Visible gap under slab edge
If you can see air under concrete, support may be missing.
Void Under Concrete Slab in Central Illinois
If you can see space under a driveway, sidewalk, patio, or slab, the concrete may need support underneath before the problem gets worse.
Start with the symptom
Sunken concrete can look like cracks, pooling water, uneven steps, trip hazards, or slabs pulling away from the house.
Seeing empty space under concrete is a support problem first. The top of the slab may still look decent, but unsupported concrete does not have much to sit on.
Voids often show up near slab edges, downspouts, driveway joints, patios, sidewalks, or places where water has washed base material away. If the slab is still intact, slab jacking, mudjacking, or polyjacking may be worth asking about.
The water source matters. If water keeps washing under the slab, filling the void without addressing the cause may only buy time.
What You Might Be Seeing
Signs there may be a void under the slab
Hollow sound
A hollow sound can point to empty space under the slab.
Settled slab
Concrete often drops after the base below it disappears.
Water washing under concrete
Moving water can carry soil away.
Cracks forming above void
Unsupported concrete is more likely to crack.
Soil missing near edge
Missing soil near the slab can be a clue.
Slab rocking or moving
Movement can mean the slab is not supported well.
Water pooling nearby
Pooling water can be part of the washout problem.
Why It Happens
What causes voids under concrete?
Most voids come back to water and support. Downspouts, poor drainage, erosion, poor compaction, age, and water movement can remove the base under a slab.
Animals can occasionally be part of the story, but water is usually the first thing I would look for. Find out why the base left before you just fill the hole.
Repair Options
Void filling, concrete leveling, mudjacking, foam lifting, or replacement?
Void filling is about supporting the slab. If the slab also settled, concrete leveling may be part of the repair. Mudjacking and foam lifting both can fill space and lift concrete in the right situation.
If you are still sorting it out, start with the Can My Concrete Be Lifted? quiz or read about water pooling on concrete.
Void Filling
Adds support where soil or base material is missing.
Mudjacking / Slab Jacking
Can fill voids and raise settled concrete with slurry or grout.
Foam Lifting / Polyjacking
Uses expanding foam to fill space and support the slab.
Replacement
May be better if the slab is broken, unstable, or badly deteriorated.
When Filling/Lifting May Work
When I'd ask about filling the void and lifting the slab
I would ask about filling and lifting when the slab is mostly intact, the void is visible or suspected, settlement has started, and the concrete still looks worth saving.
The water issue matters. If the cause is not addressed, the same spot can keep causing trouble.
When Replacement May Be Better
When the slab may be too far gone
Replacement may be better when the slab is broken, crumbling, unstable, badly cracked, or sitting over severe washout.
If water keeps running under the concrete, replacement alone may not solve everything either. The cause still matters.
Void filling or slab lifting cost depends on the size of the void, slab size, amount of settlement, repair material, access, drainage cause, cracks, and whether replacement should be compared.
If the void is related to drainage, the water pooling guide is a good companion page.
Water
Find the water before you just fill the hole
This is where people get tripped up. If water keeps washing the base out, filling the void without addressing water may only buy time.
Look for downspouts, low spots, bad grading, or water paths along the slab. A good contractor should at least talk through what caused the void.
Void FAQs
Void under concrete questions
Can a void under concrete be filled?
Often, yes, if the slab is still worth supporting. The method depends on the slab and access.
What causes a void under a slab?
Water washout, erosion, poor compaction, drainage, and age are common reasons.
Can foam fill voids under concrete?
Foam lifting can fill voids in some situations, but a contractor needs to inspect the slab.
Can mudjacking fill a void?
Mudjacking or slab jacking may fill voids and lift settled concrete when the slab is a good candidate.
Is a void dangerous?
It can leave concrete unsupported. If the slab moves, cracks, or rocks, have it looked at.
Should the slab be replaced?
If it is broken, crumbling, or unstable, replacement may be the better conversation.
Do photos help?
Photos can help later, especially if the void is visible. They are not required to start.
Request a Quote
Want someone to look at the void under your concrete?
Tell us where the void is, what slab is affected, and how soon you'd like it looked at. Photos can help once someone reviews it, but they are not required just to start.
Keep Comparing
Related concrete leveling resources
A few practical next pages if you are still sorting out what makes sense.
Can My Concrete Be Lifted?
Answer a few questions about the slab before you request a quote.
Request Quote
Tell us what is sinking, where you are located, and when you want it looked at.
Concrete Leveling Cost
Understand what can affect the price.
Concrete Trip Hazard Repair
Compare lifting, grinding, and replacement for toe-catching edges.
Why Concrete Sinks
Understand what may be happening under the slab.
Concrete Crack Repair vs Leveling
Know when cracks are the symptom and settlement is the bigger issue.
Mudjacking vs Polyjacking
Compare slurry lifting and foam lifting.
Concrete Leveling vs Replacement
Compare lifting settled concrete with tearing out and replacing it.
Concrete Leveling
Start with the broad idea of lifting and supporting settled concrete.
Mudjacking
Plain-English help for traditional slurry lifting.
Slab Jacking
Learn what slab jacking means and when it may fit.
Springfield
Concrete leveling information for Springfield.
Bloomington-Normal
Concrete leveling information for Bloomington-Normal.
Decatur
Concrete leveling information for Decatur.
Champaign-Urbana
Concrete leveling information for Champaign-Urbana.
Peoria
Concrete leveling information for Peoria.
Next step
Need someone to look at your sunken concrete?
Tell us what is sinking, where you are located, and how soon you would like it looked at. Photos are helpful later, but they are not required to start.