The Science of Soil Voids: Why the Ground Disappears Under Your Concrete

Your Concrete Is Not Always the Problem. The Ground Is.

When a driveway, sidewalk, patio, or garage floor sinks, it is easy to blame the concrete. But concrete does not normally sink on its own.

It moves because the soil underneath has settled, shifted, shrunk, or washed away. This creates an empty space between the ground and the bottom of the slab. That empty space is called a soil void.

Soil voids are common around homes and businesses throughout St. Louis and the surrounding communities. Heavy rain, water runoff, clay soil, poor drainage, and changing temperatures can all weaken the ground supporting a concrete slab.

You cannot see a void from above. By the time the concrete tilts, cracks, rocks, or creates a trip hazard, the empty space underneath may have been growing for months or even years.

Understanding why soil voids form can help you catch the problem sooner and choose a repair that addresses what is happening below the surface.

What Is a Soil Void?

A soil void is an empty space between a concrete slab and the ground that is supposed to support it.

Some voids are only a fraction of an inch deep. Others can stretch across a large part of a driveway, sidewalk, patio, porch, pool deck, or concrete floor.

The size is not the only concern. Even a small unsupported area can allow the concrete to flex when weight is placed on it.

Concrete is strong when it rests on a firm and even base. It is not designed to hang over an open space. When soil support disappears, the slab begins acting like a bridge over a hole.

The weight of a vehicle, a person, outdoor furniture, or the concrete itself can cause the unsupported section to bend. Eventually, the slab may crack, tilt, or drop into the void.

In simple terms, your concrete is only as stable as the ground beneath it.

Why Soil Voids Form Under St. Louis Concrete

Soil is not one solid mass. It contains mineral particles, moisture, air, clay, sand, and sometimes organic material. When water or weather changes any part of that mix, the soil can change shape or volume.

Most soil voids around St. Louis properties develop because of one or more of the following problems.

1. Water Erosion and Soil Washout

Water is one of the biggest causes of soil voids.

Rainwater can enter beneath a slab through open joints, cracks, or gaps along the edges. Water from a short downspout, overflowing gutter, poor grading, or leaking irrigation line can also flow toward the concrete.

As the water moves, it carries small soil particles away with it. This process can slowly create a channel under the slab.

One storm may remove only a small amount of soil. But after months or years of repeated rain, the empty space can become large enough for the concrete to sink.

Watch for soil or sand appearing along the edge of a driveway, sidewalk, or patio after heavy rain. That material may be coming from underneath the slab.

2. Poorly Compacted Fill Soil

During construction, soil is often removed to build a home, install utilities, or prepare the property. That area is later filled back in.

If the fill soil is not compacted in thin, firm layers, air pockets can remain between the soil particles. The surface may look solid, but the ground below can still be loose.

Over time, gravity and the weight of the concrete press those air pockets out. The soil settles lower while the slab temporarily stays in place. This leaves an unsupported gap underneath.

Eventually, the concrete drops into the empty space.

This type of settlement is common near foundations, garages, utility trenches, porches, patios, and newly developed areas.

3. Expansive Clay Soil

Clay soil absorbs and releases moisture much like a sponge.

During wet weather, clay can swell. During hot and dry periods, it can shrink and pull away from the bottom of a slab.

St. Louis weather can move between heavy rain and long dry periods. These moisture changes make the ground expand and contract beneath concrete.

The slab may move slightly during each cycle. Over time, the soil no longer supports the concrete evenly. Low areas and hidden voids begin to develop.

4. Freeze-and-Thaw Cycles

During cold weather, moisture in the soil can freeze and expand. This may push the soil and concrete upward. The movement is commonly called frost heave.

When temperatures rise, the frozen soil thaws. The ground can become wetter and looser than it was before. It may then settle lower, leaving an empty space below the slab.

Repeated winter freeze-and-thaw cycles can slowly change the soil underneath driveways, sidewalks, patios, steps, and other outdoor concrete.

5. Buried Organic Material

Tree roots, stumps, lumber scraps, and layers of topsoil may sometimes be buried during construction.

Organic material breaks down over time. As it decays, it takes up less space. The soil above it can then collapse into the newly formed gap.

A concrete slab sitting over that area loses support and begins to settle.

This is one reason a driveway or patio can start sinking years after it was installed.

6. Poor Base Preparation

A concrete slab should be installed over a properly prepared and compacted base.

If concrete is poured over loose native soil, soft topsoil, or an uneven gravel layer, it may have weak support from the beginning. The slab may look fine when it is new but begin moving as the ground settles.

Once the base shifts, voids can form underneath the concrete.

Why a Small Void Can Become a Bigger Problem

A soil void rarely stays the same size.

Once the empty space forms, it can start a cycle that causes more movement:

A small void allows part of the slab to flex or tilt.
That movement opens cracks, joints, and edge gaps.
Rainwater enters through the new openings.
The water carries away more soil.
The void becomes larger.
The concrete settles farther and opens wider gaps.

This cycle can turn a small dip into a raised edge, drainage problem, or serious trip hazard.

A sinking slab may also begin directing rainwater toward a garage, basement, foundation, or building entrance. The longer the underlying problem continues, the more difficult and expensive the repair may become.

