Page 182 - Water and wastewater engineering

WELLS 4-37

− 3
(225 )( .818 ×10 m/s )(4 2 . min )(60 s/min )
.
2
S = 1775 10 5

.
( (300 m ) 2

where r 300 m, the distance from the pumping well to the observation well.
(3) Estimate drawdown
Calculate the drawdown with Equation 4-1.
2
3
5
With T 2.818 10 m /s, S 1.775 10 , and a well diameter of 15 cm selected
in step (c) above ( r 15 cm/2 7.5 cm or 0.075 m), calculate u as
2
(0075m ) (1775 10 5 )
.
.
u 1025 10 12
.
2
3
,
( 4 2 818 10 m /s )(1100 d)( 86 400s/d)
.

and 12 2 12 3
.
Wu() 0 577216. ln 1 025 10 12 1 025. 10 12 (1.025 10 ) ( . 1025 10 )
22! 33!
?
?
27 .03
3
⎛ 1 ⎞
3
( 32m/h ) ⎜ ⎝ ⎟ ⎠
s 3 600 s/h, ( 27 03 ). 6 785 or. 68 m.

3
2
4 ()( )( 2 818 . 10 m/s)
0
p
The 6.8 m is measured from the static piezometric surface that is 5.2 m below grade. The
drawdown piezometric surface will be a total of 5.2 6.8 12.0 m below the ground
surface. The top of the artesian aquifer is 150.8 m below grade. Thus, there is no poten-
tial for failure of the well due to dewatering the aquifer.
e. Well depth, casing length, and grout requirements
(1) From the well log, select the water-bearing aquifer
The well penetrates an impervious layer of shale, extends through a sand layer, and
terminates in shale. From the well log and the static water level in the well, the sand
layer is determined to be an artesian aquifer.
(2) A fully penetrating well is selected. Therefore, the well depth is 189.5 m.
(3) The casing length and grout requirements are based on MSDWA rules, that is:
For artesian aquifers, the casing is sealed to the upper confining layer from within
1.5 m of the top of the aquifer to the ground surface.
f. Slot size
Plot the grain size analysis as “Cumulative % Retained” versus “Grain Size, mm.” The
points in Figure 4-12 were plotted using the data provided for this example. Note that the
data are presented as “Cumulative % passing” and that they are plotted as “Cumulative