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Water Quality Data
STANDARD GUIDELINES FOR DESIGN
AND PROPER CONSTRUCTION
OF A WATER WELL
By Roy F. Senior, Jr.*
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GRAVEL PACK
Appropriate gravel pack MUST be used and placed correctly to obtain maximum well efficiency and production. In the University of California Bulletin No. 1889, titled "Water Well and Pumps: Their Design, Construction & Maintenance," the following information is referenced:
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Grain size distribution curves are drawn for material in each water-producing zone.
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Grain size distribution curves are used to identify the aquifer with the finest material.
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The 70% retained size of the finest aquifer material is selected as a basis for design. The gravel pack to retain 70% of the aquifer material should be 4 to 6 times larger than the aquifer material. For uniform fine material, the factor should be four (4); for non-uniform coarser material, five (5); and for highly non-uniform material including fines, six (6).
The selection of gravel roundness is extremely important because it allows the use of the proper graduation to fit the finer formations and retain the maximum porosity and permeability of the gravel pack to achieve maximum well efficiency. Two suppliers who come closes to meeting the roundness criteria are "Colorado Silica" and "Heart of Texas." Both are expensive compared to less suitable gravel (due to availability and freight), but their use will pay back the added cost may times over during the life of a well.
CASING & SCREEN
Casing and screen diameter must be adequate to allow the desired amount of water to pass without friction loss. The pump chamber casing must be large enough to allow the required size column pipe and bowl assembly to be installed freely in the well to the point of anticipated future needs.
Screen opening design should retain 80% to 90% of the gravel pack. The most commonly used perforations are louvered and continuous V slot wire-wound casing. Both are resistant to gravel pack plugging and perform efficiently with most gravel pack installations.
Millslot perforation is also frequent used, but a much higher percentage of plugging by the gravel pack occurs with millslot because the perforations are straight rather than louvered or "V" shaped. Millslot can be used in some areas, but in most instances it is unwise to use it.
Although additional slot openings can be added to compensate for plugging, millslot casing is not considered as efficient as louvered or wire-wound. A word of caution….if millslot is used exclusively in a well, it will generally result in lower well efficiency, and create greater drawdown, thereby causing higher pumping cost for the entire life of the well. However, if a combination of millslot in the lesser producing areas and continuous "V" slot wire-wound in the more productive areas is used, the combination can prove satisfactory.
* Note: These remarks were presented by the author at a District
workshop in 1992.
Last updated January
2001