Expansive clay soil and North Texas sidewalks: why the ground moves

The short answer

Expansive clay is soil that swells when it takes on water and shrinks as it dries. The Texas Department of Licensing and Regulation (TDLR) describes expansive soils as "soils that shrink and swell due to changes in moisture content" and calls them a very common cause of foundation problems. A 2019 University of Texas report for the Texas Department of Transportation (TxDOT) says expansive clays are common throughout Texas and "may cause severe damage to pavements and other lightweight structures."

The Federal Highway Administration (FHWA) lists expansive native soil among the reasons a concrete sidewalk slab rises, subsides or tilts. The ground may keep moving after a repair, so the soil belongs in the decision, not only the concrete.

What makes a clay expansive

TDLR says the expansion ability of a soil is determined by the percentage of clay and the type of clay in it. A 1971 University of Texas study for the Texas Highway Department says montmorillonite clay has a high potential for swelling, and that when soaked some of it expands eight to ten times its original volume.

Soil surveys measure the effect as linear extensibility: how much an unconfined clod changes in length as it dries from moist to dry. A Natural Resources Conservation Service (NRCS) reference on soil survey tables rates shrink-swell potential low below 3 percent, moderate from 3 to 6, high from 6 to 9, and very high above 9. Above 3, it says, shrinking and swelling can damage buildings, roads and other structures, and "special design commonly is needed."

Where it is in North Texas

A USDA sheet on Houston Black, the Texas state soil, says the series occurs on about 1.5 million acres in the Blackland Prairie, which extends from north of Dallas south to San Antonio. It calls Houston Black a classic Vertisol, known worldwide for clays that shrink and swell markedly with moisture, and says that where it occurs in metropolitan areas its very high shrink-swell potential commonly is a limitation for building sites. The 2019 TxDOT report adds that most of the Texas population lives near some of the most extensive deposits of expansive clay, naming the population centers around Dallas, Fort Worth, Austin, San Antonio and Houston.

The NRCS official description of the series gives measurements from the field. In most years its dry-season cracks stay open 90 to 150 days in total, measure 0.5 to 4 inches across, and run from the surface to 12 inches deep or more. In undisturbed ground, its surface forms low mounds 3 to 18 inches higher than the hollows between them, repeating every 10 to 24 feet.

These describe a soil series, not any address. TDLR says NRCS soil survey reports can help determine the presence and location of expansive soils, and that engineering data for specific counties can be downloaded from the NRCS Web Soil Survey. A map shows an area, not what is under one walk.

How the clay moves a sidewalk

The 2019 report says natural seasonal moisture changes cause shrink-swell cycles within a depth below the surface known as the active zone. When a pavement or other lightly loaded structure sits on these clays, it says, the cycles may lead to distress "particularly towards the edges of the structure." The 1971 highway study describes one pattern: the pavement seals the surface, so the soil under its centerline grows wetter while the moisture in the shoulder soil fluctuates with the seasons.

FHWA's 2013 maintenance guide describes what that does to a sidewalk:

• A crack along the walk. Where clays swell significantly at the edges, the slab moves unequally, which "leads to longitudinal cracking." Where swelling is greater at the center than at the edges, the slab is raised, which often leads to a longitudinal crack too. See the cracks guide.
• A panel that drops. FHWA says slabs that drop below their initial grade are more accurately explained by the soil underneath shrinking as it loses moisture. The sinking panel guide covers the other causes.
• A step at a joint. Because sidewalks are built with joints to control cracking, FHWA says heaving most often occurs at the joints; see the joints guide.

Seasons, water and trees

The 1971 study says that where a wet season is clearly separated from a hot, dry season, soil moisture varies widely through the year.

The USDA sheet says water enters Houston Black rapidly when it is dry and cracked and very slowly when it is moist. The 1971 study cites research that swelling in a highly expansive clay can occur as deep as 20 feet, though rain or ponding does not usually wet more than the top 6 feet in several months, which, the 1971 study says, was then often treated as the depth of active swelling. The 2019 report says TxDOT's potential vertical rise method, Tex-124-E, is based on an active zone usually thought to correspond to a depth of 10 to 15 feet in Central Texas.

The 1971 study says tree roots tend to dry the soil to depths of 6 to 14 feet, that pavement built over such ground develops differential heave as the moisture evens out, and that trees near pavements can cause localized settling as their roots dry the surrounding soil. That is a different problem from a root pushing a panel up, which the tree roots guide covers.

For house foundations, TDLR says proper surface drainage, plant choices, sprinkling practices and long-term maintenance all help minimize changes in soil moisture. No source opened for this page gives such a rule for the ground beside a sidewalk, so treat those as questions to raise, not instructions.

How engineers work with clay

From highway research and general guidance, not a specification for any walkway:

• Measure it. The 2019 report says TxDOT's method Tex-124-E estimates a soil's potential swelling from its plasticity index and initial moisture content, using correlations from some expansive clays in Central Texas.
• Treat it with lime. In 1971, lime stabilization of about the top 6 inches of highway subgrades was "probably the most widely used method" of controlling swelling clay, and the Texas Highway Department used hydrated lime to stabilize subgrades whenever clay was present. The 2019 report's focus is hydrated lime as a chemical stabilizer.
• Remove some of it. The 1971 study says how much clay to remove depends on clay type, soil structure, permeability and climate, "and there is no definitely established rule."
• Keep water out. The same study says an impervious membrane that keeps moisture out of the soil will prevent swelling, but that most information on membranes was "generally unfavorable from the economic and long-range point of view."
• Build the slab for the soil. FHWA lists general practices from Canadian national guidance: for light axle loading, four inches of concrete over sand or gravel and five over silt or clay; for heavy axle loading, five to six inches over sand or gravel and six inches or slightly greater over silt or clay. It says subgrade design should be based on local soil conditions and policies, and that an adequate subgrade helps a sidewalk resist seasonal changes.

A contractor or engineer judges which, if any, a repair needs.

What to ask before a repair

• What soil is under this walk, and how do you know?
• Is the movement seasonal? If a step changed between a wet month and a dry one, the soil may still be moving.
• Is it clay, a tree drying the soil, or a void? Each calls for different follow-up; the sinking panel guide covers voids and compaction.
• If the panel is replaced, how will the base be prepared, and how thick will the concrete be?
• Where does water run and stand, and where do sprinklers reach the walk?

Next steps

Record each moving panel, the height of each step and the month you measured it with the walkway inspection checklist, then measure again in the opposite season. The method comparison sorts grinding, lifting and replacement; the curing guide covers new walks. Who repairs a street-side walk depends on the city: see public vs. private repairs. The inquiry preparation guide gathers what a contractor needs, and you can ask about your project when ready.