Sunken Concrete Repair: Filling Voids Under Concrete

Concrete doesn’t usually fail first. Often, the support under the concrete fails first quietly until you have cracking, settlement, or a floor that no longer feels solid.

That missing support is usually a void space. And if the void is inside a building, downstairs, far from an entrance, and with no clean way to stage material, fixing it can turn into a labor nightmare. To repair the sunken concrete, it’s important to understand what a void space is, why it matters, the main ways to fix it, and when aggregate fill (sand or pea stone) is the right solution, especially when access is the real problem.

Understanding Void Spaces

To fix void spaces, you first need to understand what they are and why they happen. Knowing the causes helps you find better ways to use the space effectively.

What is a “Void Space” Under Concrete?

 A void space is an empty gap under a concrete slab where supporting material (often sand/soil) has been washed out or settled away.

People describe and search for this problem using lots of different terms, including:

• Void under slab/void beneath slab / under-slab void
• Void under the concrete floor
• Vollow spot under the concrete
• Unsupported slab
• Subgrade washout / subbase washout
• Subgrade erosion/subbase erosion
• Slab settlement / concrete floor settlement
• Sinking slab / sunken slab
• Sasement slab void
• “Sinkhole under slab” (often used even when it’s not a true sinkhole)

Whatever you call it, the issue is the same: the slab is no longer fully supported.

Why Void Spaces Are a Problem

Concrete is strong, but it’s not designed to “span” large empty areas beneath it. When support disappears, the following issues can take place:

• Cracks form (often in patterns that spread over time)
• The slab settles and becomes uneven
• Edges break, and surfaces start to spall
• The void can grow if water continues moving through the area
• You can end up with trip hazards, equipment issues, and expensive downtime

A void also tends to be a “multiplier problem”: the longer it’s ignored, the more likely the repair expands into demo and replacement.

How Voids Happen

Void spaces under concrete occur when the supporting soil shifts, settles, or erodes, leaving the slab unsupported. This can happen for a variety of reasons, including: 

• Soil Erosion: Water runoff from rain or broken pipes washes away soil, creating gaps.
• Poor Compaction: If the ground was not properly packed during construction, it can settle over time.
• Water Drainage Issues: Downspouts directing water toward the concrete, rather than away, wash out the subgrade.
• Organic Decomposition: Tree roots or debris under the concrete can decay, leaving empty spaces.
• Animal Burrowing: Pests like moles, chipmunks, or mice can dig tunnels.
• Initial Construction: Sometimes, cardboard forms are left under concrete, which decompose over time. 

Alternatively, sometimes a void is created through the process of mechanical or utility work. For example, a plumber may need to open up a slab to repair faulty lines. This usually requires displacing the material that was originally supporting the slab. Thus, the job isn’t finished until the area is properly backfilled and supported before the new slab is poured.

How to Fix Voids Under Concrete: Five Solutions

There are multiple legitimate ways to address voids. The “best” option depends on two questions:

1) Do you need to lift/level the slab—or just restore support?

2) Is access inside the building the main constraint?

Here are the five most common approaches to fixing voids:

Option 1: Demo &Start Over (Remove slab, Rebuild base, Re-pour)

Best when: the slab is badly damaged, the base is unstable over a large area, or the project already includes major rework.

Drawbacks:

• Most disruptive solution.
• Dust, noise, hauling, and staging problems.
• Longer downtime (including cure time).
• Usually, the highest total cost if the slab is indoors.

Demo is sometimes necessary. But it’s rarely the first choice when the slab can be saved.

Option 2: Flowable fill (CLSM)

Flowable fill (often called CLSM) is a cementitious material designed to flow into place and harden.

Best when: the void is well-contained, specs require it, and the placement can be controlled.

Drawbacks (especially indoors):

• Containment is everything. If the void connects to chases, penetrations, or unknown pockets, flowable fill can run where you don’t want it.
• It’s a wet process—floor protection and cleanup matter.
• Typically used to fill/support, not to lift/level.
• Depending on the mix, it may be difficult to remove later if future access is needed.

Option 3: Mudjacking / Slabjacking

Mudjacking pumps a thick slurry under the slab to fill voids and sometimes lift/level.

