Research
The soils that break patios and retaining walls
NRCS shrink-swell classes, Soil Data Access acreage by state, named clay series, and the IRC and city provisions that mention expansive soil.
Written by HyreYard Research Desk Primary-source research, data analysis and fact checking
The finding
Patios and small walls crack on expansive clay, soil that swells when wet and shrinks when dry. NRCS rates it by linear extensibility (NSSH Part 618): Moderate and above can damage structures.
From Soil Data Access we count 37,518,710 acres of Vertisols, 38% of them in Texas, and 64,064,786 acres rated Very High. Look up your lot on Web Soil Survey; a soil test is the determination.
Read these limits first
Call 811 before anyone digs for a patio base, a wall footing or a drain.
Not a determination about one address. A soil map unit is a polygon with more than one soil in it. This page is a screen. Web Soil Survey is the public lookup; NRCS itself says engineering uses need an on-site investigation.
Not a map of patio failures. We counted soil acres, not cracked pavers. No public dataset counts hardscape construction, so we could not join the two.
Not a foundation study. Slabs, basement walls and any wall that holds soil against a house are out of scope. HyreFoundation covers that at hyrefoundation.com.
Not a design. Base depth, geogrid length, wall height and surcharge are engineering questions. HyreYard does not build patios or stamp drawings.
Why is there no map of where patios fail?
We planned to lay shrink-swell soils over where hardscape is built. No federal file supports that. Census building permits count housing units, not square feet of patio, and there is no national permit series for pavers or segmental walls. Using housing starts as a stand-in would hide a guess inside a number.
What we could compute is the soil side. Vertisols, the soil order defined by expanding clay, periodic cracks and slickensides, cover 37,518,710 acres as major components. Soils with a Very High horizon (LEP 9.0 or more) in the upper 100 centimeters cover 64,064,786 acres.
High or Very High (LEP 6.0 or more) covers 267,258,649 acres across 50 states. These are HyreYard calculations from Soil Data Access, retrieved September 5, 2026. They are not the official NRCS shrink-swell interpretation, which combines soil components under its own rules.
How does NRCS measure soil that moves?
National Soil Survey Handbook Part 618, amended August 2024. Linear extensibility percent (LEP) is “the linear expression of the volume difference of natural soil fabric at 1/3-bar or 1/10-bar water content and oven dryness.” Written as a fraction it is COLE, the coefficient of linear extensibility: LEP = 100 × COLE.
The classes: Low is LEP below 3.0. Moderate is 3.0 to 5.9. High is 6.0 to 8.9. Very High is 9.0 or more.
The handbook’s significance statement is the one that matters for hardscape: if the class is moderate to very high, shrinking and swelling can damage buildings, roads and other structures. High and Very High shrinkage can also damage plant roots, and “special design commonly is needed.”
Two other definitions sit beside that table. Keys to Soil Taxonomy puts a soil in the Vertisol order when it has slickensides or wedge-shaped peds in a layer at least 25 centimeters thick starting within 100 centimeters, 30 percent or more clay in the upper 50 centimeters, and cracks that open and close.
The International Residential Code, where adopted, calls soil expansive when its plasticity index is 15 or more and it has enough fine particles, or when its expansion index is over 20 (ASTM D4829).
HyreYard reading: a Vertisol can meet all three tests. A High-LEP soil of another order can meet the IRC test without being a Vertisol. The three systems answer different questions.
The four classes, and what each means in inches
| Class | LEP | COLE | Theoretical unconfined movement | What NRCS says |
|---|---|---|---|---|
| Low | < 3.0% | < 0.03 | < 0.36 in per foot of soil | Below the damage threshold in the handbook’s significance statement |
| Moderate | 3.0–5.9% | 0.03–0.06 | 0.36–0.71 in/ft | Can damage buildings, roads and other structures |
| High | 6.0–8.9% | 0.06–0.09 | 0.72–1.07 in/ft | Same, plus root damage from shrinkage |
| Very High | ≥ 9.0% | ≥ 0.09 | ≥ 1.08 in/ft | Same |
Class limits: NSSH Part 618. Inches per foot are a HyreYard calculation, LEP ÷ 100 × 12, for an unconfined clod taken from 1/3-bar moisture to oven dryness.
