Expansive Soil Checker: Is Your Soil Shrink-Swell Clay?
Expansive clay swells as it wets and shrinks as it dries, and USDA notes the movement can damage buildings and roads[1]. Enter an address to see how much USDA’s soil survey says the soil there shrinks and swells, layer by layer.
US addresses only: street address, city and state, or ZIP.
Example result · Washington County, Mississippi
Shrink-swell risk
HighHighest in the profile · 7.5% LEP
USDA: from moderate up, shrinking and swelling can damage buildings, roads and other structures. At high and very high, the shrinkage can also damage plant roots.
Forestdale silty clay
H10–6 in (0–15 cm)
4.5%
H26–24 in (15–61 cm)
7.5%
H324–36 in (61–91 cm)
4.5%
Low (under 3%)Moderate (3–5.9%)High (6–8.9%)Very high (9%+)
Expansive soil swells as it wets and shrinks as it dries; how much depends on the kind and amount of its clay. USDA measures this as linear extensibility (LEP): under 3% is low, 3 to 5.9% moderate, 6 to 8.9% high and 9% or more very high. From moderate up, the movement can damage buildings and roads[1].
The four shrink-swell classes
NRCS’s National Soil Survey Handbook sets the class limits and says what each class means for structures and roots[1]:
Class
LEP
What USDA says it means
Low
under 3%
Below the class where USDA warns of damage.
Moderate
3.0–5.9%
Shrinking and swelling can damage buildings, roads and other structures.
High
6.0–8.9%
Can damage buildings, roads and other structures, and the shrinkage can damage plant roots.
Very high
9% or more
Can damage buildings, roads and other structures, and the shrinkage can damage plant roots.
3% is where USDA’s moderate class starts, and with it the warning that shrinking and swelling can damage buildings, roads and other structures[1].
How LEP is measured
Linear extensibility is the change in a natural clod’s dimension between moist (water held at 1/3 or 1/10 bar) and oven-dry, taken against its dry length. As a percent it is the LEP; as a fraction it is the coefficient of linear extensibility (COLE), so an LEP of 3% is a COLE of 0.03[1]. A clod that measures 3.09 inches moist and 3.00 inches dry has shrunk 0.09 inch, 3% of its dry length: an LEP of 3%, the bottom of the moderate class (our arithmetic on that definition)[1].
Why the kind of clay matters
Shrink-swell tracks the kind and amount of clay. It is greatest in soils with a lot of 2:1 lattice clays, such as the smectite group; illitic clays are intermediate, and kaolinitic clays change volume least[1]. Clay percentage alone doesn’t decide it: in the example soil below, the H1 layer (45% clay) and the H3 layer (22.5% clay) carry the same 4.5% LEP in USDA’s data[2].
Wet, the clay swells and lifts the slab; dry, it shrinks and cracks. From the moderate class up, USDA notes the movement can damage buildings, roads and other structures[1].
Worked example
Forestdale silty clay in Washington County, Mississippi: its layers range from 4.5% to 7.5% LEP. The highest, 7.5% in the H2 layer at 6–24 in (15–61 cm), puts it in USDA’s High class[1][2].
LEP by layer for Forestdale silty clay, Washington County, Mississippi, from USDA’s soil survey (SSURGO)[2].
Why every layer matters
USDA’s house ratings assume footings 2 feet down, or at the frost depth if that’s deeper, without a basement, and about 7 feet down with one, and they count linear extensibility among the properties that decide whether the soil carries the load without moving[3]. That is below the surface layer, so the checker shows every layer USDA describes and the highest LEP sets the badge. Forestdale’s surface layer is moderate; its H2 layer is high[2]. The building site checker shows USDA’s house ratings for the same point.
What to do with your result
Low means USDA’s data puts the soil below the class where it warns of damage; moderate or higher means USDA expects enough movement to damage structures[1]. Either way, the result describes the typical soil USDA mapped for the area, at survey scales of 1:12,000 to 1:63,360 meant for planning[2], not the soil or fill under your house. Before building, adding on or buying, have the lot itself checked by a geotechnical engineer.
Ways people use this check
Before buying a house, to see whether the ground under it is rated to shrink and swell.
Before an addition, garage slab or pool.
Comparing nearby lots: switch the map to soil type and click the areas around you.
How we calculate this
We read the representative LEP USDA records for each layer of the main soil at your point[2], classify each layer with the NRCS shrink-swell classes[1], and show the highest. Layers without a value show a dash; we never estimate one. Depths are converted at 2.54 cm per inch[4].
Frequently asked questions
What is expansive soil?
Soil whose volume changes a lot as its water content changes. Shrink-swell tracks the kind and amount of clay and is greatest with smectite clays[1].
What does LEP mean?
Linear extensibility percent: how much a natural clod changes in length between moist and oven-dry, against its dry length[1]. Higher numbers mean more movement.
Is an LEP of exactly 3% low or moderate?
Moderate. USDA’s moderate class runs from 3.0% to 5.9%, so 3.0% is its first value[1].
If my result is low, is my foundation safe?
Not necessarily. The result describes the typical soil USDA mapped for the area, at planning scale[2], not the soil or fill under your house. Only a site investigation checks the lot itself.
Why does the building site checker also mention shrink-swell?
USDA counts linear extensibility among the properties that affect a soil’s capacity to carry a house or small commercial building without moving[3], so a high LEP often shows up there as a ‘Shrink-swell’ reason.
Can I check the lots around me?
Yes. The map colors the soil areas around your point; click any area to run the check there.