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July 23, 2013 Page 3 <br />Given the extensive experience with highwall mining on the property, AAI was asked to <br />develop guidelines for the current study areas using that experience as a basis. Specifically, the <br />following issues and concerns regarding highwall mining applications in the South Taylor and <br />West Pit are addressed: <br />• What specific geotechnical/geological constraints exist within the South Taylor and West <br />Pit areas that may limit highwall mining applications? <br />• For each seam to be mined, what minimum web pillar dimension (as a function of depth, <br />mining height, and material properties) will limit the risk that highwall stability is <br />affected by mining operations? <br />• Are interburden thicknesses such that web/barrier pillars should be columnized, or can <br />independent pillar designs be developed where multiple seams will be mined? <br />• What is the potential for "cascading pillar failure," and what minimum dimension and <br />spacing of barrier pillar will be required to sufficiently isolate extraction panels along the <br />highwall? <br />• What operational measures can be taken to increase highwall mining efficiency and <br />safety? <br />To address these questions, AAI conducted a multifaceted evaluation, including 1) site <br />characterization (on -site review of current conditions and comparison of target areas with <br />previous highwall mining areas), 2) engineering design (empirical evaluation of optimal mining <br />plans for each seam and area), and 3) review of operational practices and alterations (if any) <br />required for South Taylor and West Pit conditions. <br />AAI's evaluation is intended to provide Colowyo with a rational methodology for <br />initiating highwall mining in the current study areas. As with all previous designs provided for <br />Colowyo, it is intended that designs be evaluated as mining proceeds, and if necessary, refined as <br />field experience is gained. <br />2.0 SITE CHARACTERIZATION <br />The site characterization effort consisted of a site visit on June 26, 2013, and comparison <br />of available physical property data with the existing highwall mining physical property database. <br />2.1 Site Geology and Seam Settings <br />The coal deposits at Colowyo occur in Upper Cretaceous sediments of the Williams Fork <br />Formation (W.A. Wahler and Associates 1975). These sediments consist primarily of <br />interbedded sandstone, siltstone, mudstone, sandy shale, shale, and coal. In addition to the target <br />seams G78, F, E, D2, and C, numerous other coal seams and splits occur in the lithologic <br />column, but none are of sufficient thickness and/or continuity to be considered a highwall mining <br />target. <br />Agapito Associates, Inc. <br />