PRIVATE WATER SYSTEMS - Design - Strength Requirement

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.

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