How do mudjacking and polyurethane foam lift a sunken slab?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: Slabjacking raises a sunken concrete slab by pumping material underneath it — either a cement-based slurry (mudjacking) or an expanding polyurethane foam ¹. Both fill gaps and lift the slab, but they rest on the same soil that moved, so they do not stop expansive clay from shrinking or swelling again ¹³. They are mainly used for flatwork and for filling voids, not for slabs that are heaving.

What are the two methods?

Mudjacking (cement grout)

Mudjacking pressure-injects a slurry of sand, fly ash or topsoil mixed with cement between the slab and the soil ¹. The slurry fills empty spaces and, as more is pumped in, raises the slab.

• Grout can be blocked by roots, pipes or other objects under the slab, which can make the lift uneven ¹.

Polyurethane foam

Foam lifting injects a urethane-based foam under the slab, where it expands to raise low areas; the slab then rests on the foam ¹.

• An incorrect mix of the two foam chemicals can cause the repair to fail ¹.
• If the mix is too wet, water can seep into expansive soil before the foam hardens and cause local heave ¹.
• Oklahoma field practice records include foam with a density of about 1.75 pounds per cubic foot used for slab lifting ⁶.

Where are these methods used?

• Driveways, sidewalks, patios, garage slabs and pool decks — see heaved or settled flatwork and flatwork and exterior slabs.
• Filling voids under slabs after erosion or leak washout — see erosion, undermining and washout voids.
• Interior slab areas that have settled over loose fill, rather than heaved.

Why don't they fix clay movement?

Expansive clay swells when wet and shrinks when dry, and its shrinkage cracks do not close up perfectly on rewetting ³. A slab lifted with grout or foam still sits on that soil. An engineering committee review describes both methods as shallow repairs affected by active soil movement and lists these causes of renewed movement after repair ¹:

• Changes in soil moisture and seasonal swings
• Leaking sewer or water lines under the slab
• Nearby trees
• Poor drainage

This is why slab lifting is not used for perimeter drying settlement as a lasting fix, and why it is harmful for center heave or leak-induced heave: it adds more lift to an area that is already high. Telling the two apart is covered in heave vs settlement.

What are the risks during lifting?

The same review lists installation problems for both methods ¹:

What should be checked before lifting?

Plumbing. Grout and foam can enter cracked or broken pipes and plug them ¹, so under-slab drains are often checked before lifting. A hydrostatic test plugs the main drain, fills the under-slab system to slab level and watches for a drop in water level for up to 15 minutes ². Leaks also matter because sewer leakage is one of the ways expansive soils become saturated ⁵. See plumbing leak detection and repair.

Post-tensioned slabs. Many newer slabs are post-tensioned — reinforced with high-strength steel cables (tendons) stressed after the concrete cures. An engineering design review notes these slabs require specialized expertise and that tendon anchorages need attention ⁴. Drilling injection holes into a post-tensioned slab without locating the tendons risks cutting one.

Can they be combined with other repairs?

Foam or grout is sometimes used together with piers: piers carry the foundation beam, and foam fills voids or supports the slab between beams. Cost depends mainly on slab area, how much material is needed and access.

Related

• Void and Compaction Grouting
• Heaved or Settled Flatwork
• Heave vs Settlement
• Method Selection Matrix
• Erosion, Creeks and Slopes