Quote: 3' is needed to stop the ram from pushing thing out of the ground- ignoring the frost line.
Technicaly in Wisconsin 4' is the official frost line. But being in the Architecture field I know that the actual frost line varies- up in the far north, you need more than four feet, but where I am right next to the Illnois boarder, it is closer to the 36-42 mark.
Hubby actually buried them a bit deeper than 4'- he likes to err on the side of caution.