Expansive Clay Soil and the Slabs That Sit on It

What makes clay "expansive"

Clay particles are tiny and flat, with lots of surface area that attracts water. K-State Research and Extension compares clay to a sponge: fill it with water and it puffs up; dry it out and it shrinks. The technical term is shrink-swell capacity, and clay has a high one (K-State Research and Extension, Clay Soil: It Shrinks and Swells). The more clay in a soil, the more it moves as moisture changes.

The Kansas Geological Survey describes expansive soils as a pervasive problem in Kansas, common in areas underlain by clay deposits. Water trapped between clay layers causes the soils to expand, and in dry periods they shrink, so soils heave and shrink from season to season, cracking foundations, driveways and sidewalks (Kansas Geological Survey, GeoRecord: geologic hazards and expansive soils).

How it shows up in concrete

• Edge heave: a sidewalk panel lifts at one side after a wet spring because water reached the clay under that edge first.
• Drought settlement: a patio corner drops after a hot, dry summer as the clay under it shrinks.
• Diagonal cracks: a driveway cracks across panels when one part of the slab sits on wetter ground than another.
• Gaps at the house: steps and stoops pull away as the soil next to the foundation dries and shrinks.

K-State notes the pressure from shrinking and swelling is great enough to crack foundations and basement walls and to move concrete sidewalks (K-State Research and Extension, Clay Soil: It Shrinks and Swells).

Wichita's wet-dry rhythm

NOAA's 1991–2020 monthly normals for Wichita show precipitation peaking in May at about 5.2 inches and June at about 4.9, with January the driest at under an inch (NOAA NCEI 1991–2020 monthly normals, Wichita Eisenhower airport station). Summer heat dries the ground hard, and then spring storms soak it. That seasonal swing, plus the year-to-year swings between wet and drought years, keeps clay moving. The same yard can heave one year and settle the next.

Finding out what is under your yard

The USDA Natural Resources Conservation Service's Web Soil Survey lets anyone draw an area on a map and see the mapped soil types, including properties related to shrink-swell potential (USDA NRCS Web Soil Survey). It is a general map, not a test of your lot, but it shows what kind of ground a neighborhood sits on. For new building slabs and bigger projects, a soils report from a geotechnical firm is the next step.

What helps: control the water

Clay moves when its moisture changes, so the goal is to keep moisture under and around slabs as steady as possible.

1. Slope the ground away. K-State's dry-basements guide recommends a surface slope away from the foundation of about one inch for every foot over 15 feet as ideal, and at least six inches in ten feet maintained over time (K-State Research and Extension, MF2809 Dry Basements and Crawl Spaces).
2. Move downspouts. Extend them so roof water does not dump at a slab edge.
3. Seal joints. Open joints let water straight down to the clay under the slab.
4. Water evenly in drought. K-State notes the common advice to water the soil a foot or two around a home during drought to prevent shrinking that can stress a foundation (K-State Research and Extension, Clay Soil: It Shrinks and Swells).
5. Avoid soaking one spot. A sprinkler head aimed at a walk edge every night is a heave waiting to happen.

What helps: build for it

Good base preparation reduces the damage. NRMCA's cracking guidance calls for removing topsoil and soft spots and compacting the soil or granular fill under a slab, and for sloping the subgrade for drainage (NRMCA, CIP 4: Cracking Concrete Surfaces). On clay, many builders add a layer of compacted granular material so the slab sits on a more uniform base. Joints placed at sensible spacing let the slab crack along planned lines rather than wherever the ground moves. Reinforcement helps hold cracks tight.

None of this makes concrete immune to clay. It reduces how much and how unevenly the slab moves.

Trees and clay

Large trees draw a lot of water from the soil, drying the clay near them during summer. A walk or patio near a big tree can settle on the tree side in drought. Roots can also lift panels directly. Keep both in mind when planning new concrete near mature trees.

Lift, repair or replace?

Once clay has moved a slab, the options depend on how. A panel that settled intact may be a candidate for lifting. One that heaved or broke usually needs replacement with better base preparation. In every case, fix the water source first. See also cracks, joints and heaving.

When it is more than a slab

If cracks in a driveway or patio come with cracks in interior walls, sticking doors or a basement wall that bows, the soil may be affecting the house itself. That is a job for a structural or foundation specialist before any flatwork is planned.

The estimate factors page explains how base work changes the scope of a concrete job.

When a panel has dropped in one piece, slab lifting may be an option; when it has broken up, read about tear-out and replacement.

Sources: Kansas Geological Survey, GeoRecord: geologic hazards and expansive soils; K-State Research and Extension, Clay Soil: It Shrinks and Swells; K-State Research and Extension, MF2809 Dry Basements and Crawl Spaces; USDA NRCS Web Soil Survey; NRMCA, CIP 4: Cracking Concrete Surfaces; NOAA NCEI 1991–2020 monthly normals, Wichita Eisenhower airport station.