Pierce County, Washington
The soil mapped across Pierce County is predominantly classified well drained. Below is what that classification means for a basement, what it does not mean, and what the three repairs it points to cost here.
39% of the county’s mapped soil area falls in classes the USDA describes as wet enough to need artificial drainage for ordinary use. Saturated soil against a foundation wall is what creates hydrostatic pressure.
Linear extensibility 4.0% in the top metre — moderate. The soil moves measurably, but a footing tolerates it where drainage works.
Shallowest annual water table maps at about 55 cm (1.8 ft) below the surface — water table very shallow. A typical US basement floor sits just under 200 cm, which is also the deepest the soil survey reports.
Source: BasementSoil physiographic region model — region “Puget Sound and Willamette Lowland”. Not a soil survey of this county. Why.
What the ground here is made of
Puget Sound and Willamette Lowland. Vashon glacial till and lacustrine silts, with a dense hardpan below
The one part of the West where basements are genuinely common — and the reason they leak is glacial: dense Vashon till under a permeable cap perches water just above it through the wet season. Volume change is low. Drainage, and hillside seepage, decide everything.
How the county’s soil area splits between drainage classes
| USDA drainage class | Share of mapped area | |
|---|---|---|
| excessively drained | 1% | |
| somewhat excessively drained | 5% | |
| well drained | 31% | |
| moderately well drained | 23% | |
| somewhat poorly drained | 22% | |
| poorly drained | 14% | |
| very poorly drained | 3% |
Drainage class definitions: USDA NRCS SSURGO. Highlighted row is the dominant condition (drclassdcd). Wettest class present in the county: poorly drained.
Key soil values for Pierce County
| Dominant drainage class | well drained — water leaves readily; the profile is saturated only briefly after rain |
|---|---|
| Wettest class present | poorly drained |
| Area needing artificial drainage | 39% |
| Linear extensibility (shrink-swell) | 4.0% — moderate modelled |
| Shallowest annual water table | 55 cm (1.8 ft) |
| Hydrologic soil group | C |
| Physiographic region | Puget Sound and Willamette Lowland |
| Expected radon zone | Zone 2 (moderate) — predicted indoor average between 2 and 4 pCi/L expected, not the official designation |
| Basements in this state | a minority of homes have one |
What the three likely repairs cost in Washington
| Reference job | Modelled range in WA |
|---|---|
| Interior drain along two walls + sump pump | $5,850–$12,600 |
| Exterior excavation of one 40 ft wall | $4,400–$12,550 |
| Underpinning on 10 piers | $17,400–$37,100 |
Reference basement 30 × 40 ft. Price book 2026.08, updated 2026-08-11, regional factor for WA: ×1.16. Run your own dimensions.
Given this soil, the order of operations here
- Fix the cheap outside causes first. Extended downspouts, a discharge line that ends far from the house and positive grade for the first 3 metres. This is a few hundred to a few thousand dollars and it resolves a real share of wet basements.
- Then decide interior versus exterior — with the soil in mind, not the brochure. Honest comparison.
- Assume the pump is a permanent fixture, not a backup. With the water table this shallow, budget for a battery or water-powered backup on day one, and treat pump failure as a question of when. Size it properly.
- Get the second and third quote before you sign anything. Same scope, in writing, from firms that are not owned by the same parent. How to do it without paying twice.
Questions about Pierce County soil
Does “well drained” soil mean my basement will flood?
No. The class describes how long the soil profile stays saturated in an average year across mapped areas of the county. It raises or lowers the statistical odds of hydrostatic pressure against a wall. It says nothing about your grading, your footing drain or your downspouts — which is usually what decides whether water actually gets in.
Which number should I care about most?
Whichever is highest, because they point at different contractors. High moisture and water table risk is a drainage and pump problem. High crack risk with low moisture risk — common on the Colorado Front Range and in the Flint Hills — is a structural problem, and a waterproofing salesman is the wrong person to call about it.
A contractor told me my county has “the worst soil in the state”. Is that true?
Check it here and on the USDA source we link. It is a specific, checkable claim, and in our experience of the sales scripts it is used loosely. Pierce County scores 51 out of 100 overall on our combined index, which is elevated for the counties we cover.
Will my homeowners insurance pay for this?
Almost certainly not, if the cause is groundwater seeping in gradually. That is excluded from standard US homeowners policies, and the exclusion is usually the reason people find this site. What is and is not covered.
Does “well drained” soil mean my basement will flood?
No. The class describes how long the soil profile stays saturated in an average year across mapped areas of the county. It raises or lowers the statistical odds of hydrostatic pressure against a wall. It says nothing about your grading, your footing drain or your downspouts — which is usually what decides whether water actually gets in.
Which number should I care about most?
Whichever is highest, because they point at different contractors. High moisture and water table risk is a drainage and pump problem. High crack risk with low moisture risk — common on the Colorado Front Range and in the Flint Hills — is a structural problem, and a waterproofing salesman is the wrong person to call about it.
A contractor told me my county has “the worst soil in the state”. Is that true?
Check it here and on the USDA source we link. It is a specific, checkable claim, and in our experience of the sales scripts it is used loosely. Pierce County scores 51 out of 100 overall on our combined index, which is elevated for the counties we cover.
Will my homeowners insurance pay for this?
Almost certainly not, if the cause is groundwater seeping in gradually. That is excluded from standard US homeowners policies, and the exclusion is usually the reason people find this site. What is and is not covered.
Nearby counties
Neighbours in the same physiographic region behave similarly. Different region on the other side of the county line means genuinely different ground.
All 39 counties in Washington · More about “well drained” soil
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