The hollow sound under your driveway, the slab that rocks when you step on it — what's happening underneath, why it matters, and how void filling fixes it before the concrete fails.
Concrete is only as good as what's under it. A void is exactly what it sounds like — an empty pocket between the bottom of the slab and the soil, where supporting earth used to be. The slab above becomes a bridge: fine until the span gets too wide or the load gets too heavy, and then it cracks. Every settled slab in Utah started as a void nobody noticed.
Snowmelt, downspouts emptying beside the slab, and sprinklers soaking the concrete edge all move soil particle by particle. It starts invisible and ends as a cavern. If you can see soil washed out from under a slab edge, there's more you can't see.
Utah's signature mechanism: water under the slab freezes and expands, lifting the concrete microscopically — then meltwater drains, carrying fine soil with it. Each cycle pumps a little more support out from underneath.
Old tree stumps, buried construction debris, or organic fill decompose over years, leaving voids where solid material was. Common under older Salt Lake Valley flatwork and in areas with a building history.
Burrowing animals excavate under slabs more often than you'd think. Rare but serious: a broken water or sewer line can wash out dramatic voids fast — if your water bill spikes or one area stays constantly wet, call a plumber first, then the concrete contractor.
Professionals confirm with soil probes or ground-penetrating radar before quoting — anyone pricing a void job purely by eye is guessing.
An unsupported slab is living on borrowed time. Concrete is strong in compression and weak in tension — spanning a void puts the bottom of the slab in tension, which is exactly how it cracks. A void under a patio is a future cracked patio; a void under a driveway is a future cracked driveway with a car on it. Voids also channel water, so they grow: the erosion that made the void keeps working through it.
Void filling uses the same equipment as concrete lifting — the difference is intent. Small holes are drilled in a grid over the void area, and material is pumped in at low pressure until it fills the cavity and the slab is fully supported:
Cement-based slurry flows into voids and hardens into solid support. Economical for large voids — $3–$8 per square foot — but heavy, and it needs the void to be accessible to flowing material.
The expanding foam is arguably the better void filler: it expands to fill irregular cavities completely, bonds to slab and soil, weighs almost nothing, and won't wash out if water returns. $5–$25 per square foot.
Often void filling and lifting happen in the same visit — fill the void for support, then continue pumping to restore the slab to grade.
Water erosion is #1 — snowmelt, sprinklers, and downspouts washing soil out from under the slab. Also common in Utah: freeze–thaw pumping, decaying buried organic material, burrowing animals, and in rare cases a broken water or sewer line. If your water bill spikes or the soil stays constantly wet, call a plumber before a concrete contractor.
Tap the concrete with a hammer or heavy stick — a hollow thunk versus a solid thud is the classic test. Other signs: a slab section that rocks when you step on it, soil visibly washed out from the slab edge, or cracks forming where the slab is bridging empty space. Pros confirm with soil probes or ground-penetrating radar.
They can be. A slab bridging a void is unsupported — it can crack suddenly under vehicle weight, creating a collapse and a trip hazard in one moment. Voids also channel water, which worsens erosion. A suspected void is worth a professional look sooner rather than later.
The same equipment as concrete lifting: small holes are drilled and material — cement slurry (mudjacking) or expanding polyurethane foam — is pumped in until the void is full and the slab is supported. Foam is particularly good at this because it expands to fill irregular cavities completely.
Related but not identical. Mudjacking is a void-filling method — the slurry fills voids as it lifts. But void filling can also mean stabilizing a slab that hasn't visibly sunk yet: filling the empty space before it becomes a settlement problem. Think of it as preventive vs. corrective.
Small void-fill jobs often land near the contractor's $300–$500 minimum charge. Larger voids are priced like lifting — $3–$8 per square foot for slurry, $5–$25 for foam — since it's the same equipment and crew. The cost guide has full tables.
Describe the hollow sound, rocking slab, or washout — a local specialist will confirm what's underneath and quote the fix.