What You Need To Know About Elevation Measurements

What I am about to say is often misunderstood. So let me make some things clear. I am not against anyone making elevation measurements. I rarely look at a house and fail to make elevation measurements. I also believe they are often misinterpreted. While they have a place in foundation performance evaluations, they can be very misleading.

Elevation measurements are not required by TREC

There is nothing in the TREC SOP that requires inspectors to make elevation measurements. For that matter, there is no requirement that an engineer make elevation measurements either. TREC inspectors can make such measurements if they wish. Whether making such measurements is a good idea is a question for another day.

Elevation measurements are not required by any published engineering guideline

This is true for guidelines published by the Texas Section of the American Society of Civil Engineers, the guideline published by the Houston Foundation Performance Association, a Policy Advisory previously published by the Texas Board Of Professional Engineers, the US Army Corps of Engineers, and The National Association of Building Inspection Engineers.

In the context of residential slab-on-ground foundations, elevation measurements are of limited value. No one disputes that they are of some value, it is how much value and what they are valuable for that is in contention.

To understand why they are of limited value, consider the following:

The American Concrete Institute (ACI) has made studies to find out how level slab-on-ground foundations are cast. The results are published as ACI construction tolerances. Here are the key results:

The overall levelness tolerance is 1.5-inches, meaning that two points anywhere on the slab surface can have an elevation difference of as much as 1.5-inches.

The local construction tolerance is 1.25 inches, meaning that 1.25-inches, meaning that no two points 10 feet apart can have an elevation difference of more than 1.25 inches.

These construction tolerances are, according to the ACI, about the best you can hope for. They are also in the same order of magnitude as the allowable design deflections.

Elevation measurements are a snapshot in time. If I were to show you a photo of a parked car and asked you how far the car had been driven over the previous 24-hours, you would think I was making a joke. There is no way to answer the question since we do not know where the car was-24 hours ago. In the case of a slab-on-ground foundation, we also do not know where it started out.

Elevation measurements alone, never provide a good basis for reporting a foundation as in need of repair. They can legitimately be used as a factor in assessing the performance of a slab-on-ground foundation.

Elevation measurements can be helpful in discovering the apparent dominate distortion mode. Elevation measurements can be used to estimate the bending and tilt of a slab foundation

If a reliable estimate can be made of the bending and tilt exhibited by a slab-on-ground foundation, that would be very helpful in making an engineering assessment of how well or poorly a foundation is performing. Unfortunately, slab foundations are not constructed level or flat. In some cases, they are way off: six inches or more.

On the other hand, most slab foundations in the first few years measure no more out of level that what the ACI construction tolerance for overall levelness indicates.

To the authors, this indicates that using elevation measurements to estimate overall bending and tilt across a profile from front to rear and from side to side gives reliable estimates most of the time. Such estimates are helpful to an engineer in assessing the distortion mode for the foundation and the severity of the distortion.

We also believe that the estimates should be looked at with some degree of skepticism due to the fact that there are almost never any original measurements.

Elevation measurements can be used to monitor slab movement

In almost all cases, elevations are not taken in a way that facilitates monitoring foundation movement. The most obvious problem is that there is no stable benchmark. Without a stable benchmark, there is no way to know what areas are heaving and what areas are settling.

Elevation measurements can easily be misused

The most common misuse of elevation measurements is to take them at face value. A related misuse is to make the assumption that every elevation that is lower than other elevations is due to settlement. Slab foundation settlement is real, but by far the most common cause of differences in elevation is due to heave, the opposite of settlement.


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