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DEERE & AULT <br /> April 18,2008 <br /> Mr. Dan Worden <br /> Hall-Irwin Corporation <br /> 301 Centennial Drive <br /> Milliken,Colorado 80543 <br /> Re: Stability Analysis,Kirby-Dersham Mine;D&A Job No.CG0134.025.00 <br /> Dear Mr. Worden: <br /> A slope stability analysis was performed for the Kirby-Dersham Mine located in Weld County, <br /> Colorado. The purpose of this analysis was to investigate appropriate mining-highwall setback <br /> distances from a proposed slurry wall at the site. This analysis addresses areas where the slurry wall is <br /> to be installed along properties not controlled or owned by Hall-Irwin. Below we present a brief <br /> background and the results of our findings. <br /> BACKGROUND <br /> The site is located west of the South Platte River. Baseline Road(East 168th Avenue)forms the <br /> southern boundary. The Hall-Irwin Walker Pit bounds the site on the east,and Weld County Road <br /> (WCR)23.5 bounds the site on the west. <br /> The proposed gravel mine is to be mined with a highwall slope inclination of approximately 3:1 <br /> (horizontal to vertical). This mining slope will serve as the final reclamation slope. <br /> ANALYSIS <br /> Our stability analysis involved generating a computer model of limit equilibrium analysis using the <br /> Slope/W limit equilibrium computer program. <br /> The stratigraphy encountered during the September 2006 and November 2007 site investigations was <br /> used to develop the model. Frequently,sandy clay layers are encountered in the alluvial sand and gravel <br /> stratum. These layers are colloquially known as"mud lenses." Some mud lenses were encountered <br /> during the investigations of this site.These mud lenses were incorporated in the slope stability model. <br /> Two soil profiles were analyzed. <br /> The stratigraphy near TH-3,in the northwest area of the site, was used for a typical soil profile. This <br /> consisted of 1 foot of overburden overlain by 30 feet of alluvium. This alluvium had an intermediate,5- <br /> foot thick layer of mud lens located in the approximate middle of the deposit. Beneath the alluvium a <br /> layer of weathered bedrock was modeled. The bedrock encountered during drilling was generally <br /> competent;therefore,the weathered zone was modeled as only 1-foot thick. Underneath this weathered <br /> bedrock stronger bedrock was modeled using peak strength parameters. <br /> 600 S.Airport Road,Building A,Suite 205 <br /> Longmont,CO 80503 <br /> Phone: 303-651-1468 Fax: 303-651-1469 <br />