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Cell 2A <br />As specified in Exhibits E and F, Cell 2A will construct a compacted clay <br />embankment around the perimeter of the reservoir. For the purpose of mitigating <br />potential groundwater mounding up-gradient of the reservoir. The Cell 2A <br />underdrain was designed as follows: <br />Since the only historical onsite water table data in the vicinity of Cell 2A includes <br />elevation data form piezometers P4 and P5, regional water table elevation <br />contours published by the USGS Front Range Infrastructure Resources Project <br />(FRIRP) Fact Sheet 113-98, were used to determine baseline upgradient and <br />westerly water table information. This was done by developing a 3 dimensional <br />surface of the USGS data, then lowering it by a uniform value to correspond with <br />the Goose Haven site datum. The 3d surface water table surface from onsite <br />data was then merged with the USGS data to yield an approximate overall water <br />table contour map which is shown in Exhibit G Sheet 1. An analytical solution to <br />groundwater flow based on Darcy’s law was then used to determine the <br />volumetric flow requirement of the drain. Given a conservative assumption that <br />that groundwater mounding could saturate the full vertical extent of the soil profile <br />above the historical water table, the calculation is as follows; <br />Darcy’s Law ; Q=(K*dh/dl)*A <br />where; <br />Q= volumetric flow <br />K= saturated hydraulic conductivity = 28 ft/day (from NRCS web soil survey) <br />dh/dl = hydraulic gradient = .02 (from Exhibit G – Sheet-1) <br />A= Cross sectional area = 2300 ft x 4ft= 9,200 ft2 <br />Therefore Q= 28 ft/day x .02 x 9,200 ft2 = 5,152 ft3 /day = 0.06 c.f.s. <br />The underdrain alignment extends for 30+95.34 ft of which 2,300 ft are <br />perforated pipe and <br />Exhibit G, Attachment A gives the modeled flow capacity for 2300 ft of perforated <br />section and 795.34 ft of solid pipe. Both sections are calculated to exceed the <br />design flow of 0.06 c.f.s. <br />The end of the underdrain alignment ties into an existing 8” pipe which daylights <br />into the lower Boulder Ditch. This existing pipeline to Lower Boulder Ditch was <br />historically used to drain the agricultural fields occupied now by Cell 2A. <br />Underdrain Construction <br />The underdrain was constructed by excavating a trench to the elevations shown <br />in Exhibit G Cell 2A Underdrain Plan & Profile Sheets 2-7. For the first 2,300 <br />ft, 6” SDR-22 PVC pipe was perforated with 2 tows of 3/8” holes spaced 1 ft <br />apart. The pipe was backfilled with 6” of d50 = 1” rock surrounded with mirifi <br />geotextile fabric surrounded by 6” of sand to create a filter pack. The remaining <br />trench solid 795.34 ft section was bedded with 6” of sand. The entire trench was <br />then backfilled with native silty sand (ML).