Signs of a Void Under Your Concrete

You do not need to see beneath the slab to notice the warning signs. Look for changes in the surface and around the slab’s edges.

Common signs of soil voids include:

One concrete section sitting lower than the section next to it
Raised edges or lips between driveway and sidewalk panels
Cracks that continue to widen
Diagonal cracks near the corner of a slab
A gap between the concrete and a foundation, porch, garage floor, or steps
Water pooling in low areas after rain
Concrete tilting toward the house or garage
Soil or sand washing out from under an exposed edge
A hollow sound when you tap the surface
Concrete that moves or rocks when you step on it
Gaps opening underneath the edge of a patio or sidewalk
Doors or gates becoming harder to open because nearby concrete has moved

If you notice several of these warning signs, the concrete may have lost part of the support underneath it.

Saint Louis Polyjack can inspect the affected area and determine whether professional void filling or concrete lifting is the right solution.

Why Replacing the Surface May Not Fix the Real Problem

Tearing out a sunken slab removes the visible damage, but it does not automatically correct the soil problem underneath.

If new concrete is poured over the same loose, washed-out, or poorly compacted ground, the replacement slab may eventually settle too.

Replacement may be necessary when concrete is severely broken, crumbling, or structurally damaged. However, concrete that is still in solid condition can often be saved.

Polyurethane concrete lifting targets the empty space below the slab. It restores support, stabilizes weak areas, and raises the existing concrete without the disruption of tearing everything out.

Why Filling the Void Matters as Much as Lifting the Slab

Raising the slab is the part of the repair you can see. Filling the void is the part that restores support.

A slab should not simply be pushed upward while leaving open areas underneath. The empty space must be filled so the weight of the concrete can spread across a stable base.

Polyurethane foam expands into hard-to-reach gaps and channels beneath the slab. This helps create more even support while lifting the concrete.

For example, driveway leveling does more than remove a raised edge between panels. It also fills the hidden space that allowed the driveway to sink.

The same method can be used for sidewalks, patios, pool decks, garage floors, porches, steps, interior floors, and commercial slabs.

Polyurethane Foam Compared With Mudjacking

Saint Louis Polyjack uses polyurethane foam rather than traditional mudjacking.

Mudjacking pumps a heavy cement-based slurry underneath the concrete. It has been used for many years, but it normally requires larger drill holes and adds more weight to the soil.

Polyurethane foam offers several advantages:

Smaller access holes
Lightweight lifting material
Controlled expansion
Ability to reach small gaps and channels
Fast curing time
Same-day use for most surfaces
Less disruption around the repair area

Polyurethane foam is especially useful when the goal is to fill hidden voids, stabilize weak soil, and lift structurally sound concrete without demolition.

Concrete Areas That May Have Hidden Voids

Soil voids can form beneath almost any slab, including:

Driveways
Walkways
Public and private sidewalks
Patios
Porches
Concrete steps
Pool decks
Garage floors
Basement floors
Warehouse floors
Parking lots
Commercial entrances
Apartment and HOA walkways
Interior concrete slabs

Uneven walking surfaces can become safety concerns, especially near entrances, steps, and high-traffic areas. Sidewalk and walkway leveling can remove raised edges while restoring support beneath the concrete.

Frequently Asked Questions About Soil Voids

Can a soil void fill itself?

No. The missing soil will not normally return to its original position on its own. Water movement, continued settlement, and slab movement can cause the void to grow larger.

Are soil voids dangerous?

They can be. A void can cause concrete to crack, rock, or sink unexpectedly. Uneven slabs create trip hazards and may direct water toward a home or building.

A large unsupported area under a driveway or garage floor may also allow the concrete to break under the weight of a vehicle.

Does a hollow sound mean there is a void?

A hollow sound can indicate an unsupported area, but it does not confirm the size or exact location of a void. A professional inspection can help determine what is happening underneath the slab.

Will new concrete solve the problem?

Not if the weak or missing soil underneath is ignored. Replacement concrete can settle again when it is installed over the same unstable ground.

Can every sunken slab be lifted?

No. The concrete must still be in reasonably solid condition. A severely cracked, crumbling, or badly damaged slab may need replacement. Saint Louis Polyjack can inspect the concrete and explain the practical repair options.

How long does polyurethane concrete lifting take?

Many residential repairs can be completed in a few hours. The exact timeline depends on the size of the slab, the amount of settlement, and the volume of material needed.

When can the repaired concrete be used?

Polyurethane foam cures quickly. Most surfaces can be walked or driven on the same day after the repair is completed.

Is polyurethane concrete lifting covered by a warranty?

Saint Louis Polyjack backs its polyurethane concrete leveling repairs with a five-year warranty.

Get the Ground Under Your Concrete Checked

Sinking concrete is more than a surface problem. It is a warning that the soil underneath may no longer be providing proper support.

Filling the void early can help prevent further settlement, reduce trip hazards, improve drainage, and protect the existing slab from additional damage.

Saint Louis Polyjack serves St. Louis, St. Charles, Sunset Hills, Fenton, Kirkwood, Webster Groves, Ballwin, Chesterfield, Crestwood, Arnold, and surrounding communities.

Call 314-488-2993 or request your free concrete leveling estimate. You can also send photos of the affected area for a faster evaluation.