Because foundation repair investigations often do not offer objective analysis on the severity of foundation movement there needs to be a precise methodology applied. This needs to be an industry standard in order to obtain trustworthy evaluations and maintain acceptable outcomes. Below I’ll discuss some of these methods and cover more possibilities for objective results.  

Foundation Repair Investigations Aren’t Always Based in Fact

I have witnessed many foundation repair settlement investigators come to unobjective conclusions concerning the severity of the condition of a foundation. They have done things such as rate it on a scale of 1 to 10, and give ratings of 8.5 for example.  I wonder how they arrive at that particular number? I imagine their answers are just as unobjective. Perhaps they will explain their years of experience, and could be basing their conclusions on the money they’d like to make. It may be something as arbitrary as their kid needing braces. Clearly not objective!

The Objective Methodologies Attempted Just Aren’t Enough

There have been attempts at creating methodologies that ensure objective foundation repair investigations based on severity. Below you’ll find an overview of one of these attempts to lay out a clear, objective, data-based and broadly accepted analysis of the severity of foundation movement.

Foundation Performance Association (FPA)/ASCE Texas Chapter/Post Tensioning Institute (PTI) methods of severity.

The Foundation Performance Association, or FPA, is a Houston, Texas based organization that basically followed the Texas ASCE (American Society of Civil Engineers) methodology, and modified it several times over the years.  The FPA proclaimed that their method had received an industry standard peer review.  Although there was an apparent lack of evidence to support their proclamation.   

This work was an important first step, though it does have several fatal flaws. The method is without a basis in engineering or fundamental support. This is because it deviates from the classic definition of deflection, which is deviation from a straight line. 

 When challenged on this, they  responded that they  have case studies to support their conclusions. However, those case studies have never been disclosed to the public, or to those that have inquired. 

There are other flaws that should not be overlooked. The FPA’s analysis assumes that the slab on grade is a simply supported beam between any two points on the surface. This is not the case, because soil pressures from below are continuous and variable across the slab. Finally, they assert that a single slab can tilt in several directions,and in opposing directions, at the same time, which is not logically possible.

Foundation repair investigations need to factor in real world scenarios and possibilities in order to come to correct conclusions on the severity of foundation movement. 

The Methodology is Flawed Because it is Not Based on Empirical Evidence

Over the years, the modifications to the FPA methodology have included lengthening the space between points. This, again, offers no empirical support. The conclusion still makes it much more difficult, if not impossible, for a slab to “fail” or exceed their promulgated criteria. They are still leaving out important factors that have an effect on a slab. 

 It is important to note that there is a clear financial benefit to the proponents of this methodology. How do we know this? Because most of the controlling members of this group receive a proportion of their income from developers and home builders, who wield a lot of money and power. It can make you wonder if it is in their best interests to ensure an objective methodology that results in fair and accurate foundation repair investigations. 

The Post-Tensioning Institute has also adopted the FPA standard, albeit with minor modifications. But these modifications are small. They simply renamed and improperly conflated deflection to curvature in an attempt to differentiate the appearance of the work product.

It is also important to note that many of the same people who control the FPA are also the decision makers and influencers on the PTI, and the ASCE Texas Chapter committees.

Foundation Repair Inspections Need Truly Objective Methodologies 

I’ve noted some of the factors that detract from creating a truly objective methodology for determining severity of movement in foundation repair inspections. Beyond the motives behind these existing methodologies there is also the lack of logic.  

The issue that remains in these updated standards is the location of the initial neutral reference line. It is assumed to be flat, or because of assumed initial tolerances, adjusted to the overall tilt.

Without a baseline survey at the time of construction, we cannot definitively determine the neutral reference line. Can we safely assume the neutral reference line is L1 above? This assumption is on shaky ground at best. Still, intuitively, the above diagram seems like we can use a ratio of Delta 1 to L1 (from P2 to P4).

The problems come with edge conditions. For example, where there is movement near the edge which does not fit with the 3 point method accurately.

 The Science Behind it Explained

The issue is illustrated above. Regardless of how far between points— (P2 or P2A) the ratio of Delt1 to L1 or Delta 1A to L1A— both under portray the more intuitive ratio that we understand from the classic definition of deflection of Delta 2 to L2. It certainly makes more sense that the actual deflection is from P2 to P1 and is Delta2 to L2. The FPA/PTI method clearly understates the actual deflection of the edge of the foundation. This obviously favors builders and warranty companies.

Radius of Curvature Method

Another more rational method  has been presented by Chris Kahanek, S.E., PE, AIA, Vice President and a principal of  Forensix Design.  This is being called the Radius of Curvature method. Although it is fairly early in its presentation, it has the advantage of sidestepping the contentious issues of conflating tilt and deflection. The basis comes from the equation:

 And then arrives at:

Now,  I have to admit that it has been 45 years or so since I’ve taken a calculus class. As a result I am a bit rusty, but I understand the overarching concept of the radius of curvature of a slab. I also know this means that a tighter radius of curvature will result in more stress and resultant damage. But, how do we establish an accepted allowable?

Chris goes on to draw an equivalent from the L/360 standard found in ACI, IBC and other resources as the following:

Thus drawing the equivalent of a 750’ radius. However, he points out that there are other standards besides the L/360, such as L/600 for masonry. 

So clearly there is not one simple answer. In addition, other factors need to be considered such as the appropriate distances between measuring points. Considering we are not talking about a simply supported slab, this analysis may not be as easy as it seems.

 Chris presents a case study for consideration, showing how to start to get our arms around this. Since this seems to be a relatively new area of exploration, perhaps case study information may be the most appropriate, with as much data as we can muster.

Intuitive Equivalent

Intuitively, what it comes down to is the rate of change in the distance between topo lines. (Here the ASCE and PTI are saying that measuring across the lines is only measuring slope, NOT bending or curvature over that distance in their rebuttal) . This makes no sense, as the rate of change in deviation from a straight line is how curvature is established.  

Industry Standards Need to Be Set to Ensure Objective Foundation Repair Investigation Results

I have always used the rate of change in the spacing of the topo lines as a guide. Early on we realized that we needed a standard for the spacing of those lines. Sometimes we would have some plans with topo spacing at .1 inch and others with as much as .5 inch. If a person really wanted to make it look like a lot of movement, spacing at .1 inch could be very dramatic. On the other hand, spacing at .5 inch could give the impression that not much is happening in the same house. These variables don’t help when it comes to getting the most accurate results in foundation repair inspections. 

In addition, you would be surprised how the patterns differ with differing topo line spacing software. Yet no one has yet proposed such a standard.

In the end, we finally settled on .2 inches. Finding a topo spacing standard we can all agree on would go a long way towards reaching an objective methodology. This would mean not only visually agreeing on what constitutes severity, but it would also help to establish appropriate spacings for the radius of curvature points. 

Regardless of the contour spacing, competent foundation repair inspections and evaluations require good quality elevation data gathering practices. These would preferably:

•  Be on a grid equally spaced across each major dimension of the slab being evaluated.
•  Not to exceed 10’ on center, or preferably 5’ if possible.

Obviously, there is work to be done. I look forward to continuing the process of exchanging views and arriving at a true consensus that is objective, fair and unbiased.