies require a factor of safety of at least 1.5 for <br />mass stability. We believe that at least a factor of safety of 1.5 should be <br />considered for long -term mass stability. Our analysis indicates a factor of <br />• safety of 1.5 is achieved for deep seated failures using an overall slope <br />ratio of 2 horizontal to 1 vertical and the lower bound angle of internal <br />friction (34 This factor of safety considers ground water 20 feet above <br />the old pit floor. We recommend that the overall spoil slope be flattened to <br />at least 2:1 for long -term slope life. <br />There are two methods of achieving an overall slope ratio of 2:1. The <br />slope face could be flattened to a continuous ratio or intermediate benches <br />constructed and steeper intermediate slopes used. The relative economics of <br />these two slope flattening methods is beyond the scope of this work but should <br />be considered along with construction feasibility. <br />We analyzed one method for overall flattening of the slope to 2:1 using <br />30 -foot wide benches at 50 -foot vertical intervals and 1.5:1 intermediate <br />slopes. This configuration and our analysis is depicted on Fig. C -8. The <br />