Why Does Concrete Sink? The Science Behind Settling Slabs

Your concrete driveway or patio likely looked permanent when the crew finished the pour.

But now, a few years later, you are noticing a slope that wasn’t there before.

Understanding why this happens is the first step toward fixing it through concrete leveling and preventing expensive structural damage later.

The Foundation of the Problem

Concrete slabs do not technically float.

They rely entirely on the soil beneath them for structural support.

When that supporting soil moves, shrinks, or washes away, gravity takes over.

It is rarely the concrete itself that fails.

The issue almost always lies in the dynamic nature of the ground underneath.

Cause 1: Soil Erosion (The Primary Culprit)

Water is the single most destructive force against concrete stability.

It creates voids that the slab eventually falls into.

The Physics of Erosion: A single cubic foot of water weighs approximately 62.4 pounds.

When that weight moves through soil, it carries significant displacement force.

How the process works:

Entry: Water finds a path through unsealed control joints or along the slab edges.
Displacement: The flow picks up fine soil particles and carries them away.
Void Creation: Small gaps form initially, often hidden from view.
Structural Failure: The concrete creates a bridge over the empty space until the load becomes too heavy.
Settlement: The slab cracks and drops to meet the new soil level.

Common water sources we see:

Misdirected gutter downspouts dumping water right at the foundation.
Negative grading that funnels rain toward the driveway rather than away.
Plumbing leaks that wash out soil from below without showing surface signs.

The Colorado context: Our region experiences intense, rapid rainfall events.

A 20-minute storm can move a massive amount of soil if drainage is not managed correctly.

Cause 2: Soil Compaction

Every type of soil compresses under weight, but the timeline varies.

The Mechanics of Compaction: Native soil that has sat undisturbed for centuries is generally stable.

The problem arises when soil is moved, aerated, and then piled back up.

Why compaction leads to sinking:

The “Fluff” Factor: Excavated soil creates air pockets that slowly collapse over time.
Load Bearing: Heavy vehicles compress the driveway layers further than the natural settling rate.
Time delay: We often see this settling occur 5 to 10 years after a home is built.

The Builder Factor: Most residential construction involves backfilling around the foundation with loose dirt.

Unless the builder mechanically compacts this fill soil in distinct “lifts” or layers, it will naturally settle by 10% to 15% largely on its own.

This is why your front walkway might sink while the rest of the yard stays level.

Cause 3: Expansive Clay Soils

The Front Range is notorious for its expansive clay, specifically Bentonite.

The “Sponge” Effect: Clay soil acts much like a dense sponge regarding moisture.

According to the American Society of Civil Engineers, expansive soils cause more financial damage to U.S. property owners annually than floods and hurricanes combined.

How clay creates movement:

Expansion (Heave): When wet, this soil can expand volume by 10% or more, lifting the concrete.
Contraction (Settlement): As it dries, it shrinks back, often leaving a void where the concrete used to be supported.
The Cycle: This constant up-and-down movement stresses the slab until it snaps.

Local variations: Colorado Springs has a complex geological map.

One neighborhood might sit on stable sandstone, while a street two miles away sits on highly reactive Pierre Shale.

Cause 4: Tree Root Activity

Trees are aggressive when searching for moisture.

Their root systems often extend two to three times the width of the tree canopy.

Two distinct root problems:

1. The Physical Lift

Thick roots growing directly under a sidewalk or driveway exert tremendous hydraulic pressure.

This pushes the slab upward, creating a trip hazard and cracking the concrete.

2. The Decay Void

This issue is less obvious but equally damaging.

When a tree is removed or roots die naturally, the organic material decomposes.

As that wood turns to soil, it loses volume, creating a hollow space that the concrete eventually sinks into.

High-risk vegetation:

Poplar trees
Willow trees
Silver Maples
Large shrubs planted within 3 feet of a walkway

Cause 5: Poor Original Construction

Sometimes the issue was baked in the day the concrete was poured.

Common installation shortcuts:

Weak Base: Pouring directly onto dirt instead of a 4-inch compacted gravel base.
Excess Water: Adding too much water to the mix to make it easier to spread weakens the cured strength.
Incorrect Thickness: Residential driveways should typically be 4 inches thick (5 to 6 inches for heavy vehicles), but we frequently measure sinking slabs at only 2.5 to 3 inches.

Signs of installation failure:

Significant settling within the first 24 months.
Flaking or spalling on the surface.
Cracking that does not align with control joints.

Cause 6: Underground Changes

Subsurface issues can affect surface stability in surprising ways.

Hidden factors:

Infrastructure decay: Rusted corrugated metal drainage pipes that collapse.
Utility trenches: The loose dirt in sewer or water line trenches often settles differently than the surrounding ground.
Pest activity: Extensive tunneling by voles or other burrowing animals can compromise the soil density.

The Colorado Springs Factor

Our local environment acts as a stress test for concrete.

Climate Challenges

The Freeze-Thaw Cycle: Colorado Springs often sees over 100 freeze-thaw cycles per year.

Water enters the soil, freezes, and expands by about 9%.

This expansion pushes against the concrete and loosens the soil structure.

When it melts, the soil is less compact than before.

Rapid Fluctuations: We can swing from 70 degrees to freezing in 24 hours.

This thermal shock forces concrete to expand and contract rapidly, exacerbating any existing weakness in the base.

Soil Specifics

Bentonite Presence: This specific clay type is highly absorbent.

It requires careful water management to prevent massive volume changes under your home’s exterior features.

Prevention: Protecting Your Investment

You can extend the life of your concrete by managing the environment around it.

Water Management Strategy

Action Item	Why It Matters	Pro Tip
Extend Downspouts	Moves high-volume water away from the base.	Aim for at least 6 to 10 feet of distance from the slab.
Adjust Grading	Prevents pooling near the edges.	The ground should slope away at 1 inch per foot for the first 5 feet.
Seal Joints	Stops water from entering underneath.	Use a high-quality polyurethane sealant, not rigid mortar.

Joint Maintenance

Open cracks act as funnels for rain and snowmelt.

We recommend sealing these gaps to keep the soil beneath dry and stable.

Learn about our caulking service →

Root Management

Keep thirsty trees at a safe distance.

If you are planting new trees, choose deep-rooting varieties rather than surface-rooting ones like Maples.

Installing a physical root barrier 18 to 24 inches deep along the driveway edge can also deflect growth.

When Settling Happens: Comparing Your Options

If you are already dealing with uneven slabs, you have three primary paths forward.

Option 1: Ignore It

This is the default for many, but it carries risks.

Trip hazards can lead to liability issues, and small cracks allow water to accelerate the erosion process.

Option 2: Concrete Leveling (Polyjacking)

This modern method injects high-density polyurethane foam through penny-sized holes.

The advantages:

Cost: Typically 50% to 70% cheaper than replacement.
Speed: You can drive on it immediately after the crew leaves.
Stability: The foam is lightweight and does not add burden to already weak soil.

Learn more about our leveling process →

Option 3: Replacement (Remove and Repour)

This involves jackhammering the old concrete and starting over.

When this is necessary:

The slab is broken into too many small pieces (rubble).
The concrete is structurally crumbling or spalling.
You want to change the shape or design of the area.

The Bottom Line

Concrete does not sink without a reason.

Whether it is water erosion, clay soil movement, or poor compaction, the physics are predictable.

We believe that identifying the root cause is just as important as the repair itself.

If you spot uneven slabs or widening cracks, contact us for a free assessment.

Our team will diagnose exactly why your concrete is moving and provide a permanent solution to stabilize it.