Module 3 — Design and Construction of Water Wells
23
Slotted or perforated casing or liner is made by manually creating openings using a
cutting tool or drill. Pre-slotted plastic pipe is also available.
Slot openings and perforations are spaced further apart than screen openings. This
reduces the amount of open area to allow water into the well. The openings tend to vary in
size and may have rough edges depending on how they were made. This impedes the flow of
water into the well and may not be effective in holding back the formation sediments.
The licensed water well contractor examines the cuttings from the borehole and makes a
judgement whether to use a screen, or slotted or perforated casing/liner. While a screen is the
more expensive alternative, it is necessary if the aquifer is composed of loose material such
as fine sand, gravel or soft sandstone. A slotted or perforated casing/liner can be used when
the aquifer formation is more consolidated, such as hard sandstone or fractured shale.
After a choice is made between a screen, or slotted or perforated casing/liner other
decisions will be made regarding:
Size of slot openings
Total area of screen or perforation that is exposed to the aquifer
Placement of the screen or perforations within the aquifer.
Slot size openings
The slot openings must be small enough to permit easy entry of water into the well
while keeping out sediment. The slot size chosen will depend on the particle size of the earth
materials in the producing aquifer.
Typically a licensed water well contractor will select a slot size that allows 60 percent
of the aquifer material to pass through during the well development phase of drilling. The
remaining 40 percent, comprising the coarsest materials, will form a natural filter pack
around the perforations or screen.
Total open area of screen
The total area of the slot openings is dependent on the length and diameter of the screen.
While the length of the screen is variable, the diameter of the screen is determined by the
diameter of the well casing. The yield from a well increases with an increase in screen
diameter but not proportionately so.
The amount of open area in the screen or slotted or perforated casing/liner will affect how
quickly the water from the aquifer enters the well. A larger amount of open area allows the
Incrustation is a buildup that occurs
when dissolved minerals in the
groundwater come out of solution and
deposit on the screen or casing.
Ensure that the pumping water level in
the well never goes below the top of the
slot openings or perforations to prevent
oxygen exposure to the aquifer which will
enhance bacterial growth and result in
reduced well yield.