An Overview of Expansive Soils in the US

The American Society of Civil Engineers estimates that one-quarter of all homes in the U.S. have suffered damage from expansive soils.

Expansive clay soils, which swell when wet and shrink when dry – are a widespread geologic hazard in the United States. In fact, these soils are present in every state and cause billions of dollars in damage to homes annually. The American Society of Civil Engineers estimates that one-quarter of all homes in the U.S. have suffered damage from expansive soils. In an average year, the financial losses from soil expansion exceed those from earthquakes, floods, hurricanes, and tornadoes combined.

Geographic Distribution of Expansive Soils in the U.S.

Expansive soils occur throughout the country, but their prevalence is higher in certain climates and geologic settings. Generally, semi-arid regions with clay-rich bedrock or soil are most affected. The classic expansive clay is montmorillonite (smectite), which can swell to several times its dry volume when saturated. According to the U.S. Geological Survey’s swelling clays map, large portions of the central and western United States are underlain by soils with moderate to high shrink-swell potential. Over 50% of many areas in the Great Plains, Rocky Mountain foothills, and parts of the South contain abundant high-swelling clays. By contrast, New England and parts of the upper Midwest have little to no swelling clay (due to different soil parent materials).

West Coast

Expansive soils along the U.S. West Coast occur in distinct belts tied to sedimentary basins, clay-rich alluvium, and weathered volcanic deposits. While the coastal margins and mountain ranges tend to have thin or coarse-textured soils with low shrink–swell potential, several interior valleys and structural basins contain smectite-rich clays capable of moderate to significant seasonal movement. California has the most extensive shrink–swell zones on the West Coast, Oregon contains moderate and more localized areas primarily in its major valleys, and Washington has the least expansive soil activity, limited to inland basins east of the Cascades. These differences reflect the region’s varied geologic origins, ranging from marine sediments in California’s Great Valley to volcanic ash–derived clays and lakebed sediments in Oregon and Washington.

California

California has the largest and most consistent areas of expansive soils on the West Coast, with high shrink–swell potential found throughout the Central Valley and many inland valleys in both Northern and Southern California. Large parts of the Sacramento and San Joaquin Valleys contain smectite-rich alluvial clays derived from weathered marine sediments of the Great Valley Sequence, forming soil series such as Altamont, Pleasanton, and San Joaquin that undergo measurable seasonal expansion and contraction. The Bay Area, particularly the East Bay foothills and inland valleys such as Walnut Creek, San Ramon, and Pleasanton, contains claystones and mudstones that weather into highly plastic clays responsible for widespread heave-related distress. Southern California’s coastal areas generally have sandy or decomposed granite soils with low swell potential, but inland basins including parts of the Inland Empire and the San Fernando and San Gabriel Valleys contain clay-rich alluvium capable of moderate shrink–swell cycling. Overall, California’s expansive soils are broad, continuous, and closely linked to its sedimentary basin geometry.

Oregon

Oregon’s expansive soils are more patchy and moderate compared to California, occurring mostly in the Willamette Valley and select interior basins influenced by volcanic activity. In the Willamette Valley, older alluvial floodplain deposits produce clay-rich soils such as the Dayton, Woodburn, and Amity series, which exhibit moderate shrink–swell potential and respond noticeably to seasonal moisture fluctuation. Central and Eastern Oregon contain smaller but significant pockets of expansive clays tied to weathered volcanic ash, lacustrine sediments, and basin-fill deposits, such as those found near the Klamath Basin and portions of Harney County. While these regions can experience foundation movement, the affected areas are limited and discontinuous, meaning expansive soils in Oregon tend to be localized rather than large, continuous zones.

Washington

Washington has the lowest overall occurrence of expansive soils on the West Coast, with meaningful shrink–swell activity mainly present in Eastern Washington. The Columbia Basin, including areas near Yakima, Kennewick, and Walla Walla, contains fine-grained lakebed sediments and loess deposits that weather into clay-rich soils with moderate shrink–swell behavior. By contrast, Western Washington’s soils are largely the result of Pleistocene glaciation, leaving widespread glacial till, sand, and gravel with minimal plasticity and little to no significant swelling potential. As a result, expansive soils in Washington are isolated and regionalized rather than widespread, confined mainly to the dry interior basins east of the Cascades.

Midwest and Plains

Expansive clay is common in states like Missouri, Oklahoma, and Kansas, derived from clay-rich shales and ancient sea deposits. The “Pierre Shale” formation underlying the Dakotas and eastern Colorado, for example, produces very high swell potentials. One of the worst damage regions in the nation is along Colorado’s Front Range urban corridor (Denver–Colorado Springs), underlain by Cretaceous clay-shales; here expansive clays have caused widespread foundation damage . (This will be discussed in detail below.)

Southwest

Arid states such as Arizona, New Mexico, and Texas also have large expanses of shrink-swell soils. Dry spells desiccate the clays, then monsoon rains or Gulf moisture re-saturate them, causing cycles of heave and settlement. Texas in particular has infamous expansive clays – the “Blackland Prairie” soils of central and north Texas are a dark, smectite-rich clay (often called “black gumbo”) that can shrink and swell by several inches seasonally. This belt arcs from the Dallas–Fort Worth region down through Austin and into San Antonio. Foundations in this zone are under constant stress from soil movement.

