Before creating foundation repair plans, an expert must first understand the mechanisms of settlement and expansive clay heave. This requires a solid understanding of soils. To learn more about the depositional nature of soil and its variability, refer to my earlier post on the subject. Here’s the link to that post. 

It is a mistake to think that soil is simple. Let’s look at wet climates with and without clays and then dry climates.

Wet Climates with More Expansive Clays

Texas and Louisiana are good examples of wet climates with clays. These soils have been wet for millennia. When a structure is built on these wetter soils, the edges begin to dry out during dry spells over time. As the clay dries, it shrinks, causing the house to experience settlement around the perimeter. See the drawing below. 

This is one of the more dominant paradigms that is the most widely understood… both within the industry and by the general public.

Wet Climates with Less Clays

Geographies with predominantly sandy, silty, or gravelly soils in wet climates face a consolidation challenge. Water can drain through these soils, compacting them under the weight of the home and the overburden of the soil above weaker layers. As the water comes in contact with these soils, air pockets are forced out and lubrication of the particles allows them to compact. See the drawing below.

This concept is both intuitively and widely understood within the industry and, through marketing, has become more familiar to the general public. Florida serves as an excellent example of a wetter climate with non-clay soils.

Dry Climates with More Clay and Less Clay

What is less understood, both within the industry and by the public, is how clay soil interacts with drier climates. This is primarily because residential pile manufacturers are not based in these areas, and consequently, contractor training often lacks insight into these conditions. Additionally, until recently, these regions were less populated, and soil problems were generally less pervasive compared to wetter regions. I discuss differences in annual moisture precipitation on foundation performance in this post. 

Understanding the mechanisms of moisture accumulation under a foundation is a critical aspect of foundation maintenance and repair. However, it’s equally important to understand that not all foundation repair personnel have a comprehensive understanding of these mechanisms, which can lead to critical mistakes in diagnosing foundation performance.

4 Mechanisms of Moisture Accumulation

Over time, moisture can accumulate beneath a foundation, even in the absence of a plumbing leak, although leaks are a common source. Below, I outline four natural mechanisms that cause this accumulation over time.

Thermal Gradient Diffusion 

Thermal Movement of Subsurface Water Vapor: This mechanism is based on the principle of diffusion, where warmer, more energetic vapors spread to cooler, less energetic areas. This process is a natural occurrence driven by the soil’s temperature gradient. 

The thermal movement of subsurface water vapor can lead to moisture accumulation under the foundation slab, especially in areas with significant temperature fluctuations. This process is confirmed by Claudia Zapata, a Geotechnical professor at ASU, in a peer-reviewed paper on the Atlanta International Airport.

Clay Suction

The second mechanism involves the interaction between clay soil and water. Clay soil has a negative ionic charge, while water has a positive one. As a result, they get attracted to each other—a phenomenon geotechnical engineers refer to as negative pore pressure or suction. 

Although water moves very slowly through clays, at a rate of about 3 inches per year, the horizontal depositional layers in the soil allow for somewhat faster movement. During heavy rain, water soaks down to the soil below, and because the soil in the center of the slab is much drier, it is drawn to it. This suction effect can lead to significant moisture accumulation under the foundation over time.

The Stack Effect

The stack effect involves the movement of water vapor and other gases from around the perimeter of a structure to replace gases transmitted up through the foundation into the structure and by heat convection drawn up into the attic. 

This process is well-documented for certain gases, such as radon. In warmer climates, air-conditioning dries the surface of the slab, promoting vapor transmission. However, this cooling also results in condensation that blocks moisture migrating through the slab since the cooler moisture has less energy of diffusion. This effectively traps the moisture below the slab.  

The Evapotranspiration Cycle

This complex geotechnical concept allows for the upward movement of water vapor from the water table and other underground water sources. This process can lead to the accumulation of moisture under a foundation when the slab intercepts the rising water vapor. Also, this mechanism is particularly significant in areas with high water tables.

Results of Moisture Accumulation in a Drier Climate

In more arid climates where the soil has been dry for a long, long time, any addition of water to very dry expansive clays can result in extreme expansive soil swell. This swell usually manifests itself in the upward heave of any structures above it. 

This can be a very powerful force that commonly breaks concrete and bends steel and is almost impossible to restrain as a forensic remediation. I have personally witnessed a differential heave of over 9 inches from one side of the house to the other. This action is not usually immediate. 

I discuss more of this in a separate blog, especially the time usually required for the accumulation to take place. Below is a typical result. 

Notice the high area in the middle, commonly referred to as a dome heave. This happens when more moisture activates the clays in the middle areas that cannot dry back out as easily as the edges.

Because this looks very familiar to the settlement paradigm of the first drawing of this post, it is often mistaken for settlement. 

When you are standing in a room with the house perimeter being lower than the middle areas and there are lots of drywall cracks, it is easy to default to the more easily understood paradigm of settlement… And many times, this is a huge mistake.

Results of Moisture into Clay Under Foundations

Unfortunately, not all foundation repair personnel comprehensively understand these mechanisms. This lack of understanding can lead to critical mistakes in diagnosing foundation performance. 

For instance, a salesperson might misdiagnose the cause of a foundation problem, leading to ineffective or unnecessary repairs. This is why it’s crucial for homeowners to seek advice from licensed professional Forensic Geotechnical Engineers who have a deep understanding of these mechanisms and can provide accurate diagnoses and effective solutions.

Regional differences do make a difference. However, it is not a simple cut-and-dry. There is plenty of settlement in more arid environments and plenty of heaves in wetter climates. Although it may be more predominant, it always pays to look at each condition with fresh eyes and an open mind to every mechanism of soil movement.

Confirmation by Robert Day PE

Recently, Dave Deatherage of Copper State Engineering brought to my attention a portion of a book written by Robert W. Day called Foundation Engineering Handbook, sanctioned by ASCE. I studied a similar book by this author in order to pass the exam required for the CFRS designation from the National Foundation Repair Association. 

The page Dave sent me is displayed below. He recognized that it was a confirmation of what I had been saying for many years. 

Essentially, Mr. Day recognizes that moisture accumulation starts on the perimeter and works its way to the center of the foundation slab on grade where expansive soils are present, especially in more arid climates where the Thornthwaite index is lower. I have made this claim in several blogs.

Day further explains the underlying principles: warmer, more energetic vapors spread through diffusion to cooler, less energetic areas, supporting both the thermal movement and the suction mechanism. 

The foundation repair industry across North America is a $50 billion year industry, and it is estimated that 50% of it is misdiagnosed, wasting the effectiveness of it for the homeowners. 

This is why it is essential for homeowners to consult seasoned forensic foundation engineers who possess a deep understanding of these mechanisms and can deliver accurate diagnoses and effective solutions.