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June 29,2023 Mr.Chris Oestreich Operations Manager-Aggregates BURNCO, LLC 10100 Dallas Street Henderson, CO 80640 Re: Stability Analyses,Jefferson County Gravel Mine(DRMS Permit No.M-1987-113),Jeferson County, Colorado Dear Mr.Oestreich: This stability analyses letter has been prepared in support of the Change of Operator status for the Jefferson County Gravel Mine and to meet the Mined Land Reclamation Board (MLRB)Construction Materials Rule 6,Section 4, Subsection 19,Exhibit S-Permanent Man-Made Structures(6.4.19, Exhibit S). This letter describes the project and slope stability analyses utilized to evaluate the minimum distance between the edge of mining and adjacent structures to avoid damage to the structure. The site is located east of Highway 93 and west of the Rocky Flats Wildlife Refuge in Jefferson County Colorado. More specifically,the site is located in part of Section 9,Township 2 South, Range 70 West of the 6th Principal Meridian. REGIONAL GEOLOGY Regional geologic mapping(Trimble and Machette, 1978)indicates the Jefferson County Mine is located approximately 3 miles east of the eastern flank of the Rocky Mountain Front Range. Younger sedimentary strata dips eastward off the Pre-Cambrian igneous and metamorphic rocks that form the core of the Front Range into the Denver Structural Basin. The Denver Basin is an asymmetrical down-warp of sedimentary strata with a steeply dipping west limb and a gentry dipping east limb. The asymmetrical nature of the dipping sedimentary strata is observed at the mine. Bedrock,progressing from older to younger and west to east,of the Fox Hills Sandstone and the Laramie Formation lie at a shallow depth on the west side of the mine dipping steeply to the east. This land form is locally known as a"hogback". Borings at the site indicate the steeply dipping bedrock folds at depth forming a gently,eastward dipping bedrock surface. This bedrock surface is typically at depths of approximately 70 to 105 feet below the pre-mined elevation. Closer to the"hogback",the bedrock lies at shallower depths.The bedrock is overlain by the Rocky Flats Alluvium. The Rocky Flats Alluvium is described as consisting of boulders,cobbles,and gravel with a clayey sand matrix that was deposited during glacial melting periods. Aggregate deposits within the Rocky Flats Alluvium are mined by BURNCO east of the"hogback". Exploratory Drilling Five exploratory borings were drilled at the mine at the approximate locations shown on Figure 1. The borings were drilled with a truck-mounted CME-75 drill rig using the Tubex drill system in which a downhole percussion hammer is followed by advancing 4-inch casing. Cuttings were lifted to the surface with air. The borings ranged in depth from approximately 24.5 to 105.5 feet below existing ground. All of the borings extended through our interpretation of the aggregate deposit. Four of the borings extended to the underlying claystone bedrock of the Laramie Formation. Subsurface samples were obtained at various intervals during drilling by driving a standard split spoon sampler utilizing a 140-pound weight free falling 30-inches(Standard Penetration Test, (SPT)).