What Causes Voids Under a Concrete Slab?

What a "Void" Actually Is

A void forms when the soil under a slab erodes, settles, or shrinks away, leaving the concrete bridging an empty gap. This guide focuses on the void itself - how it opens up and why filling it (not just raising the slab) is what makes a lift last. For the full picture of what makes Kansas City concrete settle in the first place, see why concrete sinks in Kansas City.

A void is simply empty space that opens up between the bottom of a concrete slab and the soil it was originally poured on. Concrete itself doesn't stretch or sag - it's rigid. So when people notice a driveway, patio, or walkway that's sunk, cracked, or tilted, what actually happened is the ground underneath moved or washed away, and the slab, having nothing left to rest on, dropped into the space that opened up. Voids can be small - a few inches under one corner - or they can run several feet under a slab, invisible until the concrete above it finally settles into them. Understanding what's causing the void, not just patching the crack it left behind, is the difference between a fix that lasts and one that's cosmetic. Most of it traces back to a handful of causes.

Cause #2: Poor Compaction From the Original Build

Not every void is erosion - some were built in from day one. When a slab is poured, the soil or fill underneath is supposed to be compacted in layers before the concrete goes down. If that step was rushed, skipped, or done in wet conditions, the fill settles under its own weight and the slab's traffic load over months or years, and a gap opens up gradually as the ground compresses to where it should have been compacted to begin with. This tends to show up as a slow, even settling rather than a sudden drop, and it's common on additions, newer driveways, or any concrete poured over fill dirt rather than undisturbed native soil. It's also one of the more predictable causes - if a slab is younger than the house around it, poor original compaction is worth ruling out.

Cause #3: Expansive Clay Soil Shrinkage

Kansas City sits on a lot of clay-heavy soil, and clay behaves differently than sand or loam - it swells when it's wet and shrinks noticeably when it dries out. That swell-shrink cycle repeats every year with the seasons, and over time it can pull soil away from the underside of a slab as the clay contracts during hot, dry stretches, leaving a gap that doesn't fully close back up when the soil rehydrates. It's a major reason concrete settling is so common across the metro regardless of how well a slab was originally built or drained. Why concrete sinks in this region usually comes back to some combination of this clay behavior plus drainage, rather than one single cause - which is also why a lasting fix needs to fill the actual void, not just raise the slab and leave the gap.

Cause #4: Freeze-Thaw Cycles

Kansas City winters swing between freezing nights and 40- or 50-degree days, sometimes within the same week. Any moisture that's worked its way under a slab edge freezes and expands, then thaws and contracts, over and over through the season. That repeated cycle disturbs the soil structure under the concrete and can widen an existing void or help start a new one, especially at joints and edges where water tends to collect first. It's part of why settling issues often become more noticeable heading into spring - the freeze-thaw stress of winter has had months to work on soil that was already compromised by drainage or clay movement. None of these four causes usually act alone; erosion, compaction, clay shrinkage, and freeze-thaw tend to compound each other under the same slab over the years.

How to Tell a Void Is Actually There

You generally can't see a void directly, but there are reliable signs of one: a slab section that's visibly lower than the panel next to it, a hollow or drummy sound when you tap the concrete with something solid, cracks radiating from one corner, or water pooling in a spot that used to drain fine. A driveway or patio that rocks slightly under weight, or a step that's pulled away from the slab it's attached to, is also a strong indicator. None of these are guesswork you have to do alone - the diagnose tool walks through common signs, and a free on-site look confirms what's actually happening underground rather than leaving it a guess based on what's visible above the surface.

Filling the Void vs. Just Raising the Slab

Here's the part that matters most: raising a slab without filling the void underneath it just moves the same empty space to a different spot, waiting to cause the next settling. Polyjacking is built around that distinction - injected polyurethane foam expands to fill the actual void first, then continues expanding to lift the slab, so the concrete ends up resting on solid support instead of a hollow pocket with a raised slab sitting on top of it. That's also why the foam being waterproof matters - it won't erode out the way loose soil or heavy mudjacking slurry can over time. If a slab's been sinking and you're not sure why, a free estimate at (913) 713-7794 gets an honest read on which of these causes is actually at play under your concrete. You can also reach us through the contact form.