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When the wooden fence was built, they used metal posts but only filled the 2' hole with one bag of concrete. The posts are leaning at about a 25 degree angle. I want to dig out the hole again, then add another bag of concrete to each post.

My question is, will this work? or do I need to drill holes in the existing concrete and put in a few pieces for rebar to "marry" the new concrete to the old?

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How tall is the fence? The holes may not be deep enough. –  BMitch Oct 13 '13 at 14:26
    
The fence is 6 ft tall and the holes are about 30" deep. –  Brian Oct 13 '13 at 15:01
    
I'd recommend going deeper if it's possible. Rule of thumb is 1/3 underground, or 1/2 of the fence height above ground should be below ground. So a 6' tall fence needs 3' underground, which would be 1/3 of a 9' long fence post. –  BMitch Oct 13 '13 at 16:00
    
So your saying that the holes are 30" deep and 2' wide. With one bag of cement thats only 6" of concrete at the bottom of the hole. Thats a very wide hole for a 2" pipe. With all that loose backfill its no surprise the fence is leaning. How deep do you have to dig until you hit the concrete? –  Justin K Oct 13 '13 at 16:40
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1 Answer 1

25 degrees, that's huge!

If you go with the concrete method, no need to drill holes, however ...

Quicker and more robust than pouring more cement:

  1. dig out the top 12" inches around each post

  2. attach a 2' length of ground-contact pressure-treated 2x8 to the post a couple inches below grade, using, for example, 5/16" U-bolts if the post is pipe. The 2x8 needs to be eyeball parallel to the fence.

  3. back fill a couple inches at a time, compacting it as you go. Gravel would help, particularly on the lee side relative to prevailing winds or current lean, but is not necessary.

Going deeper per @BMitch would help if done right (it'd certainly help on a new install), but again not necessary assuming they are currently deeper than the frostline.

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