Module 3 — Design and Construction of Water Wells
21
Drilled wells
Drilled wells are smaller in diameter, usually ranging from 10-20 cm (4-8 in.), and
completed to much greater depths than bored wells, up to several hundred metres. The
producing aquifer is generally less susceptible to pollution from surface sources because of
the depth. Also, the water supply tends to be more reliable since it is less affected by seasonal
weather patterns.
There are two primary methods of drilling:
Rotary
Cable tool.
Rotary drilled
wells are constructed using a drill bit on the end of a rotating drillstem.
Drilling fluid or air is circulated down through the drillstem in the hole and back to the
surface to remove cuttings. Rotary drilling rigs operate quickly and can reach depths of over
300
m (1000 ft.), with casing diameters of 10-45 cm (4-18 in.).
Cable tool drilled
wells are constructed by lifting and dropping a heavy drill bit in the
bore hole. The resulting loose material, mixed with water, is removed using a bailer or sand
pump. This method, also called percussion drilling, reaches depths up to 300 m (1000 ft.).
Well diameters can range from 10-45 cm (4-18 in.). The drilling rate is typically much slower
than for a rotary rig, but when aquifers are low yielding, they may be more easily identified
using this method.
Materials used in the drilling and
construction of water wells must be new
and uncontaminated.
Figure 2 Well Completions