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• Section No 6 <br /> Revision No. i <br /> Date January 13, 1992 <br /> Page 39 of 55 <br /> 6 7 3 Filter Pack Design <br /> A filter pack consisting of clean imported sand or gravel will be <br /> installed in the annular space adjacent to the perforated interval in each <br /> well The filter pack design will be completed in accordance with U S. <br /> Bureau of Reclamation Groundwater Manual. Sieve analyses will be performed on <br /> representative samples of the water-bearing unit to be monitored Sieve <br /> analyses curves will be plotted for all samples tested The analyses of the <br /> stratum composed of the finest materials will be used as the basis for <br /> determining the uniformity coefficient and thus the gravel pack design The <br /> J <br /> uniformity coefficient, an indicator of the range of particle sizes in the <br /> water-bearing material, is calculated by dividing the D40 size (dimension of <br /> the mesh opening of the sieve which retains 40 percent of the sample) by the <br /> D90 size <br /> D40 uniformity coefficient (UC) <br /> D90 <br /> The approximate graduation of the gravel pack will be determined by <br /> multiplying the D70 size of the water-bearing material by a factor between 4 <br /> and 9, and then constructing a curve through this new point that is <br /> essentially parallel to the curve for the aquifer material The following <br /> guidelines will be considered <br /> ■ For fine uniform water-bearing zones, UC <2 0, use a multiplier of <br /> or near 4 maintaining a UC <2 0 for the gravel pack, <br /> ■ <br /> For coarse non-uniform water-bearing zones, UC >2 5, use a <br /> multiplier of or near six, maintaining a UC of <2 5 for the gravel <br /> pack, <br /> 40 <br /> 6-39 <br />