2024-10-08_REVISION - M2000144 - Laserfiche WebLink

Home Browse Search

CIVIL RES'° URCES, LLC ENGINEERS & PLANNERS MEMORANDUM TO: BURNCO Colorado,LLC FROM: Civil Resources, LLC DATE: March 8,2024 RE: DRMS Permit M-2000-144,North La Poudre Mine, Proposed Clay Liner This memorandum discusses the proposed clay liner to be constructed at the La Poudre North Gravel Mine. The mine is located on the southeast corner of Colorado Boulevard and State Highway 392 in Weld County, Colorado. More specifically,the mine is within of the north half of Section 19, Township 6 North, Range 67 West. Refer to the figure submitted with the Technical Revision. Background The cell to be clay lined is on the eastern part of the mine. This cell was previously mined and is currently an unlined gravel pond. The mine slopes are unmined gravel pit run graded to a slope of approximately 3 horizontal to 1 vertical(3h:1 v). Borings in the area and the as-constructed drawing of the La Poudre slurry wall excavated immediately south of the site, indicate the top of bedrock underlying the gravel lies at a depth of approximately 15 to 21.5 feet below existing grade at the mine. Review of regional geologic mapping(Colton, 1978)indicates the bedrock is blanketed with Quatemary Alluvium. Bedrock outcrops to the south of the site indicate the near surface bedrock is most likely the Upper Transition Member of the Pierre Shale. Proposed Clay Liner Construction of the clay liner will require dewatering of the site as was previously performed during site mining and mining of the remaining sand and gravel to a mine highwall of approximately 0.5h:1v. At this point,the exposed bedrock will be stripped of weathered bedrock and an 8 to10-foot wide keyway will be excavated near the base of the highwall to a depth of approximately 5-feet below the top of bedrock(after stripping of the weathered bedrock). Clayey soils,either from the site or imported to the site will be placed in the day liner. The clayey soils shall meet the following specifications: 1. At least 35-percent finer than the 200 sieve, 2. A plasticity index greater than 12 but not exceeding 25. Clay liners with these types of soils can be constructed to finished slopes of 3h:1v to heights no greater than approximately 30- feet. If plasticity indices exceed 25,the slopes will have to be flatter than 3h:1 v. Bedrock should not be used in the clay liner. Soils meeting the above specifications will be moisture conditioned to within 2-percent of optimum moisture content, placed in loose lifts no greater that 9-inches,and compacted a minimum of 95-percent of Standard Proctor Maximum Dry Density with a CAT 815 sheepsfoot(or equivalent)compactor. The final slope geometry will include a 10-foot crest extending from the top of the mine highwall with a 1-percent slope towards the highwall. From the 10-foot mark the clay liner crest will transition to a 3h:1 v slope until meeting the top of the stripped bedrock surface. The final slope will be vertically track rolled with a Cat D-8(or equivalent) bulldozer. In our experience, such a clay liner can be constructed at the site and will meet State leak test requirements. Reference Colton,R.B.,"Geologic Map of the Boulder-Ft.Collins-Greeley Area, Colorado", U.S.G.S.Map I-855-G. J:18URNCO-2131Winsdor12024 TR\Lapoudre north clay liner TR.docx 8308 COLORADO BLVD, SUITE 200 • FIRESTONE, COLORADO 80504 • PHONE: 303.833.1416