I had rejected perlite under my basement floor as too risky because of unknown compressive strength. That was premature: http://www.idosi.org/wasj/wasj25%2812%2913/19.pdf
This paper clearly shows that expanded perlite compacted slightly (10% or so) will have adequete compressive strength.
http://www.schundler.com/underfloor.htm
Shows loose expanded perlite under a concrete slab and the perlite institute has a publication showing perlite left in bags used similarly.
So my plan is to cover the excavation with a "drainage" layer of pea gravel, build the PWF walls on the pea gravel, inside the enclosed area I'll lay down geotextile filter fabric then, place unopened bags of perlite loosely, then fill the gaps with loose perlite. compact the result with a vibratory plate compactor, lay one sided corrugated cardboard then 10 mil poly, then pour the floor.
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