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June 12, 2009 <br />Mr. Robert Ray <br />Lafarge West, Inc <br />10170 Church Ranch Way, Suite 200 <br />Westminster, Colorado 80021 <br />Re: Stability Analysis for Lafarge Brown Property; Amendment to Greeley 35`h Avenue Mine <br />DRMS Permit #M-1977-036; Tetra Tech Job No. 133-23511.08005.11 <br />Dear Mr. Ray: <br />This letter has been prepared to address Construction Materials Rule 6, Section 4, Subsection 19, <br />Exhibit S - Permanent Man-Made Structures (6.4.19, Exhibit S) for the permit amendment for the <br />above-referenced aggregate mine. This analysis is for the expanded area known as the Lafarge Brown <br />Property which is included in the permit amendment. <br />The existing permitted mine will be expanded to include the Lafarge Brown Property which is located in <br />the northwestern portion of the mine (located in the northeast quarter of Section 34, 'Township 6 North, <br />Range 66 West). The site is in Greeley, Colorado. Several aggregate mines are present in the area. <br />Other land uses in the area are predominately agricultural and rural residential. The Poudre River Trail <br />recreational path, a fence, and a buried gas line are within 200 feet of the proposed mine limits of the <br />amended mine area. <br />Regional geologic mapping (see below) indicates mining will be in floodplain alluvial deposits. <br />Borehole data (Figures 1 through 4) encountered bedrock at depths ranging from a 16.5 to 40 feet below <br />the surface indicating mining conditions that result in varying highwall heights. <br />GEOLOGIC STRATA AND MINING CONDITIONS <br />Geologic mapping by Hershey and Schneider (1972) indicates the proposed mine site is south of and <br />adjacent to the Cache La Poudre River on the flood plain alluvial deposit known as the Valley Fill <br />alluvium. The deposit is composed of sands, silts, and clays. The site subsurface conditions were <br />investigated through an exploratory drilling program conducted by others. <br />The boring data supplied by Lafarge indicates the depth to bedrock along the proposed mine limit in the <br />amended area ranges from approximately 33.5 feet in the southeast corner to 40 feet near the central <br />portion of the northwestern boundary. Mining in the southeastern portion of the property will be <br />relatively shallow resulting in shorter highwalls generally 12 to 17 feet high. Mine highwall heights will <br />vary with depth to bedrock. Groundwater depths, in borings were typically 9 to 18 feet below the <br />ground surface. <br />From a geotechnical standpoint, the sand and gravel will form most of the mine slope. These soils are <br />generally strong and stable, particularly when dewatered. Minor amounts of clay will lie at the ground <br />surface and locally as lenses within the sand and gravel.