CEMP-E
TI 814-01
3 August 1998
utilizing the permeability of the water bearing materials around the screen. For a well
completed in a sand-gravel formation, use of a well screen will usually provide much more
water than if the installation is left open-ended. The screen functions to restrain sand and
gravel from entering the well, which would diminish yield, damage pumping equipment, and
deteriorate the quality of the water produced. Wells developed in hard rock areas do not need
screens if the wall is sufficiently stable and sand pumping is not a problem.
(1) Aperture size. The well screen aperture opening, called slot size, is selected
based on sieve analysis data of the aquifer material for a naturally developed well. For a
homogeneous formation, the slot size is selected as one that will retain 40 to 50 percent of the
sand. Use 40 percent where the water is not particularly corrosive and a reliable sample is
obtained. Use 50 percent where water is very corrosive and/or the sample may be
questionable. Where a formation to be screened has layers of differing grain sizes and
graduations, multiple screen slot sizes may be used. Where fine sand overlies a coarser
material, extend the fine slot size at least 3 feet into the coarser material. This reduces the
possibility that slumping of the lower material will allow finer sand to enter the coarse screen.
The coarse aperture size should not be greater than twice the fine size. For a filter packed
well, the screen should retain 85 to 100 percent of the filter material. Screen aperture size
should be determined by a laboratory experienced in this work, based on a sieve analysis of
the material to be screened. Consult manufacturer's literature for current data on screens.
(2) Length. Screen length depends on aquifer characteristics, aquifer thickness, and
available drawdown. For a homogeneous, confined, artesian aquifer, 70 to 80 percent of the
aquifer should be screened and the maximum drawdown should not exceed the distance from
the static water level to the top of the aquifer. For a nonhomogeneous, artesian aquifer, it is
usually best to screen the most permeable strata. Determinations of permeability are
conducted in the laboratory on representative samples of the various strata. Homogeneous,
unconfined (water-table) aquifers are commonly equipped with screen covering the lower
one-third to one-half of the aquifer. A water-table well is usually operated so that the pumping
water level is slightly above the top of the screen. For a screen length of one-third the aquifer
depth, the permissible drawdown will be nearly two-thirds of the maximum possible drawdown.
This draw-down corresponds to nearly 90 percent of the maximum yield. Screens for
nonhomogeneous watertable aquifers are positioned in the lower portions of the most
permeable strata in order to permit maximum available drawdown. The following equation is
used to determine screen length:
Q
L
= __________
AV (7.48)
(eq. 5-5)
where:
L
=
length of screen (feet)
Q
=
discharge (gpm)
A
=
effective open area per foot of screen length (sq. ft. per ft.) (approximately
one-half of the actual open area which can be obtained from screen
manufacturers.)
V
=
velocity (fpm) above which a sand particle is transported; is related to
permeability as follows:
5-15
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