“How old is the structure?” There’s more to this question than meets the eye. At first glance, older structures have had more time to show signs of stress and wear and tear, as well as interact with soil, wind, thermal movement, and other forces of deterioration. A thorough foundation inspection is crucial to identify these issues.

Why?

Over time, these factors accumulate and weaken the structure, leading to interactions between them.

Understanding the age of a structure is important for diagnosing foundation issues, planning effective foundation repair, and ensuring a comprehensive foundation inspection.

How Moisture Accumulation Affects Your Foundation

When the soil has been dry for many years, it typically has poor drainage. Adding watering to this mix and considering the four mechanisms of moisture accumulation, water starts to gather under the foundation system. 

Quick trivia: Robert Brown PE illustrates this in his book Foundation Engineering Handbook. Below, I discuss the four main causes of moisture accumulation in more detail.

Edge Heave in Newer Homes: What You Need to Know

It’s common for moisture accumulation to start on the edge unless there’s a plumbing leak. 

As a result, moisture added with expansive soil means heave… on the edge! Most newer foundations we’ve measured show edge heave. Here are a couple of examples of newer homes, both with conventional three-pour foundation systems experiencing edge heave.

Since most homes built in the last 18 years have post-tension slabs, edge heave has become expected. But is this due to the post-tension design or simply because they are new?

Why Post-Tensioned Foundations Experience Edge Heave

I have noticed that very few new homes have center heave, regardless of the foundation type. Most of the homes we’ve seen with center heave are older, approaching 20 years or more. 

Although this isn’t conclusive, it suggests that time, rather than the type of foundation system, might be a factor, but it doesn’t rule out design as a cause. I have an unproven theory about a possible explanation of PT foundation-induced edge heave. See the illustrations below.

This is a standard illustration of a uniform thickness PT foundation system. In most modern home foundations, the trusses help distribute the weight to the edges, so the tails (the very end of the cables) are turned down to resist the roof loads bearing down.

When the cables are tightened to 33,000 lbs a few days after the concrete pour, the tail attempts to straighten, pushing upwards and resisting the downward roof load on the wall. So far, all is well.

When tensioning the cables, two things occur. The cables typically elongate by around 6-8 inches in residential applications (and are then cut off), and the concrete compresses, meaning it is squeezed shorter. What happens when the concrete on both ends is squeezed toward the middle?

So here’s an exaggerated illustration. At this point, several things happen. As the foundation slides toward the center, it also slides up the angled turn down. 

The ABC (sand and gravel) layer below is compacted, as is the soil below it, allowing it to slide up. This leaves an opening for water to get in more easily.

Now if the soil underneath is expansive, it increases the likelihood of edge heave. Once the tails have risen enough, the ends may no longer be below the neutral zone of the slab, amplifying the upward force of the cable tensioning.
The opposing view is that time and moisture accumulation are more significant factors than design in new PT edge heave. Below are floor-level surveys of older PT foundations showing dome heaves, or as PTI calls it, center lift.

In both examples, the homes were over 20 years old. Although rare, these cases are not unheard of.

Tracking Foundation Changes Over Time

I have performed multiple floor-level surveys on the same house for more than ten years, using a subtractive topo to track the movement over time. In this specific case, the house had a conventional foundation system.

The topo on the left was the original one. 

The one on the right was a subtractive topo over time, which shows how it moved since the first topo. 

To clarify, the right doesn’t have the right elevations taken. Instead, it shows the differences between the values from the first reading and those from a reading taken 10 years later, with these subtracted values then being plotted using topo software.

It seems that initially, there was uneven movement along the edge for the first 10-15 years. But, after that period, there was a noticeable change in the pattern, showing a shift towards a center heave.

This observation underscores the importance of regular foundation inspection to monitor and address potential issues early.

There are exceptions that reflect regional conditions. For example, in Houston, I found older homes having a center drop due to large tree desiccating roots that have chased pipe leaks.  

Key Takeaway

In summary, newer homes usually experience edge heave and, after 20 years or so, tend to accumulate moisture in the center. The jury is still out on what will happen to the majority of the uniform thickness PT slabs. 

Understanding the age of your structure is crucial for diagnosing foundation issues. Regular foundation inspections and proactive maintenance can help identify potential problems early, saving you time and money in the long run.

Remember, ensuring a thorough foundation inspection is a critical step in maintaining the integrity of your property.

You can read more about all aspects of foundation inspections and repair in my blog.