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Methodology and data sources
We are not engineers and this is not an engineering opinion. We report public soil-survey data and published price ranges. Nothing here states that a specific house is or is not structurally sound; a licensed structural engineer who has seen the house can.
Soil data: USDA NRCS Soil Survey (SSURGO)
Every soil figure comes from the USDA Natural Resources Conservation Service's SSURGO database, queried live through Soil Data Access. It is public, free, and the same dataset engineers and county extension offices use. It is a survey of map units, not of individual lots: a map unit is the smallest area the survey delineates, typically a few acres, and it names the soil series found there with their measured properties.
How we compute shrink-swell
- For each major soil component in a map unit we take every horizon (layer) from 0 to 152 cm (60 inches) and compute a thickness-weighted mean of linear extensibility percent (
lep_r), the NRCS measure of volume change between moist and dry. - Components are combined by their mapped percentage of the unit. Map units are combined by area: on an address lookup, the intersected polygon area; on a county page, mapped acreage.
- Classes follow NRCS: low < 3, moderate 3–6, high 6–9, very high > 9.
The urban hole, and what we do about it
Map units named plainly “Urban land” carry no measured properties. Complexes such as “Houston Black–Urban land complex” do. When the unit under an address has no data we widen to a 400 m box, then 800, 1,600 and 3,200, and stop at the first radius where at least 25% of the area has data. The result is labelled a neighborhood estimate with the radius, the data share, and the split of surveyed ground by class. If no radius reaches 25% the result is marked low confidence.
County pages
City pages are built on county soil legends, and name every county they are built on. Counties split across several survey areas are assembled from all of them. A city page exists only where at least 70% of the county's mapped acreage carries usable LEP data across at least 20 map units; where it does not, there is no page.
Known limits
- LEP describes the soil, not your house. An expansive-clay rating means movement is common in that soil class; it does not mean your foundation has moved, and a low rating does not mean it hasn't.
- Geocoding uses the US Census geocoder, then OpenStreetMap Nominatim as a fallback. A failed geocode is reported as a failure, never as a soil result.
- SSURGO is updated roughly yearly; county pages show the date they were built.
We ran it on these addresses
Validation run of 2026-08-30: six real addresses in five states, chosen to span the classes. Reproducible with the lookup above.
| Address | Precision | Map unit | LEP | Class | Clay | Drainage |
|---|---|---|---|---|---|---|
| 100 Military Plaza, San Antonio, TX 78205 | address | Branyon clay, 0 to 1 percent slopes | 12.37% | very high | 51.78% | Moderately well drained |
| 1500 Marilla St, Dallas, TX 75201 | neighborhood (400 m) | Axtell-Urban land complex, 1 to 5 percent slopes | 4.31% | moderate | 24.1% | Moderately well drained |
| 200 W Washington St, Phoenix, AZ 85003 | address | Laveen loam, 0 to 1 percent slopes | 1.5% | low | 15.45% | Well drained |
| 1015 Cultural Park Blvd, Cape Coral, FL 33990 | address | Matlacha gravelly fine sand, limestone substratum-Urban land complex, 0 to 2 percent slopes | 0.31% | low | 3.77% | Somewhat poorly drained |
| 901 Bagby St, Houston, TX 77002 | neighborhood (400 m) | Texla-Urban land complex, 0 to 2 percent slopes | 4.09% | moderate | 21.88% | Somewhat poorly drained |
| 1200 Market St, Chattanooga, TN 37402 | — | —% | — | —% | — |
Cities we refused
The doorway guard refused Boston (Suffolk County, 69%), Milwaukee (Milwaukee County, 59%), Philadelphia (Philadelphia County, 42%): their county legends carry usable shrink-swell data on less than 70% of mapped acreage, so a page would be a template with a name on it. No data, no page.
Cost figures
Every price range on this site is quoted from a named, dated, published cost guide and linked at the point of use. The guides quoted, as fetched:
We do not have city-specific price data and say so on every city page; local prices are driven by pier depth, pier count and access, which is exactly what an independent engineer's report pins down.
Search and demand data
City selection used exact-match Google Ads search volume, county soil profiles and live search results, so that pages exist where people search and where the soil record says something specific. Cities were not chosen by hand.