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Previous stability analysis of the existing refuse pile design with 3H:1V slopes and a constructed <br />height of 150 feet indicated a factor of safety greater than 2.5. EFMC's proposed highwall <br />backfill design includes overall slopes of 3H:1V, with maximum fill height of 75 feet. Given the <br />proposed design, utilizing the same material, similar slopes, and a height which is approximately <br />one-half that of the refuse pile, the resultant factor of safety should be greater. Previous stability <br />analyses can be found in settion 2.05.3(8). <br />Approximately one half of the backfill will be placed on bedrock and the other half on <br />approximately 10 to 25 feet of overburden material. <br />Previous stability analyses were conduced for the refuse materials by Fox and Associates. Their <br />analyses assumed the following material properties for the coal refuse: <br />Friction Angle: 40 degrees <br />Cohesion: 0 degrees <br />Water Conditions: Drained <br />The stability analyses indicated a minimum factor of safety of over 2. <br />The current plans now call for placing some of this material over overburden material to backfill <br />the highwall. In order to characterize the engineering properties of the overburden materials <br />samples were obtained and tes[ed to determine their engineering classification. The results <br />indicated that the overburden materials are classified as a sand and gravel, some silt and clay to <br />a clayey sand. <br />Based on the material classification, afriction ang]e of 27 degrees was assigned to the overburden <br />material. Making the conservative assumption that the overburden material and the coal refuse <br />have a combined friction angle equal to the lower value for the overburden material and <br />utilizing an infinite slope stability analysis, the resulting slope would have a factor of safety of <br />approximately 1.5. This is considered conservative and mee[s the required factor of safety for <br />coal processing waste banks. <br />There are no seeps, springs, or natural watercourses in the highwall backfill area. Therefore, <br />an underdrain system is not reouired. Revised permit text is included wi[h these responses for <br />insertion into the permit application. <br />~ `~ <br />A:V56BTRI~RE1 513Ra Revised -June 1994 <br />OV01/9~ Jpm <br />