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Colorado Division of Reclamation, Mining and Safety <br />To: Tabetha Lynch <br />From: Zach Trujillo <br />CC: Dan Hernandez <br />Date: April 4, 2019 <br />Re: Foidel Creek Thickener Tank <br /> <br />As requested, I have reviewed Foidel Creek Mine’s (Foidel) proposed material associated <br />with the new thickener tank construction that were provided in response to the Division’s <br />second adequacy letter. <br /> <br />Foidel has provided documentation of the underdrain construction. The underdrain <br />system was observed and documented in NWCC’s field report, Subsurface and Surface <br />Drainage, dated March 20, 2019. Approximately 100 tons of limestone was placed <br />within the tank’s excavated foundation at a depth of 2 to 3 feet in depth. The underdrain <br />material placement and construction has been approved by NWCC to sufficiently collect <br />any subsurface drainage that enters the thickener tank foundation. However, at the time <br />of NWCC’s field report, the perforated drain pipe had yet to be installed and is strongly <br />recommended by NWCC. As part of Foidel’s response, they state the perforated drain <br />pipe will be installed. To ensure full completion of the underdrain system, the placement <br />of the drain pipe should be documented through a field report provided by Foidel or a <br />Division inspection report. <br /> <br />As recommended by NWCC report, Subsoil and Foundation Investigation, Wash Plant – <br />Thickener Tank (the Report), a perimeter drain should be located around the entire <br />perimeter of the tank to help mitigate groundwater intrusion into the tank foundation. As <br />part of Foidel’s wash plant and support facility design, a french drain is to be installed <br />south of the pump house. However, the drain only extends partially along the length of <br />the proposed thickener tank foundation. It is my understanding that the groundwater in <br />the area generally moves in a northerly direction and as proposed, the french drain will <br />only partially intercept groundwater moving in that direction. Given the Division’s <br />concerns with groundwater and the potential of associated hydrostatic uplift, as proposed, <br />the french drain may not satisfy these concerns with Foidel’s selection in a more liberal <br />alternative foundation design as recommended in the Report. <br /> <br />Please have Foidel provide additional information that would ensure the currently <br />proposed french drain is sufficient in assisting the alleviation of groundwater in <br />combination with the underdrain. Alternatively, extending the french drain in a