2011-11-28_REVISION - M1986015 - Laserfiche WebLink

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Mr. Laroche and Mr. Tuttle Page 4 November 28, 2011 6.5 Geotechnical Stability Exhibit 9. Pursuant to Rule 6.5(3), the Applicant shall demonstrate that off -site areas will be protected with appropriate factors of safety (FOS) incorporated into the analysis. The minimum acceptable safety factors will be subject to approval by the Division, on a case by -case basis. The assumed minimum FOS of 1.01 is unacceptable. Standard engineering practice /standard of care uses a FOS between 1.25 and 2.0 depending on the certainty of the strength data and the consequences of failure. Furthermore, the analysis does not provide sufficient discussion of the zone of influence or gradient of the phreatic surface necessary to evaluate the groundwater level assumed in the SlopeW model. a. The Division will accept a FOS = 1.25 for fences, but will require a FOS = 1.5 for buildings. Please provide the appropriate revised slope stability analyses. b. The surcharge load resulting from the topsoil stockpiles on the edge of the pit should be included in the stability analyses. c. Discuss potential offsite consequences of a diversion channel failure with respect to: i. Stormwater retention onsite and the SEO's 72 -hour release requirement, ii. Water quality at the discharge (i.e., if the channel were to fail and flood the pit, would the settling basin be adequate to control suspended solids ?). If either items i or ii above pose an offsite consequence, the Applicant shall provide a stability analysis demonstrating an FOS = 1.25 against channel failure (i.e. pit -side bank sloughing into the pit) with the channel at the 100 -year design capacity. d. Provide phreatic surface discussion for that surface used in the analyses. SECTION V, EXHIBIT E — AMENDMENT 4 RECLAMATION PLAN FOR PHASE 1 10. Page 2, Materials Handling — General slope from northwest to southeast of 4 to 5 feet (across approximately 2600 horizontal feet) results in a final slope of 0.15 to 0.2 percent. This is an order of magnitude less than the one percent slope proposed for Phase 7 reclamation. a. Please provide some discussion comparing the pre -mine slopes with the proposed post -mine slopes and why the difference from the one percent slope proposed for Phase 7 reclamation. b. Also address the slope with respect to promoting drainage to prevent the basal rot of the smaller cottonwood trees. 11. Page 3, Topsoiling and Growth Media — If it is unsafe to get heavy equipment on the Phase 1 sediment ponds, how is the topsoil to be distributed? 12. ExhibitF-1A — Please show: a. Proposed final reclamation contours, b. Proposed final reclamation channels (between sediment basins).