Grouting and Sealing
Grouting is the seal between casing and borehole wall. It blocks surface water and shallow groundwater from running down the annulus, stabilizes the formation, and prevents cross-connection between aquifers. Exam items focus on materials, placement method, and volume.
Approved materials and methods
- Neat cement, cement-bentonite, sand-cement, and high-solids bentonite grout are the usual choices; additives must be suitable for potable water contact.
- Grout is placed by tremie pipe or positive-displacement pump from the bottom of the annulus upward, continuously. The discharge end stays submerged in grout; free-falling grout from the top bridges and dilutes.
- If the hole caves before grouting, grout through an inner string or grout shoe instead of abandoning the seal.
- Grout after the casing is set and before development, and record the volume and method on the well record.
Worked example
A 10-in. borehole holds 6.625-in. casing to 60 ft. Annular area equals 0.7854 times (100 minus 43.89) square inches, about 44 in2, or 0.306 ft2. Times 60 ft gives 18.4 ft3. Neat cement yields about 1.18 ft3 per 94-lb sack, so the neat requirement is roughly 16 sacks; adding 30 percent for washouts and lost circulation gives about 20 to 21 sacks. Order the larger figure and pump until grout of the same mix returns at the surface.
Traps
- Pouring grout from the top and calling it sealed.
- Mixing slurry so thin that it bleeds water into the formation and shrinks.
- Grouting only the upper few feet when full-length sealing is required.
- Pulling the tremie out of the grout column mid-placement, trapping a void.