Whether expansive or collapsible soil will damage your foundation could depend on the kind of foundation system you have.
If you have a conventional foundation system—the kind that was most prevalent until 2001 or 2001—soils aren’t much of an issue with the foundation. But they can damage the floor slab, which is constructed independently of the foundation. The water can’t lift the foundation because it’s solid, but the slab will move and can cause some cracking of the drywall and tile in the home.
A post-tension foundation system is different. Everything is connected with steel tendons and strengthened through the steel tendons. It’s all one unit. Still, those things can move if enough water gets under them, and a lot of cracking can occur. The difference: A post-tension foundation can tolerate the movement better than the conventional foundation can.
Around 1995-96, a handful of builders started using post-tension slabs. But the majority of builders didn’t switch over until the early 2000s. If you have post-tension slab built 10 years ago, it won’t be as strong as the ones built more recently.
Post-tension systems overcame a lot of foundation problems we were having in Arizona.
A tip: Know which kind of foundation you have. Look in the garage. If you have a stamp in the garage that says “post-tension foundation system,” that’s the easiest way to know. With a slab, you can see it’s not connected to the foundation.
No matter what kind of foundation or slab you have, you still need to keep the water away from the house. People have a false sense of security if they have a post-tension floor slab. You still shouldn’t put plants and trees right next to the foundation.