Concrete Settling: Causes, Signs, and How to Fix It
Concrete settling happens when the soil beneath a slab compacts, erodes, or shifts, causing sections to sink. A well-installed slab on properly prepared soil shouldn't settle—but shortcuts in base preparation, poor drainage, or natural soil changes can undermine even good concrete.
Concrete Settling: Causes, Signs, and How to Fix It
Concrete settling happens when the soil beneath a slab compacts, erodes, or shifts, causing sections to sink. A well-installed slab on properly prepared soil shouldn't settle—but shortcuts in base preparation, poor drainage, or natural soil changes can undermine even good concrete.
Understanding why your concrete settled determines whether lifting makes sense or if you're just postponing replacement. Address the cause, then fix the symptom.
Why Concrete Settles
1. Poorly Compacted Fill Soil
What happens: When excavation is backfilled, soil isn't compacted properly. Over time (months to years), loose soil settles under the slab's weight.
Common situations:
- New construction on filled lots
- Areas around foundation walls
- Utility trenches backfilled improperly
- Pool removal sites
2. Soil Erosion and Washout
What happens: Water flowing beneath the slab washes away soil, creating voids. The slab eventually drops into the void.
Common situations:
- Downspouts discharging near slabs
- Poor grading directing water toward concrete
- Broken underground pipes
- Natural drainage patterns
3. Organic Material Decomposition
What happens: Buried organic matter (roots, stumps, vegetation) decomposes over years, leaving voids.
Common situations:
- Slabs poured over old tree roots
- Areas where vegetation wasn't properly cleared
- Former garden beds
4. Drought and Soil Shrinkage
What happens: Clay soils shrink significantly during dry periods, creating gaps beneath slabs.
Common situations:
- Regions with expansive clay soils
- Areas near large trees (root moisture competition)
- Extended drought periods
5. Inadequate Base Preparation
What happens: No gravel base was installed, or it was insufficient. Native soil doesn't provide stable support.
Common situations:
- DIY projects skipping proper prep
- Contractors cutting corners
- Older construction with different standards
For proper base preparation, see do you need gravel under concrete.
Signs of Concrete Settling
| Sign | What It Indicates |
|---|---|
| Visible sinking | One section lower than adjacent areas |
| Cracks with displacement | Settlement stress breaking the slab |
| Pooling water | Low spots where water collects |
| Uneven surfaces | Sections tilting or rocking |
| Gaps at edges | Space between slab and structure |
| Trip hazards | Height differences at joints |
When Settling Becomes a Safety Issue
Trip hazard threshold: Height differences greater than 1/4" between sections create trip hazards. Many jurisdictions require correction of public walkways with larger offsets.
Drainage concerns: Slabs settling toward structures can direct water into foundations or basements.
Repair Option 1: Mudjacking
Mudjacking (slabjacking) pumps a cement-based slurry beneath the slab to fill voids and lift it back to level.
How It Works
- Drill 1-2" holes through slab
- Insert injection pipe
- Pump slurry (cement, sand, water mix) under pressure
- Slurry fills voids, lifts slab
- Patch drill holes
- Ready for use in 24-48 hours
Cost
| Project | Typical Cost |
|---|---|
| Per square foot | $3-6 |
| Minimum job | $500-800 |
| Sidewalk section (50 sqft) | $150-300 |
| Patio (200 sqft) | $600-1,200 |
| Driveway (400 sqft) | $1,200-2,400 |
Pros
- 50-70% cheaper than replacement
- Same-day completion
- Proven technology (50+ years of use)
- Fills large voids effectively
Cons
- Heavy material may cause re-settling
- 1-2" holes are visible (though patched)
- May not last as long as foam (5-10 years typical)
- Not suitable for areas with active water flow
Repair Option 2: Polyurethane Foam Lifting
Foam lifting injects expanding polyurethane foam beneath slabs—a newer, often superior alternative to mudjacking.
