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The base (You call it "foundation" ) of patio is normally made of 4 inches gravel (grain size 3/4" to 1") and one or two inches bedding sand on top. That is what suggested by Homedepot DIY web.
However, the depth of base depends on your patio's traffic. A paving to drive a truck need need be deeper and stronger than that of backyard patio. I would say 4 inches base is good enough for most cases (patio, home drive way). Remember using landscape fabric to separate soil and base.
I used 3 to 4 inches rock dust (no gravel) as base last time and the patio is very stable.