Expansive Soil in California: County Ratings
9 of the 57 rated counties in California have a dominant shrink-swell rating of High or Very High. Each rating below is the NRCS shrink-swell class covering the largest share of the county's mapped soil acres, computed from USDA SSURGO data. Open a county for the full class breakdown and what it means for a slab foundation.
| County | Dominant class | High + Very High share | Survey coverage |
|---|---|---|---|
| Alameda County | High | 48% | 85% |
| Alpine County | Low | 0% | 87% |
| Amador County | Low | 2% | 86% |
| Butte County | Low | 41% | 97% |
| Calaveras County | Low | 3% | 96% |
| Colusa County | High | 51% | 100% |
| Contra Costa County | High | 68% | 83% |
| Del Norte County | Low | 4% | 97% |
| El Dorado County | Low | 1% | 86% |
| Fresno County | Low | 16% | 95% |
| Glenn County | Low | 43% | 96% |
| Humboldt County | Low | 8% | 100% |
| Imperial County | High | 81% | 94% |
| Inyo County | Low | 15% | 99% |
| Kern County | Low | 8% | 96% |
| Kings County | Low | 40% | 97% |
| Lake County | Low | 13% | 93% |
| Lassen County | Moderate | 20% | 97% |
| Los Angeles County | Low | 2% | 99% |
| Madera County | Low | 10% | 98% |
| Mariposa County | Low | 3% | 94% |
| Mendocino County | Moderate | 14% | 97% |
| Merced County | Moderate | 34% | 97% |
| Modoc County | High | 37% | 91% |
| Mono County | Low | 3% | 99% |
| Monterey County | Low | 17% | 98% |
| Napa County | Low | 10% | 93% |
| Nevada County | Low | 3% | 96% |
| Orange County | Low | 22% | 98% |
| Placer County | Low | 18% | 87% |
| Plumas County | Low | 6% | 95% |
| Riverside County | Low | 7% | 79% * |
| Sacramento County | High | 50% | 93% |
| San Benito County | Low | 5% | 100% |
| San Bernardino County | Low | 1% | 90% |
| San Diego County | Low | 25% | 91% |
| San Francisco County | Low | 0% | 21% * |
| San Joaquin County | Moderate | 39% | 93% |
| San Luis Obispo County | Moderate | 21% | 97% |
| San Mateo County | Low | 19% | 57% * |
| Santa Barbara County | Low | 15% | 87% |
| Santa Clara County | Low | 19% | 98% |
| Santa Cruz County | Low | 11% | 99% |
| Shasta County | Low | 11% | 96% |
| Sierra County | Low | 7% | 89% |
| Siskiyou County | Low | 16% | 95% |
| Solano County | High | 68% | 86% |
| Sonoma County | Moderate | 29% | 96% |
| Stanislaus County | Low | 21% | 98% |
| Sutter County | High | 54% | 99% |
| Tehama County | Low | 17% | 98% |
| Trinity County | Low | 12% | 100% |
| Tulare County | Moderate | 21% | 98% |
| Tuolumne County | Low | 2% | 91% |
| Ventura County | Low | 12% | 89% |
| Yolo County | High | 61% | 93% |
| Yuba County | Moderate | 28% | 97% |
* Less than 80% of this county's map acres have completed soil survey data; treat its rating as provisional.
How these ratings are computed
Ratings come from USDA NRCS SSURGO soil survey data: for each soil component we take the maximum linear extensibility percent (lep_r) in the top 100 cm, apply the NRCS Handbook Part 618 class limits (Low under 3 percent, Moderate 3 to 6, High 6 to 9, Very High 9 and above), assign map units by plurality of component percent, and roll acres up to the county. Full details on the methodology section of the lookup page. A county rating is not a parcel-level geotechnical assessment.