Proper Subgrade Prep| Concrete Construction Magazine | Sitework .Sep 20, 2007 . With slab-on-grade foundations, an alternative to native soil material may be used to restore grade. These imported fills may include granular materials such as sand, gravel, or crushed stone. These high-strength and permeable materials offer low compressibility, which is ideal for a slab-support system.pad concrete or crushed stone,Shed Base: Concrete Slab or Gravel? | DoItYourselfConcrete shed base or gravel shed base? Which is best? It all depends. Both foundations have their pros and cons of course. Which foundation works best depends on your situation, land and needs, not necessarily on the foundation itself. Things to Consider. Is this a permanent or temporary location? If you're planning on.