A patio is confined, a yard never dries to oven dryness, and a season cycles only part of the range, so field heave is smaller. The figure gives the class names a size.
Over 4 feet of soil that cycles, the Very High floor is 4.3 inches of theoretical unconfined movement and the High floor 2.9 inches. These illustrate the class bounds; they do not predict a yard.
Houston Black: what a Very High clay looks like in numbers
Official Series Description, revised August 2017. Houston Black, the Texas state soil, is a Fine, smectitic, thermic Udic Haplusterts with 40 to 60 percent clay in the control section.
When dry, cracks 0.5 to 4 inches wide run from the surface to 12 inches or more and stay open 90 to 150 cumulative days “in most years.” Slickensides appear by 13 to 24 inches. Water enters rapidly when the soil is dry and cracked, and very slowly when it is moist.
HyreYard calculation from Soil Data Access, September 5, 2026. Houston Black is a major component on 1,289,920 acres, all in Texas, across 29 soil survey areas. The Texas Water Development Board’s state-soil card says “about 1.5 million acres,” counting the series more loosely.
Across 220 horizons with a published LEP, values run from 7.5 to 17 percent, averaging 12.6, which is Very High. Clay on those horizons is 54 to 55 percent.
At 12.6 percent, an unconfined clod changes length by 1.51 inches per foot of soil between 1/3-bar moisture and oven dryness: 6.1 inches over four feet.
At the 17 percent maximum it is 2.04 inches per foot. A patio does not move that far, but that range is why a 4-inch slab on native Blackland clay is not a small job.
Where are the cracking clays?
NRCS’s own Vertisol page names the regions. Aquerts in Texas, the lower Mississippi Valley and the Red River of the North. Torrerts in western Texas, Arizona and New Mexico. Uderts in Texas, the lower Mississippi, the Red River Valley, Alabama and Mississippi. Usterts in Texas, South Dakota and Montana.
Xererts occur in California, Oregon and Idaho. On those, NRCS says damage to structures and roads is very significant, because the soils dry every summer and wet every winter.
HyreYard calculation, September 5, 2026. Major components only, national overlay excluded: 37,518,710 acres of Vertisols. Texas holds 14,123,823 acres, 37.6% of the total, followed by South Dakota, Louisiana, California, Montana, Mississippi and Arkansas.
Colorado has only 355,214 Vertisol acres as major components but 8,621,572 High or Very High LEP acres. A soil-order map and a city’s geotechnical rule can tell different stories.
Vertisol acres by state
| State | Acres (major components) | Share of US total |
|---|---|---|
| Texas | 14,123,823 | 37.6% |
| South Dakota | 3,374,433 | 9.0% |
| Louisiana | 3,244,838 | 8.6% |
| California | 2,705,381 | 7.2% |
| Montana | 2,305,138 | 6.1% |
| Mississippi | 2,207,229 | 5.9% |
| Arkansas | 2,024,588 | 5.4% |
| North Dakota | 1,196,661 | 3.2% |
| Oklahoma | 1,082,438 | 2.9% |
| Minnesota | 811,804 | 2.2% |
| Arizona | 807,925 | 2.2% |
| Alabama | 806,586 | 2.1% |
| New Mexico | 581,388 | 1.5% |
| Missouri | 507,267 | 1.4% |
| Kansas | 466,327 | 1.2% |
| Colorado | 355,214 | 0.9% |
| Oregon | 267,507 | 0.7% |
| Idaho | 137,348 | 0.4% |
| Nevada | 119,712 | 0.3% |
| Iowa | 80,309 | 0.2% |
| Utah | 74,236 | 0.2% |
| Nebraska | 71,566 | 0.2% |
| Wyoming | 61,565 | 0.2% |
| Tennessee | 59,552 | 0.2% |
States with at least 50,000 Vertisol acres as major components. National total 37,518,710. HyreYard calculation from Soil Data Access, September 5, 2026.
