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Little water percolation or migration is expected through the waste rock pile. The HELP model <br />predicts no drainage from the waste rock pile to the underdrain under average and peak daily <br />conditions. HELP model results are summarized in Table 6. <br />The waste rock pile foundation surface is higher than groundwater that was found during the site <br />investigation (see boring logs, Appendix A, and permit drawing C-06, Appendix D), and a canyon to <br />the south cuts off groundwater flow upgradient (from the south-southwest) of the waste rock pile <br />location (see Figure 1-2). As a result, any seeps that are encountered in the construction of the waste <br />rock pile would be the result of local hydrologic conditions. Seepage rates and quantities would be <br />low as they would need to be fed from local infiltration of rain or snow. Based on these observations, <br />a design flow rate of 10 gpm has been selected for the design of the underdrain. <br />Underdrain slope, materials used to construct the underdrain, and cross-section area were considered <br />in designing the underdrain to be appropriately sized for the design flow rate. The average slope of <br />the native thalweg from the rock buttress to the upper end of the waste rock pile is about 29 percent. <br />Drainage rock for underdrain construction is specified to consist of competent rock ranging in size <br />from 6 inches to 30 inches nominal diameter. A transmissivity value of 0.1 cm/sec has been assumed <br />for the drainage rock. Based on these values, an underdrain with nominal cross-sectional dimensions <br />of 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 design flow rate of 10 gpm that is assumed to drain to the <br />underdrain. The underdrain has been and will continue to be constructed as shown on permit drawing <br />C-08, Appendix D. <br />The horizontal alignment of the underdrain beneath the waste rock 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 />Permit drawing C-07, Appendix D, shows a lateral drain extension (also called a seepage interceptor <br />trench) that will be constructed to drain any seeps that may be uncovered in the foundation of the <br />waste rock pile as it is developed. Seepage interceptor trenches will be constructed from seep <br />locations approximately perpendicular to the waste rock pile foundation surface contours to the <br />thalweg underdrain. The specific dimensions of seepage interceptor trenches will be determined <br />based on estimated seepage rates. <br />21 <br />P:\Mpls\06 CO\26\06261003 MCC Refuse Pile Site Review & Permit\WorkFiles\Permit Application\2014 Revision\Exhibit 82 2014-05- <br />29.docx <br />