PRIVATE WATER SYSTEMS - Design - Strength Requirement
There are three types of forces that act on the screen and
casing during installation. They are tension, column loading, and
collapse pressure. Once the well is installed and developed and the
grout is set, nearly all these forces become eliminated or stabilized.
Tension
This vertical force is due to the weight of free
hanging casing as it is lowered into the well, and is strongest
on the uppermost portion of the casing. The maximum tensile force
to which the casing and couplings is subjected is equal to the
weight of the longest string of casing that is suspended in the
borehole without resting on the bottom.
For optimum casing design, compare the actual tensile force
that is anticipated to that recommended by the manufacturer of the
casing. Adjustments to the casing diameter and wall thickness can
then be made to obtain the most cost effective design.
Column Loading
Column loading is the opposite of tensile forces. If a
column is allowed to set at the bottom of the borehole, the force
exerted is downward acting on the lower portion of the casing and
screen.
The maximum column loading that will take place at the bottom
of the casing string is equal to the weight of casing that it is
supporting. Calculating this force for deep wells, however, is complicated by bowing and bending of the casing. Therefore,
consult the manufacturer of the casing or screen for assistance in calculating the column loading force and for the placement of centering
guides on casing and screen.
Collapse Pressure
These forces are primarily horizontal and can cause the
casing or screen to fold inward. They are the result of the
difference between the elevation of water inside the casing and
outside the casing. One way to eliminate this force is to keep
the casing and screen full of liquid during installation.