WARNING: Never Allow Void-Filling Products (Polyurethane Foam or Grout) Under Your Home’s Slabs or Footings Without First Consulting a Licensed Geotechnical Engineer

By Michael R. Simpson, P.E., Geotechnical and Forensic Engineer

A problematic trend is spreading in the foundation repair industry: many companies are routinely recommending polyurethane foam injections under interior slabs and “deep compaction grouting” without first determining whether the soil beneath the home is expansive (shrink-swell) clay.

Here’s why this is a serious problem:

When the soil under a house is dense, dry, and consists of a high-plasticity clay (very common in the Phoenix metro area and much of the Southwest), injecting polyurethane foam or grout to “fill voids” or “stabilize” the soil can create a time bomb of sorts. As soon as that dry clay eventually gets wet (seasonal moisture, plumbing leak, landscape irrigation, or even normal humidity changes), the clay swells. Because the injected material is rigid and incompressible, the swelling clay has nowhere to go except upward; forcing the slab to heave. The result is often severe cracking in floors, walls, drywall, and ceilings; sometimes far worse than the original settlement problem the repair was supposed to fix.

The responsible approach:

Before any void-filling or soil-stabilization injection (polyurethane, grout, etc.) is performed under or around a home’s foundation, a licensed geotechnical engineer should be consulted. Proper soil sampling and laboratory testing (Atterberg limits, swell/consolidation tests, moisture content, etc.) are required to determine the shrink-swell potential of the soil and whether injection methods are appropriate or likely to cause future damage.

As I’ve written before, most foundation repair companies send salespeople; not engineers; to your “free inspection.” Those inspections frequently turn into five- and six-figure repair proposals, whether the house truly needs the work or not (see my earlier article: Does Your House Really Need Foundation Repair?). Installing piers in the wrong location may be expensive and occasionally unnecessary, but it rarely makes the problem worse. Injecting rigid materials into dry, expansive clay, however, can turn a manageable settlement issue into major structural damage when the soils inevitably re-wet.

Bottom line: 

If a contractor tells you that polyurethane foam or compaction grouting is the solution without geotechnical testing, get a second opinion from an independent, licensed geotechnical engineer before you allow anyone to pump anything under your slab. Your home’s long-term stability may depend on it.