Guidelines for Minimum Standards in Water Well Construction, Province of British Columbia — June 1982
APPENDIX 2: WELL SCREENS AND ARTIFICIAL FILTER PACK CONSTRUCTION
1. Well Screens
There are two basic types of wells in unconsolidated materials:
(i) naturally developed wells
(ii) artificially introduced filter pack wells
If the unconsolidated material is nonhomogeneous, has a uniformity coefficient1 greater than 3.0, and has an effective grain size2 greater than 0.01 inches then a naturally developed well should be constructed.
The choice of well screen material should be selected on the basis of chemical analyses of the water or prior knowledge of the water quality. Where well treatment with strong chemicals may be required or where waters are corrosive or incrustation may be a problem, stainless steel is recommended. However, there are many situations where other materials will give very satisfactory results.
The well screen aperture openings, screen lengths and diameter should be selected so that sufficient open area is available to give the required well yield without exceeding an entrance velocity into the screens of 0.1 foot per second.
Screen sections can be joined together by welding or by threaded connections. The welding rod material must be suitable for joining corrosion resistant materials so as not to reduce that resistance. Where dissimilar metals are joined, a dielectric coupling can be used. Screen settings will depend on analyses of the unconsolidated formation aquifer materials, the driller's log, stratigraphic log and/or electric log if the latter two are available.
According to EPA 570/9-75-001, screen aperture size selection should be based on the following criteria:
A. Where the uniformity coefficient of the aquifer is greater than 6 and the aquifer is overlain by an essentially non-caving formation, the aperture size should be that which retains 30 percent of the aquifer sample.
B. Where the uniformity coefficient of the aquifer is greater than 6 and the aquifer is overlain by a readily caving formation, the aperture size should be that which retains 50 percent of the aquifer sample. C. Where the uniformity coefficient of the aquifer is 3 or lower and the aquifer is overlain by an essentially non-caving formation, the aperture size should be that which retains 40 percent of the aquifer sample.
D. Where the uniformity coefficient of the aquifer is 3 or lower and the aquifer is overlain by a caving formation, the aperture size should be that which retains 60 percent of the aquifer sample.
E. For conditions between the extremes listed, the contractor should interpolate to obtain the proper screen aperture size.
F. Where a formation to be screened has layers of different grain sizes and graduations, use the following rule: If the 50 percent size of the coarsest layer .is less than 4 times the 50 percent size of' the finest layer, the aperture size should be selected on the basis of the finest layer, or for each specific layer as indicated in A, B, C, D, or E.
G. If the water is corrosive or the accuracy of the chemical analysis is in doubt, select an aperture size that will retain 10 percent more than is indicated in the above paragraphs.
H. Where fine sand overlies coarse sand, use the fine sand size aperture for the top two feet of the underlying coarse sand. The coarse size aperture should not be larger than twice the fine sand size. For further details on screen apertures size selection the reader is referred to the recognized text books and publications on the subject.
I. Where an artificial filter is to be used the aperture size selection criteria should be in accordance with Section 2 Artificial Filter Pack Construction. Recommended methods of connecting screen to casing will depend on whether nominal or pipe size screen is used and the method of screen installation. Methods include neoprene or rubber seal and threaded, coupled or welded joints. Recommended methods of sealing the bottom of the screen include welded plate (screen material), fabricated plug threaded or welded (screen material), self closing valve, bag cement methods in extension pipe at bottom of screen.
2. Artificial Filter Pack Construction
2.1 Introduction
Well screens set within aquifers consisting of unconsolidated materials with a uniformity coefficient less than 3 and an effective grain size less than 0.01 inches are recommended to be surrounded by an artificial filter pack. According to EPA 570/9-75-001, an artificial filter pack consists of clean sand or gravel of selected grain sizes and gradations which is installed in the annular space between the screen and the wall of the well bore. The filter has a larger average grain size and usually a smaller coefficient of uniformity than the aquifer material. This permits use of a larger screen slot size and consequent larger open area so that entrance velocity is lowered and head losses to the well are reduced. The filter has a considerably higher permeability than the formation so that the effective diameter of the well is increased to some extent. Both these factors tend to increase the efficiency and specific capacity of a well and tend to reduce the possibility of excess sand production.
The grain size and gradation of the filter are selected to stabilize the aquifer material and to permit only the fine fractions to move into the well during development. Thus after development a correctly filtered well is relatively sand-free, and a narrow annulus of the formation adjacent to the filter has its permeability increased to some degree. Generally, the thinner the filter the better. Actually a correctly designed filter 1/2-inch thick would be adequate, but the mechanical difficulties of satisfactorily placing such a filter preclude its use. From a practical stand-point, filter packs are usually about 4 to 8 inches thick.
In a shallow well with 6 inches or more of annular space and 5 to 6 feet of screen the filter material can be easily dumped in to give a satisfactory installation. In deep wells with longer screens, to avoid bridging and segregation of the filter material, it should be placed with great care. If placed by gravity the material should be introduced at a metered, uniform rate. Frequently it is placed via a tremie pipe. Other practices include washing or pumping the filter material in with water (as a slurry) which is an effective way of placing the filter pack. Once installation of the filter material is started it should proceed at a uniform rate until completed from the bottom of the well to a selected point above the screen or perforations.
2.2 Selection of Artificial Filter Pack Grain Size and Screen Aperture Size
According to EPA 570/9-75-001 "The filter grain size should be determined by taking the 70 percent retained grain size of the finest formation to be filtered and multiplying it by 4, 5 or 6. This is the 70 percent retained grain size of the filter material to be used. The uniformity coefficient (the size of sieve that retains 40 percent of the sample divided by the size that retains 90 percent) should not be greater than 2.5. The gradation of the filter material should form a smooth and gradual size distribution curve when plotted. The screen aperture openings should be of such size as to retain between 85 and 100 percent of the filter material. The thickness of the filter should range from a minimum of 3 inches to approximately 8 inches."
2.3 Length of Artificial Filter Pack
In general the filter pack material should extend a distance of 2 1/2 times the largest diameter of the well above any screen. Sand, cement or additional filter material should be placed between the filter material and the lower limit of the sanitary seal. The size of the sand should be such that it will not infiltrate into the filter material.
2.4 Storage of Filter Material
If the filter pack material is delivered in bulk it should be placed on a protective sheet to prevent contact with the ground and also covered.
2.5 Disinfection of Filter Material
Procedures for adequate disinfection are given under Well Disinfection (Appendix 6).
1The uniformity coefficient is the size of sieve that retains 40% of the sample divided by the size that retains 90%.
2The effective grain size is the sieve size that retains 90% of the sample.
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