Page 176 - Water and wastewater engineering
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WELLS 4-31

                               Slot number
                       10    20  3040 50 60 80 100
            100

            90

            80

            70
           Cumulative percent retained  60


            50

            40

            30

            20
            10

             0
              0.1   0.2  0.3 0.4 0.5  0.7 1.0  2.0  3.0 4.0 5.0  7.0  10
                                Grain size, mm
            FIGURE 4-12
           Grain size analysis for selection of screen slot size.


            Screen Slot Size
           The size of the screen openings, commonly called the  slot size,  is selected on the basis of a sieve
          analysis of the aquifer material. A plot of the results of a sieve analysis is shown in  Figure 4-12 .
           Table 4-6  is an example of slot size options that are obtained from screen manufacturers. For
          relatively fine and uniform materials ( uniformity coefficient   *     3), the slot size may be taken as
          the size of the sieve opening that will retain 40 percent of the material ( D   40%  ) if the groundwater
          is noncorrosive and  D   50%   if the groundwater is corrosive. If the aquifer is coarse sand and gravel,
          the slot size may be  D   30%   to  D   50%   of the sand fraction. For nonuniform materials (uniformity
          coefficient   6), slot sizes should be about equal to  D   30%   if the overlying material is stable. If it
          is unstable, the slot size should be  D   60%  .
                  The grain size of the gravel pack is selected to retain the grains of native material while allow-
          ing the maximum amount of water into the pump. A typical approach is to select the 70  percent
          retained size of the unconsolidated aquifer material and then multiply that grain size by 4 to 6 in
          specifying the grain size of the gravel. The screen opening is then sized to retain 100 percent of
          the gravel.





              * The uniformity coefficient is defined as the quotient of the 40 percent size ( D   40%  ) of the sand divided by the 90 percent size
          ( D   90%  ). The  D   40%   is the size of the sieve opening that retains 40 percent of the sand upon sieving. U.S. Standard sieve sizes are
          given in Appendix B.
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