Page 97 - Water Engineering Hydraulics, Distribution and Treatment

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3.17 Well Construction
3.17.1 Dug Wells
deeper and deeper, sections of rod are added to the auger
stem. Bits up to 36 in. (914 mm) in diameter have been used
Small dug wells are generally excavated by hand. In loose
successfully, and wells have been enlarged in diameter up
overburden, they are cribbed with timber; lined with brick,
to 48 in. (1219 mm) by reaming. A concrete, tile, or metal
rubble, or concrete; or cased with large-diameter vitrified tile
casing is inserted in the hole and cemented in place before
or concrete pipe. In rock, they are commonly left unlined.
the strainer is installed.
Excavation is continued until water flows in more rapidly
than it can be bailed out. Dug wells should be completed
when the water table is at or near its lowest level. Otherwise,
3.17.4 Drilled Wells
they may have to be deepened at a later date.
High-capacity, deep wells are constructed by drilling.
Large and deep dug wells are often constructed by sink-
Because the water-bearing materials vary so widely, no one
ing their liners as excavation proceeds. The lead ring has a
method of drilling can be adopted under all conditions. The
steel cutting edge; new rings are added as excavation pro-
method of drilling is selected to suit the particular condi-
gresses.
tions of a site. The systems of drilling used in water-well
construction are based on either the percussion or the rotary
3.17.2 Driven and Jetted Wells principle.
Wells can be driven into relatively shallow sand formations.
As shown in Fig. 3.14, the driving point is attached to a 3.17.5 Collector Wells
strainer or perforated section of pipe. To reduce friction, the
point is somewhat larger than the casing. The driving weight A collector well consists of a central shaft of concrete cais-
is commonly suspended from a block attached to a tripod. son some 15 ft (4.57 m) in internal diameter and finished off
In hard ground a cylindrical shoe equipped with water jets below the water table with a thick concrete plug. From this
loosens the soil and washes it to the surface. Batteries of shaft, perforated radial pipes 6 or 8 in. (150 or 200 mm) in
driven wells may be connected to a suction header to supply diameter and 100–250 ft (30–76 m) long are jacked horizon-
enough water. tally into a water-bearing formation through ports near the
bottom of a caisson. The collector pipes may be installed and
developed in the same manner as for ordinary wells.
3.17.3 Bored Wells
Wells can be bored with hand or power augers into suf- 3.17.6 Pumps
ficiently cohesive (noncaving) soils. Above the water table,
the soil is usually held in the auger, which must then be raised Many types of well pumps are on the market to suit the wide
from time to time to be cleaned. Below the water table, sand variety of capacity requirements, depths to water, and sources
may wash out of the auger and have to be removed from the of power. Figure 3.15 shows a well pump and 190,000 gal
bore hole by a bailer or sand pump. As the well becomes (719,150 L) water tank. In the United States almost all well
pumps are driven by electric motors.
Pump head
Watertight flange Casing pipe
Plastic cement
Compression nut
Platform (applied or removed
by means of a
Steel or iron foked wrench)
casing pipe
Frost vent
Rubber packer
Retainer plate
Coupling
Seal
Driven pipe Driven pipe
Pump pipe
Drive point
Figure 3.14 Driven well and its sanitary protection (After Iowa Figure 3.15 Well and 190,000 gal (719,150 L) Tank, Maui, HI
State Department of Health). (Courtesy of the Department of Water Supply, Maui County, HI).