In my last few blogs I’ve covered soil conditions and the importance of analyzing them during the site inspection process.

But ,the type of foundation in a structure makes a critical difference in how it interacts with the soil. Foundation types play a significant role in helping determine whether heave or settlement occurs. Here are four common types of foundations:

• Floating Floor Slab (Conventional)
• Post-Tensioned Foundation (uniform thickness type)
• Pier and Beam (crawlspace)
• Basements
• Variations of the above types

I would also like to thank the following individuals for helping me with this article:

• Jonathan Pease, P.E., in Reno, NV, at Reno Tahoe Geo Associates
• Kyle Young, P.E., PMP, in South Dakota at Bice, Young & Associates, LLC

Foundation Types and Their Effect on Settlement

Above, we see that the heavier forces in the downward direction are on the footings. Additionally, the area of the footings is relatively small in comparison to the floor slab and, therefore, can push into the soil easier than the floor slab. The floor slab is a large area with very little weight on it… it is comparatively more difficult for it to settle.

Now, let’s look at the opposite scenario (?!) The forces that push up have much more influence on a floor slab than they do on a footing. The area of the floor is large, with very little weight on it, leaving it susceptible to soil heaving. Accordingly, the footings have a smaller area, sit lower with more load on them, and are much less susceptible to heave.

To summarize, slabs heave more easily than footings, and footings settle more easily than slabs. If the footing settles and creates a void, the slab may follow, but usually only if the footing leads the way.

Of course, there are always going to be exceptions. I show two exceptions below. On occasion, we have found a slab settling when a deep soil problem caused the soil to settle in the middle area of a home, where soil overburden on a weakened layer induced settlement and the slab followed it, but again, that is unusual. (see below sketch).

Another exception may occur when there are high stem walls (often more than 24”) and poor backfilling inside the stem wall. This may result in floor slabs that settle without footing settlement, particularly where local practices use less-than-ideal fill, such as cinder rock. See below.

Turndown Mono Variation

One variation of the slab on grade is the monolithic turndown found commonly in additions. This is usually done to limit the number of small, short-load orders of concrete. The forces acting on it are similar to those of classical conventional foundations, with the exception that the floor slab does not generally float separately from the footing. 

Post Tensioned Foundations

Post-tensioned foundations have gained in popularity over the last 30 years. In parts of the Southwest, a particular type has gained enormous popularity: The Uniform Thickness Post Tensioned (PT) Foundation. 

This type of foundation has high-strength cables placed in both directions (north/south and east/west) and spaced about every 30” each way (give or take a few inches) throughout the slab. These cables are tightened up to about 33,000 pounds a few days after the concrete is poured. This has the advantage of creating a very stiff slab (see picture below).

The Uniform Thickness Post Tensioned (PT) Foundation has a uniform thickness system. It is most common in the Southwest. Originally, PT systems had deep ribs with cables at both the top and bottom. Since the cables are limited in stretching, this helped resist the foundation system from deflection or bending. 

The uniform thickness system is less robust than the rib type, in that it has one cable in the middle (which, as it turns out, is the neutral zone). The concrete above and below the cable is not held in tension by any cables and can crack in tension (and thus bend the slab) more easily.

The other disadvantage of this particular system is that it has very little edge protection. Since it projects down very little, water can penetrate around the edge and go underneath very easily. As a result, soils are more easily affected by poor drainage.

You will notice that there are no isolated footings. The downward forces of the roof loads are spread across a larger area, making it harder for settlement to happen. Because the slab is tied into the foundation monolithically, the entire system is much more at risk for heave.

On a conventional foundation, the footings are not likely to heave. However, the (monolithic) footings are more likely to heave with the Uniform Thickness Post Tensioned (PT) Foundation.

The overwhelming type of foundation failure I see with a uniform thickness PT foundation is edge heave. It is not clear at this time whether that is a function of the time required for moisture to migrate to the center, or if it is more of a function of design.

To summarize, the Uniform Thickness Post Tensioned (PT) Foundation systems are more susceptible to heave, although we are seeing mostly edge heave at this time.

Mat Slabs

Another similar type of foundation is the mat slab. It is similar to a Uniform Thickness PT slab but substitutes two or more mats of rebar for the post-tensioned cables. A mat of rebar consists of a layer of rebar built in both directions creating a mat. 

