Expansive Soil in Montana: County Ratings
12 of the 55 rated counties in Montana 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 |
|---|---|---|---|
| Beaverhead County | Low | 7% | 98% |
| Big Horn County | Moderate | 27% | 98% |
| Blaine County | High | 48% | 99% |
| Broadwater County | Low | 1% | 95% |
| Carbon County | High | 38% | 88% |
| Carter County | High | 53% | 100% |
| Cascade County | High | 37% | 98% |
| Chouteau County | Moderate | 46% | 98% |
| Custer County | Low | 18% | 95% |
| Daniels County | Moderate | 3% | 94% |
| Dawson County | Moderate | 4% | 96% |
| Deer Lodge County | Low | 2% | 94% |
| Fallon County | Low | 36% | 100% |
| Fergus County | High | 60% | 97% |
| Flathead County | Low | 1% | 94% |
| Gallatin County | Low | 12% | 96% |
| Garfield County | Low | 21% | 88% |
| Glacier County | Moderate | 23% | 99% |
| Golden Valley County | Low | 27% | 100% |
| Granite County | Low | 5% | 99% |
| Hill County | Moderate | 35% | 99% |
| Jefferson County | Low | 1% | 99% |
| Judith Basin County | High | 45% | 100% |
| Lake County | Low | 0% | 83% |
| Lewis and Clark County | Low | 6% | 97% |
| Liberty County | Moderate | 36% | 99% |
| Lincoln County | Low | 0% | 91% |
| Madison County | Low | 3% | 98% |
| McCone County | Moderate | 11% | 93% |
| Meagher County | Low | 0% | 96% |
| Mineral County | Low | 0% | 100% |
| Missoula County | Low | 1% | 95% |
| Musselshell County | Low | 26% | 98% |
| Park County | Low | 8% | 43% * |
| Petroleum County | High | 63% | 98% |
| Phillips County | High | 68% | 99% |
| Pondera County | High | 43% | 98% |
| Powder River County | High | 48% | 99% |
| Powell County | Low | 7% | 99% |
| Prairie County | Low | 8% | 94% |
| Ravalli County | Low | 0% | 100% |
| Richland County | Moderate | 3% | 97% |
| Roosevelt County | Moderate | 9% | 95% |
| Rosebud County | Moderate | 31% | 94% |
| Sanders County | Low | 2% | 98% |
| Sheridan County | Moderate | 4% | 97% |
| Silver Bow County | Low | 2% | 98% |
| Stillwater County | Moderate | 26% | 98% |
| Sweet Grass County | Low | 22% | 91% |
| Teton County | Moderate | 29% | 99% |
| Toole County | Moderate | 32% | 98% |
| Treasure County | High | 57% | 99% |
| Valley County | High | 58% | 98% |
| Wibaux County | Moderate | 24% | 95% |
| Yellowstone County | Low | 34% | 90% |
* 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.