Grout Volume and Lost Circulation in a Till-to-Sandstone Well
A contractor is drilling a residential well with 6-inch-diameter PVC casing in glacial till over fractured sandstone. The borehole is 8 inches in diameter to 140 ft, and the casing is set to 120 ft. The plan calls for grouting the annular space from the bottom of the casing to the ground surface. The driller has 25 sacks of neat cement grout, each 94 lb and yielding about 1.15 ft³. The static water level is 35 ft. During grouting, the crew pumps 40 gallons and sees no grout return at the surface; pump pressure rises slowly. The owner suggests stopping and filling the upper annulus with bentonite chips because the till is 'tight.' The site is 60 ft from a septic tank and 25 ft from the property line. The driller wants to know whether the grout job can be accepted and whether the chips are an acceptable substitute.
1. Calculate the annular grout volume for 120 ft of casing in an 8-inch borehole, compare it with the 25 sacks on site, and state whether the supply is adequate. Show your units.
Read the worked answer
The annular area is the difference between the borehole and casing areas:
Area = π/4 × (8² − 6²) = π/4 × 28 = 21.99 in².
Volume per foot = 21.99 in² × 12 in/ft = 263.9 in³/ft.
Convert to gallons: 263.9 ÷ 231 in³/gal = 1.14 gal/ft.
For 120 ft: 1.14 × 120 = 137 gal. Add 10–20% for hole roughness and minor loss: about 151–164 gal.
The 25 sacks yield 25 × 1.15 = 28.75 ft³; 28.75 × 7.48 = 215 gal. The supply is adequate for the calculated volume, but volume alone does not prove the annulus is sealed.
2. The crew has pumped only 40 gallons and has no returns. What should the driller do before accepting the grout seal? Explain why stopping at 40 gallons is not acceptable.
Read the worked answer
Forty gallons is only about 29% of the calculated 137 gallons needed to fill the annulus. No return means the grout is going into the formation, bridging, or channeling. That is not a completed seal. The driller should keep the tremie pipe submerged in grout, slow the pump, check for a bridge, and use approved lost-circulation control or pressure grouting if the formation is taking grout. Grout must be confirmed at the surface before the job is accepted. If returns cannot be established, the well may need re-grouting or replacement; do not sign off on a partial annular seal.
3. Evaluate the owner’s proposal to use bentonite chips in the upper annulus instead of continuing tremie grouting.
Read the worked answer
Bentonite chips are not an equivalent substitute for continuous tremie grout from the bottom of the casing to the surface. Chips can bridge, leave voids, and fail to bond to the borehole wall, especially where the hole is rough or the formation is losing fluid. The code requires a continuous low-permeability annular seal placed by an approved method, normally tremie grout. The 25-ft property-line distance does not change the grouting requirement. The driller should finish the grout seal and document returns; chips should not be used to shortcut a failed grout job.