A Guide To Water Well Casing and Screen Selection   -   4

3.0  GENERAL CASING AND SCREEN CONSIDERATIONS AND MATERIALS

3.1 Strength and Durability

Regardless of construction method, water well casings and screens have some common requirements. Strength must be adequate to withstand not only the stresses of installation, but also other forces which may be applied during well completion, development and use. The forces of installation which tend to pull the casing and screen apart must be exceeded by the tensile strength of the material.* The resistance of the casing to collapse (see page 27) must be greater than the external hydrostatic forces calculated.

Radial stresses of the wall of a small diameter hole in a consolidated formation are negligible. However, it is impossible to calculate the load on casings and screens in unconsolidated formations. Unknown are forces from formation sloughing, caving and subsidence, or the sudden downward movement of filter pack material. These stresses can rupture casings and screens.

Another requirement related to strength is durability. Small increases in the wall thickness of ordinary low carbon steel casings not only improve strength, but under most conditions, extend well life, from a corrosion standpoint, by a factor greater than the percentage thickness difference. Corrosive environments may require the use of special corrosion-resistant material.

3.2  Handling and Maintenance

Ease of transport, handling and installation are important considerations. These not only influence cost but are also relevant to the selection of the type of casing and screen field connections.

Casing and screen should be designed to facilitate future rehabilitation including cleaning, incrustation removal, redevelopment and repair. Another consideration is the possibility of future well deepening. Casing and screen diameter as well as type and material selected are influenced by the parameters.

3.3  Economy

Economy plays an important role in the engineering of any ground-water producing installation. Considering water well materials, however, lower cost does not necessarily mean the least durable or efficient. On the other hand, the most expensive do not always produce the best results. Optimum balance between design and price requires a knowledge of those special conditions pertaining to the well and comparison of available products. An example is analysis of the required well life. This is becoming more important, particularly in the case of municipal wells. Inflation, the lack of proper sites, and the difficulty of transporting, setting up and operating water well construction equipment in restricted urban locations, have dictated the use of better well designs, and in many cases more durable materials for extra longevity.

*When casings are driven into place, a more critical and complicated condition occurs which is discussed in Appendix III.

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