20
Module 3 — Design and Construction of Water Wells
Well Design Considerations
Well design and construction details are determined after a test hole has been completed
and the geological zones have been logged. There are many components to well design the
driller must take into account. Decisions will be made about:
Type of well
Intended use
Well depth
Casing material, size and wall thickness
Intake design
Annular seal
Monitoring and preventive maintenance provisions.
Well Depth
During the test hole drilling, the licensed water well contractor will complete a lithologic
or formation log. Soil and rock samples are taken at various depths and the type of geologic
material is recorded. This allows the driller to identify zones with the best potential for water
supply. Some drillers also run a geophysical (electric) log in the test hole to further define the
geology. This gives them more accurate information about aquifer location.
Generally a well is completed to the bottom of the aquifer. This allows more of the aquifer
to be utilized and ensures the highest possible production from the well.
Types of Wells
There are two main types of wells, each distinguished by the diameter of the bore hole.
The two types are bored wells and drilled wells.
Bored wells
Bored wells are constructed when low yielding groundwater sources are found relatively
close to the surface, usually under 30 m (100 ft.). Bored wells are constructed using a rotary
bucket auger. They are usually completed by perforating the casing (also called cribbing) or
using a sand screen with continuous slot openings (see Figure 1, Bored Well).
One advantage of bored wells is the large diameter of the casing, from 45-90 cm
(18-36
in.). It provides a water storage reservoir for use during peak demand periods.
A disadvantage of utilizing a shallow groundwater aquifer is that it generally relies on
annual precipitation for recharge. Water shortages may occur following long dry periods in
summer and extended freeze up during winter months. It can also be more susceptible to
contamination from surface land-use activities.
Figure 1 Bored Well