2011-09-09_APPLICATION CORRESPONDENCE - C2010089 (2)

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C- 2010 -089 PAR No. 2 New Horizon North Mine 9- Sep -2011 Page 14 of 25 4. As required under Rule 2.05.3(4)(a)(i)(B) and Rule 2.10(2), please provide certified cross sections of each sediment pond, oriented both lengthwise and widthwise and which include each primary discharge structure. DRMS: In the submittal dated May 26, 2011, a revised Arcadis report with the requested cross sections was provided in the renamed Appendix 2.05.3(4) -1. However, the Division has additional questions based on this recent submittal of the pond cross sections. a) There does not appear to be a discussion in the permit application concerning the construction methods that will be used for the three sediment ponds. Rule 2.05.3(4)(a)(ii)(B) requires construction specifications for the sediment ponds in order to ensure that the factor of safety has been met. Please revise the permit text, perhaps in Section 2.05.3(4) or Appendix 2.05.3(4) -1, to include a description of the construction techniques that will be used for building the sediment ponds, pond embankments and pond discharge structures. b) Please add a discussion in the permit application as to whether any of the proposed sediment ponds can be classified as having Class B or Class C dams, as defined in the U.S. Department of Agriculture, Soil Conservation Service Technical Release No. 60 "Earth Dams and Reservoirs ", October, 1985. If any of the sediment ponds can qualify as having a Class B or Class C dam, please revise the permit application to demonstrate compliance with those Rules which apply to Class B or Class C dams. c) As required under Rule 4.05.9(8)(a) or (b), please revise the permit application after construction of the three sediment ponds to include the appropriate geotechnical demonstrations that the applicable factor of safety required under Rule 4.05.9(8)(a) or (b) had been achieved for each sediment pond embankment. d) Appendix 2.05.3(4) -1, "Arcadis Report on Sediment Control ", contains cross sections for the three sediment ponds. The cross sections show that ponds NHN -001 and NHN - 003 will have 10 foot wide embankment tops. However, the cross section for pond NHN -002 shows that it will have a 5 foot wide embankment top. The Division is concerned, especially since a public road is adjacent to the length of the pond, that such a narrow embankment may not be stable enough to achieve the applicable factor of safety. The Division requests that the permit application designs be revised to show that the embankment top width for pond NHN -002 will be at least 10 feet. e) The proposed sediment pond embankment heights, measured from the pond bottom to the emergency spillway bottom, are listed in Table 3 and shown on the cross sections, in Appendix 2.05.3(4) -1. The proposed embankment heights for ponds NHN -001 and NHN -003 are less than 10 feet but for pond NHN -002, the proposed embankment height is 11.5 feet. Rule 4.05.9(6) requires a foundation investigation for those pond embankments that exceed 10 feet in height. Please provide the foundation investigation required under Rule 4.05.9(6) or lower the height of the pond embankment for NHN - 002 to 10 feet or less. f) The certified pond cross sections in the Arcadis report in Appendix 2.05.3(4) -1 show that the pond embankment inslopes and outslopes for sediment ponds NHN -002 and NHN -003 are both 2H:1 V. However, Rule 4.05.9(7)(e) requires that the combination of the inslope and outslope of a pond embankment not be steeper than 5H:1 V unless a demonstration is provided to show that the steeper slopes will be stable. Please provide