Let’s say you are looking to recommend a pressure grout solution to stabilize a structure on problematic soil. In many cases, this involves a structure resting on consolidating, unstable soil, and this is where pressure grouting is often considered as a potential solution. 

Heaving or swelling soils are usually not candidates for grouting, so, for practical purposes, I will limit this blog to the treatment of consolidating soils. However, you must identify and know what soil type you are dealing with before undertaking any kind of soil-related problem.

The purpose of this blog is to point out the smoke and mirrors that exist in the grouting world. My goal is to share technical information to help clarify issues plaguing the grouting industry. This includes clearing up misconceptions about when polyurethane soil injection works—and when it absolutely does not.

I would like to start by pointing out that the greatest need is to “educate/inform” engineers, contractors, and clients of the fundamentals of grouting and how it relates to the most basic science of soil mechanics. 

Soils are made up of solids, water, and air. The ratio of these three elements determines density, bearing capacity, permeability, and weight/volume relationships. The mineralogy of the solids determines plasticity and other characteristics that influence whether a soil consolidates or swells.

Consolidated soils are usually the result of too much water and air within a massive volume, and not enough solids. This typically results in a low weight-to-volume ratio, so when a load is imposed on it or water is extracted, the solids come closer together and consolidation occurs. 

The Main Types of Grouting Used in Soil Stabilization

To maintain the same massive volume without consolidation problems, grouting can be approached in three primary ways:

Permeation Pretreatment of the Soil

Pretreat the soil with a permeation-type grout that fills all air/water voids between the solid particles and essentially glues the particles together. This is a very precarious undertaking because:

• If the grout particles are larger than the soil apertures, penetration will not occur.
• Instead, the grout may fracture the soil and migrate away from the intended treatment zone.
• Even when particles are fine enough, an injection rate that is too fast can create back-pressure and cause hydrofracturing, producing the same undesirable result.

This is one reason polyurethane soil injection is often marketed incorrectly—its low viscosity can cause uncontrolled migration rather than consolidation.

Soil-Mixing Operations (Mechanical or Jet Grouting)

This involves mixing grout with soil particles using mechanical or high-pressure jetting, thereby gluing everything together into a conglomerate mass. These applications are typically very invasive and expensive to perform.

Compaction Grouting

Compaction grouting is a widely used and highly effective soil stabilization method. It involves:

• Slow injection of a low-mobility, stiff cement-based grout (less than 2-inch slump)
• A specifically designed gradation to maintain internal shear strength
• Formation of round, dense grout bulbs that compact the surrounding soil

When done properly, this type of pressure grouting will create solid round bulbs in the soil and compact the problematic soil mass between injection points. It can also provide the bonus of creating columns for additional support. This non-destructive method is typically performed with a grout casing between 2 and 3 inches in diameter, under pressures typically between 100 and 700 psi.

Additional Grouting Techniques for Specialized Conditions

When the issue of basic soil mechanics is applied to this system, it should be obvious that the injection rate cannot exceed three cubic ft. per minute because the soil being treated must give up its water and/or air without taking the solids with it, which would result in an induced “quick” condition.

Intrusion Grouting

This method involves the injection of a thinner (fluid) type grout. The mix can be cementitious, chemical, or any fine material such as fly ash or screened soil. It is most effective in rocky soils or rock formations where the goal is to seek out and fill voids. There is a risk of the grout traveling into places you don’t want it to go, such as conduits, pipes, or adjacent properties. 

These grouts can be injected with smaller casings, lower pressures, and relatively simple pumping equipment. Many contractors call this “compaction grouting” simply because they pumped grout into the ground, but again, I refer you to “Basic Soil Mechanics,” and you will understand why this is not compaction grouting.  

Permeation Grouting

This uses super-thin grouts (ultrafine cement, chemicals, silica fume, etc.) designed to flow between existing soil particles and fill microscopic voids. It creates a matrix similar to concrete or mortar using the existing soil as aggregates. This can be done with casing as small as 1 inch in diameter and usually at pressures under 100 psi. Tube-a-manchettes (a double-pipe system) are often used.

Slab Jacking (Mud Jacking)

The process of lifting concrete slabs by injecting grout under the slab and using the resultant hydraulic pressure for lifting. Holes are typically drilled on a 3- to 4-ft grid, ranging from ¾ inch to 2 inches in diameter. The grout can be chemical or a fine cementitious mix.