Hawaii is included; Puerto Rico, the Virgin Islands and the “US” national overlay are not. Minor components are not in the acres.
Which states have the most Very High shrink-swell soil?
Applying the Very High class (LEP 9.0 or more) to any horizon starting in the upper 100 centimeters gives 64,064,786 acres in 42 states. Texas is first again, at 15,508,587 acres, 24.2% of the total.
South Dakota, Louisiana, Kansas and Missouri bring the top five to 61.7%. Kansas ranks 15th on the Vertisol table and 4th here: soils of other orders can carry Very High LEP and still move.
One class wider, High or Very High (LEP 6.0 or more), gives 267,258,649 acres across 50 states, 47,045,887 of them in Texas. HyreYard analysis: the NSSH class is a screen. The IRC plasticity and expansion tests, and a city’s geotechnical rule, can catch soils the soil order does not.
Named clay series, as examples, not a danger ranking
| Series | Major-component acres | Survey areas | LEP range | Average LEP | Region |
|---|---|---|---|---|---|
| Sharkey | 4,146,622 | 63 | 1.5–20.8% | 13.8% | Lower Mississippi Valley |
| Houston Black | 1,289,920 | 29 | 7.5–17% | 12.6% | Texas Blackland Prairie |
| Pierre | 1,232,037 | 24 | 0.3–17% | 9.4% | Northern Great Plains / Front Range parent |
| Fargo | 890,903 | 32 | 0.4–12% | 8.9% | Red River Valley |
| Heiden | 872,322 | 36 | 7.5–17% | 13.3% | Texas Blackland Prairie |
| Summit | 413,595 | 31 | 3.6–17% | 6.9% | Southern Great Plains |
| Branyon | 403,699 | 22 | 9.9–17% | 12.4% | Texas Blackland Prairie |
| Capay | 261,366 | 16 | 0–14% | 9.0% | California valleys |
| Iredell | 234,916 | 54 | 0–17% | 7.1% | Piedmont |
| Clear Lake | 214,433 | 25 | 0–14.8% | 9.2% | California |
| Houston | 72,213 | 8 | 4.5–17% | 12.9% | Alabama / Mississippi Blackland |
| Denver | 24,355 | 8 | 4–8.3% | 6.4% | Colorado Front Range |
HyreYard calculation from Soil Data Access, September 5, 2026. LEP is the representative value stored on major-component horizons, not a swell test on a lot. A range starting near zero means some horizons in the series are not expansive.
Houston Black, Heiden and Branyon are Blackland Prairie clays. Pierre is a northern Great Plains clay also found along the Front Range. Capay and Clear Lake are California valley clays.
Iredell is a Piedmont example that is not a Vertisol everywhere. The Denver series is small and averages High, not Very High; that says nothing about the city of Denver.
Why does clay soil crack a patio?
The mechanism below is HyreYard analysis, not a federal finding. NRCS publishes no patio-failure table; the mechanism follows from the measurement.
LEP is a change in length with a change in water. A patio concentrates water at its edges: downspouts, irrigation, the unpaved strip where slab meets lawn. The middle stays at a steadier moisture. The edge wets and dries.
That uneven movement is what opens cracks, tilts pavers and pops edge restraints. An even inch of heave would be a nuisance. An inch at the drip line and a quarter inch under the table is a failed patio.
High-LEP clay also drains slowly: the OSD calls Houston Black “very slowly permeable.” Water that reaches it sits. When the clay dries, its cracks let the next storm in deep and fast. A patio that sheds onto native Blackland clay is watering the soil that will lift it.
ICPI Tech Spec 2 puts a minimum compacted base of 4 inches under patios and sidewalks and 6 inches under residential driveways, on well-drained soils. It calls for 2 to 4 inches more on continually wet or weak soils in colder climates, and notes that “greater thicknesses are often used” on expansive soils.
