Page 137 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch03_061-117.qxd 8/17/10 7:48 PM Page 99

3.16 Well Design 99

Solution 2 (SI System):
The specific capacity of a 305-mm, fully penetrating well after 40 days of pumping can be calcu-
lated from Eqs. 3.12 and 3.13:
s = (Q>4pT) W(u)
2
u = (r s)>(4Tt)
Given data are
t = 10 d
3
T = 124.18 m /d/m
S = 4 * 10 -4
r = (305 mm>2) = 0.1525 m
Then
-4
2
u = (0.1525) * (4 * 10 )>(4 * 124.18 * 10)
= 1.87 * 10 -9
From Table 3.2, W(u) 19.52
3
(Q>s) = 4pT>W(u) = 4 * 3.14 * 124.18>19.52 = 80 m /d/m
The percentage of aquifer screened is K p 6.1>15.24 = 40%
The slenderness of the well factor is b>r w 15.24>0.1525 100
The value of F p from Fig. 3.12 is 0.65
3
The expected specific capacity of the well (Q>s p ) (Q>s) F p 80 0.65 51.9 m /d/m)
The maximum available drawdown 45.72 6.1 39.62 m
3
3
The maximum yield of the well (51.9 m /d/m)(39.62 m) 2,057 m /d
3
The actual maximum yield will be between 1,635 and 2,457 m /d

3.16 WELL DESIGN
From the standpoint of well design, it is useful to think of a well as consisting of two parts:
(a) the conduit portion of the well, which houses the pumping equipment and provides the
passage for the upward flow of water to the pumping intake; and (b) the intake portion,
where the water from the aquifer enters the well. In consolidated water-bearing materials,
the conduit portion is usually cased from the surface to the top of the aquifer, and the intake
portion is an uncased, open hole. In unconsolidated aquifers, a perforated casing or a screen
is required to hold back the water-bearing material and to allow water to flow into the well.
The depth of a well depends on the anticipated drawdown for the design yield, the ver-
tical position of the more permeable strata, and the length of the intake portion of the well.
The well size affects the cost of construction substantially. The well need not be of the
same size from top to bottom. The diameter of a well is governed by (a) the proposed yield
of the well, (b) entrance velocity and loss, and (c) the method of construction. The control-
ling factor is usually the size of the pump that will be required to deliver the design yield.
The diameter of the casing should be two nominal sizes larger than the size of the pump
bowls, to prevent the pump shaft from binding and to reduce well losses. Table 3.5 gives
the casing sizes recommended for various pumping rates.
The selection of the well size may depend on the size of the open area desired to keep
entrance velocities and well losses at a reasonable value. In deep-drilled wells, the minimum