Short definition
A well casing is the steel or PVC liner installed inside the borehole. It serves two jobs: structural support to keep the bore open against soil collapse, and sanitary isolation of the producing aquifer from non-target surface and shallow strata. The casing terminates at a well screen or open hole into the production zone at the bottom and a sanitary cap at the top.
What it is
A driller advances casing as the borehole deepens. The annular space — the gap between the casing and the surrounding soil — gets sealed with bentonite or grout to prevent surface water from short-circuiting down to the aquifer. At the bottom, a slotted well screen or open hole admits water from the producing zone. At the top, the casing extends above grade and is sealed with a sanitary well cap.
Casing material in Washington is typically steel (older wells, sometimes thin-walled and prone to corrosion over decades) or PVC (modern installations where soil and depth allow). A pitless adapter penetrates the casing wall below the frost line to bring the supply line out horizontally toward the house.
Why it matters to a homeowner
You’ll hear “casing” in three conversations:
- Real-estate inspection on a rural Washington well. Inspector confirms the casing extends at least 6 inches above grade and is properly capped — both code-compliance items.
- Well rehabilitation. Corroded steel casing can be re-lined or replaced in sections; an old well with failed grouting may need re-grouting of the annulus.
- Well log review. Every Washington drilled well has a well construction report on file with the Department of Ecology, listing casing material, diameter, depth, and grouting. Worth pulling when buying property or planning pump work.
Washington note
Washington wells are constructed under WAC 173-160 by drillers licensed under RCW 18.104. Specific requirements relevant to casing:
- WAC 173-160-291 — casing must extend at least 6 inches above grade, and at least 2 feet above 100-year flood elevation in flood plains. The cap must be mechanically secured or welded.
- WAC 173-160-201 / -211 — casing material and grouting standards (specific section text not pulled in research; reachable through Ecology’s WAC navigation).
A well log filed with Ecology after construction documents the casing details. Homeowners can request the log from Ecology’s well log database.
Common failure modes
- Steel casing corrosion in older wells with thin-wall steel — collapsed casing or contamination.
- Casing perforation by drilling vibration through hard strata — admits non-target water.
- Annular grout failure — gap between casing and soil lets surface water short-circuit to the production zone.
- Tilted or misaligned casing — pump installation issues, drop-pipe wear.