Typically, these are spaced 12”-30” in the center each way, similar to post-tensioned slabs. These foundations perform similarly to a uniform thickness PT slab in terms of soil interaction.

Waffle Slabs

Waffle slabs are built with concrete ribs in both directions. These ribs function as grade beams. They offer the advantage of having reinforcement at the top and bottom of each beam to resist the bending of the beams without having to have all the weight and expense of solid concrete between the beams. However, they are very labor-intensive to construct. See the thickened illustration below for better understanding.

Voided Slab Variation

Voided slabs are a variation of the waffle slab. They are constructed with cardboard or plastic rectangular units laid down in a grid pattern before the concrete is placed. They usually have smaller spacing between grade beams. Voided slabs are typically designed to reduce the intense labor of installing traditional waffle slabs while retaining some benefits such as stiffness, lighter weight, and concrete savings.

Crawl Space Elevated Foundations

As you might deduce from prior discussions, because crawl space foundations have only concentrated loads with no slabs, the opportunities for heave are greatly diminished, making settlement a more likely outcome and simplifying diagnoses. There are portions of these homes that have slabs-on-grade that connect up to them, such as garages, patios, and sidewalks. 

These slabs are more vulnerable to heave and could move differently than the portions of the home without slabs on grade. Sometimes, the footings of the crawl space adjacent to the slabs can heave together with the slab, which can sometimes be confusing. See the drawing below.

I have rarely seen footings on the perimeter heave with these foundation types. However, as noted above, footings have concentrated roof loads and are not as wide as slabs on grade, making them less likely to heave.

If the footings are heaving, then it is due to either very expansive soil and/or the footing sitting lower down, directly on top of a layer of very expansive soil.

This type of foundation is common in older homes where there are no trusses to spread the roof loads only to the perimeter footings. In those situations, interior walls transfer the roof load down to jacks under the crawl space in the middle areas. Because of this, interior floors can experience settlement more easily than homes with trusses.

Also, with the roof loads spread out to many more points in non-truss types, the loads are not as concentrated and may be more likely to experience heave than if there were trusses concentrating the loads only on the perimeter. 

With trusses, the interior jacks under the floor have only floor loads on them, making them more likely to heave than settle. Follow the weight! Where there are concentrated loads, settlement is more likely than heave—the heavier the load, the greater the likelihood of settlement.

Conversely, with lighter, more widely spread-out loads, heave is more likely. When slabs are connected to a structure with only footings touching the soil, there may be opportunities for separate or even opposite movement.

Pier & Beam

Pier-and-beam foundation types are commonly built on very problematic soils. They typically have piles of some sort to bypass problematic soils and reach a safer depth. The piles can be made of reinforced concrete or, increasingly today, steel helical or micro piles. The piles sit under the foundation to support it.

The foundation itself can be similar to a crawl space foundation on top of the piles… or a waffle slab (discussed above). On a waffle slab the piles would be placed at the intersections of the grade beams.

Basements

In colder climates, particularly in northern latitudes, basements are more common. Frost depths require footings to be dug deeper to resist the heaving that occurs from freezing moisture in the soil. Since you are digging deep, you might as well dig just a little more and get a basement for only an incremental amount of work. Basements also help control temperatures against freezing weather.

In some cases, insulation down the stem wall and out into the dirt can help protect against frost heave on shallow cold weather footings. 

Basements commonly suffer from leaking and lateral movement of the basement wall,  particularly older homes that may have been built with rubble, clay tile, or masonry walls, as well as without much reinforcing to resist horizontal soil pressure, as opposed to reinforced concrete.

Mono Footing Variation

A slight variation of the footings on a basement slab is where the slab and footings are monolithic as shown below.

Understanding Foundation Behavior: Key to Diagnosing Movement

Knowing the type of foundation in a home is essential for diagnosing movement—whether up or down or lateral. By assessing how much weight the foundation carries, the extent of its soil contact, and its ability to distribute loads, we can better predict and address potential issues like settlement or heave. 

Different foundation types interact with soil in unique ways, and factors such as drainage, construction methods, and environmental conditions all play a role in their long-term stability. Identifying these elements early can help homeowners and professionals implement the right solutions to maintain a strong and durable foundation.