One of the most important factors of foundation analysis is the floor elevation in a structure. But this is often used alone as a quick and dirty analysis. This is a fatal mistake. Floor elevation is only one of the many useful factors used to obtain accurate conclusions. I will cover what is involved when considering floor elevation in foundation analysis below. 

Topological Mapping of Floor Elevation in Foundation Analysis

In a previous blog on foundation types, I provided section views of a home indicating where the problem is and where it is not.  You can reference those images here.

These types of floor level surveys are often displayed with topo software that visually helps interpret the numerical data. This is one way we arrive at necessary conclusions when factoring in floor elevation in foundation analysis. 

So, how do we arrive at these conclusions and know where the problem is and where it is not?  We arrive at it using topological mapping. If you draw an accurate floor plan and take accurate floor elevation readings (taking care to adjust for changes in floor coverings) then apply a good topo-mapping software, you’ll arrive at something like the drawing below. 

If you took a giant saw and cut down the turquoise line and looked at it sideways, you would see the view on the right.

The Level Doesn’t Always Indicate the Cause

The first thing to understand about considering floor elevations in foundation analysis is that highs and lows don’t tell us automatically what has moved up or what has moved down. These readings are only relative, not absolute elevations. In other words, the numbers have meaning only in how they relate to each other. 

This being the case, what we need to do is look at the topo line patterns and understand what they typically mean. 

The black lines in the image above show graduations of 2/10”. This is a standard I recommend. 

The colors indicate graduations to higher vs lower. The closer the lines are to each other the steeper the grade.

Is it Settlement or Heave?

The above example is of a typical settlement pattern. It represents a relatively flat slab with a low area on the perimeter which bears the roof loads and is most subject to moisture change. This is one area that has damage associated with settlement.

If most of the slab is relatively flat, and has only one lower area, it is reasonable to assume that the lower area has settled, rather than there being heave in the entire rest of the slab and foundation in uniform way. 

So, you can see why the topo map images alone don’t tell us the cause of these differences in elevation. This is why, when considering floor elevation in foundation analysis, we must also utilize supplemental data. For example, damage to the area and the soil type surrounding the foundation will need to be considered to be conclusive. 

Now, contrast this to a pattern that shows lower levels around the entire perimeter. Is it reasonable to assume that the entire perimeter settled uniformly in perfect harmony? Again, this is really unlikely. It is more likely that the middle area has risen up, or heaved.

The pattern above is generally assumed to depict a pattern of heave, although, as often mentioned, there are exceptions to every rule.

This topo survey shows one side as low and one side as high. So has the high side moved up? Or has the low side moved down? At this point, without more data, there is no compelling interpretation.

Sometimes the topo survey looks like a bunch of amoebas floating around.

We try not to get excited about small changes such as two tenths of an inch (usually one topo line) as usually seen in the amoeba. Rather, we look for a significant consistent change as evidenced by a series of topo lines in close proximity. These look like zebra stripes. 

These, as shown above, indicate significant movement that is worth paying attention to.

Often with newer homes, particularly post-tensioned foundations, we see a particular pattern that shows higher areas all around the perimeter. 

Studying Floor Elevation in Foundation Analysis Requires More than Topo Patterns Alone

Topo patterns alone are useful, but they shouldn’t be your only source of data when studying floor elevation in foundation analysis. 

These are just some common patterns observed over years of collecting results. Of course, I want to repeat that topo patterns by themselves can never be conclusive. They can be a strong indicator of the issue but without all the other points of data it is dangerous to reach conclusions solely based on this.  Later on, I will demonstrate how a topo pattern can fit in with other points of data.

Many foundations present both patterns of expansive clay heave and settlement at the same time in different areas. Take a look at the sample below.

Fitting Topo Patterns in with Other Points of Data

This particular view is in 3D to help visualize the contours better. In looking at this particular profile, we first notice a high area in the middle, which was mentioned earlier as representative of a center heave. 

Simultaneously, we also see the northeast corner is lower than the others. This is representative only of the settlement in that particular corner. Our engineering team encountered this regularly. The big question becomes: At what point does the heave end and the settlement begin? In other words, where do we stop and start the piers? 

As you can see, when using topo patterns of floor elevations in foundation analysis, there are many factors beyond what we see to consider.  I’ll discuss this in more detail in an upcoming blog on the topic of piering, or underpinning, rules.