Expansive Soils and the Cracks They Cause
Why this matters
On expansive clay, the house is not sitting still. It rises a little when the ground gets wet and drops a little when it dries, and it does not do either evenly. A tech who does not know that will read a sticking door as a hinge problem, level a condenser pad in April and get a callback in September, or hard-pipe a gas line through a wall that is going to move. A tech who does know it sees the same symptoms as a seasonal pattern, builds connections that tolerate the movement, and hands the structural question to the person who should answer it.
What makes a clay expansive
All clay holds water, but some clay minerals, the smectite family and montmorillonite in particular, take water into the structure of the particle itself and swell. When they dry they shrink back and crack open. The movement is a change of volume, and it pushes in every direction, including up.
Three facts follow from that, and they explain almost everything you see in the field:
- Weight does not hold it down. The pressure a swelling clay can exert commonly exceeds what a one or two story house puts on its slab, so the house rides on the soil rather than resisting it.
- It moves with moisture, not with temperature. Expansive soil movement follows rain, drought, irrigation, leaks and tree roots. Frost heave is a different mechanism and is covered in Frost Depth and What It Sets.
- The edges move more than the middle. Soil under the center of a slab is shielded from drying and wetting. Soil at the perimeter is not. So the perimeter drops in a dry season (the slab "domes," high in the center) and can rise in a wet one (the slab "dishes," low in the center relative to its edges). A leak under the slab swells the soil around it and lifts that area on its own schedule.
Expansive clays are mapped across large parts of Texas, the Colorado Front Range, the northern and central Plains, parts of California, the Gulf states and many other areas, but they occur in patches inside regions that are otherwise fine. The USDA Web Soil Survey rates mapped soils for shrink-swell potential, which is a quick first check before a job, and the local building department often knows the neighborhoods where it is a problem.
What the code says, and who decides
The IRC, as adopted by your jurisdiction, addresses this in its section on foundations on expansive soils (R403.1.8 in the 2021 edition). That section treats soil as expansive when testing shows an expansion index greater than 20 under ASTM D4829, and it requires foundations on such soil to be designed for it, with an exception your edition states for soil that has been removed or stabilized under the conditions it lists. The general soil-test provision (R401.4) lets the building official require testing where expansive soil is likely.
What that means for a service trade: the structural response to expansive soil is a design decision made by an engineer for new construction and a repair decision made by a foundation professional for an existing house. Your part is to recognize the pattern, build your own work so it survives it, and route the structure question.
The pattern of the symptoms
A single crack tells you very little. The pattern, and whether it changes with the season, tells you a lot.
| What you see | What the movement is doing | Seasonal tell |
|---|---|---|
| Doors that stick at the top latch corner, then free up months later | Frame racking as one side of the house moves relative to the other | Comes and goes with wet and dry months |
| Diagonal cracks running off the corners of doors and windows | Wall bending at its weakest points | Width changes between seasons |
| Stair-step cracks in brick veneer | Masonry following uneven movement | Often wider in late summer |
| Gaps opening between baseboard and floor, or at the top of cabinets | Floor dropping or rising relative to the wall | Opens and closes |
| Cracks in a slab floor, sometimes with one side higher | Slab bending over uneven support | Offset can grow over years |
| Sewer line bellies, repeated drain clogs at the same spot | Line following soil movement under or beside the slab | Builds over years |
| Outdoor pads tilted toward or away from the house | Perimeter soil dropping or rising | Worst at the end of the dry season |
The useful habit is to note the date with every observation. "Back door sticks at top latch corner, 4 August" means something to the next visit in March. "Door sticks" does not.
Building your work to move with the house
The rule for any connection that crosses from the house to the ground, or from the ground into the house, is that it must tolerate a few inches of relative movement without breaking, pulling apart or loading the equipment. Specifically:
Outdoor equipment pads. Set condensers, heat pumps, generators and pool equipment on a pad that floats independently of the foundation, on a compacted base, not doweled or bolted to the house's footing. A pad tied to the house moves with the perimeter; a pad tied to nothing moves less and can be re-leveled.
Refrigerant lines. Leave a gentle service loop between the outdoor unit and the wall penetration, so a few inches of pad movement bends the loop rather than pulling on a brazed joint or a service valve. Seal the penetration with a flexible sealant, not mortar, so the line can slide.
