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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.