2006-04-12_REVISION - M1983067 (5)

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Mr. Ken Lambrecht April 4, 2006 Page 3 The setback distance in the table is from the crest of the highwall to the slurry wall centerline. The slurry wall centerline is 30 feet from existing utilities orrights-of--way, so the total setbacks from highwall crest to property lines aze 67 and 46 feet, respectively, for the cases analyzed. Each of the owner's of the existing structures (in this case the utilities in theright-of--way) can be approached to see if a less conservative analysis can be used. However, unless site-specific strength data is obtained indicating smaller setbacks are appropriate, the set back distance from the slurry wall to the steep mine highwall will need to be 37 feet or more, based on the DMG parameters. if the highwall is mined at a 3:1 slope the set back only needs to 16 feet. HIGHWALL TO SLURRY WALL SETBACKENGINEERINGANALYSlS The appropriate distance frorn the slurry wall to the crest of the highwall was analyzed by using the stratigraphy encountered in the borings. The soil strength parameters used in the model were derived from typical, similar sites and engineering judgment. Table 3 highlights the strength parameters of the soils used in the model. Table 3: Assumed Model Parameters Soli Tjpe Effective Friction Angle ' (de rees Effective Cohesion c' (psJ) Moist Unit Weight Ya (Pcn Overburden 24 50 114 Gravel 35 0 130 Bedrock (Peak S4en 26 700 124 Co acted Cla Liner 24 100 115 Bedrock Full Softened S[ren 26 0 124 analysis involved varying the offset distance from the highwall to the slurry wall and evaluating the ~r of safety and failure surface orientation at each offset distance. Shallow surfaces (infinite slope or eling failures")were excluded from the analysis, as they should not impact the slurry wall. evaluated two types of reclaimed slopes. The first type consists of mining a steep highwall (0.5:1) placing overburden soils compacted to 45 percent of the maximum dry density per ASTM D-698 for 3:1 reclamation slope. The second type of slope evaluated consists of mining a 3:1 highwall slope leaving the native soils in-place as the final reclamation slope. highwall was modeled with a slope of 0.5: I typical for gravel mines in the South Platte River bedrock was encountered in the exploratory borings in the shallow and intermediate sections. bedrock strength parameters were used for all sections.