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I will be pouring a garage slab sooner or later, and was hoping someone would weigh in on your question.

I did do some research and the results are to take out all the topsoil and use a crushed rock as a fill.
Sand shifts too much and is hard to compact.
Maybe crushed concrete could be good too.
I'm breaking up the slab and placing it in the bottem and will fill in with a crushed product. Another week to remove the slab and probably a week to fill and level.

We scraped away the topsoil and brought in some forty loads of a red clay (that has some sand in it) mixture for the foundation and the road. It came from the biggest provider of foundation dirt around here so if it is improper, alot of buildings here are in trouble.

The three big piles of topsoil will be bulldozed back around the garage when all is finished.