Settled concrete slabs, driveways, pathways, garage floors, and in some cases structural house slabs, can sometimes be raised back toward their original level by injecting material beneath them. Two methods dominate: the traditional mudjacking technique and the modern polyurethane foam injection process.
The honest caveat upfront: neither method addresses the underlying soil problem that caused the slab to sink. They are most useful for non-structural concrete (paths, driveways, pool surrounds) or as a short-term measure where full replacement is not justified. For settled house slabs with structural consequences, underpinning with screw piles is usually the right answer.
Mudjacking (Slab Jacking)
How It Works
Mudjacking involves drilling 50-75 mm holes through the settled slab, then pumping a slurry mixture under pressure into the void space beneath. The slurry, typically a mixture of Portland cement, soil, and water, fills voids and generates pressure that lifts the slab. Once the slab is raised to the desired level, the drill holes are patched.
The material sets over 24-72 hours, after which the area can be used.
Where Mudjacking Works
- Driveways and paths that have settled into voids (not on reactive clay)
- Garage aprons that have sunk at the edge
- Steps and pool surrounds that have settled
- Industrial floor panels that have lost uniform support
Limitations on Wagga’s Reactive Clay
Mudjacking is designed for situations where the slab has settled into a void beneath it, for example, where a water line eroded soil away or where a tree root decayed and left a hollow. In Wagga’s reactive clay, slabs do not typically settle into voids. Instead, the clay contracts away from the slab unevenly, leaving no single void but a complex pattern of partial separation between the slab soffit and the clay.
The mudjacking slurry, once pumped in, adds weight to the clay below. On reactive clay, adding weight to already-stressed soil is generally counterproductive. And because the lifting force is a fluid pressure, it cannot selectively lift specific points, it tends to lift the whole slab (including areas you don’t want raised).
Mudjacking is rarely appropriate for structural house slabs in Wagga Wagga and is more suited to non-clay situations with discrete voids.
Polyurethane Foam Injection
How It Works
Polyurethane foam injection (also called foam lifting, poly-lift, or geo-foam injection) uses a two-component expanding polyurethane foam. Small holes (typically 16-25 mm diameter) are drilled through the slab at calculated positions. Two-component resin components are injected through a mixing nozzle. The two components react, expand into foam, and fill the void below the slab. Controlled injection allows gradual, targeted lifting of specific areas.
The foam cures rapidly, typically within minutes, allowing the slab to be put back into use almost immediately. Hole sizes are much smaller than mudjacking, meaning patches are less visible.
Advantages over Mudjacking
| Feature | Mudjacking | Polyurethane Foam |
|---|---|---|
| Hole size | 50-75 mm | 16-25 mm |
| Weight added below slab | High (heavy slurry) | Very low (lightweight foam) |
| Cure time | 24-72 hours | 15-30 minutes |
| Precision of lifting | Moderate | High, targeted lifting |
| Minimum void requirement | Needs accessible void | Works with partial separation |
| Cost | Lower | Higher |
| Durability in wet conditions | Slurry can erode over time | Foam is water-resistant |
Where Polyurethane Foam Is Appropriate in Wagga
Polyurethane foam injection is increasingly used for:
- Path and driveway panels that have settled differentially
- Commercial warehouse floors with settled panels (creating trip hazards)
- Pool surrounds and outdoor entertaining areas
- Paths adjacent to doorways where a step has developed creating accessibility issues
- Garage aprons
Limitations for House Slabs in Wagga
Like mudjacking, polyurethane foam injection does not address the underlying clay movement. For a house slab that has settled because perimeter clay has dried and contracted, foam injection can partially lift the settled edge, but the clay will continue to move. The foam is rigid and does not adapt to future clay movement. What was temporarily raised may re-settle, or adjacent unsupported areas may settle faster once the immediate area is stiffened.
Some practitioners offer polyurethane foam as a “lightweight underpinning” for house slabs. This framing is misleading. Foam provides slab support by filling voids but does not provide the deep bearing that screw pile underpinning provides. It is not equivalent to proper underpinning.
For house slabs with measurable structural movement and associated wall cracking, underpinning with screw piles remains the standard of care. Foam injection for structural house slabs should only be considered in conjunction with a structural engineer’s assessment.
Cost Comparison in Wagga Wagga (2026)
| Method | Typical Application | Approximate Cost (AUD) |
|---|---|---|
| Mudjacking | Driveway/path (per m²) | $100 to $200/m² |
| Polyurethane foam | Path/driveway (per panel) | $300 to $800 per panel |
| Polyurethane foam | Warehouse floor (per 10 m²) | $2,000 to $5,000 |
| Structural underpinning (screw piles) | House slab | $15,000 to $45,000+ |
The cost differential between foam lifting and screw pile underpinning is significant. However, for structural house slabs, the foam approach may not provide a durable solution, meaning you end up spending money on foam and then spending again on proper underpinning later.
Decision Guide: Which Option Is Right for You?
Work through these questions:
1. Is the concrete structural or non-structural?
- Non-structural (path, driveway, garage floor, pool surround): Foam injection may be appropriate
- Structural (house slab, foundation): Consult a structural engineer; underpinning likely needed
2. Is the settlement on reactive clay or a discrete void?
- Discrete void (drain collapse, root decay, erosion): Mudjacking or foam may work
- Reactive clay (seasonal shrink-swell): Neither mudjacking nor foam addresses the cause
3. How much settlement has occurred?
- Less than 25 mm, non-structural: Foam injection may be cost-effective
- More than 25 mm, or any structural settlement: Professional engineering assessment needed
4. Is there ongoing movement?
- Settlement has stopped (e.g., post-flood stabilisation): Foam may be appropriate to restore level
- Settlement is ongoing (seasonal clay movement): Any slab lift will re-settle without addressing the cause
What Wagga Foundation Repair Recommends
We are honest about what these techniques can and cannot do:
- For driveways, paths, and non-structural outdoor concrete: foam injection is a cost-effective solution in appropriate conditions
- For garage floors with minor settlement over discrete voids: foam may be adequate
- For house slabs with structural consequences: get an engineering assessment before committing to any injection method
- Do not accept foam injection as a substitute for proper underpinning on a house foundation without independent structural engineering advice
Frequently Asked Questions
Q: Can foam lifting fix my settled verandah slab? A: Possibly, if the settlement is discrete and the slab is not load-bearing structural. Verandah slabs are generally non-structural, making them a reasonable candidate for foam lifting. However, if the verandah is attached to the house and pulling the main structure, the issue is structural and needs engineering.
Q: How long does polyurethane foam last under a slab? A: High-quality closed-cell polyurethane foam used for slab lifting has an estimated service life of 50+ years. It does not biodegrade, does not erode in water, and does not compress significantly under typical slab loads.
Q: Is foam injection available in Wagga Wagga? A: Yes, several contractors offer polyurethane foam lifting in the Wagga region. Make sure the contractor is experienced with the specific product and can show previous project references.
Q: My pathway has sunk 80 mm, can foam fix it? A: 80 mm of settlement is significant. Foam injection can potentially recover most of this level, but the cause of 80 mm settlement (a significant void or substantial clay movement) should be identified before injecting. A very large void may require significant foam volume, increasing cost.
Q: The foam injection company says they can fix my house slab, should I proceed? A: Get a second opinion from a structural engineer first. An engineer can assess whether foam provides adequate support for your specific structural configuration, or whether screw pile underpinning is needed. Do not proceed with foam injection on a house slab without engineering advice.
Not sure whether you need foam lifting or proper underpinning? Book a free assessment, we’ll give you an honest recommendation based on what’s actually happening under your Wagga slab.