Gas piping. Gas piping that passes through a foundation wall is sleeved or otherwise protected under the fuel gas code as adopted, and the connection to outdoor equipment uses an appliance connector listed for that use. If you smell gas at a connection that has been strained by movement, stop work, get everyone out without operating switches or using a phone inside, and call the gas utility from outside; a strained joint is not a repair to attempt with gas in the line.
Water, drain and sewer lines. Where pipe passes through a foundation wall or grade beam, the plumbing code as adopted requires protection, commonly a sleeve with clearance around the pipe. At the building drain's exit, a listed flexible coupling tolerates movement that a rigid joint will not.
Electrical. A liquidtight flexible conduit whip to outdoor equipment already tolerates movement; rigid conduit from a wall to a pad does not.
Drainage around the house. Keep downspouts discharging well away from the foundation and the grade falling away from it, which the IRC (R401.3, as adopted) sets at a minimum fall of 6 inches within the first 10 feet, under the exceptions your edition lists. Consistent moisture is the goal on expansive clay. Big swings are what move the house.
Worked example: a condenser replacement in late summer
An HVAC crew replaces a failed condenser at a single-story slab-on-grade house in late August, after a dry summer. The lot is mapped as high shrink-swell clay.
What they find. The existing pad sits against the foundation and is tilted toward the house: the house edge is 1.5 inches lower than the outer edge across a 36 inch pad, a slope of 1.5 divided by 36, about 4 percent. The line set enters through the brick veneer in a mortared hole, and a stair-step crack runs up from that hole. Inside, the back hall door sticks at the top latch corner, and a crack off the corner of the nearby window measures 3/16 inch at its widest.
Reading it. The perimeter has dropped in the dry season, and the pad has dropped with it because it sits on the same edge soil. The mortared line set has been carrying that movement into the brick. None of this, by itself, says the foundation is failing; it is what a house on this soil does in August.
What they do with their own work. They set the new condenser on a new pad placed on a compacted base, not touching the foundation, and level it within the tolerance the unit's installation instructions give. They leave a service loop in the line set between the unit and the wall and replace the mortar in the penetration with a flexible sealant. They do not try to "fix" the old pad's tilt by shimming the new unit, because that only resets the problem for the next dry season.
What they tell the owner. That the tilt, the door and the window crack are consistent with seasonal clay movement; that the crew has set up the new equipment so it can move without damage; and that the way to find out whether the house is doing more than seasonal movement is to watch it. They suggest marking each end of the window crack with a pencil and the date, measuring the width at the same point in late winter and again next August, and noting whether the door frees up.
When it becomes someone else's job. If the crack is wider next August than this August, or if either side of it moves out of plane with the other, or if a slab crack shows one side higher, the owner should bring in a foundation professional. There are two kinds and they answer different questions: a structural or geotechnical engineer gives an independent assessment of whether repair is needed and what kind, while a foundation repair contractor proposes and sells a repair. An owner who calls only the second is asking a seller whether they need the product. Suggesting the engineer first, with the dated crack measurements in hand, is the more useful advice.
What would change the call. The same tilt found in March, after a wet winter, means the perimeter is near its high point, so the pad is likely to drop further by summer, and the line set loop matters even more. A tilt away from the house, with the pad's outer edge low, is more often settlement of a pad on loose fill than clay movement, and belongs to Soil Types Around the House. And a slab crack with one side high near a plumbing line on a wet lot suggests swelling from a leak, which needs a plumber's leak test before anyone talks about the foundation, because the cause is water the soil should not be getting.
How to verify you handled it right
Look at the ticket. It should carry the dated observations with measurements (tilt over run, crack width at a named point), state that the new connections were built flexible and how, and record that the owner was told how to monitor and who to call if the monitoring shows growth. A later tech reading it should be able to tell in one pass whether this year's crack is new or last year's crack grown.
References
- International Residential Code, Sections R401.3 drainage, R401.4 soil tests and R403.1.8 foundations on expansive soils, 2021 edition, as adopted by your jurisdiction
- ASTM D4829, standard test method for expansion index of soils, as referenced by the adopted IRC
- USDA Natural Resources Conservation Service, Web Soil Survey shrink-swell ratings
- See related: Soil Types Around the House
- See related: Frost Depth and What It Sets