St. Martin Parish, LA Shrink-Swell Risk
Shrink-swell potential in St. Martin Parish, LA
Very High
Soil across 64.7% of St. Martin Parish, LA comes back "Very High" in USDA's shrink-swell data, averaging 11.5% linear extensibility. Ground that moves this much routinely produces the classic expansive-soil symptoms: diagonal stair-step cracking, doors that stick seasonally, uneven floors. A standard footing is a poor bet here.
What this number does and doesn't tell you
This is a countywide, area-weighted figure across every soil survey map unit in St. Martin Parish, LA - never a reading for one specific parcel. The real answer for one lot is still a site-specific geotechnical report, running about $1,000-$5,000 (typically near $2,700).
St. Martin Parish: the real numbers
Given St. Martin Parish's rating, spending a few thousand now is the way to avoid spending tens of thousands later. A geotechnical report runs $1,000-$5,000 (typically $2,700); an actual foundation repair, once cracking sets in, typically runs $5,000-$15,000 (around $10,000 typical) - roughly 4x the report.
Where this sits nationally
Among the 3,085 rated counties on this site, St. Martin Parish, LA ranks 14 most-constrained nationally, and 10 of 64 within Louisiana. St. Martin Parish covers 738 square miles and counts 51,218 residents.
For contrast
Orleans Parish, LA carries this site's most-constrained reading. San Saba County, TX and Montgomery County, NY are worth a look for comparison.
How this reading is built, and its real limits · The warning signs of expansive-clay damage, explained
Elsewhere in Louisiana
Every county in Louisiana is on this site, including Webster Parish, LA and St. Tammany Parish, LA. The full underlying dataset is downloadable directly, one row per county.