Annular Seal in a Caving Sand and Gravel Hole
A driller is completing a 6-inch Schedule 40 PVC casing in a 10-inch borehole to 85 feet. The upper 20 feet is sandy fill, 20 to 55 feet is caving sand and gravel, and 55 to 85 feet is fractured shale. The static water level is 22 feet. The driller sets the casing to 80 feet, leaving a 5-foot sump, and plans to pour 3/4-inch bentonite chips from the surface into the annular space until the hole looks full, then cut off the casing and install a cap. A neighboring irrigation well 200 feet away has had muddy water since a previous well was drilled nearby. The local health department requires a continuous annular seal. The driller has a positive-displacement grout pump and a tremie pipe on the rig but wants to save time with chips.
1. Evaluate the proposed bentonite-chip method. Will it create a continuous annular seal in this borehole? Explain the failure modes and why the 22-foot water level matters.
Read the worked answer
The chip method will not reliably produce a continuous seal in this hole. It may look full at the surface while leaving long unsealed intervals downhole.
Failure modes:
- The borehole is caving between 20 and 55 feet, so the annulus is irregular. Chips bridge and arch at ledges, at the casing shoe, and at the top of the caving zone instead of falling to the bottom.
- The water level is only 22 feet deep. Chips free-fall through water, hydrate, swell, and can bridge near the water surface or at 30 to 40 feet. The lower annulus may remain open.
- Hydrated chips form a soft, non-uniform mass that does not displace water or press against the formation. They do not provide the density and continuity needed to stop vertical migration.
- The fractured shale below 55 feet and the nearby irrigation well suggest a possible connection between aquifers. An unsealed annulus could let muddy water or surface contamination move along the casing.
A surface inspection cannot prove continuity. The driller would be accepting the seal on faith, which does not satisfy the local health department's requirement or Part 127. Chips can be acceptable in some dry, stable, shallow holes when placed with a tremie and verified, but not in a caving hole with a shallow water level and a suspected cross-connection.
2. Design the grout program. Include material, volume estimate, placement method, and acceptance criteria. Show the calculation.
Read the worked answer
Use a tremie-placed grout designed for annular sealing, not surface-poured chips.
Material:
- Neat cement slurry at about 15 to 16 pounds per gallon, or a high-solids bentonite grout at about 9 to 10 pounds per gallon. Neat cement is preferred where the caving zone needs density and strength; bentonite is acceptable if mixed to high solids and placed under pressure.
Volume calculation:
- Hole diameter = 10 inches; casing outside diameter for 6-inch Schedule 40 PVC = 6.625 inches.
- Annular area = 0.7854 x (10^2 - 6.625^2) = 44.1 square inches = 0.306 square feet.
- Grouted length = 80 feet.
- Theoretical volume = 0.306 x 80 = 24.5 cubic feet = 183 gallons.
- Add 25 percent for the caving zone and normal loss: about 30.6 cubic feet, or 229 gallons.
- For neat cement yielding about 1.15 cubic feet per 94-pound sack: 24.5 / 1.15 = 21.3 sacks theoretical; with 25 percent excess, about 27 sacks. The caving zone may take much more, so the driller should have at least 35 to 40 sacks on site.
Placement:
- Lower a tremie pipe to within a few feet of the bottom, attach centralizers to the casing, and pump grout from the bottom up.
- Keep the tremie end submerged in grout and maintain positive pressure. Do not pull the tremie out of the grout column.
- If returns are lost before grout reaches the surface, stop, wait, and investigate. Do not continue pumping blindly.
Acceptance criteria:
- Grout returns at the surface.
- Actual volume used is at least the theoretical volume plus 20 to 25 percent, with no unexplained loss.
- The annulus is full after settling; top off if needed. Document volumes, pressures, and returns.
3. The hole collapses at 45 feet before grouting can start. What are the options, and what would you refuse to do?
Read the worked answer
A collapse at 45 feet means the annulus is no longer a continuous open space, so the grout program must be changed before any sealing is attempted.
Acceptable options:
- Ream the hole with drilling fluid to clear the collapse and keep the annulus open long enough to run the tremie.
- Set a temporary surface casing through the caving zone, grout the upper section outside that casing, then grout the lower section in a second stage.
- Use a packer and pressure-grout the annulus in stages from the bottom upward.
- If the collapse cannot be stabilized, redrill the hole or use a larger borehole and a grout mixture with lost-circulation material.
What to refuse:
- Do not pour bentonite chips or dump sacks of material from the surface and call the hole sealed. The material will bridge at the collapse and leave the lower annulus open.
- Do not accept a full-looking hole at the surface as proof of a seal.
- Do not grout through a plugged tremie or with the tremie end above the collapse zone.
The driller should document the collapse, notify the owner, and contact the local health department if the approved construction method must change. The final seal must be continuous from the bottom of the casing to the surface, and the record must show how that was achieved.