My builder will be pouring two grade beams in order to support my garage slab (new addition). As I understand it, there should be a void under the grade beams (using cardboard void forms) since I have a clay soil. Should there also be a void under the slab itself? There are products on the market specifically for this, but my builder didn't mention it. I believe they are going to pour the slab directly over 4" aggregate, which doesn't seem like the right approach. If the grade beams need a void underneath, surely the slab itself does as well.

That bring me to my next question. They had to remove a section of my porch slab, which is supported by a few short concrete stem walls that protrude perpendicularly from the main foundation. The builder said they are going to back fill underneath so the slab is sitting on a solid base, as opposed to having a void underneath. Don't I want a void underneath?

 

It's not your fault that you had a series of mishaps from the first day when the ribbon footings didn't match the plans. IMO, that should have been corrected first. I don't know anything about building on clay but I don't trust anything that guy says..

 

I really don't think that filling in under the stem walls is a good idea. If that works lab have held up good in the past why change it now? They just need to put corrugated metal across the stem walls in order to pour new slab