Low risk About 3% of Clark County's soil area is high shrink-swell (expansive) clay — 2.0 times the Washington average of 1%, and far below the national average of 17%. That places it #8 of 33 Washington counties for foundation soil risk.
Share of the county's ~404,544 acres of USDA-mapped soil with linear extensibility ≥ 6% in the top meter (SSURGO).
| High shrink-swell (expansive) clay | 3% |
| Moderately expansive | 39% |
| Low / non-expansive | 58% |
| Foundation risk tier | Low |
| Rank in Washington | #8 of 33 counties |
| Higher-risk than | 35% of all U.S. counties |
Expansive clay swells as it takes on water and shrinks as it dries, and that repeated movement is what lifts and drops a foundation unevenly — opening stair-step cracks, racking door and window frames, and, left unmanaged, cracking slabs and footings. Clark County's exposure is low-to-moderate. With just 3% high-expansive soil, expansive clay is unlikely to be the main driver of foundation movement in Clark County. Settlement here more often traces to drainage, fill, tree roots, or original construction — worth a diagnosis before paying for clay fixes.
Clark County's shrink-swell risk is concentrated in the Cove soil series alongside Sara and Minniece variant — clays the USDA maps as strongly expansive, swelling and shrinking with every wet–dry cycle. Homes built on these series most need the drainage and moisture discipline above; a lot-level soil report (or the county NRCS survey) shows which one sits under a given address.
| County | High-risk soil | |
|---|---|---|
| Higher risk → | Thurston County | 3% |
| This county | Clark County (#8 of 33) | 3% |
| Lower risk → | Whitman County | 3% |
For context, the average Washington county is 1% high-expansive soil and the average U.S. county is 17%.
Foundation problems get more expensive the longer they wait. Get an assessment and repair quotes from independent local pros.
Get repair quotes →Costs follow the same structure everywhere — from a few hundred dollars for a single crack injection to $8,000–$25,000+ for pier stabilization on a settling home. At this risk level the clay is rarely the culprit, so a proper diagnosis is the first dollar to spend. See the full foundation repair cost guide for method-by-method pricing.
Risk metrics are computed from USDA SSURGO soil survey data (linear extensibility of soil components, area-weighted by county). Soil varies lot to lot — this is county-scale context, not a substitute for a site-specific geotechnical or structural assessment.