TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
creased and the above-described procedure repeated
enough to equal the rate of well flow (e.g., curve c). In
until the well has been pumped at three or four rates.
many areas, formation boundary conditions exist that
The drawdown from each step should be plotted as a
limit the area1 extent of aquifers. The effect of such a
continuous time-drawdown curve as illustrated in fig-
boundary on an H' versus (log) t graph is in reverse to
ure C-12. The straight-line portion of the time-draw-
the effect of recharge. Thus, when an impermeable
down curves is extended as shown by the dashed lines
boundary is encountered, the slope of the H' versus
in figure C-12, and the incremental drawdown AH'
(log) t curve steepens as illustrated by curve d. It
for each step is determined as the difference between
should be noted that a nonequilibrium analysis of a
the plotted and extended curves at an equal time after
pumping test is valid only for the first segment of a
each step in pumping. The drawdown H' for each step
time-drawdown curve.
is the sum of the preceding incremental drawdowns
b. Step-drawdown pump test.
and can be plotted versus the pumping rate as shown in
(1) The efficiency of a well with respect to en-
figure C-13. If the flow is entirely laminar, the draw-
trance losses and friction losses can be determined
down (H-h for artesian flow and H2-h2 for gravity
from a step-drawdown pumping test, in which the well
flow) versus pumping rate will plot as a straight line; if
is pumped at a constant rate of flow until either the
any of the flow is turbulent, the plot will be curved.
(2) The well-entrance loss He, consisting of fric-
drawdown becomes stabilized or a straight-line rela-
tion losses at the aquifer and filter interface through
tion of the time-drawdown curve plotted to a semilog
the filter and through the well screen, can be deter-
scale is established. Then, the rate of pumping is in-
U.S. Army Corps of Engineers
Figure C-12. Drawdown versus elapsed pumping time for a step-drawdown test.
C-12
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