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TETRA TECH <br /> MEMO <br /> To: Christine Felz, Aggregate Industries, Inc. <br /> From: Derek Foster, PE, Tetra Tech July 3, 2019 <br /> Updated: Jeffrey Butson PE, Tetra Tech November 8, 2019 <br /> Subject: Slope Stability and Setback Updates, July 3, 2019; Tetra Tech Job No. 200-23514-18004 <br /> The memo describes the updated analysis performed to evaluate the minimum setback distance between the <br /> proposed mine limit and identified critical manmade structures near the site as required per the Mine Land <br /> Reclamation Board (MLRB) Construction Materials Rules 6.3.12(b) and 6.4.19(b) and the memorandum from the <br /> DRMS dated May 16, 20181. This slope stability analysis was not performed to evaluated the stability of <br /> highwalls, proposed mine slopes, proposed reclamation slopes, or infrastructure owned by Aggregate Industries. <br /> The setbacks reported in this memo represent the minimum setback based on the physical and geotechnical input <br /> parameters for each model. The required setback may be greater based on regulatory requirements. <br /> 1.0 SITE LAYOUT <br /> The Tucson South Mine is comprised of approximately 250 acres, located a half-mile west of the City of Brighton, <br /> Colorado. The site lies within the south half of Section 1 and the northwest quarter of Section 12 in Township 1 <br /> South, Range 67, West of the 61h Principal Meridian. Land use around the site is aggregate mining, residential, <br /> and agricultural. Tucson Street, a paved north-south road, bisects the proposed East and West Pits of the mine. <br /> Colorado Highway 7 (E 1601h Avenue) separates the proposed East and West Pits from the proposed Southwest <br /> Pit. The South Platte River forms the eastern border for the proposed mine and the Brighton Ditch forms the <br /> western border of the proposed mine. <br /> 2.0 GEOLOGY <br /> The site is in the alluvial valley of the South Platte River. Geologic mapping indicates that the mine is located on <br /> an alluvial terrace corresponding with the Piney Creek and Post Piney Creek alluvium2. The bedrock underlying <br /> the alluvium may be the Arapahoe Formation, which consists of claystone and sandstones. <br /> A total of 130 boreholes have been drilled on the site. Drilling logs indicate the general subsurface profile consists <br /> of an average of three feet (one to ten feet) of silty sand overburden, overlying an average of 23 feet(five to 46 <br /> feet) of sand and gravel, overlying the bedrock. A mud lens was found in the area, mainly in the West and <br /> Southwest Pits of the proposed mine. Groundwater was encountered while drilling in 2004 at depths between <br /> eight and 12 feet below the ground surface. Monitoring well readings from December 2003 to August 2004 have <br /> water levels between four and 16 feet below ground surface. <br /> Cazier, T. (2018, May 16). Re: Factors of Safety for Slope Stability/Geotechnical Analyses Associated with <br /> Mining Operations, Denver, CO: Division of Reclamation, Mining and Safety Minerals Program. <br /> 2 Trimble, E. D., & Machette, N. M. (1979). Geologic Map of the Greater Denver Area, Front Range Urban <br /> Corridor, Colorado. USGS Map 1-856-H, Version 1.1. <br /> Page 1 <br />