TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
U.S. Army Corps of Engineers
Figure 4-6. Flow and head for fully and partially penetrating circular slots; circular source; artesian flow.
be selected conservatively from pumping test data or,
from the equation and plots in figure 4-23. Where
there is little or no recharge to an aquifer, the radius of
if necessary, from figure 4-23.
influence will become greater with pumping time and
(4) Wetted screen. There should always be suffi-
with increased drawdown in the area being dewatered.
cient well and screen length below the required draw-
Generally, R is greater for coarse, very pervious sands
down in a well in the formation being dewatered so that
than for finer soils. If the value of R is large relative to
the design or required pumping rate does not produce
the size of the excavation, a reasonably good approxi-
a gradient at the interface of the formation and the
well filter (or screen) or at the screen and filter that
mation of R will serve adequately for design because
starts to cause the flow to become turbulent. There-
flow and drawdown for such a condition are not espe-
cially sensitive to the actual value of R. As it is usually
fore, the design of a dewatering system should always
impossible to determine R accurately, the value should
be checked to see that the well or wellpoints have ade-
4-7
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