2006-04-12_REVISION - M1983067 (5)

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~~_~ ~. DEERE & AULT w`~s~ ~~~ ~ ~~ ,,! ~ ~ ~~'~' RECEIVED Apri14, 2006 I' ~ ~ ~ / APR 12 2U06 } Division of Minerals and Geology Mr. Ken Lambrecht Hall-Irwin Construction, Inc. ~ n /c~ 301 Centennial Drive I "f -~ t ~~ lC/~~ Milliken, Colorado 80543 Re: Stagecoach Pit Slurry WalUMining Limit Setback Slope Stability Analysis; //~ D&A Job No. CG0134.007.00 Dear Mr. Lambrecht: This letter presents a summary of a slope stability analysis performed to investigate various mining- highwall setback distances from the slurry wall at the Stagecoach Pit aggregate mine. The goal of this analysis was to determine the appropriate spacing between the gravel mine highwall and the slurry wall at the pit. Below we present a brief background and the results of our findings. BAC%GROUND The site is located neaz Brighton in Adams County, Colorado. The South Platte River flows adjacent to the site on the western boundary. A slurry wall was recently constructed around the perimeter on the site. The two primary concerns are that a stability failure of the mine highwall could impact the exisfing nearby utilities or the newly constructed slurry wall. This site has several utilities east of the site ~iconstructed parallel to Old Brighton Road, approximately 30 feet from the slurry wall. two possible mining scenarios were proposed. One approach was to mine the gravel pit with a slope inclination of approximately 0.5:1 (horizontal to vertical). After mining, a final on slope of 3:1 will be constructed out of compacted soils. her alternative is to mine the highwal] at the final reclamafion slope of 3:1. If this alternative is we understand that the slurry wall to mine highwall crest distance is to be 16 feet. stability analysis involved generating a computer model of limit equilibrium analysis using the ~e/W computer program. In addition to the mining highwall, long-term stability of the reclaimed e subject to water storage loadings was also evaluated. stratigraphy encountered during the August 2005, Site Investigation was used to develop the model. uently, sandy clay layers are encountered in the alluvial sand and gravel stratum. These layers are known as "mud lenses." No significant mud lenses were encountered during the t of this site. Therefore, mud lenses were not incorporated in the slope stability model. 600 S. Airport Road, Building A, Sui[e 205 Longmont, CO 80503 Phone: 303-651.14b8 Fax: 303-651-1469