2009-07-06_REVISION - M1977036 (24)

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Mr. Robert Ray June 12, 2009 Page 2 KNOWN STRUCTURES WITHIN 200 FEET OF MINING LIMITS Known man-made structures within 200 feet of the amended area mine limits, excluding those owned by Lafarge, Inc., are listed below: ¦ A buried 4" gas flowline owned by DCP Midstream located approximately 150 feet southwest of the southern property line in the southwest corner. ¦ A 10' wide recreational trail owned by Weld County, located along the northwestern border. ¦ A fence, also owned by Weld County, located at the boundary between the mine and trail corridor. The east and south property lines of the amended area abut the previously permitted mine area. No structures are within 200 feet of the east property line of the mine expansion. STABILITY ANALYSES Stability analyses were performed in order to evaluate potential for damage to existing permanent structures due to mine highwall slope failures. Analyses were performed with the Slope-W 2007 computer program. For each case we analyzed the most critical soil profile(s) based on exploratory boring data (i.e. the tallest highwall, thickest overburden layer, and associated weaker clay lenses). The stability analyses were run in accordance with accepted geotechnical standards and DRMS requirements for temporary pit slopes summarized as follows: • Mine highwalls modeled as vertical slopes • Residual soil strength parameters used for the upper 2.5 feet of weathered bedrock • Factor of Safety must be greater than 1.0 Groundwater levels on the profiles were input to portray site dewatering. No laboratory strength tests were performed on soils at the site. The soil strength parameters used were based on typical values for the anticipated soils and our experience working with DRMS at other sites in the area. The input strength parameters were: Material Moist Unit Saturated Unit Effective Cohesion Effective Friction Weight c Weight c c Angle (degrees) Overburden clay 114 126 150 28 Sand and gravel 130 137 0 37 Cla Lense 116 119 200 22 Weathered Residual Strength Bedrock 124 134 0 14 Cla stone Bedrock 124 134 500 22