Posted 9/13/2021 12:41 (#9218123 - in reply to #9218087) Subject: RE: Tile and sub soiling.
North Central Illinois
30" depth seems to be the common target for contractors who have worked for me. Some as shallow as 24" if necessary to keep grade without going super deep other areas. That said, there's some old stuff that barely has a foot of cover in some low areas.
I'd be concerned if you were warned that it is "Fairly shallow." If you snag it, you'll be replacing whatever lines you snag.