The fact that your soil is very hard clay can also have challenges for a slab. In the summer during drought times the clay dries out severely and shrinks away from the foundation which is supposed to be around the foundation to support it. With the clay shrinking, it can cause your slab to settle/crack. Some people go out and water their house around the foundation to prevent this. Sometimes the type of slab used for this kind of soil is a Monolithic slab. It has a footer poured every so many feet to support the span. Of course chat and a supportive type of metal (sometimes re-bar) is also used. Whatever you do I would for sure dig the roots out around the area and fill w clay and pack this before pouring over it.
I am just going out on a limb here and assuming some things about your barn. I assume that it is a pole barn of sorts? Just because those are very popular and affordable. The thing with a pole barn is that the concrete is poured as a floor only, it is not intended to support any walls. The walls are free standing of themselves. Many many pole barns are put up on just dirt- dirt floor. Of course if the building is intended to house vehicles or tractors ect, then the concrete would have to support the weight of those. I'm just saying, an actual footer to support the building is not necessary in most cases. So the slab design for your barn could be very different than what you need to support a home's walls/weight.
Since your post is a month old, how is your project coming along?
