You do not need to go thicker than 4" on the slab. Every driveway across this country that is concrete is formed with 2x4's and unless you plan on parking a Semi on it 4" is plenty thick. A typical concrete 6 bag mix will yield about 4000 pounds per square inch of compressive strength after 28 day's. I doubt you will be placing more than 4000 pounds of anything in a single square inch anywhere on your slab.
Yes, compact, and then compact again. Also that's a big 10-4 on the gravel, compact that too.
Yes, compact, and then compact again. Also that's a big 10-4 on the gravel, compact that too.
