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S • Balaz & Assceiatcs New Coal Waste Pile and Haul Road Project, Sanborn Creek Mine, Somerset, CO June 18, 1997 Page 2 It muss be noted the geologic section in this area could only be approximated Gom existing road cuts and surface outcrops oC the Bowie Member. For the most critical trial failure surfaces, the safety factor calculated by the Modified Bishop Method for global stability (fill height increased to 52') was found to be approximately 1.8. The most critical trial failure surfaces safety Caclor calculated by the Modified Bishop Method for the fill portion was found to be approximately 1.7. The most critical trial failure surface, as calculated by the Modified Bishop Method for the cut scetion, above the proposed road was found to have a safety factor of approximately 2.4. It should be noted the fill section was placed at a slope of l: l (H:V), with the tce of this slope placed on the existing bench, kno++n as the Wcst Yard. An additional requirement on this fill is that a minimum distance of 25 feet from the lce oC the new hiiul road fill to the edge of the existing West Yard bench crest must be maintained. This particular requirement is for slope stability concerns for the upper fill, to allow su(ficicnt room to construct the sediment pond road and to minirrtize the amount of fill used to construct the sediment pond road which may adversely alTect the global stability of the entire slope section. The most recent road alignment and sections observed by personnel of Lincoln DeVore on 6-2-97 indicate that the fdl at station 27+00 will be the most critical in this portion of the haul road, due to fill height and possible colluvium supporting the fill base and toe. The base of the fill at 27+00 will require a bench to be cut into the existing slopes through the assumed colluvium and into the Bowie Member. It should be noted the highest portion of this fill should be founded on the existing road section, which will require additional excavation prior to beginning the earthen fill for the haul road. The tce oCthis slope is to be set back 7 feet Gom the existing outside shoulder/fill crest. The fill slope in this area will beat approximately 1.7: I (H:V). It must be noted the geologic section in this area +vas approximated from existing road cuts and surface outcrops of the Bowie Mcmbcr. For the most critical trial failure surfaces, the safety factor calculated by the Modified Bishop Method for global stabilih• +vas found to be approximately 1.7. `Select' colluvial fill was used, due to higher unit weight to be conservative. The most critical trial failure surfaces safety factor calculated by the Modified Bishop Method for the fill portion was found to be approximately I.S. The most critical trial failure surface, as calculated b}' the Modified Bishop Mclhod for the cut scetion, about the proposed road was found to have a safety factor of approximately 6, which is very high, but natural Gacture patterns will probably decrease the safety factor to less than 2. If these road sations can be constructed ++~th the silty, sandy gravel colluvium and alluvium, in theory, little or no soil reinforcement (mechanically stabilized earth) will be required. In reality, it is anticipated that, even ++ith selective usage of on site materials, that the clayey colluvium and "rocky" portions of excavations Gom the Bo++~e Member ++~II undoubtably be utilized in the earth fills. Unfortunately, the amount oCclayey materials and the qualih• of the actual fill cannot be readily assessed +vithout a significant number oCexploration pits. It is recommended that the higher fills placed in the vicinity of station 8+00 be mechanically stabilized Cor at Icast the bottom 20 feet. For preliminary design purposes, it is assumed that the mechanical reinforcing can be accomplishcci using a gcotcxlile. non-woven needle punched fabric ++~th a minimum ++~de strip tensile strength of 65 pounds per inch in the +vcakest direction. The fabric is to extend into the fill a minimum of 12 Ceet and is to be placed in the fill, ceerv 3 feet of vertical height. For case of conswction and to help control the slope face during construction, fabric in excess of 12 feet could be wrapped up and around to partially enclose the fill fact. The fill fact and fabric must be covered with a rocky soil, which could be revegetated.