Frost line depth in Piatt County, IL
How deep the ground freezes in Piatt County, IL
29 in. Dig to 30 in, just below the frost. Nearest NOAA station: Champaign 9 Sw, 12 mi.
Your building department sets the legal frost line depth (IRC Table R301.2(1)); footings must go below it (R403.1.4.1). Call them before you dig.