Is overlaying concrete a lasting solution to address foundation heave? No.
Most expansive clays are able to break a 4” unreinforced concrete slab and push it up.
Recently, I watched a YouTube video of a self-proclaimed “Foundation Repair Expert” who, visually shows cracked up shrunken clay soil and large gaps of the soil at the adjacent to the stem wall. With a false sense of bravado, he claims that this is not expansive soil, and that the way to fix these problems is with a concrete overlay in the low areas on the floor slab.
To fix expansive soil heave, you first need to understand what it really is and how it affects homes and structures. It’s not enough to parrot that expansive soil is real….. you must understand how it works to apply a lasting solution.
This same “Foundation Repair Expert” has almost no understanding of soil mechanics and unsaturated soil properties. He has misidentified expansive soil with high moisture content as “wet”. Expansive clays with high plasticity have a tremendous capacity to attract and absorb moisture and still appear to be very dry to the naked eye. Geotechnical engineers measure an expansive soil’s plasticity (its ability to swell when moisture is added to it) as the difference between its liquid limit and its plastic limit. The liquid limit is how much moisture is added until it effectively runs downhill with a little vibration. The plastic limit is how much moisture it takes to make the soil plastic…. sort of like pottery clay ready to mold into a shape.
Floor overlays are a temporary fix for expansive soil heave and provide no long term assurance of any future performance.
In very expansive clay soils, the liquid limit is very high. It can absorb a lot of moisture and still not be liquid. And in these stages, it can swell a lot. Enough to push up slabs and even sometimes footings. I saw one report in the Litchfield Park area that had 28,000 pounds per square foot of force. That is a lot of force! Enough to break almost any reinforced concrete and bend steel. In fact, typical concrete only has about 900 lbs. of tensile capacity. Most expansive clays are able to break a 4” unreinforced concrete slab and push it up.
This same “Foundation Repair expert” claims that high areas in the middle of a house are not from heaving soils, but rather the 4” unreinforced concrete cantilevering up from the footings pushing the slab down on the edge. It is ludicrous to think that 4” unreinforced concrete can be pushed down on one end span 20 feet without breaking and lift up on the other end. Most structural engineers would estimate that a 4” unreinforced slab (like common home floors) would span about 4 feet or so before cracking and deflecting. Not to mention that the slab is not even physically attached to the footing in any way!
To fix expansive soil heave, you first need to understand what it really is and how it affects homes and structures.
This Expert says a “surefire way” of determining between expansive soil heave and “heave from cantilevering” is to see if there is void under the high area. Wow! Every forensic geotech in the world must have missed this one! As mentioned, expanding clay soil is still appears dry to the naked eye and behaves as a solid, not a liquid. It will not expand upward to fill every void, rather it will push upward as a solid mass leaving voids under the slab in high areas.
Moisture accumulation under a foundation resulting in slab heave with expansive soils is a well-documented and common phenomenon. It is something any Google Search can verify and support with a plethora of technical, peer reviewed papers on the subject and field verified in countless observations. Something a real “Foundation Repair Expert” would at least have a basic understanding of.
In order to help evaluate the floor overlay method of repairing expansive soil heaved slabs, take a look at the diagram below:
Look at the blue wedges on top of the floor that essentially make it flat. This is good for the present. However, what happens when the soil continues to heave the slab up in the middle? Remember, the best predictor of future behavior is a continuation of the past behavior. The problems will continue. Also, to put a few inches of grout on the floor, all the baseboards need to be removed and new ones installed at the higher level, not to mention all the floor coverings. What do you do when you reach doorways? Make a little ramp? Raise the threshold? Add a few steps… maybe a ladder?
Floor overlays are a temporary fix for expansive soil heave and provide no long term assurance of any future performance.