Chemical Grouting (Polyurethane Soil Injection)

Usually done with polyurethanes. When injected, the material has very low viscosity, and as it reacts, it foams to a calculated density. Much of the time, this is used as a waterproofing technique; the other common use is slab jacking.

Undersealing

Grouting under a slab to fill a void without raising it. This is done with a low-viscosity grout that flows easily. Careful monitoring of the slab will indicate when to stop as soon as movement begins.

Choosing the Right Grouting Method Matters

Each of these techniques will deliver great results when used for the applications for which they are designed. Grouters and engineers with the proper experience know when and where to use these methods to achieve effective and cost-efficient results. Using the wrong technique in the wrong application, however, almost always leads to an unsatisfactory—and often disastrous—outcome.

When property owners or engineers are unfamiliar with these techniques, they typically leave the decision up to the contractor. The only way this approach works is to require the contractor to:

• Perform the project at his or her own expense (no mobilization fee allowed)
• Proceed only after a third-party engineer can sign off on both the technical and non-technical aspects of the work
• Receive payment only after that sign-off occurs

If the contractor truly understands grouting applications, this arrangement should not be a problem.

Just know that the amount of up-front investigative information will be reflected in the contractor’s bid. The more risk the contractor is forced to assume due to a lack of soil investigation, the higher the bid will be. Problems arise when contractors try to make their single preferred method the solution to every soil improvement challenge.

I will write more details on each technique, including the pros, cons, applications, and important considerations for properly managing your grouting contractor. There are other techniques, such as fracture grouting, that are not commonly used, so they fall outside the scope of this discussion.

I will also highlight the drawbacks and dangers of using any of these methods in the wrong application. Here is a hint:

• If your engineer tells you that the technique for a specific application should be determined solely by the contractor, your engineer lacks the necessary experience.

Pressure grouting is not a black art. It is a science that can be measured and controlled using consensus guidelines. In fact, the ASCE and the Geo-Institute have developed a grouting guide with contributions from some of the best grouting engineers and professionals in the country. They have performed hundreds of field tests and identified which techniques give the best results in each soil condition. You can purchase the guide from the ASCE website here.

Focus on “Deep” Chemical Grouting

I would like to focus in particular on Chemical Grouting, also commonly referred to as Foam Grouting, and outline the uses, advantages, and limitations of this specific form of grouting.

Foam grouting is usually done with a polyurethane material that can be either a 1-part or 2-part mix. The urethane can be hydrophobic or hydrophilic. Hydrophobic urethanes are averse to mixing with water and are usually more dimensionally stable in the absence of water. Hydrophilic urethanes seek out water and react with it, and in the absence of water, they can shrink.

Foam grouts are best used in:

• Waterproofing applications, such as curtain walls in soil
• Crack injections in concrete structures
• Mud jacking, where small holes and small equipment are helpful
• Areas with restricted access
• Projects where downtime must be minimal
• Locations where floor finishes cannot be damaged

However, some contractors and suppliers claim that foam grouting is suitable for deep soil improvement, and some state road agencies have even been convinced that it is appropriate in that application. I respectfully disagree, and I believe the following points make it clear why polyurethane soil injection should never be used for deep soil stabilization:

• It does not follow basic soil mechanics science. (I will explain this in detail below.)
• It is a very expensive grout material for an application it is not suited for.

Geo-Institute Verifications and Publication

The Geo-Institute, in conjunction with the ASCE, has conducted numerous tests of grout mixes by injecting them into the soil and then testing the soil and excavating the treated areas to observe the geometry of the grout. These observations were cross-referenced with SPT blow counts taken before and after grouting.

What they consistently found was this: when the gradation of the mix results in low internal shear strength, the grout does not form round, bulbous shapes but instead spreads out into thin fins. These fins are clear indicators that no compaction has occurred. Instead, the soil has been fractured, which results in a loss of soil bearing capacity. This typically happens when viscosity modifiers such as bentonite or lime are added, or too much water is introduced into the mix.

Both conditions reduce the shear characteristics needed for proper compaction.

As a result of extensive field testing, the Grouting Committee of the Geo-Institute developed a Compaction Grouting gradation envelope as displayed below.

