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comparison to the proper Moisture / Density curve. In the Division's cursory review of past <br />compaction testing results reported for SCC, it is not readily apparent that these procedures <br />have been fully implemented. The purpose of this comment was to ensure that compaction <br />testing will be conducted in full accordance with standard materials engineering practice, <br />from the laboratory to the field to the certified report following completion of the project. <br />3. In response to the Division's comment regarding level of compaction, SCC refers to the permit <br />[Tab 13, page 331, which requires a minimum of 90 percent of Standard Proctor density. <br />While this may be the approved permit condition here, the Division strongly recommends that <br />a minimum of 95% MDD be achieved, in accordance with standard engineering practice for <br />fine-grained, cohesive soils. The Division is aware of other mines that require 95% MDD. <br />Compaction in cohesive soils is best achieved by the use of a sheepsfoot roller. Please revise <br />the language of Attachment 20-D to include a statement that a sheepsfoot roller will be used, <br />if necessary, to achieve the required level of compaction. <br />In response to the Division's comment regarding moisture content, SCC replies that NWCC <br />believes that acceptable compaction can be obtained at OMC +/- 4%. Again, the Division <br />strongly recommends that a narrower range of moisture contents be targeted. This will be <br />especially true if the 95% MDD compaction is to be achieved. A range of OMC +/- 2% would <br />be preferred, and is standard engineering practice. When fine-grained, cohesive soils are <br />placed in a drier than optimum condition, with reduced levels of compaction, moisture tends <br />to be drawn in from adjacent areas. This moisture can contribute to volume changes, piping, <br />and potential instability. <br />With respect to the lift thicknesses, neither the Rules (4.03.2(3)(f)) nor the PAP (Tab 13, page <br />33) allow for lift thicknesses to be increased beyond 12 inches, unless sufficient large size rock <br />is present. Please revise the proposed lift thickness to comply with the rules and the language <br />of the PAP. <br />4. Response accepted. <br />5. Response accepted. <br />6. Response accepted. <br />7. Response accepted. <br />8. SCC responds that the depth to the subsurface water level is greater than 20 feet at that <br />iucarion, and a cutoff channel is not feasible. This question has been adequateiy answered. <br />9. Response accepted: <br />Page 2 of 4