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<br />DEERE 6i AULT <br />January 17, 2006 <br />Mr. Ken Lambrecht <br />Hall-Irwin Construction, Inc. <br />301 Centennial Drive <br />Milliken, Colorado 80543 <br />Re: Stagecoach Pit Slurry Wall/Mining Limit Setback Slope Stability Analysis; <br />D&A Job No. CG-0134.007.00 <br />Deaz Mr. Lambrecht: <br />At your request, we performed a slope stability analysis to investigate various mining-highwall <br />setback distances from the slurry wall at the Stagecoach Pit aggregate mine. This letter presents <br />our fmdings of that analysis. The goal of this analysis was to determine the appropriate spacing <br />between the giavel mine highwall and the.slurry wall at the pit. Below we present a brief <br />background and the results of our fmdings. <br />BACKGROUND <br />The site is located near Brighton in Adams County, Colorado. The South Platte River flows <br />adjacent to the site on the western boundazy. A slurry wall was recently constructed azound the <br />perimeter on the site. The chief concern is that a stability failure of the mine highwall could <br />impact the existing, newly constructed slurry wall. Long-term stability of the reclaimed slope <br />subject to water storage loadings was also evaluated. <br />ANALYSIS <br />Our stability analysis involved generating a computer model of limit equilibrium analysis using <br />the Slope/W computer program. Table 1 highlights the parameters of the soils used in the model. <br />Table 1: Model Parameters <br />Soil type ~' (degrees) e' (psi Ym (Pct <br />Overburden 24 50 114 <br />Gravel 35 0 130 <br />Bedrock 26 700 124 <br />Com acted Clay Liner 24 100 115 <br />600 S. Airport Road, Building A, Suite 205 <br />Longmont, CO 80503 <br />Phone: 303-651-1468 Fax: 303-651-1469 <br />