Why it lifts unevenly instead of all at once
Water expands as it turns to ice, and soil holding more water in one spot than another freezes and expands unevenly across a slab or a wall. Wet clay, shaded ground that stays colder longer, and a spot where water collects instead of draining away all freeze and lift more than the dry, well drained ground next to it. That uneven lift is what separates frost heave from a slab that is simply settling. Settlement sinks, usually because the ground underneath compressed or washed out. Frost heave pushes up, and it does it in a pattern that follows moisture in the soil rather than load on the surface.
Why footings sit below it and flatwork sits above it
A footing is set below the depth that freezes in winter for exactly this reason, so the ground underneath a foundation stays below the frost line and never gets the chance to heave. Flatwork is different. A driveway, a sidewalk, a patio, a garage floor without a foundation under it, all sit on or just below grade, inside the zone that freezes every winter. Some seasonal movement in a slab on grade is normal and expected. The goal on flatwork is not zero movement, it is a base built to move evenly, not a base that heaves one corner more than the rest.
What uneven heave looks like on a real slab
A little seasonal movement in a slab on grade rarely bothers anybody. What is worth a second look is heave concentrated in one spot rather than spread evenly, which usually points at a wet base or poor drainage under that one section rather than the whole slab. A few signs worth paying attention to:
That pattern almost always traces back to water getting into the base under that one area and having nowhere to drain before it freezes, which is a base and drainage problem more than a concrete problem.
- A slab or a section of sidewalk noticeably higher in winter than it sat the rest of the year
- A joint that lifts enough to catch a plow blade or a shovel edge
- A panel that heaves at one corner while the rest of the slab stays flat
- A spot near a downspout or a low corner of the yard where water collects before a freeze