2018-05-22_PERMIT FILE - M2018027 (4)

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May 13, 2018 Mr. David W. Hunt 1440 Weld County Road 40 Platteville, CO 80651 Re: Stability Analysis for the Hunt Water, LLC Gravel Mine, Dear Mr. Hunt: This letter has been prepared to address 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)for the proposed DHF Gravel Mine located near the intersection of U.S. Highway 85 and State Highway 60 in Weld County, Colorado. 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 between the towns of Platteville(to the south) and Gilcrest(to the northeast). More specifically, the site is within part of Section 6, Township 3 North, Range 66 West of the 61h Principal Meridian. The mine is in an area of irrigated agricultural land with common oil and gas wells and related infrastructure and is bound by Weld County Road (WCR)36 on the south,WCR 25.5 on the west, agricultural land on the north, State Highway 60 on the east, and U.S. Highway 85 on the southwest. The site and vicinity are shown on Figure 1. The site will be mined in two cells. The main cell will be in parts mined above the bedrock and in some areas will be mined to the top of bedrock. Plans are to construct a vertical slurry wall around this cell with perimeter mine slope of 3:1 (horizontal to vertical), as indicated in the DRMS permit application. The second cell will be mined above the water table to a maximum depth of 10 feet and will also have a perimeter mine slope of 3HAV. Based on the stability analyses, this report indicates that the setbacks and perimeter slopes in the mining plan are sufficient to protect structures within 200 feet of the permitted mining boundary. Actual setbacks may be greater due to permit limitations, zoning requirements, construction issues, agreements with owners, and extent of economically mineable aggregate or other issues. GEOLOGY The Site is located approximately 21 miles east of the eastern flank of the Rocky Mountain Front Range. Younger sedimentary strata dip 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 downwarp of sedimentary strata with a steeply dipping west limb and a gently dipping east limb. Bedrock does not crop out at the site, however regional geologic mapping of the area(Colton, 1978) indicates the near surface bedrock at the site is most likely the Laramie Formation. Colton (1978)describes the Laramie as mostly claystone, shale, sandy shale, and lenticular sandstone. The Lower Laramie is described as sandstone, sandy shale, and claystone with several coal beds. The regional mapping indicates the bedrock is overlain by the Broadway Alluvium. Colton (1978)describes the Broadway as sand and gravel deposited by the South Platte River and its tributaries that is terraced typically lying about 40 feet above major streams. Along the South Platte River,the Broadway is typically on the order of 35 feet thick but can range up to 125 feet thick. GEOTECHNICAL CONDITIONS Based on the site investigations, the natural site stratigraphy generally consists of three main units: 1)Overburden generally consisting of near surface silty to clayey sand; 2)sand and gravel alluvial deposits that underlie the overburden