By David Tuck, Guest Blogger from StableSoils of Florida

Over the past several years, StableSoils of Florida has been very active in the injection of many polyurethane soil enhancement products from several different manufacturers and blending companies. We have completed some very technical repairs including fuel/water separation tanks, commercial seawalls, severely leaking (1 million gallons per day) storm drain pipes; lifted swimming pools, railroad tracks, entire buildings and foundations using polyurethane injection.

We have completed many soil stabilization projects, many of which were residential applications, spec’d by Engineering firms, involving a technique of “soil densification/soil compaction” in the shallow soil zones (10’ and shallower) around foundations. This procedure typically involves injecting 20-25-30 pound “pillows” of polyurethane foam on 4-6-8 foot centers. The theory of this type of injection is that of “tightening” the adjacent soils between each injection. To better illustrate, if you took a 10 gallon empty glass aquarium and filled it with soil and then injected this material in the center, the polyurethane will chemically expand, thus pushing the contained soils outward, eventually shattering the glass enclosure. When these injections are made in close proximity to each other, like underneath a building or house foundation, the soil zone  between the injections will condense and compress. Or at least that is the theory behind this practice.

What concerns me about this practice, taking the rational approach of real world results and after some basic research, I have found that because these injections are made extremely close to the existing drip-line of the overhead roof, these once compacted/densified soils will eventually “relax” their tension and compaction qualities due to rain water events and the water flows that penetrate these upper soil zones quite easily.

Hand cone penetrometer (understanding that consistent results are very difficult to obtain) tests have shown that post injection readings and readings taken only months after injection reveal that most soils relax to their pre-injection densities. If the soils are susceptible to extreme rainfall events (as we have here in Florida), results of these injections are very short-lived. This product’s useful life is very limited by pre-injection soil density, soil composition and water in-flows. Installing proper drainage systems (gutters, trench drains, etc.) will prolong the life, but even then, the failure is only delayed.

I have found that using the 1-part, catalyzed, polyurethane soil “binding” product is far superior and perform exceptionally better, with little to no possibility of future failure.

These products are injected in a similar fashion but are typically placed directly adjacent and below the foundation to provide chemically treated, bound, waterproof soil columns that directly support the foundation and are installed to depths specified by competent engineering. These resin columns are injected at a rate of 10 pounds per vertical foot, extracted slowly while pumping is in progress and brought all the way up to the foundation. This method will produce an approximate 18” column (depends on soil composition/particle size).  The products that we have found to produce the best results are #910 and #920, produced by Prime Resins of Conyers, GA.  Both of these products expand upon injection only to facilitate soil penetration and fill small voids, but do not generate lift when pumping as directed; a benefit as all 2-part polyurethane product injections are monitored for unwanted lift characteristics.

Third-party testing has shown that typical “ball-bearing” type soils will bind to form a 1200-1400 p.s.i. mass, and, the support facial friction of these created columns generate approximately 1600 pounds  per vertical foot in the same soils.

Although very unlikely that great depths would be required to support a residential structure, these products have been successfully injected to depths of 20’-30’, whereas 2-part polyurethane injection depth is limited due to chemical expansion within the injection “needle”.

Post injection penetrometer tests reveal, even years later after many inches of recorded rainfall, that these columns remain intact, just as the day they were installed. Unaffected by any of the elements and without any “relaxing” as the treated soils are “bonded”. These created, chemically bound soil columns perform very much like an auger-cast concrete piling structure, without introducing unwanted additional weights on already weak soils as concrete (3800+lbs/cubic yard) would.

Tests on polyurethane products have shown, once injected sub-grade, have a useful chemical half-life of 115 years. Once cured (cure times are minutes), these products are inert to our environment or water table.

Pricing for the 2 different procedures can vary. Typically, the 1-part system is a marginally higher priced material, but the timeless benefits for soil stabilization or foundation support far outweigh the final cost. 2-part polyurethane has its definite role in repairing some structures. Its rapid chemical expansion rate is a very desired property when lifting of the structure or slab is required. In addition, it is very lightweight, extremely strong and very controllable.

Each of these 2 products have a definite place in the stabilization of structures and buildings. But both product types produce different results. The 2-part polyurethane best suits lifting and larger void filling while the 1-part encapsulates and strengthens loose soils and fills smaller voids.