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-55- <br /> In preparing the stability analysis, Morrison Knudsen, Inc. employed material <br /> strength test results obtained by testing performed by Whaller and Associates, <br /> Inc. These analyses were performed on specific samples of waste and <br /> foundational materials obtained at the site through test augering and pit <br /> sampling. The material strength testing was performed in accordance with <br /> acceptable testing methods. The analysis assumed that the refuse pile had a <br /> sufficiently high permeability to prevent pore pressure buildup. This <br /> assumption has not been completely verified. The applicant has installed <br /> monitoring wells above, within, and downslope from the coal processing waste <br /> pile. These locations should provide the necessary data. <br /> The applicant has agreed that if the monitoring results demonstrate that the <br /> ground water conditions encountered within the coal processing waste pile <br /> differ from those assumptions made in completing the stability analysis the <br /> analysis will be revised. If a revised stability analysis suggests that <br /> design revisions are necessary to achieve a static safety factor of 1 .5, the <br /> appropriate design revisions will be made. <br /> Mid-Continent Resources, Inc. proposes the construction of a new coal <br /> processing waste pile at their Coal Basin Mine. The pile would be located <br /> adjacent to Coal Creek on a parcel referred to as the Sutey Parcel . The <br /> applicant proposes to construct a 7.8 million cubic yard structure during a 47 <br /> year project life span. The application contains a phased general plan for <br /> the entire life of the structure and a detailed engineering plan for the <br /> initial 920,000 cubic yard five-year configuration of the pile. <br /> The coal processing waste pile's design incorporates a dendritic underdrain <br /> system. Typical underdrains are to be a minimum of 4 feet and 6 feet deep <br /> filter fabric lined, gravel-packed trench. Waste material is to be placed in <br /> horizontal lifts not to exceed 24 inches in thickness. Compactive effort is <br /> to be applied to achieve a Standard Proctor relative density of 90 percent. <br /> The pile' s outslopes are not to exceed 2h:ly (horizontal to vertical grade), <br /> with a top slope of 3 percent to promote drainage of the pile. <br /> The applicant has completed a detailed geotechnical site investigation and <br /> stability analyses of the proposed structure. In response to Division <br /> comment, the amended stability analysis which applies a horizontal <br /> acceleration factor of 0.05, resulting in a pseudo-static slope stability <br /> factor of 1 .3. <br /> The coal processing waste structure will be founded on an assemblage of <br /> low-permeability alluvial and colluvial surficial materials adjoining Coal <br /> Creek. As a result of its preliminary adequacy review, the Division also <br /> expressed a concern that low-permeability foundation materials might undergo <br /> an undrained failure, if loading rates exceeded those materials' ability to <br /> drain and consolidate. The applicant had not performed effective stress <br /> triaxial strength testing on the foundational materials and could not derive <br /> the parameters necessary to complete a sophisticated undrained effective <br /> stress analysis of the foundational materials beneath the proposed pile. As <br /> an alternative, the amended static slope stability analysis is based on a <br />