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This winter the Minnesota frost heaved up 5 fence posts in my yard.

I've pulled out the 5 posts, and have found basically puddles of water and very very wet mud. I shop-vac'd out the standing water, and have let the holes air out on nice days. When it has rained I've tarped a 8x10ft area to try and get the water down hill. This is what I have been doing for the past week.

Some of the holes are still filled with 3-4inches of water, others are very wet but not filled. The holes right now are around 2ft deep, and I need to dig them to 4.5ft or so.

Should I just dig the holes with standing water? Some of the holes are so wet I'm afraid a deeper hole would just collapse. Or how can I dry them out?

Image of fence

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2 Answers

I would just dig it out while it is wet. If you really want a clean hole, you could rent a 2 man post hole digger.

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That way you can stand far enough away from the hole to keep from caving it in. Personally, I think if you are going that deep, then it does not matter how clean the hole is, just leave some room for grass to grow over the concrete.

The new concrete will still set up with water in the hole.

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Post hole auger is probably out of the running. I have a few utilities going right beside two of the posts. So at least those two need to be hand dug. I'm not sure what you mean by "if you are going that deep, then it does not matter how clean the hole is". –  dpollitt May 4 '11 at 20:45
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You said you are going 4.5ft. That is is pretty deep. With the upside down bell shape that @DA01 suggested, a little over 2' should be sufficient. But, back to the question, you can dig it and pour the concrete while it is wet. –  Tatton Chantry May 5 '11 at 18:17
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In MN here the frost line is quite deep. I'm no expert, but from what I understand 4.5ft is kind of the standard in these parts. The existing posts were at 2 ft, and as you can see they are no longer in the ground. –  dpollitt May 6 '11 at 21:36
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Yea, code for structural footings is pretty deep in MN...I think 54 inches. That said, that's for structural footings (decks, foundations, etc.). It's probably a bit overkill for fence posts. I've had wood 6' posts in the ground 2' for about 4 winters now with no visible heaving (though they are pretreated lumber so warped like crazy...never again. Next time it'll be metal or cedar) –  DA01 Jun 15 '11 at 19:50
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Are you in the Twin Cities? We have lots of clay here. Clay never really dries out ;)

Looking at the concrete, notice how it's flared at the top, this is a nice 'handle' for the frost to grab on to and push up. Ideally, your flare would be the complete opposite...wide at the base of the hole, narrow at the top.

If it's really heavy clay, you may want to forgo concrete completely and instead use compacted crushed rock.

If you stick with concrete, be sure to dig deeper than you need to and put gravel at the bottom of the hole for drainage before putting in the post and concerete. Also be sure to get at least a good 2' of the post into the ground.

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Yes I am. Honestly I have no idea if it is clay or not. I'm not up on my geology so much. It's soupy like soft serve ice cream at this point. From what I've read I'm going to try to reverse bell shape, bigger flare at the bottom, and add 2-3inches of gravel at the bottom. –  dpollitt May 5 '11 at 2:33
    
+1 for the shape of the concrete. It's not deep enough, too. –  Alex Feinman Jun 15 '11 at 19:43
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