Reinforced conc. SOG over Geofoam - How thick?

I have no access to AASHTO specs or books regarding the design of floor slabs on grade so I am asking this very simple question here. Situation is: I am designing a small garage (24'x24')on ground that falls away at a 4:1 slope so I will have foundation walls around 7' tall at one end. I want to avoid hauling and compacting fill. Also to alleviate pressure and avoid breaking the "pill box" I am considering Geofoam EPS blocks to fill the pill box and pour the slab on, thus avoiding the need for external fill. Now the design problem: UBC code requires 50psf dead load and 2000lbs/20 sq. in. LL capacity for residential garage floors. The EPS has various ratings of 3.2psi, 4.6psi, 6.2psi, etc. up to about 11psi. Is there a simple rule I can apply to determine slab thickness, rebar schedule, etc. I need to convince the county building department too since they have never heard of such a thing.

 

[ishvaaag]

The calculation assuming an elastic behaviour (what is much, since geofoam is plastic) can be made asuming a modulus of young and considering so the axial stiffness of geofoam. It may turn that even having more than enough capacity is so compressible that you end having settelement-squashing problems over big thicknesses of the material...what may be compounded with differential settlement in the case you dare to make the foundation with different thicknesses of geofoam infill. Yet in any case a proper calculation starting from the engineering stress-strain diagram of the used geofoam may provide with design information to choose the reinforcement...what won't solve the settlement issue that can cause both a step in the access and some inclination (even only from continued unsymmetrical loading of the sog).