CEMP-E
TI 814-01
3 August 1998
well, the yield will be increased by 19 percent. The values in the above table are valid only for wells in
unconfined aquifers (water table wells) and are based on the following equation:
Y2/Y1
=
(log R/r1) / (log R/r2)
where:
Y2
=
yield of new well
Y1
=
yield of original well
R
=
radius of cone of depression, in mm (the value of R used for this table is 120
m).
r2
=
diameter of new well, in mm
r1
=
diameter of original well, in mm
b. Depth. Depth of a well is usually determined from the logs of test holes or from logs of
other nearby wells that utilize the same aquifer. A well that is screened the full length of the
water bearing stratum has a potential for greater discharge than a unit that is not fully
screened. Where the water bearing formations are thick, cost may be the deciding factor in
how deep the wells are installed. Cost, however, usually is balanced by the savings from a
potentially long-term source of water.
c. Casing. In a well developed in a sand and gravel formation, the casing should extend
to a minimum of 1,500 mm (5 ft) below the lowest estimated pumping level. In consolidated
formations, the casing should be driven 1,500 mm (5 ft) into bedrock and cemented in place
for its full depth. The minimum wall thickness for steel pipe used for casing is 8 mm (1/4-inch).
For various diameters, the EPA recommends the following wall thicknesses:
Nominal Diameter, mm (in)
Wall Thickness, mm (in)
150
(6)
8
(.250)
200
(8)
8
(.250)
250
(10)
8
(.279)
300
(12)
9
(.330)
350
(14)
10
(.375)
400
(16)
10
(.375)
450
(18)
10
(.375)
500
(20)
10
(.375)
In the percussion method of drilling, and where sloughing is a problem, it is customary to drill
and drive the casing to the lower extremity of the aquifer to be screened and then install the
appropriate size screen inside the casing before pulling the casing back and exposing the
screen to the water bearing formation.
d. Screens. Wells completed in sand and gravel with open-end casings, not equipped
with a screen on the bottom, usually have limited capacity due to the small intake area (open
end of casing pipe) and tend to pump large amounts of sand. A well designed screen permits
5-14
Previous Page
