Sealing a Flowing Abandoned Well with Gravel Pack
A property owner has an abandoned 4-inch steel-cased well, 85 ft deep, with an open annular space and a gravel pack. The well flows at about 2 gpm at land surface, and the casing stands 6 inches above grade. The owner wants to cut the casing at ground level and fill the well with bentonite chips poured from the top. There is a septic system 40 ft away and a wetland 30 ft away. The local government unit has ordered the well sealed before a building permit is issued. The driller proposes to dump 25 bags of bentonite chips, each 50 lb and yielding about 0.75 ft³, into the casing and leave the upper casing in place. The driller argues that because the well is flowing, the water will carry the chips down and seal the hole.
1. Calculate the approximate volume needed to fill the 4-inch casing to its full 85-ft depth and compare it with the proposed 25 bags of chips.
Read the worked answer
A 4-inch inside diameter is 0.333 ft. Area = π/4 × (0.333)² = 0.0873 ft².
Volume for 85 ft = 0.0873 × 85 = 7.42 ft³.
In gallons: 7.42 × 7.48 = 55.5 gal.
Add 10–25% because the gravel pack and open annulus may take material: about 61–69 gal.
The 25 bags yield 25 × 0.75 = 18.75 ft³, or about 140 gal. The bulk volume is more than enough, but that does not make the method acceptable. The gravel pack may take extra grout, and the actual sealed volume could be larger than the casing volume alone.
2. Is pouring bentonite chips from the top acceptable in this flowing well? Explain the failure modes and the procedure the driller should use.
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No. In a flowing well, chips poured from the top can be carried upward or sideways, bridge near the top, and leave the lower well and annulus unsealed. The flow also keeps the borehole unstable. The driller should control the flow first, for example with a packer or by extending casing, then place an approved grout by tremie from the bottom upward. Neat cement or bentonite grout is preferred over chips. The grout should fill the entire well and annular space, with returns checked and the seal documented. If flow cannot be controlled, a heavier grout or staged sealing method may be needed.
3. What is wrong with cutting the casing at ground level and leaving the upper casing in place? What should be done with the casing and the site?
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Cutting at grade leaves a direct pathway and does not meet abandonment practice. The casing should be removed or cut off at least 3 ft below grade where possible, then the remaining hole should be sealed to the surface with approved grout. The site should be backfilled and marked or recorded as required. Leaving a flowing casing at grade also creates a physical hazard and can allow surface water to enter the seal. The driller should report the sealing to the local unit and keep a record of depth, materials, and grout volumes.