Porch post problems

Q: Why are my porch posts rising out of the ground? About four years ago I installed two posts, setting them 50 inches below the surface in bell-shaped holes to prevent heaving. I also put a layer of pebbles at the bottom of the holes to aid drainage, and topped them off with concrete, to form a volcano-shaped rain cap. I’m now noticing that the cap on one post is an inch above ground. I hoped this might be due to soil settling, but since the porch is no longer level, I know heaving is the cause. Should I wait another year to see if it heaves more? What's to stop a new post from doing the same thing?

A: It sounds like you've got a case of frost jacking. This happens when frost grips the outside of posts like yours and lifts them, despite most of the right precautions having been taken. It also sounds like your concrete cap is part of the problem. Frost probably got underneath and lifted it and the post together. One way to avoid this is by using cylindrical cardboard forms for concrete that go all the way to the bottom of the hole, with the post sitting in a metal bracket set on top. Sonotube is one brand name of cylindrical form. By wrapping the outside of the tubes with several layers of black plastic (stapled in place), you’ll make them slippery, stopping the insidious frost jacking process. I’ve seen this system work perfectly, even in heave-prone, heavy clay soils.