Best when: you need to lift the slab, and the site is accessible enough to manage the equipment and slurry.

Drawbacks when indoors:

• Heavier and messier logistics (slurry handling inside finished spaces).
• Often larger holes and more visible patching.
• Material takes the path of least resistance unless the void geometry is straightforward.
• Doesn’t solve the cause (a leak must be fixed first).

Option 4: Foam Injection (Polyurethane)

Foam injection can fill voids and lift/level with small holes and a fast cure.

Best when: you need lift/leveling, the void volume is moderate, and quick return to service matters.

Drawbacks:

• Cost can climb quickly for large void volumes.
• Ventilation/odor planning may be needed indoors.
• If voids connect into larger unknown areas, you can end up “chasing” the problem.
• The cause (leak/washout) must be addressed.

Option 5: Aggregate Installation (Sand or Pea Stone)

Aggregate fill means placing sand or pea stone into the void to restore support.

Best when: the goal is fill + stabilization/support, not lifting. This is often the most practical approach for interior voids—if you can place the material correctly.

Honest limitation: If you need to raise a settled slab back to elevation, aggregate fill by itself is usually not the right tool. Foam or mudjacking may be better for that specific goal.

Why aggregate still matters: Many interior jobs are primarily about restoring support, preventing further settlement, and doing it with minimal disruption. Aggregate does that well—provided you can deliver it to the void efficiently and cleanly.

Aggregate Installation: Traditional Methods vs Our Sand Truck

Traditional aggregate installation (without specialty equipment)

This is what many crews try first:

• Buckets, wheelbarrows, funnels through drilled holes
• Staging piles indoors and moving material by hand
• Opening the slab larger than desired just to get access

Problems with traditional methods (especially indoors):

• Slow and labor-heavy (the “wheelbarrow marathon” problem)
• Hard to reach far corners of the void (incomplete fill is common)
• Mess and tracking through finished areas
• Downtown/occupied buildings often have no good staging area

In other words, aggregate may be the right material, but the installation method can be the deal-breaker.

Our Method: SGC’s Sand Truck

At Superior Groundcover, we use a specialized truck we call our sand truck for controlled aggregate placement up to ~350 feet. It blows sand or pea stone through a long application hose so we can place material where equipment can’t reach.

Sand Truck Benefits:

• Long reach: hose can run deep into buildings (up to ~350 ft)
• Minimal disruption: we can often fill through small holes rather than tearing up the slab
• Cleaner: material stays in the truck (less staging, less mess)
• Faster: avoids manual hauling
• Better placement: we can direct material into the void area, not just dump it nearby

This method is especially valuable if the slab is still intact because of our ability to better reach the corners of the void. Once small access holes are drilled, we can blow the pea stone or sand through the holes and restore support with minimal disruption. 

Even if it is necessary to open up the slab for utility work, our truck is still useful for efficiently backfilling the space with sand or aggregate.

In-Field Implementation: Two Real Examples

Discover two real examples of Superior Groundcover’s aggregate installation using our sand truck.

 Example 1: Leaking Floor Drain → Washout → Pea Stone Under a Slab

A leaking floor drain can quietly wash out supporting sand over time. In this job, the void was in a basement utility area far from any entrance—no good way to haul material in, and even less of a way to get it back under the slab once inside.

What we did:

• Ran the hose down about 16 feet, then through about 100 feet of interior utility space
• Blew sand through small holes to fill the void and support the slab again

This is a classic “support restoration” job where access is the hardest part.

Example 2: Downtown Grand Rapids Renovation → Under-floor Void Fill with No Staging

Downtown projects often create a different problem: even if you could stage material, you may not have room to do it without making a mess or disrupting traffic and neighbors.

Challenges:

• No equipment access into the building
• Manual hauling would be brutal
• Busy downtown frontage = nowhere to stage material cleanly

What we did:

• Snaked the hose through the building
• Placed pea stone where it was needed, including hard-to-reach areas
• Kept the job cleaner and faster for the contractor

Contact Superior Groundcover to Make Material Placement Easy

If you have sunken concrete in a hard-to-reach area that needs repair, we’re here to help. Contact us to request an estimate for your project and receive dependable service and support from start to finish. 

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FAQ: Filling Voids under Concrete Slabs