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Technical Memo <br /> <br />TETRA TECH <br />351 Coffman Street, Suite 200, Longmont, CO 80501 <br />Tel +1.303.772.5282 Fax +1.303.772.7039 | tetratech.com <br />To: Julie Mikulas, Martin Marietta <br />From: Aric Hotaling, PE - Geotechnical Engineer, Tetra Tech <br />Caleb Stock, PE – Project Geotechnical Engineer, Tetra Tech <br />Date: September 21, 2022 <br />Subject: Slope Stability and Setback Exhibit – Windsor East Mine; Tetra Tech Job No. 117-8741006 <br />This Technical Memorandum (Tech Memo) describes the analysis performed to evaluate the slope stability of proposed <br />reclamation slopes and minimum setback distance requirements between the proposed mine limit and identified <br />structures or utilities as required per the Mine Land Reclamation Board (MLRB) Construction Materials Rules 6.3.12(b) <br />and 6.4.19(b) and the memorandum from the DRMS dated May 16, 2018 (Cazier & Wait, 2018). The setbacks reported in <br />this memo represent the minimum setback based on the physical and geotechnical input parameters for each model. <br />The required setback may be greater based on regulatory requirements. <br />1.0 SITE LAYOUT <br />The Windsor East Mine site is 150.25 acres. The mine is located in the Town of Windsor, Colorado. The site lies within the <br />northwest quarter of Section 36 in Township 6 North, Range 67 West. Land use around the site is aggregate mining, <br />residential, heavy industrial, and agricultural. County Road 23, a gravel north-south road, forms the western border of <br />the mine property. An unnamed gravel farm access road forms the northern border, and the Cache la Poudre River <br />corridor is along the southern border of the site. Martin Marietta’s Parsons Mine site is adjacent to the site on the east. <br />2.0 GEOLOGY <br />The site is in the alluvial valley of the Cache la Poudre River. Geologic mapping indicates that the mine is within the Post- <br />Piney Creek alluvium (Colton, 1978). The bedrock underlying the alluvium may be the Arapahoe Formation, which <br />consists of claystone and sandstones. <br />Deere & Ault Consultants, Inc. drilled eight borings in 2017 for the property owner (Attachment A), and Martin Marietta <br />drilled 16 borings across the parcel in December 2019 (Attachment B) with similar findings. This information was used to <br />determine the subsurface conditions across the Windsor East Mine area. Drilling logs indicate the general subsurface <br />profile consists of approximately 3 to 9 feet of silty to clayey sand overburden, overlying approximately 6 to 17 feet of <br />well-graded gravel with varying amounts of sand and silt, overlying claystone bedrock. Groundwater was encountered at <br />depths ranging from 4 to 7 feet in the most recent 2019 Martin Marietta borings. <br />3.0 SLOPE STABILITY ANALYSES <br />This slope stability analysis was performed using the computer program Slide2 (2021), developed by Rocscience, Inc., to <br />determine the factors of safety of critical slip surfaces. The software is capable of computing factors of safety for a range <br />of materials using finite element analysis and limit equilibrium principles. Spencer’s method of slices (Spencer, 1967) <br />was used to calculate the safety factor for the individual failure surfaces. The Slide2 program incorporates a search <br />routine to locate those failure surfaces with the lowest factor of safety within user-defined search limits. Trial failure <br />surfaces were defined with “entry and exit” parameters, resulting in a range of possible locations to search for the critical <br />potential failure surface (lowest factor of safety). <br />Several assumptions were made for the models including: <br />Surficial geology was modeled based on the 2017 Deere & Ault report and 2019 Martin Marietta borings. <br />Geologic layers are generally horizontal in the subsurface. <br />Groundwater elevations from 2019 are representative of field conditions. <br />The highwall will be concurrently reclaimed.