Laserfiche WebLink
Slope Stability — Union Mill Tailings Pond <br />Page 2 <br />August 12, 2011 <br />Colorado State Engineers, Ground Water Section. The second bullet on the following page <br />states "The pond excavation at a maximum 21 ft depth does not impact the known aquifers at <br />80 -100 feet below the surface." Based on the water level encountered in TH -2, the <br />separation between bottom of the pond and groundwater is on the order of four feet. Please <br />address this discrepancy, the gradient of the water table encountered in TH -2, and how the <br />applicant can ensure groundwater separation. <br />4. Page 6, Direct Shear Testing: The second paragraph indicates cyclone tailings will be placed <br />adjacent to the embankment. This statement contradicts the statement on page 1 about the <br />clay liner (see comment No. 1 above). The TSF cross - sections on Drawing TSF 04 do not <br />differentiate between slimes and cyclone sands as does Figures B -3 and B -6 in the slope <br />stability analyses. Please provide DRMS with consistent narrative, drawings and figures, <br />including a detail section of the interior face of the tailings pond showing expected thickness <br />and order of the liner and cyclone sand layers proposed on the embankment. This will also be <br />helpful in understanding and evaluating the proposed leak detection system. <br />5. Page 8, Table B: Based on Tables 2.8 and 2.9 in the referenced text by Vick, cycloned <br />tailings could be expected to have a slightly lower friction angle than tailings slimes. Please <br />comment on using the same friction angle for both fine and coarse fractions of tailings. <br />6. Appendix B: Slope stability analyses: <br />a. Figure B -2: Comparing B -2 with B -3, B -2 does not include a layer of cycloned <br />sand that is included in B -3. Please explain why the sand layer was left out of B -2. <br />b. Figure B -5: Comparing B -5 with B -6, B -5 does not include a layer of cycloned <br />sand that is included in B -6. Please explain why the sand layer was left out of B -5. <br />c. Figure B -5: The cohesion used for the tailings in B -5 is 0 psf. However, the <br />cohesion for tailings in B -2 is 50 psf. Please provide an explanation as to why two <br />different cohesion values were used for presumably the same material. <br />m: \min \tcl \m- 1990 -057 union milling \stabilitycomments_mem 12augl 1.docx <br />