2019-03-22_REVISION - C1982056 (5)

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March 21, 2019 Ms. Tabetha Lynch, Environmental Protection Specialist Division of Reclamation, Mining, and Safety 1313 Sherman Street, Room 215 Denver, Colorado 80203 (303) 866-3567 X 8147 TwenVmile_Coal,_LLC Foidel Creels Mine 29515 RCR 27 Oak Creek, CO 80467 970.879.3800 RE: Twentymile Coal, LLC - Foidel Creek Mine (Permit No. C-82-056), Technical Revision (TR19-93) Wash Plant Additional Thickener Tank System Design Modification, Adequacy Response Dear Ms. Lynch: Below you will find responses to the adequacy review items that were noted in the adequacy review dated March 12, 2019 • Item #1: Please provide an updated cost estimate for the revised plan a. An updated cost estimate was submitted in the original plan and has also been attached to this adequacy response. Item #2: Please review Zach's letter and provide additional requested information a. Twentymile has reviewed the letter provided by Zach Trujillo in regards to the thickener tank foundation and base fill material drainage and has the following responses: i. Only one lateral perforated drain pipe installed... 1. With the approval of NWCC, Twentymile installed only one perforated drain due to the specification of the fill material being free draining. By using free draining fill the water will be able to drain from under the tank area freely. Ideally the water will drain to the path of least resistance which will be towards the limestone drain that Twentymile placed on the northwest side of the tank foundation. Twentymile will then place a 6 inch perforated pipe that will daylight into Pond D allowing the water to drain freely and not cause any hydrostatic pressure or uplifting. ii. No perimeter drainage system... 1. As indicated on the site plan submitted for TRI 9-93 Twentymile plans to place a french drain on south side of the pump building to direct any surface water from the south into pond D. A ditch will also be placed on the north side of the tank to direct any water from north of the thickener tank and the west side of the wash plant to drain into the constructed sump. In addition any run off from the east side of the thickener tank and the south side of the wash plant will be captured by the drainage culverts and discharged into the pre -sump before draining into pond D. iii. Provide additional information that ensures proposed design to be sufficient.. 1. Twentymile has attached a letter from NWCC stating that the current design of drainage is sufficient and that precautions will be followed during construction. iv. Provide documentation of the underdrain system aside from drawings... 1. At the end of this response, Twentymile has provided a picture that documents the limestone drain that was placed on the northwest side of thickener foundation. Approximately 100 tons of limestone was placed to allow for adequate drainage under the thickener tank.