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• EXECUTIVE SUMMARY <br />Buckhom Geotech has been retained to conduct a stability evaluation of the proposed coal waste <br />(gob) pile No. 4 at Bowie Mine No. 2, near Paonia, Colorado. Total and effective stress slope <br />stability evaluations were conducted for this analysis. The evaluations were based on <br />information gathered from drilling investigations, laboratory testing of gob and native foundation <br />soil, and our professional experience. <br />Gob Pile No. 4 is located on the slopes of the north side of the North Fork of the Gunnison River <br />valley, between the existing Gob Pile No. 1 and Gob Pile No. 2. The Gob Pile No. 4 footprint is <br />currently being used as a soil borrow area for cover material for Gob Pile No. 2. The excavation <br />at the borrow area also provides additional capacity for gob. It is anticipated that excavation of <br />native soils at Pile No. 4 will continue concurrently with gob placement. <br />Soils at the proposed Gob Pile No. 4 consist of colluvium and slope wash originating from <br />sedimentary bedrock units upslope of the site. Two boreholes were drilled at the gob pile <br />footprint to characterize the foundation soils. Soils encountered in the boreholes consisted of dry <br />to damp, hard, moderate brown clayey gravel, cobbles, and boulders. The colluvial soils overlie <br />Mancos Shale bedrock. However, Mancos Shale was not encountered in the boreholes to depths <br />up to 57 feet. No groundwater or evidence of groundwater was observed in the boreholes. <br />Additionally, three boreholes were drilled within gob at Gob Pile No. 2. Samples of compacted <br />gob were taken for triaxial shear testing. Undrained and drained shear strength pazameters for <br />• gob were estimated based on triaxial shear testing. <br />Limit equilibrium stability evaluations for the foundation excavation and ultimate gob <br />configuration were conducted using total-stress (undrained) and effective stress (drained) <br />analyses. Excavation slopes in native material were first evaluated to determine stable slope <br />angles. Excavations with overall slopes not exceeding 2H:1 V resulted in a factor of safety of I.5 <br />or greater. Given the uncertainty of native soil strength pazameters, we recommend a minimum <br />factor of safety of 1.50 for the overall temporary excavation slope. Steeper excavations may be <br />made for short slopes, but the excavations should be benched into the slope to provide an overall <br />slope of 2H:1 V or flatter for the approximate 118 vertical foot high excavation footprint. <br />Stability was subsequently evaluated for the ultimate gob pile (ultimate height approximately <br />118 feet), considering 2.SH:1 V and 2H:1 V overall excavation slopes. For the model, geometry <br />of the gob was assumed to consist of a 2.SH:1 V slope for the lower 50 vertical feet, with variable <br />slopes above this level. For an excavation slope of 2H:1 V, a gob slope of 3H:1 V was found to <br />result in critical factors of safety greater than 1.50 for effective stress and total stress analyses. <br />Both circulaz and block failures were considered. For an excavation slope of 2.SH:1 V, a gob <br />slope of 2.SH:1 V was found to result in critical factors of safety greater than 1.50 for effective <br />and total stress analyses. <br />If construction plans or waste pile configurations vary from those described and evaluated in this <br />report, we must be notified to evaluate stability for the actual conditions. <br />Bowie Gob Pile No. 4 stability report <br />Project #OS-126-GEO <br />Page 3 of I S <br />