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July 23, 2013 <br />Page v <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, 2) engineering design, and 3) review of operational practices and alterations (if <br />any) 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 />Summary of Findings and Design Recommendations <br />The analyses performed by AAI support the conclusion that highwall mining is <br />geotechnically feasible for the target seams identified in both the South Taylor and West <br />Pit study areas. Indications are that the mudstone parting between the G7 and G8 seams, <br />and the D2 and E seams in South Taylor are competent, and that pillar designs for these <br />seams can be based on the coal strength, without special consideration of the parting. For <br />areas where multiple seams will be mined in the same vertical section, AAI recommends <br />that an interburden -to -lower seam mining height ratio of 2.0 or greater be maintained. <br />This will allow panels to be laid out independently, without the requirement of pillar <br />columnization. <br />Numerical modeling indicates that future mining of the F Seam in the West Pit below E <br />Seam panels that were highwall mined several years ago engenders considerable risk due <br />to the uncertain condition of the E Seam pillars. If E Seam pillar failure has occurred, <br />load transfer from failed web pillars to adjacent abutment zones could cause considerable <br />stress concentration in the F Seam, well beyond the overburden stresses that are normally <br />assumed in the design method. E Seam pillar design could be modified to account for <br />worst -case stress concentrations, but the result would be low overall recovery. Unless <br />Colowyo needs to maximize highwall miner tonnage within the study area, avoidance of <br />the old E Seam highwall miner workings is recommended when designing F Seam <br />panels. <br />The pillar design curves presented in Figures 29 through 33 reflect design criteria that <br />have evolved since the earliest highwall mining at Colowyo. Web and barrier pillars are <br />designed for a 1.5 safety factor using one of two pillar strength formulas. For pillars with <br />width -to- height (w/h) ratios less than 1.4, the modified van der Merwe formula is used, <br />and for pillars with w/h ratios greater than 1.4, the Mark - Bieniawki formula is used. In <br />both cases, in -situ coal strength is estimated based on laboratory strength testing, with a <br />maximum in -situ coal strength of 900 pounds per square inch (psi). Barrier pillars are <br />designed assuming that they would be placed after every 20 highwall openings. <br />• While the majority of the rock mass appears to consist of competent strata, it has been the <br />experience in previous highwall mining at Colowyo that weak mudstone roof tends to fall <br />Agapito Associates, Inc. <br />