Well Casings, Seals, and Annular Space Requirements
What this domain tests
Casing, seals, and annular space are the well's containment system. The exam checks that you know what casing must be, how deep and with what material the annulus is sealed, how grout is placed and verified, and how the surface completion keeps surface water out.
Casing
- Casing may be steel or plastic but must be sound, straight, and of adequate wall thickness for the depth and load; used pipe is acceptable only if it is in good condition. Many local ordinances set minimum wall thickness and specify Schedule 40 or heavier plastic.
- The casing top must terminate at least 12 inches above finished grade and above expected flood level, fitted with a watertight, vermin-proof sanitary cap with a downward-facing screened vent.
Depth and material of the annular seal
- DWR Bulletin 74-90 requires a continuous annular seal from land surface to a minimum depth of 20 feet. Deeper seals, commonly 50 feet or to below the shallowest protected aquifer, are required by many local ordinances and by site conditions.
- Acceptable materials include neat cement grout, cement-and-sand grout with limited sand, bentonite grout, bentonite pellets placed so they cannot bridge, and concrete. Drill cuttings and drilling mud are not seals.
- Mix neat cement at roughly 5 to 6 gallons of clean water per 94-pound sack; extra water weakens the grout and increases shrinkage and permeability.
Placement and surface completion
- Place grout by tremie pipe from the bottom of the seal upward, keeping the discharge below the grout surface. Never pour from the top.
- Use one continuous placement. Estimate volume from the theoretical annulus and add 25 to 50 percent for washouts and overbreak.
- Finish with a concrete surface seal at least 4 inches thick, extending at least 2 feet from the casing in all directions and sloped away from the well.
Worked example
An 8-inch hole with 4-inch casing sealed to 20 feet. Annulus area = 0.7854 x (0.667 squared minus 0.333 squared) = 0.262 square feet. Volume = 0.262 x 20 = 5.2 cubic feet. Neat cement yields about 1.15 cubic feet per sack, so 5.2 divided by 1.15 is about 4.6 sacks; adding 25 percent for waste, order 6 sacks.
Traps that cost marks
- Computing annulus volume with radii where diameters belong.
- Topping off a thin cap of cement instead of running a continuous tremie seal.
- Grouting in lifts, then drilling the seal out later to fit an adapter.