High-density polyurethane foam is injected through small holes to fill voids beneath a slab or to raise it. Because the method is fast and needs little access, it is often proposed for conditions it cannot correct. Ground Force Engineering writes the specification when foam is the right tool, and says when it is not: under a slab that carries bearing walls or columns, which is a foundation repair, or over a subgrade that is still washing out.
The specification sets injection hole spacing, the foam density and compressive strength, the lift target and the maximum lift per pass, the sequence across the slab, and what the contractor must record. Void filling under sidewalks, garage floors, warehouse slabs and pool decks is the common use; lifting a settled slab back to grade is possible when the cause of settlement has already been dealt with.
Scope of services
- Suitability review covering slab condition, void stability and the loads the slab carries
- Void volume estimate from probing, coring or ground penetrating radar results
- Injection hole pattern, spacing and depth drawn on a slab plan
- Foam density, compressive strength and cure time requirements
- Lift targets by station with a maximum per pass and a stop rule
- Monitoring during injection: dial gauges or laser at set points
- Post-injection check of elevations and of the support under the slab, and a completion letter
How we approach it
Before writing a foam specification we check that the concrete is sound, that the cause of the settlement has been dealt with, and that no bearing wall or column depends on the slab. The specification is written around measured elevations rather than the contractor's judgment, with a lift limit per pass to keep the foam from cracking the slab. We attend the injection where the lift is significant and issue a letter once the final elevations and the support under the slab are checked.