(4) Drilled wells. Drilled wells may be constructed in virtually any type of soil
and at depths to 3,000 feet or more. Drilled wells may be drilled either by the rotary
method or by percussion. Either method requires a derrick and hoist. The primary
difference in techniques is whether the drilling is accomplished by a rotating drilling pipe
with a bit at the bottom, or whether the bit is suspended by a cable and alternately
raised and dropped in the hole, thus digging by breaking up the soil and rock formations
in a pulverizing action. When the depth of the hole reaches the water-bearing strata, a
casing is placed. In cases where caving occurs or is likely to occur, the casing may be
set as the well is drilled. In addition to the grouting of the annular space around the top
of the casing, a grout seal may be pumped through the casing to seal off the aquifer
from seepage through other earth layers (see figure 6-7).
Figure 6-7. Drilled well.