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May 12,2023 <br /> Mr.Chris Oestreich <br /> Operations Manager-Aggregates <br /> BURNCO,LLC <br /> 10100 Dallas Street <br /> Henderson,CO 80640 <br /> Re: Stability Analyses,Nissen Farms Gravel Mine(DRMS Permit No.M•2003.001),Weld County,Colorado <br /> Dear Mr.Oestreich: <br /> This stability analyses letter has been prepared in support of the Change of Operator status for the Nissen Gravel Mine <br /> and to meet the Mined Land Reclamation Board(MLRB)Construction Materials Rule 6,Section 4,Subsection 19, <br /> Exhibit S-Permanent Man-Made Structures(6.4.19, Exhibit S). This letter describes the project and slope stability <br /> analyses utilized to evaluate the minimum distance between the edge of mining and adjacent structures to avoid damage <br /> to the structure. <br /> The site is located in the South Platte River Valley upstream of its confluence with the Cache La Poudre River,east of <br /> Greeley,Colorado,along Weld County Parkway. More specifically,the site occupies 89 acres in the South Half of <br /> Section 12,Township 5 North,Range 65 West of the 6th Principal Meridian. <br /> PREVIOUS STABILITYANALYSES and REVISED NINE PLAN <br /> Previous stability analyses were performed when the mine was originally permitted. The previous analyses are <br /> presented in Attachment A. The previous analyses modeled a 46-foot high,vertical high wall located 55-feet interior to <br /> the site slurry wall with the nearest structure(not owned by the miner)20-feet exterior to the slurry wall. The previous <br /> analyses also modeled unmined areas interior to the slurry wall which provided access to site oil/gas wells,associated <br /> pipe lines and related oil/gas facilities. These interior wells have been plugged and abandoned and the related gas/oil <br /> infrastructure facilities have been removed from the site. <br /> As a result of the removal of these interior structures,BURNCO LLC.(BURNCO)plans to mine the site as one cell to be <br /> reclaimed as a reservoir. Exterior mine slopes will be mined at slope of 3 horizontal to 1 vertical(3h:1v)with a mine limit <br /> 20 feet from the slurry wall. The stability analyses discussed herein(Attachment B)show that known structures within <br /> 200-feet of the mine limit will be stable. <br /> GEOLOGY <br /> The Mine Site is located in the alluvial valley of the South Platte River. Regional geologic mapping at the site(Smith,et <br /> al, 1972)indicates the mine lies on the recent alluvial terrace of the South Platte River. Other regional geologic mapping <br /> approximately 3,000 feet west of the site(Colton, 1978)indicates this terrace corresponds with the Post Piney Creek <br /> Alluvium. The mapping indicates the bedrock underlying the alluvium is most likely claystone,sandstones,siltstones, <br /> and possibly coal of the Laramie Formation. <br /> GEOTECHNICAL CONDITIONS <br /> Data for these analyses were extracted from eight(8)exploratory borings drilled at the site in January 2000 by Terracon <br /> (2000). For reference a test Boring Map showing the boring locations with corresponding overburden and sand and <br /> gravel thickness is included as part of the original analyses in Attachment A. The Terracon logs indicate that the general <br /> subsurface profile across the site consists of approximately two(2)to five(5)of overburden silty sand overlying <br /> approximately 34.5 to 44 feet of sand and gravel on top of claystone bedrock. The logs indicate the borings encountered <br /> approximately one and a half(1.5)to seven(7)feet of weathered claystone. However,the bedrock was not sampled <br />