Lost Grout Returns and a Brick Dug Well
A 350-ft, 6-inch bedrock well is being completed with 60 ft of casing, 20 ft into bedrock. The annulus is 1.5 inches. The driller mixes neat cement and tremies from the bottom of the casing. At 20 ft of grout placement, returns are lost; the hole takes 25 sacks of cement without any return to surface. The driller proposes to stop, wait, and top off the annulus from the surface with a sack of bentonite. Nearby, a 24-inch-diameter brick-lined dug well, 25 ft deep, is to be abandoned; it has an old hand pump and 2 ft of sediment. The property owner wants both done 'by Friday' and asks whether the dug well can simply be filled with stone and covered with a concrete cap.
1. What does lost returns at 20 ft mean, and is topping off with bentonite acceptable?
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Lost returns mean the grout is going into a void, fracture, or permeable zone rather than rising in the annulus. Topping off from the surface with one sack of bentonite is not acceptable because it may bridge and leave a continuous unsealed channel. The driller should stop, record the volume placed, and use stage grouting: place grout by tremie to the top of the loss zone, wait for initial set, then re-tremie and top off. If losses continue, use a thicker cement-bentonite mix or approved lost-circulation material, and verify the final volume against the theoretical annular volume. The goal is a continuous grout column from the bottom of the casing to ground surface.
2. How should the 24-inch dug well be abandoned?
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Remove the hand pump, piping, and debris; disinfect the well; then fill from the bottom up with an approved grout, such as cement-bentonite or neat cement, using a tremie. A 24-inch by 25-ft well has a large volume, so calculate the volume and order enough material. Filling with stone and a concrete cap is not enough: stone leaves void spaces and a vertical pathway for surface water and contaminants. If the brick lining is unstable, clean out loose material first or use an approved bridging or backfill method below the grout, but the upper seal and the full vertical interval must be grouted. File the abandonment report with the state and give the owner a copy.
3. What records should be kept for both the grouting and the abandonment?
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Keep a grout log for both wells: date, mix design, sacks used, volume mixed, volume placed, tremie depth, returns, loss zones, waiting periods, and final top-off. For the dug well, record depth, diameter, volume calculation, debris removed, disinfection, and final grout volume. Photograph the completed surface seal. These records are the driller's defense if the seal is questioned and are needed for the state report.