capable of supporting the weight of the casing string when suspended from the surface in an

air filled borehole. The tensile strength of the casing joints is equally as important as the

tensile strength of the casing. Because the joint is generally the weakest point in a casing

string, the joint strength will determine the maximum axial load that can be placed on the

casing. By dividing the tensile strength by the linear weight of casing, the maximum

theoretical depth to which a dry string of casing can be suspended in a borehole can be

calculated. When the casing is in a borehole partially filled with water, the buoyant force of

the water increases the length of casing that can be suspended. The additional length of

casing that can be suspended depends on the specific gravity of the casing material.

The compressive strength of a material is defined as the greatest compressive stress

that a substance can bear without deformation. Unsupported casing has a much lower

compressive strength than installed casing that has been properly grouted and/or backfilled,

because vertical forces are greatly diminished by soil friction. This friction component means

that the casing material properties are more significant to compressive strength than wall

thickness. Casing failure due to compressive strength limitation is generally not an important

factor in a properly installed monitoring well.

As important as tensile strength is the final strength related property considered in

casing and screen selection    collapse strength. Collapse strength is defined as the capability

of a casing to resist collapse by any and all external loads to which it is subjected both during

and after installation. The resistance of casing to collapse is determined primarily by outside

diameter and wall thickness. Casing collapse strength is proportional to the cube of the wall

thickness. Therefore, a small increase in wall thickness provides a substantial increase in

collapse strength. Collapse strength is also influenced by other physical properties of the

casing material including stiffness and yield strength.

Casings and screens are most susceptible to collapse during installation before the

placement of the filter pack or annular seal materials around the casing. Although the casing

may collapse during development, once a casing is properly installed, collapse is seldom a

concern (National Water Well Association and Plastic Pipe Institute, 1981). External loadings

on casing that may contribute to collapse include:

Net external hydrostatic pressure produced when the static water level outside

of the casing is higher than the water level on the inside;

Unsymmetrical loads resulting from uneven placement of backfill and/or filter

pack materials;

Uneven collapse of unstable formations;

Sudden release of backfill materials that have temporarily bridged in the

annulus;

November 1992

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