high density rigid insulation used as fill

huangyhg

high density rigid insulation used as fill?
i am currently working on a project which is going to convert an old (1920s) warehouse into a parking garage.  the floor of the structure is a beam/ rib joist system with a thin slab between the joist.  we need to create a ramp within the building rising approx 3.5 feet over approx 40 feet.  my thought was to use rigid insulation (blue board) rated at either 60 psi or 40 psi to create this ramp.  this system should be the lightest system that would create the required ramp, as well as uniformly loading the slab below.  after placing the foam, a concrete slab would be designed similiar to a slab on grade for a driving surface.  i was wondering if anyone out there has done anything like this before.  how high can i stack this 2 to 3 inch foam safety?  how often should a pin the layers together, or should the be glued? there will be a slab below, slab above, and walls on both sides of the fill so hopefully it will be completely encapsulated and deformations will not an issue.  how can i analyze the structural ingretity of this system?  i would really appreciate any help that i can get.  
taldridg
we have used rigid insulation under floors of refrigerated warehouses - usually this is a couple layers of 4" board with a 6" slab on top.
for your case, what we've done is use the foam simply as void forms, and we space them out such that the new floor slab actually is supported by a series of 6" thick rib-walls spaced at 24" o.c. or 36" o.c.  the new slab is reinforced so that it structurally spans from rib-wall to rib-wall.  you can orient the rib-walls in either orthogonal direction depending on how you want to transfer the load to the structure below.
the insulation boards are usually glued to the the existing floor and to each other so as not to float up during concrete placement.
for 3.5 ft., i would worry that the insulation stiffness would be too low - under a set pressure, the resulting strains might provide more sponginess at the high end and result in cracks.
i re  
geo foam, can be specified in different compressive strengths. like jae said comes in big blocks.
do a search of "geofoam horvath" - you should get dr. horvath's site and a wealth of information on use of eps foam.  i-15 in utah was built with it on soft soils; many other applications too.  80 to 100 m of geofoam approx equivalent to 1 m of soil fill.
if your slab is spanning wall-to-wall, you will have different strain conditions for bending.  if you design the slab for full span and two-way bending, you don't need the foam.  if you design it as a "soil-supported" slab, you will need to consider the foam as an "equivalent" soil structure.  in order to do this, it is necessary that the foam be completely stable and prevented from movement so that your vertical strains can be equilibrated.  horvath and others have given info on the property correspondence.
you should glue the foam together or use a cubic gridwork of foam and cellular concrete, with a final topping of cellular concrete prior to the placement of your slab.  this will likely give the better uniform load distribution you desire, while keeping the "overburden" weight low.
i have used such combinations for pavement design over very soft soils and for the structural remediation of large fake "rock" structures at "a major theme park" in florida, so that the structures didn't sink into oblivion from the extra weight of soil fill.
thanks everyone for the responses.