Southeast

Even the humid Southeast has pockets of expansive clay. For instance, parts of Mississippi (the Yazoo Clay) and Georgia have clay soils that stay wet for long periods and then dry, causing foundation shifts. A construction law source notes that expansive soil is “common in the Southeast” U.S. (as well as in Southern California). In Virginia, expansive marine clays in the Coastal Plain and clay-rich sediments in certain inland basins have caused significant home foundation damage. The Virginia Geological Survey reports that these problem soils occur around Richmond and in parts of Northern Virginia’s suburbs (Culpeper basin), among other areas.

Expansive Soils in High-Risk Metro Areas

Below we identify key metropolitan regions where expansive soils are common and housing values are high. In these hotspots, homeowners frequently encounter foundation cracking, differential settlement, and other structural issues due to soil expansion.

San Francisco Bay Area (Northern California)

The Bay Area exemplifies the hazards of expansive soil in a high-value housing market. Much of the region’s soil contains significant clay content that swells with winter rains and shrinks during the dry summers. In particular, the bay muds around the margins of San Francisco Bay are highly expansive clays. These soft, fine-grained soils underlie areas of Silicon Valley and the East Bay; when they dry out, they develop deep cracks, and when re-wetted, they heave upward.

From a housing standpoint, the Bay Area is one of the most expensive markets in the country. San Francisco and San Jose metro areas have typical single-family home prices roughly 150–235% above the U.S. average. This means any structural damage can translate to very costly repairs and value loss. For example, in parts of Santa Clara County (San Jose area), tract homes built on expansive clay have seen cracked slabs and sticking doors after drought years. Local planners recognize the risk: soils with high clay content (Bay mud) are flagged as “highly expansive” in Bay Area development plans.

Sacramento & Central Valley (Northern California)

Just inland, the Sacramento region and parts of California’s Central Valley also sit on expansive clays – albeit with generally lower home prices than the Bay Area. Sacramento’s growth has pushed into former farmlands underlain by clay-rich alluvium. Notably, in the Natomas area of north Sacramento (a major zone of new housing development), roughly 75% of the area is underlain by expansive soils. Geotechnical studies of sites near Sacramento found surface clays with “medium to highly” plastic characteristics, meaning they undergo significant volume change as moisture varies. Homeowners in suburbs like Natomas and Elk Grove often see their foundations heave during winter rains and settle in the dry hot summers.

Southern California (Los Angeles, Orange County, San Diego)

Many are surprised to learn that parts of Southern California – despite its reputation for sandy beaches – do have expansive clay soils. In the Los Angeles Basin, pockets of old lakebed clay and marine clay underlie certain neighborhoods. For example, the City of Torrance (in LA’s South Bay) warns that “expansive soil is found in most of North Torrance and in the Walteria Lake area”. These clays can push foundations and basement walls outward if not properly managed. Likewise, inland areas of Orange County and the Inland Empire (Riverside/San Bernardino) include fine-grained soils that swell during the winter rainy season. A construction defect attorney in California notes that expansive soil is “a very common issue” causing damage to homes, and specifically “common in Southern California” (as well as the Southeast U.S.) . Many southern California homeowners have observed cracks in foundations or stucco that align with drought and re-wetting cycles of the soil.

Colorado Front Range (Denver & Colorado Springs Corridor)

The Colorado Front Range is a textbook case of geology colliding with development. The urban corridor from Fort Collins through Denver to Colorado Springs sits on clay-rich bedrock (including the Pierre Shale) that is notorious for swelling. As one scientific study observes, “one of the worst swelling soils damage regions in the United States occurs along the 300 km Front Range Urban Corridor in Colorado”. Here, entire subdivisions have experienced heaving foundations – for example, parts of the Denver suburb Aurora were built on expansive clays that literally lifted garage slabs several inches. The clays can exert pressures up to 20,000–30,000 pounds per square foot when expanding, enough to crack basement walls. Colorado’s semi-arid climate (wet springs and summers punctuated by drought) exacerbates the issue: soils that were relatively dry can gain moisture from lawn irrigation or rainy periods and then swell dramatically.

Texas: High-Growth Cities

Texas arguably has the largest expanse of problematic expansive soils in the U.S., along with several fast-growing, high-value housing markets. The focus is on the Blackland Prairie, a band of heavy clay soil that runs through central and north Texas. This soil – often called “Houston Black” or “black gumbo” – is a dark, highly plastic clay that can shrink and swell dramatically with moisture changes. In North Texas (Dallas–Fort Worth region), the Blackland clay is notorious for cracking slabs; it can “shrink or swell up to seven inches”, exerting up to 15,000 psf pressure on foundations. Many homeowners in Dallas suburbs use soaker hoses to keep soil moisture constant around their foundations as a preventative measure. The issue is so pervasive that FEMA has estimated annual damages over $7 billion from expansive soils – much of that likely in Texas and similar states.

Implications of Climate Change: Future Risk Projections

Looking ahead, climate change may intensify the shrink-swell cycles of expansive soils, worsening the risks to homes. Climate models project more frequent and severe droughts interspersed with extreme rainfall events in many U.S. regions. This is a perfect recipe for expansive soil problems: prolonged drying causes clay soils to contract and develop deep fissures, then sudden heavy rains rapidly re-saturate the ground, causing abrupt swelling. Geologists in Virginia have observed that these hazards are “exacerbated by prolonged drought followed by soil-saturating precipitation events.” In practical terms, a warmer climate could lead to deeper drying of soils under foundations during heatwaves, meaning when rains do arrive, the uplift pressures on slabs and footings will be even greater than in the past.

All of this implies that expansive soil-related damage may worsen in coming decades. Home foundations that might have been borderline stable under historical climate patterns could fail under more extreme moisture fluctuations.

(note: created with the help of online research & AI)