ASK STEVE: Frost jacking causes heaving, help!

About 5 years ago I installed two posts under a porch and now one seems to be rising out of the ground. I set the bottoms 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 the backfill off with sloped concrete, to shed rain. I’m now noticing that this concrete 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. Any explanations about why it happened? Should I wait another year to see if it heaves more? What’s to stop a new post from doing the same thing? I thought I did everything right.

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 during installation. It sounds like your concrete was only installed as a cap. Is that right? If so, frost may have gotten underneath and lifted it and the post. One way to avoid this is by using cylindrical cardboard forms for concrete that go all the way to the bottom of the hole. Sonotube is a common brand name. By wrapping the outside of these tubes with several layers of black polyethylene plastic, you’ll make them slippery, stopping the insidious frost jacking process. I think you’ll find that your post will go down a bit as spring turns to summer, though probably not all the way. That’s why you should consider piling extra snow over the area this winter, to act as insulation that limits frost penetration and heaving. You may find that the post can be pounded down in the spring, when the soil is soft and wet. If that fails, then post replacement (with plastic-wrapped Sonotubes) is a permanent, though troublesome, cure.