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ground interface. 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 augered to determine if the groundwater table <br />was near the surface 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 augering, 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 />of.groundwater at the surface. 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 are the primary sources <br />of water reaching the pile. A portion of this water will infiltrate the surface 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 move 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 6.38 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 />