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Discussion Starter #1
Two years ago I built a shed, and used 8-inch tube forms to make foundation piers. I used those J-bolts to connect the piers to the shed. For some reason which I cannot now remember, I decided to lift one of the floor frame pieces off its bolt (this was after the concrete was well hardened). I used a big steel bar as a lever, and as I leaned on the bar to lift the wood off the bolt, the whole pier in its tube form began to come out of the hole.

Obviously there wasn't much friction between the soil and the tubular form. I had drilled the holes with a rented gasoline powered auger, so there wasn't much clearance between tube and hole wall.

This makes me wonder if, I would have been as well off to not use a tube form, and just toss the concrete into the hole. The hole is about 4 feet deep because I am in North Dakota.
 

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The main purpose of the Sonotubes is for the concrete to be placed above grade to get level piers or to keep the structure above grade a certain distance.
 

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That's one reason, another is so as the soil above the frost line moves up and down due to moisture content or freezing the building does not move, the soil just slips around the tube.
 

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Discussion Starter #4
Ah.

That makes sense! I wanted to have the wood above the soil, and that's why I used the forms. At least I think that was my reason for using them. Within a month, I usually forget nearly every detail of how I built a project or why I did things this way or that.

When the concrete was cured, I tore the tube form off the part of the pier showing above the ground. I wonder if that is standard practice for builders? Maybe if the form was standing free, a person could tear off the tube the whole length of the pier and get the advantage of more soil friction after backfilling.
 

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By now, I would think that the ground has settled enough around the pier that it's not as likely to "slip" upward. That and the fact that the cardboard is probably nearly deteriorated under ground.....
 

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Discussion Starter #8
Yes, I put the tube all the way down, leaving about 6-inches above ground. I suppose if I had widened the bottom of the hole for a footing the cylinder of concrete and the footing would have connected and the wider footing would have resisted being lifted out of the hole.
Or even if the tube hadn't been all the way to the bottom, the concrete that poured out of the tube down there would have contacted the soil and gave more friction.

I hadn't really thought of the tube decomposing with time, but it is reasonable that would happen and the soil would slump toward the cylinder of concrete and give more resistance to lifting.

Lots of good comments and food for thought.
 

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Yes, I put the tube all the way down, leaving about 6-inches above ground. I suppose if I had widened the bottom of the hole for a footing the cylinder of concrete and the footing would have connected and the wider footing would have resisted being lifted out of the hole.
Or even if the tube hadn't been all the way to the bottom, the concrete that poured out of the tube down there would have contacted the soil and gave more friction.

I hadn't really thought of the tube decomposing with time, but it is reasonable that would happen and the soil would slump toward the cylinder of concrete and give more resistance to lifting.

Lots of good comments and food for thought.


Usually you only leave the sonotube on the portion of the pier that will be exposed, but as jomama said it will eventually break down,no harm no foul.

As to making the bottom of the hole wider, I don't know how you would accomplish that, but they do widen the bottoms of caissons,and it's done with a tool called a belling bucket,trouble is no such tool exists for small holes like 8,10,12,inch diameter holes.
 

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As JOE has explained.... the sono tube in frost country is made expressely to allow slipping.

Your instance is normally a relatively strange occurance.

Sometimes... when soil conditions require, you run your sonotube into a "big foot" (which is just a bell bottom. ...yes you oversize your pier hole.)

(In non-frost country, I just use the sonetube above grade and fill the pier hole.)

Best
 

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Discussion Starter #11
If the sono tube decomposes in the soil, how can it for many years allow slipping past the concrete pier in cold weather country?
 

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If the sono tube decomposes in the soil, how can it for many years allow slipping past the concrete pier in cold weather country?
It is not the sonotube itself allowing the slipping.... it is the perfectly round smoth pier, without any irregularities that prevent frost heaving.

.
 

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Discussion Starter #13
That thought had occurred to me after I posted the question. I suppose it is important, then, to make sure the concrete is well-plunged with a piece of wood or a piece of rebar to eliminate any air or stone pockets so that the pier surface is indeed smooth at the wall of the sono tube.
 

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That thought had occurred to me after I posted the question. I suppose it is important, then, to make sure the concrete is well-plunged with a piece of wood or a piece of rebar to eliminate any air or stone pockets so that the pier surface is indeed smooth at the wall of the sono tube.
Yep... I think it is better to.... I always take the time to "rod" it.

(In Co obviously frost heave is an important/critical building consideration... so we pay good attention to those issues. Been buiding with my kid in SoCal... and our pier holes can be sloughed out and any shape..... some were so irregular that we used sonotubes to save concrete.)
 
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