*TM 5-813-1/AFM-88-10, Vol. 1
(1)
Size.
Gravel size is based on
approximately 8 inches. A gravel envelope thicker than
information obtained by sieve analyses of the material in
about 8 inches will not greatly improve yield and can
the aquifer. The well screen aperture size will be
adversely affect removal of fines, at the aquifer-gravel
selected so that between 85 and 100 percent of the
interface, during well development.
gravel is larger than the screen openings. Criteria for
(3)
Pack length. Gravel pack will extend
sizing the gravel are as follows:
a minimum of 10 feet above the top of the screen. If
(a) Perform sieve analyses on all
possible, well development should be completed before
strata within the aquifer. The sieve sizes to be used in
additional material is placed above the gravel pack. That
performing these analyses are:
way, gravel can be added as the pack consolidates. If
3 in.
No. 10
this is not possible, a tremie may be placed prior to filler
2 in.
No. 20
material being added. Then additional gravel can be
11/2 in.
No. 40
added through the tremie to maintain gravel above the
1 in.
No. 60
top of the screen. A bentonite seal should be placed
3/4 in.
No. 140
directly above the gravel pack to prevent infiltration from
No. 4
filter material. A gravel-pack well has been shown
The results of the analysis of any particular sample
schematically in figure 5-6.
should be recorded as the percent (by weight) of the
(4)
Disinfection. It is important that the
sample retained on each sieve and the cumulative
gravel used for packing be clean and that it also be
percent retained on each sieve (i.e., the total of the
disinfected by immersion in strong chlorine solution (200
percentages for that sieve and all larger sieve sizes).
mg/l or greater available chlorine concentration,
Based on these sieve analyses, determine the aquifer
prepared by dissolving fresh chlorinated lime or other
stratum which is composed of the finest material.
chlorine compound in water) just prior to placement.
(b) Using the results of the sieve
Dirty gravel must be thoroughly washed with clean water
analysis for the finest aquifer material, plot the
prior to disinfection and then handled in a manner that
cumulative percent of the aquifer material retained
will maintain it in as clean a state as possible.
f.
Grouting and sealing. Grouting and sealing
versus the size of the mesh for each sieve. Fit a smooth
curve to these points. Find the size corresponding to a
of wells are necessary to protect the water supply from
70 percent cumulative retention of aquifer material. This
pollution, to seal out water of unsatisfactory chemical
size should be multiplied by a factor between 4 and 6, 4
quality, to protect the casing from exterior corrosion and
if the formation is fine and uniform and 6 if the formation
to stabilize soil, sand or rock formations which tend to
is coarse and nonuniform. Use 9 if the formation
cave. When a well is constructed there is normally
includes silt. The product is the 70 percent retained size
produced an annular space between the drill hole and
(i.e., the sieve size on which a cumulative 70 percent of
the casing, which, unless sealed by grouting, provides a
the sample would be retained) of the gravel to be used in
potential pollution channel.
the gravel pack.
(1)
Prevention of contamination from
(c) The uniformity coefficient of the
surface. The well casing and the grout seal should
gravel will be 2.5 or less, where the uniformity coefficient
extend from the surface to the depth necessary to
is defined as the ratio of the grain size for 40 percent
prevent surface contamination via channels through soil
retention to the grain size for 90 percent retention.
and rock strata. The depth required is dependent on the
(d) The plot of cumulative percent
character of the formations involved and the proximity of
retention versus grain size for the gravel should be
sources of pollution, such as sink holes and sewage
approximately parallel to same plot for the aquifer
disposal systems. The grout seal around the casing
material, should pass through the 70 percent retention
should have a thickness of at least 2 inches and a
value and should have 40 and 90 percent retention
greater thickness is recommended where severe
values such that the uniformity coefficient is less than
corrosive conditions are known to exist.
Local
2.5.
Gravel pack material will be specified by
regulations may govern the grout length and thickness.
determining the sieve sizes that cover the range of the
Materials for sealing and grouting should be durable and
curve and then defining an allowable range for the
readily placed. Normally, Portland cement grout will
percent retention on each sieve.
meet these requirements. Grout is customarily specified
(2)
Thickness.
The thickness of the
as a neat cement mixture having a water-cement ration
gravel pack will range from a minimum of 3 inches to
5-14
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