Frost line depth in Shelby County, IL
How deep the ground freezes in Shelby County, IL
24 in. Dig to 24 in, just below the frost. Nearest NOAA station: Shelbyville Dam, 2.1 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.