How It Works
- Drill small holes (dime-sized, ~5/8")
- Insert injection nozzle
- Inject two-part polyurethane foam
- Foam expands to 15-20× volume, filling voids
- Slab lifts within seconds
- Material sets in minutes
- Ready for use almost immediately
Cost
| Project | Typical Cost |
|---|---|
| Per square foot | $5-15 |
| Minimum job | $800-1,200 |
| Sidewalk section (50 sqft) | $250-750 |
| Patio (200 sqft) | $1,000-3,000 |
| Driveway (400 sqft) | $2,000-6,000 |
Pros
- Lightweight (won't cause additional settling)
- Waterproof (won't wash out)
- Smaller holes, less visible
- Sets in minutes (immediate use)
- Longer lasting than mudjacking (10-20+ years)
- Precise control during lifting
Cons
- More expensive than mudjacking
- Cannot be adjusted after setting
- Material expands rapidly (operator skill matters)
- Limited void-filling capacity for very large gaps
Repair Option 3: Replacement
Sometimes lifting isn't practical. Full or partial replacement makes more sense when:
Replace When:
- Settling exceeds 2-4 inches—lifting this much stresses the slab
- Slab is badly cracked—lifted sections won't reconnect properly
- Underlying problem can't be fixed—it'll settle again
- Lifting cost approaches 60%+ of replacement—diminishing returns
- Concrete is severely deteriorated—other problems beyond settling
Replacement Cost
| Project | Cost |
|---|---|
| Per square foot | $8-15 |
| Sidewalk section (50 sqft) | $400-750 |
| Patio (200 sqft) | $1,600-3,000 |
| Driveway (400 sqft) | $3,200-6,000 |
Choosing the Right Fix
| Situation | Best Option |
|---|---|
| Minor settling (<1"), budget limited | Mudjacking |
| Moderate settling, long-term fix | Foam lifting |
| Severe settling (>3"), cracked slab | Replacement |
| Active water problem | Fix drainage, then lift or replace |
| Slab over 25 years, multiple issues | Replacement |
| Localized void (small area) | Either lifting method |
Preventing Future Settling
Whether you lift or replace, prevent recurrence:
Drainage
- Direct downspouts away from slabs (minimum 4-6 feet)
- Ensure grading slopes away from structures
- Fix any leaking underground pipes
- Install French drains if needed
Soil Stability
- Compact fill soil in layers (6" lifts)
- Allow filled areas to settle before pouring
- Remove all organic material from base
- Install proper gravel base (4-6")
During New Construction
- Inspect subbase before pour
- Verify compaction testing on fill areas
- Document any concerns before concrete arrives
Frequently Asked Questions
Why is my concrete slab sinking?
The most common causes are poorly compacted fill soil, water erosion beneath the slab, decomposing organic material, or drought-related soil shrinkage. The cause determines whether lifting will provide a lasting fix or just delay the inevitable.
Is mudjacking or foam lifting better?
Foam lifting is generally better—it's lighter (reducing re-settling risk), waterproof, faster, and longer lasting. However, it costs 50-100% more. For budget-conscious repairs where soil is stable, mudjacking works well. For premium, long-term results, foam lifting is worth the investment.
How much does it cost to fix settled concrete?
Mudjacking costs $3-6 per square foot; foam lifting costs $5-15 per square foot. A settled 200-sqft patio typically runs $600-1,200 for mudjacking or $1,000-3,000 for foam lifting. Replacement costs $8-15 per square foot.
Can settled concrete be a safety hazard?
Yes. Height differences exceeding 1/4" create trip hazards. Settling toward structures can direct water into foundations. Severely settled driveways can damage vehicles or create drainage toward garages. Address settling before it becomes a liability issue.
Key Takeaways
- Diagnose the cause before choosing a fix—or it'll happen again
- Mudjacking ($3-6/sqft) is budget-friendly but may not last
- Foam lifting ($5-15/sqft) is more expensive but longer lasting
- Replace when settling exceeds 3-4 inches or the slab is badly damaged
- Fix drainage regardless of repair method—water causes most settling
- Trip hazards (>1/4" offset) need prompt attention
For related problems, see why concrete cracks. For leveling options, visit how to level concrete. For new projects, use our concrete calculator.