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at the borrow area also provides additional capacity for gob. It is anticipated that excavation of native soils at Pile No. 4 will continue concurrently with gob placement. Excavation slopes in native material were first evaluated to determine stable slope angles. Excavations with overall slopes not exceeding 2H:1 V resulted in a factor of safety of 1.5 or greater. Given the uncertainty of native soil strength parameters, we recommend a minimum factor of safety of 1.50 for the overall temporary excavation slope. Steeper excavations maybe made for short slopes, but the excavations should be benched into the slope to provide an overall slope of 2H:1 V or flatter for the approximate 118 vertical foot high excavation footprint. Stability was subsequently evaluated for the ultimate gob pile (ultimate height approximately 118 feet), considering 2.SH:1 V and 2H:1 V overall excavation slopes. For the model, geometry of the gob was assumed to consist of a 2.SH:1 V slope for the lower 50 vertical feet, with variable slopes above this level. For an excavation slope of 2H:1 V, a gob slope of 3H:1 V was found to result in critical factors of safety greater than 1.50 for effective stress and total stress analyses. Both circulaz and block failures were considered. For an excavation slope of 2.SH:1 V, a gob slope of 2.SH:1 V was found to result in critical factors of safety greater than 1.50 for effective and total stress analyses. These analyses assume that foundation underdrains will be designed and constructed to drain the upper foundation soils, surface water diversion channels will be designed and constructed to convey water azound and away from the gob pile, and that surface water channels will be designed and constructed on the gob pile itself to shed surface water off and away from the gob. It is critical that gob materials aze not placed wet of the optimum moisture content. Gob should be sealed (by compaction) to minimize infiltration from precipitation and should be graded to shed water per the drainage plan. The findings and recommendations provided in this report aze contingent on careful and proper fill placement; inspection, in-situ density testing, and on-going laboratory testing of gob material, as appropriate. 7.0 Buckhom Geotech (2006). Geotechnical Report: Stability Evaluation of Coal Waste Disposal Area, Bowie No. 2 Mine, Delta County, Colorado. February I5, 2006. Dunrud, C. Richard (1989). Geologic Map and Coal Stratigraphic Framework of the Paonia Area, Delta and Gunnison Counties, Colorado, United States Geological Survey, Map C- 115, 1989. Harr, M.E. (1987). Reliability-Based Design in Civil Engineering, McGraw-Hill, New York. Kulhawy, F.H. (1992). On the Evaluation of Soil Properties, Geotechnical Special Publication 31, ASCE, Reston, VA, pp. 95-115. I• Bowie Gob Pile No. 4 stability report Project #OS-126-GEO Page 14 of I S