Fine Sand, Screen Slot, and Filter-Pack Design
At a site in the Connecticut River valley, a 12-inch borehole penetrates 40 ft of fine to medium sand and gravel above bedrock. The water table is 12 ft. Sieve analysis of the formation gives D50 = 0.15 mm, uniformity coefficient 3.0, 35% passing 0.075 mm, and 10% gravel. The driller proposes a 6-inch Schedule 40 PVC screen with 0.040-inch slots and no filter pack, then plans to develop by surging and airlifting until sand clears. The well must produce 20 gpm for a 6-unit motel. A nearby well yielded 25 gpm with 8 ft of drawdown in a 24-hour test. The local board of health requires a minimum 4-inch annular seal and grout to surface.
1. Is a 0.040-inch slotted PVC screen with no filter pack appropriate? Specify a defensible screen and filter-pack design.
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No. Formation D50 is 0.15 mm, with 35% fines, so a 0.040-inch (about 1.0-mm) slot is far too coarse; it will pass formation sand and cannot be developed into a stable natural pack. A filter pack is needed. A common design rule is a filter-pack D50 of 4-6 times formation D50, or 0.6-0.9 mm, using clean, washed, well-rounded quartz with a uniformity coefficient less than 2.5. The screen slot should retain about 90% of the filter pack; for a 0.7-mm pack, a slot around 0.015-0.020 inch (0.38-0.51 mm) is reasonable. Confirm by sieve analysis of the actual pack, not a catalog number. The pack should extend at least 5 ft above the screen and be placed by tremie from the bottom, not dumped from the surface. With 35% fines, development will be slow; avoid over-surging. If the pack cannot be placed properly, use a smaller-diameter screen or a different completion method.
2. Describe the annular seal and grouting sequence for the 12-inch borehole and 6-inch screen, including what must not be done.
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The annular space is 3 inches, which is adequate for a tremie. Centralize the screen and casing. Place the filter pack from bottom up with a tremie pipe, rising in stages, and compare actual volume with theoretical volume. Above the top of the pack, install a bentonite pellet or bentonite-cement seal, then grout the remaining annulus to surface with a low-permeability grout such as neat cement or cement-bentonite by tremie from the bottom. Do not pour bentonite chips from the surface and assume they form a continuous seal; they can bridge and leave voids. Do not grout through the screen interval or allow grout to mix with the filter pack. Provide a surface sanitary seal and a proper well cap. If the borehole is unstable, case it or use a temporary liner. Record pack and grout volumes and any loss of circulation.
3. What development and testing criteria should be met before declaring the well successful?
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Develop by gentle surging and pumping, not prolonged high-rate airlifting, until sand content is minimal. A practical guideline is no more than a trace of sand in a clear sample after 30 minutes, or less than about 1 teaspoon per 5 gallons; local criteria may be stricter. Measure sand with an Imhoff cone or filter. Then run a constant-rate test at 20 gpm for at least 4 hours, longer if the well will serve a public system, with drawdown and recovery readings. Acceptance should include: pumping level stabilizes, specific capacity is adequate, sand production is negligible, and water quality meets the intended use. If the well cannot make 20 gpm without excessive drawdown, the motel needs storage or a second well. Do not accept a well merely because it 'clears up' at the end of a short airlift; fine sand can migrate later and damage pumps and fixtures.