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• pressure and lower strength values, were used in the stability analyses. The <br />recommendations from the MESA coat refuse engineering and design manual <br />(Ref. 7) were also incorporated in this design. <br />Tests will be conducted on actual preparation plant coal processing waste to deter <br />mine actual physical properties of this kind of waste. If these actual properties <br />are significantly different from those used in this report, then appropriate <br />analytical and design changes will be made to the refuse pile stability analysis and <br />design so that the entire pile will be designed based upon a composition of 100% <br />preparation plant coal processing waste. <br />The permanent site for the mine's refuse disposal area is located southwest of the <br />mine portal, between elevations 6660 and 6900 ft. The geological and <br />topographical conditions provide a colluvial foundation that is 15 to 80 ft thick <br />above the competent bedrock. However, the presence of an old landslide requires <br />that the refuse pile be above the 6650 ft elevation. The lower slopes of the area <br />north and east of the disposal site are dotted with minor slope washes and slides <br />that become active only during the wet months. <br />• <br />The properties of the colluvium have been measured in the laboratory and by back <br />analyses. The properties of the refuse pile were derived from laboratory tests of <br />samples from the mine and compared to published data. Review of the tests and <br />analyses of the available refuse properties shows a friction angle of 30° and <br />cohesion of 288 pcf which were used for the stability analysis. <br />The optimum dimensions based on the known properties and drainage conditions <br />require that the refuse pile slope be 20° but not more than 2S°. The bench height <br />with a 20° slope should not exceed 6880 ft elevation for a toe at 6660 ft. This <br />will provide a total storage capacity of 9 million tons. The critical factors <br />required to maintain a factor of safety of 1.5 or better are proper construction <br />procedures and provisions for effective drainage. Drainage control features to be <br />installed are good sub-base preparation, diversion ditches, general drains and <br />embankment height control. <br />• -13- <br />