CEMP-E
TI 814-01
3 August 1998
(1) Type of aquifer
-water table
-confined
-artesian
(2) Slope of aquifer
(3) Direction of flow
(4) Boundary effects
(5) Influence of recharge
-stream or river
-lake
(6) Nonhomogeneity
(7) Leaks from aquifer
5-6. WELL DESIGN AND CONSTRUCTION. Well design methods and construction
techniques are basically the same for wells constructed in consolidated or unconsolidated
formations and only one aquifer is being penetrated. Typically, wells constructed in an
unconsolidated formation require a screen to line the lower portion of the borehole. An
artificial gravel pack may or may not be required. A diagrammatic section of a gravel packed
well is shown on figure 5-5. Wells constructed in sandstone, limestone or other creviced rock
formations can utilize an uncased borehole in the aquifer. Screens and the gravel pack are
not usually required. A well in rock formation is shown in figure 5-6. Additional well designs for
consolidated and unconsolidated formations are shown in AWWA A100.
a. Diameter. The diameter of a well has a significant effect on the well's construction
cost. The diameter need not be uniform from top to bottom. Construction may be initiated with
a certain size casing, but drilling conditions may make it desirable to reduce the casing size at
some depth. However, the diameter must be large enough to accommodate the pump and the
diameter of the intake section must be consistent with hydraulic efficiency. The well shall be
designed to be straight and plumb. The factors that control diameter are (1) yield of the well,
(2) intake entrance velocity, (3) pump size, and (4) construction method. The pump size,
which is related to yield, usually dominates. Approximate well diameters for various yields are
shown in table 5-3. Well diameter affects well yield but not to a major degree. Doubling the
diameter of the well will produce only about 10-15 percent more water. Table 5-4 gives the
theoretical change in yield that results from changing from one well diameter to a new well
diameter. For artesian wells, the yield increase resulting from diameter doubling is generally
less than 10 percent. Consideration should be given to future expansion and installation of a
larger pump. This may be likely in cases where the capacity of the aquifer is greater than the
yield required.
5-10
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