That is practice, not a section HyreYard will write for a lot. The point is the qualifier: the 4-inch patio base is a well-drained-soil number, and high-plasticity clay is not well drained.
Recommendation, not a spec. On Moderate, High or Very High LEP, ask for the base depth in writing, whether native clay stays under the pavement, how the section drains and how the patio is isolated from the house. The paver calculator shows the base as a volume once you have a depth.
Why does the same clay tip a small wall?
Again HyreYard analysis. A freestanding segmental wall fails by sliding, overturning, bearing or water, and high-plasticity clay makes all four worse. It bears poorly when wet, it does not drain, and its swell pressure is a sideways load a gravity wall was not sized for.
It is also a poor fill behind the wall. Concrete Masonry and Hardscapes Association practice (successor to NCMA) does not recommend high-plasticity or organic soils (MH, CH, OH, OL, PT) as infill for segmental retaining walls, and it tightens plasticity and fines limits as walls get taller.
CMHA practice puts at least 12 inches of free-draining gravel behind the facing, a drain pipe that outfalls, and geogrid whose length is designed, not copied from a product page. Geogrid length, wall height and surcharge are engineering questions.
The 2024 IRC, where adopted, sends unbraced walls retaining more than 48 inches of unbalanced fill, or walls over 24 inches resisting extra lateral load, to accepted engineering practice with a 1.5 safety factor. That section does not apply to foundation walls supporting buildings, which is this page’s boundary.
Irrigation above a clay-backed wall adds water; a patio on the high side adds weight. Either can move a wall into the engineered category. The retaining wall calculator keeps drainage as its own line and flags 4 feet, the IRC model number. It does not certify a wall on Houston Black.
What high-plasticity clay changes in the work
ICPI’s 4-inch patio minimum is for well-drained soils. High and Very High LEP clays are not. Put the depth and compaction in the bid. A recommendation, not a federal thickness.
Slope the surface away from the house (IRC R401.3: 2 percent on impervious surfaces within 10 feet of the foundation, where adopted). Keep downspouts off the pavement edge. Do not irrigate the strip that will heave.
A patio poured against a foundation passes its movement into the building. Recommendation: ask for an isolation joint between the two.
CMHA practice excludes CH and MH soils from the reinforced zone. Imported granular fill and a drainage layer are part of the cost of a wall on this soil.
Once a wall holds soil, its failure modes are structural. Manufacturer charts assume a soil and a drainage detail. If you cannot name both, you are not reading a design.
LEP is potential; moisture change uses it. A dry year after a wet spring drives shrinkage. A new irrigation zone on dry clay drives swelling.
What do building codes say about expansive soil?
Model codes are adopted and amended locally; none of this is a national hardscape law. Each provision below was read in the cited document on the retrieval date.
IRC R401.4 lets the building official require a soil test where “quantifiable data created by accepted soil science methodologies indicate expansive soils, compressible soils, shifting soils or other questionable soil characteristics are likely to be present.” Web Soil Survey is that kind of data. The section does not require a test for every patio.
IRC R403.1.8 sends building foundations and floor slabs on expansive soils to IBC 1808.6. It is a building provision, cited here as the line this page does not cross. R403.1.8.1 defines expansive soil by plasticity index, fines and particle size, or by expansion index.
IRC R401.3 requires 6 inches of fall in the first 10 feet from foundation walls, and a 2 percent slope away on impervious surfaces within 10 feet. A patio that drains toward the house fails it where adopted. On expansive soil it also waters the clay.
IRC R404.4 is the freestanding-wall engineering trigger, and cities amend it. Fort Collins (Ordinance 213, 2025) measures from low-side grade to the top and requires a permit above 4 feet or with a surcharge, with uphill tiers spaced at least the lower wall’s height.
Larimer County requires engineered footings in its Front Range “area C” because of shrink-swell soils, and engineering on walls 4 feet and taller countywide. Denver’s 2025 policy on DRC R401.4 requires a geotechnical report for new dwellings, townhouses and conditioned accessory structures, and whenever expansive soils are likely.
