Civil Engineering Reference
In-Depth Information
Ground Surface
2r
Diameter of Wall
R
Radius of Influence
Cone of
Depression
Drawdown
Curve
Pumping Level
Well Screen
Impervious Stratum
Fig. 8-20.
Water-table well (Courtesy of US Filter / Johnson Screens)
All other terms are as defined in Equation (8-3) for a water-table well. The above
formulas show how the well diameter affects yield:
1
Q
(8-5)
log
R
/
r
In other words, well yield is not a direct function of well diameter. For example, if a
6-inch (0.15-m) well will yield 100 gpm (6.3 L / s) with a certain drawdown, a 12-
inch (0.31-m) well constructed at the same spot will yield 110 gpm (6.94-m) with the
same drawdown, and an 18-inch (0.46-m) well will yield 117 gpm (7.38 L / s). A 48-
inch (1.22-m) well will yield 137 gpm (8.65 L / s), or 37 percent more water than a 6-
inch (0.15-m) well at the same drawdown.
Derivations of the above formulas are based on the following simplifying assump-
tions:
25
1. The water-bearing materials are of uniform permeability within the radius of
influence of the well.
