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Little water percolation or migration is expected through the refuse pile. The HELP model predicts a <br />peak daily discharge of less than five cubic feet per day, or approximately 0.003 gallons per minute <br />(gpm). HELP model results are summarized in Table 4. <br />The refuse pile foundation surface is higher than groundwater found in the site investigation (see <br />boring logs, Appendix A, and slope stability model Figures 2 -1 through 2 -6), and a canyon to the <br />south cuts off groundwater flow upgradient of the refuse pile location at elevations higher than the <br />base of the refuse pile (see Figure 1 -2). As a result, any seeps that are encountered in the <br />construction of the refuse pile would be the result of local hydrologic conditions. Seepage rates and <br />quantities would be low as they would need to be fed from local infiltration of rain or snow. Based <br />on these observations, a design flow rate of 10 gpm has been selected for the design of the <br />underdrain. <br />Underdrain slope, materials used to construct the underdrain, and cross - section area must be <br />considered in designing an underdrain that is appropriately sized for the design flow rate. The <br />average slope of the native thalweg from the rock buttress to the upper end of the refuse pile is about <br />29 %. Drainage rock for underdrain construction is specified to consist of stones ranging in size from <br />6 inches to 30 inches nominal diameter. A transmissivity value of 0.1 cm /sec has been assumed for <br />the drainage rock. Based on these values, an underdrain with nominal cross - sectional dimensions of <br />14 feet wide by 4 feet deep provides a capacity of approximately 24 gpm, resulting in a factor of <br />safety of 2.4 when compared to the assumed flow rate of 10 gpm that may drain to the underdrain. <br />Filter rock will be placed over the drainage rock, which will be covered with a geotextile to prevent <br />intermixing of refuse and underdrain materials. <br />The horizontal alignment of the underdrain beneath the refuse pile will follow the thalweg of the <br />gulch and may be adjusted based on the type of material encountered in the area of the underdrain <br />and the ability to excavate the drain without hardship. <br />Drawing C -07 shows a lateral drain extension that will be constructed to drain any seeps that may be <br />uncovered in the foundation of the refuse pile as it is developed. Lateral drains will be constructed <br />from seep locations approximately perpendicular to the refuse pile foundation surface contours to the <br />Revised July 21, 2011 <br />2i��� <br />P.\Mpls \06 CO \26 \06261003 RPE E Permit Mod Support \WorkFiles\Permit Application \Text \Exhibit 82 RPEE design 2011 -07 -21 TR 127 doc 19 <br />