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1100318 R0T0 0811 JRW1 38 <br />laboratory test results from this region were used in the interpretation of the slope stability <br />analyses when the failure plane passed through this region of the limestone. <br />Drilling and blasting of Area H altered the material properties of the near surface rock as a result <br />of overshoot (please refer to Section 5.2.2.2). The overshoot significantly fractured the rock. <br />Trenches TR-2 andTR-3, located on the floor of Area H, established that the depth of fractured <br />rock extended to a depth of approximately 4 to 5 feet below the floor of Area H. Therefore, the <br />top 4.5 foot layer of rock at the bottom of Area H is assigned strength properties consistent with <br />the severe fracturing from overshoot observed in the trenches. A friction angle of 25 degrees <br />and no cohesion were utilized for this layer. <br />5.3.2 Slope Stability Results <br />The results of the slope stability analyses are contained in Appendix 47. The analyses indicate <br />the following: <br /> The presence of the clay bed is the single most significant destabilizing <br />condition present in the slope. The toe geometry of the critical failure surface <br />passing through the clay bed in the Sawatch Formation and into Area H is <br />consistent with the eyewitness accounts that described the pit floor appearing <br />to heave 1 to 1½ feet at the moment of failure.54 <br /> In the absence of the clay bed, the slope would have been stable with a factor <br />of safety in excess of 4.55 The significance of this result is that the <br />December 2, 2008 landslide would not have occurred without the presence of <br />the clay bed. <br /> The different groundwater assumptions result in varying back calculated <br />Manitou Formation limestone rock strengths. Higher groundwater levels <br /> <br />54 United States Department of Labor, Mine Safety and Health Administration, Pittsburgh Safety and Health <br />Technology Center, Mine Waste and Geotechnical Engineering Division, Report No. MW09-008, <br />February 23, 2009, pages 4-5 (Appendix 36) <br />55 A slope stability analysis Factor of Safety is a measure of stability of a slope. Factors of Safety less than <br />one (1.0) indicate an unstable slope, i.e., one that will fail. Standard Factor of Safety for slopes used by <br />reviewing agencies and municipalities for static conditions varies between 1.3 and 1.5. A Factor of Safety of 4 <br />far exceeds minimum acceptable Factors of Safety.