Rapid City, South Dakota (Ordinance 6050) requires a professional engineer to design foundations and structural slabs on expansive soils: a foundation rule, not a patio code. Austin’s Transportation Criteria Manual treats pavement subgrade as high swell at PI over 35, liquid limit over 60 and over 50 percent passing the No. 200 sieve.
HyreYard analysis. The codes that mention expansive soil are building-foundation codes. Hardscape meets them at the edges: drainage away from the house and walls above a local height. That is why a federal soil class plus a local wall rule is more useful than an imagined national patio code.
Provisions we read
| Authority | What it says | Applies to hardscape? | Retrieved |
|---|---|---|---|
| NSSH Part 618, Figure 618A-3 | Four shrink-swell classes on LEP / COLE. Moderate and above can damage buildings, roads and other structures. | The soil rating. Not a construction spec. | Aug. 2024 handbook / Sep. 5, 2026 |
| 2024 IRC R401.4 | Building official may require a soil test where expansive or other questionable soils are likely. | Only if the official says so. Web Soil Survey is the kind of data the section describes. | Sep. 5, 2026 |
| 2024 IRC R403.1.8 / R403.1.8.1 | Building foundations and floor slabs on expansive soils designed to IBC 1808.6. PI ≥ 15 plus fines, or EI > 20. | No. A building provision. | Sep. 5, 2026 |
| 2024 IRC R401.3 | 6 inches of fall in 10 feet; 2% slope on impervious surfaces within 10 feet of the foundation. | Yes, where adopted: a patio is an impervious surface. | Sep. 5, 2026 |
| 2024 IRC R404.4 | Unbraced walls retaining > 48 in of unbalanced fill, or > 24 in with extra lateral load, need engineering. Safety factor 1.5. Not foundation walls. | Yes, freestanding walls. Local amendments change the number. | Sep. 5, 2026 |
| Denver DRC R401.4 policy, 2025 | Geotech report for new dwellings, townhouses, conditioned accessory structures, and whenever expansive soils are likely. | Accessory structures, not a patio slab by itself. | Sep. 5, 2026 |
| Larimer County foundations policy | Engineered footing in Front Range area C because of shrink-swell. Retaining walls 4 ft and up engineered countywide. | Walls at 4 feet. Unenclosed patio covers are a listed exception to the foundation rule. | Sep. 5, 2026 |
| Fort Collins Ord. 213, 2025 | 2024 IRC adopted. Walls over 4 ft measured from low-side grade to top, or with a surcharge, are not permit-exempt. | Yes, walls. Measured from low-side grade, not exposed face. | Sep. 5, 2026 |
| Austin Transportation Criteria Manual, App. B | High-swell subgrade: PI > 35, LL > 60, > 50% passing No. 200. | Pavement, not patios. A city swell screen on Blackland clay. | Sep. 5, 2026 |
| Rapid City Ord. 6050 | Local amendment: engineer design of foundations and structural floor slabs on expansive soils. | No. A foundation rule. | Sep. 5, 2026 |
Each row is a document we opened. Model code is not national law; cities amend it. Whether an at-grade patio needs a permit is a local question, and a wall is a different one.
Our retaining wall permit study has the city-by-city height table; this page does not repeat it.
What this study covers, and what it does not
Covered here
Linear extensibility, NSSH classes, Vertisol and Very High LEP acres, named series, and how high-plasticity clay heaves pavers and tips freestanding walls.
Patios, walkways, driveways and garden walls that do not hold up a house.
For standing water rather than clay movement, see soil drainage and yard flooding.
Not this study
House foundations, basement walls, slabs that carry the house, and any wall that holds soil against it. That is a different trade: hyrefoundation.com.
A county map of patio failures. We do not have the failures, so we do not color counties as if we did.
Permit height for walls, city by city: retaining wall permit thresholds.
What to ask before anyone digs
Utility locates come before any base excavation, footing or drain trench.
The map is the federal screen IRC R401.4 describes, not a geotech report. Note the map unit, its components, linear extensibility and the ratings for dwellings without basements. Then decide whether the job needs a test.
