We get the same call every spring once the ground thaws. A homeowner in Crystal Lake, or somewhere else in McHenry County, notices the slab by the garage sits lower than it did last year, or a sidewalk panel has tipped just enough to catch a toe. Nothing dramatic happened. No flood, no truck parked on it. The concrete just moved. Understanding why comes down to two things we deal with constantly up here: the soil under Northern Illinois, and what our winters do to it.
The ground we build on isn’t stable to begin with
Most of McHenry County sits on glacial till. When the glaciers pulled back, they left behind a mixed deposit of clay, silt, sand, gravel, and the occasional boulder, dropped in no particular order. Around Crystal Lake you also find silty lacustrine soils, the fine material that settled out of old glacial lakes. Both share a trait that matters for anything you pour concrete on: they hold water, and they don’t drain well.
Clay-heavy soil behaves differently than the free-draining sand you might picture under a slab. It swells when it takes on water and shrinks as it dries out. Over a wet spring and a dry August, the same patch of ground can change volume more than once. Concrete is rigid. The soil under it is not. When the support underneath expands, contracts, and settles unevenly, the slab has to follow, and it doesn’t bend gracefully. It cracks, it tips, or it drops.
Freeze-thaw is the part that does the real damage
Now add the winter. In Northern Illinois the frost line runs down around 42 inches, which is why footings here are required that deep. But your driveway, sidewalk, and patio aren’t footings. They sit near the surface, right in the zone that freezes and thaws over and over from November through March.
Here’s what actually happens under there. Water in the soil freezes and expands, roughly nine percent by volume. In clay soils that hold a lot of moisture, that expansion lifts the slab. It’s called frost heave, and it’s why you’ll sometimes see a walkway sit higher in January than it did in October. Then the thaw comes. The ice melts, the soil loses that volume, and the ground drops back down. Except it rarely settles back to exactly where it was. Water drains out, voids open up, loose soil consolidates, and the slab comes to rest a little lower, or a little more crooked, than before.
Do that for one winter and you might not notice. Do it for ten or fifteen winters, which is what every slab in this area goes through, and small movements stack up. A patio that was dead level when it was poured ends up pitched toward the house. A driveway develops that low spot near the apron where water now pools and refreezes, which only speeds the cycle up.
Why the low spots keep getting lower
Once a void forms under a slab, water goes looking for it. Rain and snowmelt run off the concrete, find the gap at the edge or through a crack, and wash more soil away. The unsupported section gets heavier relative to what’s holding it up, and it sinks further. This is the pattern we see most often on service calls. It isn’t one big failure. It’s a slow feedback loop: the settling opens a path for water, the water removes more support, and the settling accelerates.
Downspouts make it worse. We’ve lifted plenty of slabs in Crystal Lake where the real culprit was a downspout dumping roof water right at the edge of the driveway for years. The concrete wasn’t the problem. The saturated, eroded soil under one corner of it was.
What this means for your concrete
The reason we lead with soil and water instead of the slab itself is that replacing the concrete without addressing what’s underneath just resets the clock. Tear out a sunken sidewalk, pour a new one over the same unstable, void-riddled base, and the same freeze-thaw cycle goes to work on it. The new slab looks great for a season or two, then starts moving again.
That’s the case for lifting and leveling instead of replacement. When we raise a slab, we’re filling the voids under it and stabilizing the soil at the same time. Polyurethane foam injection goes in through small ports, expands to fill the gaps, and lifts the concrete back to grade while adding support the ground had lost. Mudjacking does similar work with a soil-based slurry, but does not last as long. Either way the point is the same: fix the base, not just the surface.
Signs the cycle is already working on your slab
You don’t need to wait until you trip on something. Around here the early tells are pretty consistent:
- A panel that sounds hollow when you tap it usually has a void under it.
- Gaps opening between the slab and the foundation, or between panels, mean sections are moving on their own.
- Water pooling where it used to run off is a grading change that points to settling.
- Any lip between two panels, even half an inch, is a trip hazard that tends to grow, not shrink.
None of this means something is about to collapse. It’s the opposite kind of problem. It develops quietly over years, and the longer the soil keeps washing out and refreezing, the more it costs to deal with. If your concrete is showing any of these signs, it’s worth having us take a look and tell you whether it’s something to watch or something to lift. We’ll give you a straight answer either way.
Questions we get
How deep does the ground freeze in Northern Illinois?
Around 42 inches, which is why footings here are set that deep. Your driveway and sidewalk sit well above that, in the layer that freezes and thaws all winter, so they take the movement.
If my concrete settled, do I have to replace it?
Usually not. If the slab is still sound and it dropped because the soil under it settled, we can lift it back to grade and fill the voids underneath. Replacement is for concrete that’s actually broken up.
Will lifting fix it for good, or will it settle again?
Lifting fills the voids and supports the slab, and correcting drainage at the same time keeps water from washing the soil out again. Settling comes back when the water problem that caused it is left in place.
When’s the best time to have settled concrete looked at?
Spring and fall are when the movement shows up most, but any time works. Catching it early, while it’s a small lip and not a broken slab, keeps it a smaller job.