Texas has a region of highly expansive soils called The Blackland Prairie which covers a large part of North Texas but also extends down into and around Austin where this Inspector is located. These black soils (black gumbo or black clay) are why foundation repair companies have been so successful here! Also very successful are a lot of licensed Engineers who specialize in residential and commercial foundation design and problem remediation.
These soils are notorious for movement and have literally lifted large commercial buildings causing large foundation problems and failures. This building being built is a relatively lightweight structure compared to even an average sized house. AHJ’s in these areas typically all require a designed foundation for any large structure. Active soils where moisture content can vary significantly can be from 8’ - 15’+ deep. Building in these soils typically does require a clean slate and soil preparation. Even among the black clay there are pockets of sandy loam and heavy rock formations but in this case there is no idea what might be beneath this old slab and around it without proper soil sampling.
Add to the soils problem is what they plan to do with the old slab? What are the chances they would blow it clean and add a bonding agent to try making it a part of the new slab? Even if they claim to be doing that what are the chances of doing it right? Looking at the pictures it appears they may try to cover the old slab with soil and pour on it. These are some of the issues with that.
- Even if they are lucky enough to compact the soil properly if it becomes wet. which it can easily do, the weight of the structure above can settle pushing those soils out leaving problem voids under the slab. I’ve seen where problems have been caused by mud packing piers causing settlement issues. This is no different.
- If voids are caused they are collection points for water and that means uneven moisture content of the soils. This is a problem with expansion and movement.
- These soils can cause the old pad to shift and heave under the new foundation. That can damage anything near it or even above it.
Can nothing at all happen? Absolutely and hopefully it never does. However I suspect they are putting in at least a 30’ X 40’ building. A basic building of that size, without plumbing or utilities, is a healthy $25K - $30K. I know I sure as he11 would not take that chance if it were my building and would ensure my client is well aware of the potentials! All we can do is educate the client and make sure they know what they are getting into.