Annular Grout Volume Shortfall on a 120-ft Domestic Well
You are grouting the annulus of a 120-ft domestic well. The borehole is 10 inches in diameter; the 6.625-inch OD PVC casing is set at 120 ft. The driller mixes neat cement (1 sack = 94 lb, yield about 1.18 ft³) and pumps through a tremie. After 180 gallons the grout level in the annulus has not reached the surface; the driller wants to stop and top off later. The formation from 0-30 ft is sandy clay, 30-80 ft is saprolite with voids, and 80-120 ft is fractured rock. The casing has centralizers at 20 ft and 100 ft. Georgia requires the annular space to be grouted to land surface.
1. Calculate the theoretical annular grout volume and the number of sacks required, showing your units.
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Annular area equals π/4 times the difference of the squares of the hole and casing diameters: π/4 × (10² - 6.625²) = 0.7854 × (100 - 43.89) = 44.06 in². Volume per foot is 44.06 in² × 12 in/ft = 528.7 in³/ft, which is 528.7 ÷ 231 = 2.29 gal/ft. For 120 ft, the theoretical volume is 2.29 × 120 = 274.6 gal, or 274.6 ÷ 7.48 = 36.7 ft³. At 1.18 ft³ per sack, that is 36.7 ÷ 1.18 = 31.1 sacks. In practice add 10-25 percent for hole enlargement, centralizer displacement, and loss to voids, so plan on roughly 300-345 gal and 35-39 sacks. The 180 gal already pumped is only about 65 percent of the theoretical volume, so the annulus is not full and the shortfall is not a rounding error.
2. Diagnose the likely causes of the shortfall and explain why stopping and topping off later is unacceptable.
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The saprolite with voids from 30-80 ft is the most likely thief zone; grout can flow into open voids rather than rising uniformly, especially if the tremie is not kept submerged. A second possibility is that the grout channeled through a bridge or around a centralizer and left an ungrouted pocket on one side. A third is that the tremie was raised above the grout surface and the new grout fell through water, diluted, and set as a weak plug. Topping off later cannot fix any of these: pouring grout from the surface creates a bridge at the top and leaves the lower annulus unsealed, which is exactly where the fractured rock and saprolite need isolation. The result would be a well that passes a surface inspection but still has a vertical pathway. The correct response is to keep pumping until returns reach land surface, using a thixotropic or cement-bentonite mix if losses continue, or to pressure-grout the void zone through a packer. Verify by sounding or by observing return, not by estimating from sacks.
3. Describe the proper tremie placement procedure and the verification you would require before accepting the seal.
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Lower the tremie to within a few feet of the bottom, pump a water-free grout mix continuously, and keep the discharge end submerged at least 5 ft in fresh grout as the column rises. Do not raise the tremie faster than the grout rises, and never let it come out of the grout; if the pump stops, the tremie may have to be pulled and the hole reamed before regrouting. Centralizers keep the casing off the wall so grout can flow around it. When returns reach land surface, continue pumping long enough to confirm the returns are the same mix, not drilling fluid. Verification includes measuring the volume placed against the theoretical volume, observing return at surface, and checking the grout level after the initial set. If the well is in a sensitive area, a pressure test or a downhole camera can confirm continuity. The driller should record the final volume, mix design, and any loss zone on the completion report.