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_ Little water migration is expected through the refuse pile as indicated by the HELP model results. <br />Therefore, a geotextile will not be needed to prevent migration of refuse particles into the drainage <br />layer. An additional consideration is that the design of the proposed CPP is anticipated to process a <br />refuse material that will be approximately 93 percent particles between approximately 8 millimeters <br />up to approximately 200 millimeters (0.3 inch to 8 inches) in diameter, and approximately 7 percent <br />less than 1 millimeter (0.04 inch) in diameter. A refuse material with this gradation should naturally <br />filter against the ripped sandstone drainage material. <br />4.4 Underdrain <br />The underdrain will be constructed in accordance with Rule 4.09.3. The refuse pile is situated above <br />the uppermost coal and shale seams. A box canyon is situated south of the ridge at the south end of <br />No Name Gulch, which cuts off virtually all potential for groundwater flow to the base of the refuse <br />pile. Clearwater drainage diversions will be maintained above the working bench of the refuse pile <br />to divert surface water away from the refuse pile. Therefore, the only drainage that must be <br />accommodated by the underdrain is the water that is percolating through the refuse pile. Based on <br />the HELP model, the peak daily flow collected in the drainage blanket is approximately 51.5 cubic <br />feet per day, or approximately 0.0006 cubic feet per second (cfs) (0.27 gallons per minute — gpm). <br />To provide a factor of safety, the discharge was rounded up to 0.01 cfs (4.5 gpm) for underdrain size <br />calculations. <br />An underdrain of ripped sandstone with a cross sectional area of 11 square feet and a hydraulic <br />conductivity of 1 cm/sec will accommodate drainage needs within the refuse pile footprint (based on <br />the HELP model results and site hydrology combined with clearwater diversions). Additional cross <br />sectional area (28 square feet) is provided by the design (see permit drawings) to provide a factor of <br />safety of over 2. The horizontal alignment of the underdrain beneath the refuse pile will follow the <br />thalweg of the gulch and may also be adjusted based on the type of material encountered in the area <br />of the underdrain and the ability to excavate the drain without hardship. <br />4.5 Compaction <br />Refuse will be compacted to 90 percent (or greater) of the maximum standard Proctor dry density and <br />lifts will be no greater than 24 inches in thickness, as required by Rule 4.10.4 (3). <br />Revised March 19, 2010 <br />P:\Mpls \06 CO\26 \06261003 MCC Refuse Pile Site Review & Permit\WorkFilesTermit Application \Text \2010- 04- 19_RPE East Permit Application.doc15 <br />