2019-11-27_PERMIT FILE - M2019058 (19)

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CTL I THOMPSON November 20, 2019 Schedio Group, LLC 808 9th Street Greeley, Colorado 80631 Attention: Tim McCarthy Ben Sommers Subject: Geotechnical Evaluation Proposed Keenesburg No. 2 Mine Keenesburg, Colorado CTLIT Project Number: FC08689-125-L3 CTLIThompson, Inc. was requested to provide setback and cut slope recommendations for various existing features for the proposed Guttersen Ranch industrial sand mine project located north of Keenesburg, Colorado. Based on our understanding of the project, existing features on the site include oil and gas well heads, water wells, and oil, gas and water pipelines. The depth and size of the well heads, water wells and pipelines has not been provided. We understand current plans consist of providing a 50-foot buffer from all pipelines and that the anticipated maximum excavation depth is 20 to 30 feet. CTL-Thompson investigated subsurface conditions at the site by drilling 24 borings in December 2018 and January 2019. Results of the field and labora- tory investigation were presented in two letters dated February 27, 2019 and March 27, 2019. Soils encountered in our borings generally consisted of a thin topsoil veneer overlying loose to medium dense, interlayered clean to slightly clayey sand and clayey sand with occasional sandy clay. Bedrock was encoun- tered in only two borings at depths of 24 and 35 feet. Water was measured in 11 borings at erratic locations and depths across the site. For the purposes of our analysis, we anticipate the subject pipelines are not deeper than 10 feet. If deeper pipelines are known to exist on site, we should be notified to reevaluate our recommendations. The 50-foot buffer discussed above is more than enough to prevent disturbance to pipelines or well heads. We understand that post-mining slopes with a 3HAV (horizontal:vertical) orientation are planned. For excavation depths on the order of 30 feet deep and using an infinite slope condition, we calculate 3:1 slope orientation will have factor of safety of at least 1.7 using conservative strength parameters. Typically, a mini- mum factor of safety of 1.5 is considered acceptable for long-term slopes. If seepage into excavations is encountered, then shallower slope inclinations may be necessary. This will need to be determined on a case-by-case basis. Stock- 400 North Link Lane I Fort Collins,Colorado 80524 Telephone:970-206-9455 Fax:970-206-9441