Some years ago, a friend was installing a fence. He dug holes 4 feet deep and anchored the posts in concrete. But first he put an 8" Sonotube (a heavy cardboard tube 8" dia., 4' long) in the hole. I had never seen Sonotubes used this way. He said it made the concrete with smooth sides, so when the ground froze and heaved up, it would not lift the post up too. Made sense to me. I never had to install a fence, so didn't know if it was really necessary. Found out last spring it IS a good idea. Saw a fence down the street where they had not used Sonotubes. We had a mild winter, lots of freeze-at-night/thaw-during-day cycles, and now some of the fenceposts are about 4" higher than they were. See photos.