Understanding the Compaction Grouting Gradation Envelope

You can see by this gradation envelope that there needs to be a variety of aggregate sizes, with 10%–20% gravel at the larger end and approximately 20% passing the #200 sieve at the finer end. The fines ensure that the mix will be pumpable. Without the fines, the material would simply squeeze out its water content and plug the pump hoses.

The gravel and larger concrete sand-sized aggregates are what provide the internal shear strength of the mix. In good compaction grouting, the operator always keeps the mix right on the edge of being non-pumpable, maintaining just enough internal shear for it to behave as a true compaction grout.

A simple field test is this: take a ball of grout from the hose and squeeze it in your hand. If it extrudes between your fingers, it does not have sufficient internal shear strength. It is not just the slump that matters; toothpaste is near zero slump, but it would not have the shear strength needed to properly grout. It is too slippery and lacks the required structure.

The conclusion of the Geo-Institute studies was clear: round, bulbous grout geometries correspond directly with improvements in the SPT blow counts. From the geometry, we can infer the level of improvement achieved in the soil.

This is the type of grout geometry that is typically associated with increased SPT blow counts:

How Do Urethanes Compare to the Established Gradation Envelope?

By definition, urethanes in their liquid state—before activation—lack the internal shear strength needed to form round, bulbous formations in in-situ soils. Instead of compacting the soil, they tend to fracture it, which leads to a loss of soil bearing capacity.

The question then becomes: Does this happen in practice, not just in theory?

It has been requested numerous times that suppliers or contractors provide evidence of deep urethane foam forming true compaction bulbs rather than thin fractures. 

It so happens that I do have pictures showing evidence of thin fin formations of urethane foams attached here for your viewing. These pictures conclusively demonstrate that the foam is fracturing the soil rather than compacting it.

Can Urethanes Show Round Bulbous Geometries?

These two pictures above vividly illustrate the geometry of urethane grouts injected into in-situ soils. Often, when suppliers and contractors share pictures, they share something like the picture below.

These pictures are, in fact, from injections into voids near the surface. It is natural to have these types of geometries in voids near the surface. Because the material is not restricted, it easily forms this way. And, by the way, this is a great application for this technology.

However, this is entirely different in deep soil injection, where the soil is semi-compacted, with natural joints and fractures that occur in almost all soils. These joints happen in many directions and varying lengths for a variety of reasons, which I won’t go into at this point.

What Happens to Low Shear Strength Grouts, Including Urethanes?

So what happens in particular with low-viscosity, low internal-friction grouts when they are injected into semi-compacted soils with natural joints and cracks? (Almost all in-situ soils have these conditions.) As I point out in a previous blog, soil consists of many layers with differing densities, mineral content, water affinities, and other characteristics, each with boundaries that facilitate natural joints and cracks.

When a low-viscosity, low internal-friction grout is injected, it behaves like water—it follows the path of least resistance. And what is that path of least resistance? Those existing cracks and joints. The grout finds them and splits them wider, which in turn extends those paths even farther for the material to travel down. In fact, this behavior is commonly referred to as “traveling.”

This results in weakening the soil, as opposed to strengthening it.

Final Thoughts, Real Life Example

I vividly recall a state bidding project that was soliciting requests for proposals. This was an overpass that had typical settlement in the bridge abutment pads. The project had been urethane-injected twice and was still settling. It also had MSE vertical walls with joints. There, in one of the joints—years later—urethane from the previous injections was still poking out. I asked if the injections were done on the edge; the answer was no… which demonstrated how far it had traveled. Traveling of the grout indicates soil fracture and no improvement.

Some manufacturers demonstrate their product in loosely compacted soils. This does not in any way reflect real-world deep-injection conditions.

For deep injections, the use of urethanes is demonstrably ineffective.

Despite the clear and compelling evidence, people and agencies (DOTs in particular) still cling to the smoke-and-mirrors hype peddled by very good sales teams, relying on anecdotal claims of success. These anecdotal examples are repeatedly contradicted by stronger, documented evidence of failure, both in private and public works projects.

Most manufacturers of urethane will not support claims of deep soil improvement with their products. A few—one in particular—still claim they can reliably improve deep soil conditions to support foundations, roads, and other infrastructure. It is time that, as an industry, we stand up and demand accountability for the claims being made and insist that the right products are used in the right applications.

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