A Guide To Water Well Casing and Screen Selection   -   24

8.6  Compression Sections

In many arid and semi-arid regions of the world, a growing cause of well failure is the rupture of casing from subsidence of the ground because of depletion of water from the surrounding aquifer systems. Where the drop in artesian head greatly exceeds the water table decline, stresses are developed by the resulting hydraulic gradient that are often sufficient to cause one or more breaks in the well casing and screen. These breaks occur as the casing shortens by rupture. The broken sections usually divide into segments which slip past each other to produce a telescoping compression break. Experience has shown that such breaks and deformations cannot be prevented by employing stronger, heavier wall casing. Use of special telescoping joints, known as compression sections, installed in the casing and screen string, has solved this problem in many cases.

Compression sections are composed of three 6 foot lengths of casing, two of which are the same diameter and wall thickness as the well casing. These two joints are equipped with beveled steel rings welded to the lower end of the upper section and upper end of the lower section. Thus the joints are free to move or telescope within the length of an outer section of shell which is similarly equipped with rings at each end acting as stops and stabilizers. This shell is usually two inches larger than the parent casing. The rings are manufactured form ½ x 2 inch steel and beveled to a 45º angle.

Location of compression sections should be given some study since compression failures have been known to occur in intervals between the joints. The user is advised to check the history of the area. Sections should be located at the depth or formation where breaks in surrounding wells have been obtained by locating a compression section at the bottom of the pump housing casing. If the well exceeds 1,500 feet in depth, another section may be placed in the middle of the screen string.

Clay is able to exert greater compressive force than coarser sediments, particularly in strata thicker than eight feet. This should also be considered when locating compression sections.

Despite the fact that in many areas surface levels have declined more than six feet, the sic foot potential travel of the standard compression section has been found satisfactory.

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