Home Made Cellar: Suspended Slab on a Culvert

We are finally getting around to forming and pouring a suspended slab on a culvert, thats right, a home made cellar. We got a great deal on these stubby culvert ends and figured they'd make great buried root cellars. The front one is 12' diameter and about 9' tall while the back one is 10' diameter and 6' tall. The little one will be more of a typical root cellar, totally buried and totally freeze proof while the one in front will be only mostly buried and may be a bit more multipurpose. We kinda guessed at the design for the suspended slab, out first ever. the concrete in the bigger slab will weight approx. 7,000lbs and after it's set it will need to hold a pile of earth as well. The front slab: spans 12' diameter 5" thick 15mm (5/8") rebar 12" on centre plus some bonus steel! The back slab: spans 10' diameter 4" thick 15mm (5/8") rebar 12" on centre plus some bonus steel! The suspended slab on a culvert is the secret to these root cellars, they'll keep out critters and remain structurally strong for ever underground.