Not just “is it clay.” Kaolinitic clays swell little; smectitic clays like Houston Black swell a lot. The class is the number.
Depth, material, compaction and whether native clay is removed. Bids that omit this cannot be compared. The paver calculator shows the volume once you have a depth; the patio size planner checks the area.
Downspouts, irrigation, slope and the outlet of any wall drain. The drainage diagnostic asks the questions; it does not prescribe a French drain.
If any is unnamed, the bid is incomplete. Above the local trigger, or with a driveway on the high side, it is an engineer’s wall. The wall calculator does not stamp it.
How we built this study
NSSH Part 618, Keys to Soil Taxonomy 13th edition, the Houston Black Official Series Description, the NRCS Vertisol page, Soil Data Access queries run September 5, 2026, the 2024 IRC sections cited, and the city documents in the code table. We analyzed NRCS data; we collected no soil samples.
Component acres = map-unit acres × component percent / 100, major components only. Vertisols filtered on taxorder. Very High: any horizon with hzdept_r < 100 and lep_r ≥ 9; High or Very High: lep_r ≥ 6. The “US” legend, Puerto Rico, the Virgin Islands and Pacific territories are excluded.
NRCS publishes its own rating with its own aggregation. We applied the handbook’s class limits to stored representative values, so a Web Soil Survey shrink-swell map can differ from our totals without either being wrong.
Fill, grading, imported topsoil and an old patio are invisible to SSURGO. NRCS’s Web Soil Survey home page says engineering applications need on-site investigation.
Housing permits are not patio counts, so we did not substitute one for the other.
LEP is measured unconfined, oven-dry to 1/3-bar. Confinement, partial drying, load and a draining base all reduce real movement. The inches give the classes a size; they are not a swell test.
IRC text is model language. Denver, Fort Collins, Larimer County, Austin and Rapid City are the jurisdictions we opened, not a national sample.
Questions
Why did my patio crack?
What is expansive soil, for a patio?
What is shrink-swell soil?
Does clay soil make retaining walls fail?
How do I know if my yard is on expansive soil?
Is extra base enough on expansive clay?
Can I build a small retaining wall myself on clay?
Is this a foundation problem?
Did HyreYard sample soils?
Written and audited by
HyreYard Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a landscaping company. We read the statute, the municipal code, the water-district rule or the federal soil file ourselves, and we publish the figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify. This desk is new.
The counts below are what we can already stand behind, and every one of them is counted from the studies on this site.
- 255
- source documents cited, each linked
- 112
- government (.gov) domains among them
- 258
- citations carrying a retrieval date
- 10
- research studies published
How this desk works
- Primary sources only. A rule comes from the statute, ordinance, board or agency that issued it, cited by section, with the URL and the retrieval date. We do not cite a blog that cites a code.
- Coverage is a number. A table titled to fifty states has fifty sourced rows. A table of cities is titled to the cities we actually read. Gaps are published as gaps.
- Company counts stay inside seven states. Texas, Arizona, Washington, California, Oregon, Florida and North Carolina. Texas is the largest roster on the weakest evidence, a self-reported NAICS sales-tax permit, because Texas licenses no landscaper. National claims rest on federal or state agency data, never on our counts.
- Fact, calculation, analysis and recommendation stay labeled. "According to…" is a source. "HyreYard analyzed… and calculated…" is ours. "This suggests…" is interpretation. We do not present our reading as something the source stated.
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Data as of NRCS Soil Data Access queried September 5, 2026; NSSH Part 618 amended August 2024. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold.
Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
Price the part under the pavers
HyreYard does not build patios. The paver calculator puts base volume on the same page as the bid, which is the part that fails first on this soil.
HyreYard does not perform landscaping, hardscape construction or geotechnical engineering, and has no commercial relationship with NRCS, ICC, CMHA, ICPI or any municipality named here. This page is general information, not a design, not a permit, and not advice about a named lot. Foundation-bearing work is out of scope.