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ground intertace. Sizing of the underdrain should consider both of these water <br /> sources. No detailed guidelines for determining the size are provided by the state. <br /> The following factors were used in setting the underdrain size. <br /> Groundwater: <br /> Two observation wells were angered to determine if the groundwater table <br /> was near the surtace in the area of the proposed waste pile. The first well was <br /> located just upvalley of the top of the 2.5:1 slope on the face of the pile. It was <br /> centered in the.valley. The boring log showed that the top 22 feet of natural <br /> material consists of silty and slightly sandy clay with some gravel. Water was <br /> encountered at a depth of 20 ft. and bedrock at 22 ft. The second boring was <br /> located approximately 600 ft, upvalley of the first. The tpp 22 ft consisted of the <br /> same material as in the first bore. From 22 ft. to 36 ft. the material is siltstone and <br /> sandstone. No water was encountered. The total depth was 36 feet. <br /> In addition to the augenng, a detailed search of both sides of the drainage <br /> from bottom to top was completed for signs of seeps, springs or other indications <br /> ofgroundwater at the surtace. None was found. This supports historic <br /> observations from routine inspections made over the.last several years. Based on <br /> these results it is concluded that groundwater does not significantly contribute to <br /> the water within the waste bank. <br /> Underdrain Size: <br /> Precipitation falling directly on the pile and snowmelt era the primary sources <br /> of water reaching the pile. A portion of this water will infiltrate the surtace and <br /> make its way either to.1) the natural ground-waste pile interface where it will <br /> continue to move down to the groundwater table or 2) it wilt�mbve to the <br /> underdrain located at the bottom and center of the waste pile. The underdrain <br /> should be sized to adequately transport this latter portion out of the waste pile. <br /> The size of the underdrain was estimated based on a rough approximation of <br /> the hydraulic routing of water through the pile. The next table shows the <br /> geometric, hydraulic and hydrologic parameters used in the analysis. <br /> The first component is the design precipitation. NOAA climatologic data for <br /> Durango were reviewed and the maximum monthly total for each month was <br /> selected from the 97 years of record. These were averaged over the 12 months <br /> to get a maximum average monthly value of 6.35 inches. This depth was spread <br /> out over 15 days instead of 31 to reflect the discontinuous nature of local rainfall. <br /> The resulting rate was used as the precipitation input for all drain sizing <br /> estimates. <br /> The waste pile was divided into five equal zones along its length. Routing of <br /> the infiltrated precipitation through each zone was approximated based on the <br /> geometry of that zone. Once the flow moves vertically through the zone it enters <br /> the drain. Note that the hydraulic conductivity of the gravel used to fill the drain is <br /> January 1998 <br />