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• 7.0 CONCLDSIONS AND RECOMMENDATIONS <br />7.1 SLOPE STABILITY <br />The stability analysis indicated the coal refuse piles will have <br />a factor of safety greater than 1.5 based on the proposed <br />configuration supplied to us by Powderhorn Coal Company. This <br />factor of safety assumes that no underdrains will be built as <br />originally shown on the plans and cross-sections. <br />Since the test borings and the field investigation did not show <br />any water in either the slope or the underlying alluvium, it is <br />not anticipated that the refuse piles will have slope instability <br />due to the development of a high phreatic surface. Also, it is <br />• planned that a diversion ditch and berm be built on the slope <br />above the coal refuse pile to divert all surface runoff from <br />entering the coal refuse pile. Therefore, the only water to <br />enter the refuse pile will be through precipitation events only. <br />A maximum groundwater mound approximately 10 feet high was <br />calculated to be the extreme phreatic surface which is very <br />conservative, since it assumes the foundation material is <br />impervious. Actually, the underlying alluvial soils should act <br />as an underdrain. <br />The slope stability analysis was performed using three different <br />. phreatic surfaces which are very conservative. All of these <br />phreatic surfaces, along with their respective slope stability <br />14 <br />