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February 1, 2021 <br /> Mr. Jeremy Dueto <br /> LafargeHolcim/Aggregate Industries <br /> 1687 Cole Blvd., Suite 300 <br /> Golden, CO 80401 <br /> Re: Stability Analysis for Gravel Mine, East of Greeley <br /> Dear Mr. Dueto: <br /> Aggregate Industries(Al) proposes to mine gravel at a site located east of Greeley, upstream of the confluence of the <br /> Cache la Poudre and South Platte Rivers. The mine will be reclaimed as a slurry wall lined reservoir. Two(2)silt ponds <br /> will be located at the mine in an area not enclosed by the slurry walls. This letter summarizes stability analysis <br /> performed in the subject area. <br /> GEOLOGY <br /> The Site is located approximately 23 miles east of the eastern flank of the Rocky Mountain Front Range. Younger <br /> sedimentary strata dip eastward off the Pre-Cambrian igneous and metamorphic rocks that form the core of the Front <br /> Range into the Denver Structural Basin. The Denver Basin is an asymmetrical downwarp of sedimentary strata with a <br /> steeply dipping west limb and a gently dipping east limb. <br /> Bedrock does not crop out at the site, however regional geologic mapping of the area(Colton, 1978 and Braddock and <br /> Cole, 1978)indicates the near surface bedrock at the site is most likely the Laramie Formation. Colton(1978)describes <br /> the Laramie as mostly claystone, shale, sandy shale,and lenticular sandstone with common coal beds in its lower <br /> section. The regional mapping indicates the bedrock is overlain by the Post-Piney Creek. Colton(1978)describes these <br /> alluvial deposits as sandy to gravelly alluvium. <br /> GEOTECHNICAL CONDITIONS <br /> Civil Resources(CR) reviewed records on file at the Colorado Division of Reclamation and Mining Safety(DRMS). The <br /> records indicate that J.T. Consulting had worked on the permit documents at the site and that Cesare and Associates <br /> had performed previous geotechnical studies at the site. In addition,Al provided plan and cross sectional view of the silt <br /> ponds and mine area to be reclaimed as a slurry wall lined reservoir. <br /> Based on the site investigations by J.T. and Cesare,the natural site stratigraphy generally consists of three main units: <br /> 1)Overburden generally consisting of clayey and silty sands locally grading to sandy clay; 2)sand and gravel alluvial <br /> deposits that underlie the overburden and overlie the bedrock;and 3) bedrock consisting of sandstone and claystone. <br /> From a geotechnical standpoint,the sand and gravel will form the majority of the mine slopes. These soils are generally <br /> strong and stable, particularly when dewatered. Dry mining is planned as the slurry wall will control water level in the <br /> main reservoir cell. CR assumes the silt ponds will be mined in dry conditions as the operator will dewater during mining. <br />