Grouting a Narrow Annulus and Abandoning a Corroded Old Well
A replacement well is planned on a lot with an old 2-inch galvanized well. The old well is 90 ft deep, its casing is corroded through at about 12 ft, and the top is at grade in a proposed patio. The new well will use a 6-inch borehole with 4-inch PVC casing to 120 ft. The driller plans to pour bentonite pellets from land surface into the 1-inch annular space, then install a pitless adapter and a sanitary seal. He also plans to leave the old well open because it is only 2 inches and will be covered by the patio. The local health department permit requires abandonment of the old well before the new well is placed in service.
1. Is the 1-inch annulus acceptable for grouting? What should be done?
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The 1-inch annulus is too narrow for reliable tremie grouting, and surface-poured bentonite pellets are not an acceptable substitute. NC grouting rules require a continuous seal placed from the bottom of the casing upward, usually with a tremie pipe and positive-displacement pump. In a 1-inch annulus, the tremie and grout flow are easily bridged, especially with pellets or chips. The driller should enlarge the borehole, typically to 8 inches for 4-inch casing, to provide a 2-inch annulus, or use another method specifically approved by the health department. The grout should be neat cement or bentonite slurry, pumped from the casing shoe to land surface, with volume checked against the annular area. A sanitary seal and pitless adapter do not substitute for a full-length annular seal; they protect the top of the well only.
2. Write the abandonment procedure for the old well.
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The old well must be abandoned, not covered by a patio. The procedure is to remove the pump, drop pipe, and debris; disinfect the well; and fill the entire well from bottom to land surface with an approved grout, such as neat cement or bentonite slurry, placed by tremie. Because the casing is corroded at 12 ft, the driller should not assume a simple fill will stay in place. If grout escapes into the formation, the driller should use a thicker grout, add sand, or pressure-grout in stages until the well is sealed. The casing should then be cut off below land surface, capped, and the site restored. The driller should record the abandonment depth, grout volume, method, and any problems, and submit the required abandonment record to the health department. Leaving the old well open creates a direct pathway for surface water and contaminants to enter the same aquifer the new well will use.
3. What if the old well flows artesian or intersects a large bedrock fracture?
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An artesian flow or large fracture changes the abandonment design because grout can be diluted, washed away, or carried into the fracture before it sets. The driller should first control the flow and evaluate whether the well can be filled with a heavier, faster-setting grout or a cement-sand mix. In some cases, the driller may need to pressure-grout the fracture zone, use a packer, or place grout in stages and let each stage set before adding more. The objective is still a verified seal from bottom to land surface. The driller should not stop at a calculated volume if grout is being lost; the fill should continue until returns or refusal indicate the well is sealed. Because flowing wells can affect nearby wells and surface water, the health department should be notified of the condition and the final abandonment method should be documented.