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Section 8

Gravel Filter Pack line


A filter pack is coarse sand or fine gravel (2-6 mm diameter) that is placed between the borehole wall and screen (see Section 9 - Figure 15). Filter packs are used to settle-out fine grained particles that may otherwise enter the well and to increase the effective hydraulic diameter of the well. A filter pack is like a wells "lungs" passing water to the "heart of the well" (the screen). A filter pack should be installed in all wells except those completed in rock, coarse sand or gravel.

Filter Material: Ideally, the filter media should consist of silica-based material since it will not dissolve over time; this ensures that the integrity of the filter pack is maintained and that harmful or unpleasant substances are not leached into the water (Ives and Coad, 1987).

Material Cleanliness: Filter material must be clean to minimize filter pack collapse and reduce well development time (Driscoll, 1986). Treat all filter material with 50 mg/L chlorine solution prior to placement to ensure it doesn't contaminate the well.

Grain Size/Uniformity: The sand or gravel should be of a uniform size which is just slightly larger than the size of the slots in the well screen. This is the main factor controlling the possibility of sand pumping! Desert sand, for example, should be avoided because it is too fine and will block the screen or wash into the well causing turbid water and rapid pump cylinder wear. If there is a wide range of particle sizes, the filter pack can become severely blocked (Ives and Coad, 1987). Alternatively, the coarse material may separate as it is poured into the well resulting in possible continued sand pumping (Driscoll, 1986).

Grain Shape: Well-rounded and sorted grains from river or ocean deposits should be used since they will reduce drawdown, increase yield and allow for more effective development (Driscoll, 1986). Even if rounded gravel must be transported from far away, local angular rock should not be used since it will compact when the well is pumped and can severely restrict the flow of water (Moffat, 198?). Finally, flake-like grains are unacceptable because they settle to form a filter media with a low permeability (Ives and Coad, 1987).

Finding Material: The best material is coarse silica sand and fine gravel material which is usually found in river-beds or ocean beaches. Separate the desired size fraction by using two screens which have slot sizes of 3 and 6 mm (1/8 and 1/4 in). Put the screens on the top and bottom of a strong wooden frame with the coarse slot screen on top. Suspend the frame in the water and scoop sand and gravel onto the top screen. After rinsing, suitable material will be trapped between the two screens. Suitably sized, strong window screen may not be readily available overseas, but can be easily flattened and brought with you in your suitcase.

Filter Volume: Calculate the volume of filter material (see Appendix B) necessary to fill the well annulus to 2 metres above the top of the well screen - this allows for areas of borehole washout to be filled without exposing the upper screen to formation stabilizer or borehole fines. Whenever possible, however, do not place gravel within 3-6 m (10-20 ft) of ground surface.

Installation: After the casing and screen have been installed, continue to rinse the borehole by circulating clean water through the bottom of the casing (see Section 7). Slowly pour the filter media into the annular space and let it settle into the upward flowing water (0.19 - 0.25 L/s or 3 - 4 gpm). This process, called "floating in the gravel pack" helps prevent the filter material from bridging and keeps the fines from settling. A feeler line or weighted measuring tape should be used to confirm where the top of the filter pack is.

After the filter pack is installed, place a cap over the casing so that nothing can fall in the well during grouting and cement pad construction. The LS-100 can now be removed.

References

Driscoll, F. (1986) Groundwater and Wells, St. Paul: Johnson Division

Ives, K and A. Coad (1987) "Selecting Filter Media", Developing World Water, Hong Kong: Grosvenor Press Int'l, pp. 202-203.

Moffat, B. (198?) "Efficient Water Wells", Developing World Water", Hong Kong: Grosvenor Press Int'l, pp. 36-37.

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