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September 30, 2022 <br /> Mr. Brennan Middleton <br /> Water Resources Manager <br /> City and County of Broomfield <br /> 3951 West 14411 Avenue <br /> Broomfield, CO 80023 <br /> Re: Heit Gravel Mine,Geotechnical Stability Analysis <br /> Dear Mr. Middleton: <br /> This letter has been prepared to address the Mined Land Reclamation Board (MLRB)Construction Materials Rule 6, <br /> Section 4, Subsection 19, Exhibit S-Permanent Man-Made Structures(6.4.19, Exhibit S)for the Heit Gravel Mine. This <br /> letter describes the project and slope stability analyses utilized to evaluate the minimum distance between the edge of <br /> mining and adjacent structures to avoid damage to the structure. <br /> The site is located in the Platte River Valley,north of Fort Lupton, Colorado,on the northwest comer of Weld County <br /> Roads 20 and 25. More specifically,the site is within part of Section 13,Township 2 North, Range 67 West of the 61, <br /> Principal Meridian. <br /> The mine consists of one(1)cell mined at a slope of 3 horizontal to 1 vertical(3h:1v). Final reclamation will be a slung <br /> wall lined water storage reservoir. The City and County of Broomfield will operate the reservoir. <br /> Based on the stability analyses,this report indicates that the setbacks and perimeter slopes in the mining plan are <br /> sufficient to protect structures within 200 feet of the completed mining boundary. Actual setbacks may be greater due to <br /> permit limitations,zoning requirements,construction issues,agreements with owners,and extent of economically <br /> mineable aggregate or other issues. <br /> GEOLOGY <br /> The Site is located approximately 24 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. <br /> Bedrock does not crop out at the site, however regional geologic mapping of the area(Colton, 1978)indicates the near <br /> surface bedrock beneath the site is most likely the Laramie Formation. Colton (1978)describes the Laramie Formation <br /> as mostly grey claystone,shale,sandy shale,and scattered beds of sandstone and lignite overlying a lower part <br /> consisting sandstone and sandy shale,interbedded with clay,shale and several beds of coal. The regional mapping <br /> indicates the bedrock is overlain by modem alluvium known as the Piney Creek and Post-Piney Creek deposits. <br /> GEOTECHNICAL CONDITIONS <br /> Based on site investigations performed during slurry wall design and the reservoir upgrade design,the natural site <br /> stratigraphy generally consists of three main units: 1)Overburden:generally consisting of near surface,sandy clay and <br /> clayey sand;2)Sand and Gravel: alluvial deposits that underlie the overburden and overlie the bedrock;occasional <br /> lenses of silty sand lie within this deposit; and 3)Bedrock usually consisting of claystone. <br /> Overburden <br /> Overburden was encountered in the borings drilled at the mine. Overburden is generally soft to medium stiff with <br /> thicknesses ranging from approximately three(3)to six(6)feet. Geotechnical laboratory testing resulted in percentages <br />