*TM 5-813-1/AFM-88-10, Vol. 1
Table 5-4. Change in Yield for Variation in Well
The minimum wall thickness for steel pipe used for
Diameter.
casing is V/4-inch. For various diameters, EPA
recommends the following wall thicknesses:
Original
New Well Diameter
Well
Nominal Diameter (inch)
Wall Thickness (inch)
Diameter
6"
12"
18"
24"
30"
36"
48"
6
.250
6"
100% 110% 117% 122% 127% 131% 137%
8
.250
12"
90 100 106 111 116 119 125
10
.279
18"
84
93 100 104 108 112 117
12
.330
24"
79
88
95 100 104 107 112
14
.375
30"
76
85
91
96 100 103 108
16
.375
36"
73
82
88
92
96 100 105
18
.375
48"
69
77
82
87
91
94 100
20
.375
Note: The above gives the theoretical increase or
In the percussion method of drilling, and where sloughing
decrease in yield that results from changing
is a problem, it is customary to drill and drive the casing
the original well diameter to the new well
to the lower extremity of the aquifer to be screened and
diameter. For example, if a 12-inch well is
then install the appropriate size screen inside the casing
enlarged to a 36-inch well, the yield will be
before pulling the casing back and exposing the screen
increased by 19 percent. The values in the
to the water bearing formation.
above table are valid only for wells in
d.
Screens. Wells completed in sand and
unconfined aquifers (water table wells) and
gravel with open-end casings, not equipped with a
are based on the following equation:
screen on the bottom, usually have limited capacity due
(Y2/Y1) = (log R/r1)/(log R/r2)
to the small intake area (open end of casing pipe) and
where:
tend to pump large amounts of sand. A well designed
= yield of new well
Y2
screen permits utilizing the permeability of the water
= yield of original well
Y1
bearing materials around the screen.
For a well
R
= radius of cone of depression,
completed in a sand-gravel formation, use of a well
in feet (the value of R used for
screen will usually provide much more water than if the
this table is 400 feet).
installation is left open-ended. The screen functions to
= diameter of new well, in feet
r2
restrain sand and gravel from entering the well, which
= diameter of original well, in feet
r1
would diminish yield, damage pumping equipment, and
b.
Depth. Depth of a well is usually determined
deteriorate the quality of the water produced. Wells
from the logs of test holes or from logs of other nearby
developed in hard rock areas do not need screens if the
wells that utilize the same aquifer. The deeper the well is
wall is sufficiently stable and sand pumping is not a
driven into a water bearing stratum, the greater the
problem.
discharge for a given drawdown. Where the water
(1)
Aperture size.
The well screen
bearing formations are thick, there is a tendency to limit
aperture opening, called slot size, is selected based on
the depth of wells due to the cost. This cost, however,
sieve analysis data of the aquifer material for a naturally
usually is balanced by the savings in operations resulting
developed well. For a homogeneous formation, the slot
from the decreased drawdown. Construction should seal
size is selected as one that will retain 40 to 50 percent of
off water bearing formations that are or may be polluted
the sand. Use 40 percent where the water is not
or of poor mineral quality. A sealed, grouted casing will
particularly corrosive and a reliable sample is obtained.
extend to a depth of 20 feet or more from the ground
Use 50 percent where water is very corrosive and/or the
surface. Check local regulations to determine minimum
sample may be questionable. Where a formation to be
requirements. Where the depth of water of poor quality
screened has layers of differing grain sizes and
is known, terminate the well above the zone of poor
graduations, multiple screen slot sizes may be used.
quality water.
Where fine sand overlies a coarser material, extend the
c.
Casing. In a well developed in a sand and
fine slot size at least 3 feet into the coarser material.
gravel formation, the casing should extend to a minimum
This reduces the possibility that slumping of the lower
of 5 feet below the lowest estimated pumping level. In
material will allow finer sand to enter the coarse screen.
consolidated formations, the casing should be driven 5
feet into bedrock and cemented in place for its full depth.
5-12
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