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Data study · Updated August 2026

Texas Foundation Soil Risk Index

All 254 Texas counties, mapped to their real soil regions and ranked by foundation movement risk. 46 counties sit on very-high-risk expansive clay; 17,583,722 Texans live in the 106 counties rated high or very high.

Counties
254
Soil regions
10
Very-high risk
46 counties
Pop. at high+ risk
17.6M

How this index works

Each county is assigned to one of 10 soil regions following USDA Natural Resources Conservation Service (NRCS) land resource regionalization, and each region carries a foundation movement risk rating derived from the shrink-swell (expansive clay) behavior documented for its dominant soil series — the Houston Black and Heiden clays of the Blackland Prairie, the Lake Charles and Beaumont "gumbo" of the Gulf Coast, and their counterparts statewide, as characterized in NRCS soil surveys and Texas A&M AgriLife Extension research on expansive soils.

What this is: a planning-level index — which soil world a county sits in, and how aggressively that soil moves houses. What it is not: a parcel-level engineering assessment. Soils vary within counties, and a specific lot can behave better or worse than its region; a geotechnical or structural engineer is the authority for any individual property.

The 10 most exposed counties in Texas

Very-high-risk soil, ranked by the number of people living on it. This is where expansive clay meets the most homes:

Highest-risk Texas counties by population
# County Population Soil region Dominant behavior
1 Harris County 3,041,431 Gulf Coast Prairie coastal gumbo
2 Dallas County 2,812,788 Blackland Prairie black gumbo country
3 Bexar County 1,647,483 Blackland Prairie black gumbo country
4 Travis County 1,075,040 Blackland Prairie black gumbo country
5 Collin County 846,681 Blackland Prairie black gumbo country
6 Fort Bend County 392,853 Gulf Coast Prairie coastal gumbo
7 Williamson County 376,780 Blackland Prairie black gumbo country
8 Nueces County 342,755 Gulf Coast Prairie coastal gumbo
9 Bell County 313,798 Blackland Prairie black gumbo country
10 Galveston County 312,124 Gulf Coast Prairie coastal gumbo

All 254 counties by risk tier

Very high risk

46 counties · Blackland Prairie · Gulf Coast Prairie

Blackland Prairie: The Texas Blackland Prairie runs in a band from near the Red River south through Dallas, Waco, Temple and Austin toward San Antonio. Its signature soils — the Houston Black series is the best known — are deep Vertisols formed over chalk, marl and calcareous shale, dominated by smectite (montmorillonite) clays. These are among the most expansive soils mapped anywhere in the United States: they swell substantially when wet and shrink and crack deeply during dry summers.

Gulf Coast Prairie: The Gulf Coast Prairie — greater Houston, Beaumont, Victoria and the coastal bend — sits on young, flat coastal-plain deposits. The dominant soils, including the Beaumont and Lake Charles series, are deep, poorly drained clays with high plasticity: the local "gumbo." The land is extremely flat, water tables are shallow, and drainage is slow, so soil moisture around foundations is driven as much by ponding and drainage as by rainfall itself.

High risk

60 counties · Cross Timbers & Grand Prairie · Post Oak Savannah · South Texas Plains & Rio Grande Valley

Cross Timbers & Grand Prairie: West and southwest of Fort Worth — Weatherford, Cleburne, Granbury, Mineral Wells — the landscape alternates in narrow north-south bands: sandy soils weathered from sandstone in the Cross Timbers (Windthorst, Duffau series) and calcareous clays over limestone and marl in the Grand Prairie (San Saba, Denton, Sanger series). A single neighborhood, even a single lot, can cross from sandy ground onto expansive clay.

Post Oak Savannah: Between the Blackland Prairie and the East Texas forests — Bryan-College Station, Bastrop, Palestine, Athens — lies the Post Oak Savannah. Its typical soils (Alfisols such as the Axtell, Crockett and Wilson series) have a loamy or sandy surface over a dense, very slowly permeable clay subsoil: the local "claypan." That subsoil is genuinely expansive even though the surface looks sandy, which surprises homeowners who assume they don't have clay problems.

South Texas Plains & Rio Grande Valley: From San Antonio's southern edge through Laredo to the Rio Grande Valley, the brush country and delta lands carry heavy clay and clay-loam soils — series like Victoria, Harlingen, Montell and Maverick. Valley soils formed in Rio Grande delta clays are strongly expansive; upland brush-country soils range from moderately to highly expansive, all under an intensely hot, evaporation-dominated climate.

Moderate-high risk

22 counties · Hill Country & Edwards Plateau

Hill Country & Edwards Plateau: The Hill Country — from western Travis and Hays counties (Austin's west side, Dripping Springs, Wimberley) out through Kerrville and Fredericksburg — sits on Cretaceous limestone: Edwards and Glen Rose formations. Soils are typically thin, dark, clayey and calcareous (Mollisols such as the Tarrant, Brackett and Eckrant series) over hard or marly rock, with pockets of deeper expansive clay in valleys and on marl benches.

Moderate risk

110 counties · East Texas Piney Woods · Rolling Plains · High Plains & Panhandle

East Texas Piney Woods: East Texas — Tyler, Longview, Lufkin, Nacogdoches — is forested, gently rolling country over Eocene sands and clays. Typical upland soils have a sandy or loamy topsoil over a clayey subsoil (Alfisols and Ultisols such as the Bowie, Cuthbert and Nacogdoches series). The clay subsoils are less expansive than Blackland Vertisols, but they still shrink and swell, and the sandy caps let moisture reach them quickly.

Rolling Plains: The Rolling Plains — Abilene, Wichita Falls, San Angelo's northern reaches — are underlain largely by Permian-age red beds: red shales, clays and sandstones. Soils such as the Tillman, Vernon and Miles series range from moderately expansive red clay loams to sandier ground, under a climate with long dry spells and hard, fast rains.

High Plains & Panhandle: The Llano Estacado and Panhandle — Lubbock, Amarillo, Plainview — sit on the Ogallala Formation. Soils like the Pullman, Amarillo and Olton series are clay loams and sandy clay loams, commonly over a calcium-carbonate-cemented caliche layer within a few feet of the surface. The Pullman series in particular has genuinely expansive clay subsoil, though the dry climate keeps it from cycling as violently as Blackland clays.

Low risk

16 counties · Trans-Pecos & Far West Texas

Trans-Pecos & Far West Texas: Far West Texas — El Paso and the basin-and-range country — is arid, with soils formed in desert basin fill: gravelly, sandy and gypsic soils (Aridisols such as the Delnorte, Hueco and Turney series), often with calcic or gypsum-cemented layers. Expansive clay is limited; the region's distinctive risk is collapsible and gypsiferous soil — loose, dry deposits that suddenly compress or dissolve when they get wet for the first time.

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