*TM 5-813-1/AFM-88-10, Vol. 1
e.
Testing objectives. A simplified example is
borehole in the aquifer. Screens and the gravel pack are
not usually required. A well in rock formation is shown in
given in appendix B. When conducting tests by methods
such as the drawdown method, it is important to note
unconsolidated formations are shown in AWWA A100.
accurately the yield and corresponding drawdown. A
a.
Diameter. The diameter of a well has a
good testing program, conducted by an experienced
significant effect on the well's construction cost. The
geologists, will account for, or help to define, the
diameter need not be uniform from top to bottom.
following aquifer characteristics:
Construction may be initiated with a certain size casing,
(1)
Type of aquifer
but drilling conditions may make it desirable to reduce
-water table
the casing size at some depth. However, the diameter
-confined
must be large enough to accommodate the pump and
-artesian
the diameter of the intake section must be consistent
(2)
Slope of aquifer
with hydraulic efficiency. The well shall be designed to
(3)
Direction of flow
be straight and plump. The factors that control diameter
(4)
Boundary effects
are (1) yield of the well, (2) intake entrance velocity, (3)
(5)
Influence of recharge
pump size and (4) construction method. The pump size,
-stream or river
which is related to yield, usually dominates. Approximate
-lake
well diameters for various yields are shown in table 5-3.
(6)
Nonhomogeneity
Well diameter affects well yield but not to a major
(7)
Leaks from aquifer
degree. Doubling the diameter of the well will produce
only about 10-15 percent more water. Table 5-4 gives
5-6. Well design and construction
the theoretical change in yield that results from changing
Well design methods and construction techniques are
from one well diameter to a new well diameter. For
basically the same for wells constructed in consolidated
artesian wells, the yield increase resulting from diameter
or unconsolidated formations.
Typically, wells
doubling is generally less than 10 percent.
constructed in an unconsolidated formation require a
Consideration should be given to future expansion and
screen to line the lower portion of the borehole. An
installation of a larger pump. This may be likely in cases
artificial gravel pack may or may not be required. A
where the capacity of the aquifer is greater than the yield
diagrammatic section of a gravel packed well is shown
required.
on figure 5-6. Wells constructed in sandstone, limestone
or other creviced rock formations can utilize an uncased
Table 5-3. Well Diameter vs. Anticipated Yield.
Anticipated
Nominal Size of
Optimum Size
Smallest Size
Well Yield
Pump Bowls
Well Casing
Well Casing
(gallons/minute)
(inches)
(inches)
(inches)
<100
4
6 ID
5 ID
75-175
5
8 ID
6 ID
150-00
6
10 ID
8 ID
350-650
8
12 ID
10 ID
600-900
10
14 OD
12 ID
850-1300
12
16 OD
14 OD
1200-1800
14
20 OD
16 OD
1600-3000
16
24 OD
20 OD
5-9
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