Annular Seal Where the First Clay Layer Is Above 50 Feet
A 5-inch PVC well for a 40-connection public water system is proposed to 80 ft in an alluvial valley. The log shows 0–10 ft sandy loam, 10–35 ft sand and gravel with the water table at 14 ft, 35–42 ft blue clay, and 42–80 ft gravel with a confined aquifer. The proposed design is an 8-inch borehole, screen from 45 to 75 ft, gravel pack from 42 to 80 ft, and an annular seal from 0 to 50 ft using dry bentonite chips. The county requires a 50-ft minimum seal but allows termination at the first confining layer. The owner wants maximum yield and asks whether the seal can be shortened to 30 ft to save money.
1. Evaluate the proposed 0–50 ft bentonite-chip seal and give a corrected seal design.
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The proposed seal is unacceptable in two ways. First, the first confining layer is the blue clay at 35–42 ft; the standards allow the seal to terminate at that layer, but they do not permit a seal that stops above it or that leaves a gravel-filled annulus across it. A 0–50 ft chip seal would place chips through the clay interval and into the gravel pack, so chips can bridge, mix with gravel, and leave voids. Second, dry bentonite chips placed in a water-filled annulus may swell and seal the hole before reaching depth. Design: set the gravel pack from 80 ft to about 42 ft; place a 2-ft bentonite plug or grout cap at the top of the pack; then seal the annulus from 42 ft to ground surface with cement-bentonite grout or a bentonite slurry placed by tremie from the bottom up. Include a surface seal of at least 20 ft. The seal must be continuous and bonded to the borehole wall.
2. How would you verify the seal before accepting the well?
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Calculate the theoretical annular volume from the 8-in borehole and 5-in casing and compare it with grout placed; a significant excess suggests a void or lost grout, while a shortfall suggests bridging. Measure the grout density and keep placement records. After the grout sets, tag the top of the seal with a weighted line to confirm it is at the designed depth, and run a video camera if available. Check that no gravel appears in the seal interval and that the gravel pack top is below the seal. For a public-system well, verify the surface seal and casing centralizers; if the volume or tag result is questionable, ream out the suspect interval and regrout rather than accept it. Do not rely on a dye test alone, because a dye can travel through a small channel without proving a continuous seal.
3. The owner asks to shorten the seal to 30 ft to save money. What is your response?
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Refuse the change. At 30 ft the seal would stop 5 ft above the top of the blue clay and leave a sand/gravel connection around the casing from 30 to 42 ft. That is a direct path between the shallow water-table aquifer and the confined aquifer, and it defeats the purpose of the confining layer. The standards allow a seal shorter than 50 ft only because the confining layer is shallower; they do not allow the seal to stop above that layer. Shortening the seal would also risk drawing shallow, possibly contaminated water into the public-supply well and could cause the county to reject the well. If cost is the issue, reduce the seal only to the top of the clay with written county approval, never to 30 ft.