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I've read through countless posts but couldn't find a straight answer for my particular situation.

My property is all clay. 5 years ago I built a fence around 2 sides of my property, digging down 4 foot holes (our frost line is around 54 inches, but various contractors told me you only need 4 ft for fences since the snow acts like insulation around them, so frost line doesn't get as deep), inserting post, and compacting gravel around the post.

Although most contractors said they use concrete, I decided to go with crushed gravel after doing some research, since the consensus seems that no matter what you do, if you have clay water will get in. Crushed gravel seems easier to repair posts in the future as opposed to concrete.

5 years later that fence is still quite sturdy, but a few posts have heaved a few inches, and in one section it has developed a bit of a lean to it.

I'm now looking to do the third side, and hoping to improve a bit. I'm doing 6x6 in the corners, hoping to go a little bit deeper and possibly try to make it bell shaped (though not sure how easy this will be with an auger?). I'm also questioning the gravel method, as now that I know more about frost heave, I don't know what is actually holding the post down with compacted gravel? Where with a concrete bell shape, I can see the frost heave not affecting the post.

My question is, what if I dig the hole, put a small amount of concrete in the bottom (maybe half a bag), then do compacted gravel for the rest of the hole? Would this provide more of an anchor for the post, preventing frost heave and maybe preventing the eventual lean from compacted gravel?

I also thought about just putting a threaded rod through the bottom of post before putting it in the hole, and wondered if that would do the same thing as the bell shape?

Thanks

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    Not much will prevent frost heave except going below the frost line. Check with your local building department and see if footings need to go to 54 inches or four feet are good enough.
    – crip659
    Jul 10, 2023 at 13:25
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    It seems odd to count on a layer of snow being present to act as insulation. It can get cold without snow, and "a layer" seems like an imprecise unit of R value. ;)
    – spuck
    Jul 10, 2023 at 14:30
  • @spuck True for many parts of Ontario, but where I am up in the North, I can't really think of any time where it was cold enough to send frost 4 feet deep without having at least a few feet of snow! This past winter had many freeze/thaw cycles, much more than normal. I'm thinking that might be why a few posts heaved My local building department says 54 inches now, so I will be sure to go at least to that depth. Do you think it's fine to go down that low, use half a bag of concrete to hopefully stop it from leaning in the future, then compacted gravel for the rest?
    – LBJ33
    Jul 11, 2023 at 0:09
  • @crip659 My local building department says 54 inches now, so I will be sure to go at least to that depth. Do you think it's fine to go down that low, use half a bag of concrete to hopefully stop it from leaning in the future, then compacted gravel for the rest?
    – LBJ33
    Jul 11, 2023 at 0:09
  • I would use concrete at the bottom and the top, if you want to prevent leaning. Concrete in just one location does nothing to stabilize the other.
    – Huesmann
    Jul 11, 2023 at 13:35

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