Explain concrete sonotubes to me

[oosik]

The log cabin I built in Alaska in 1976 stands on raised sonotubes. Dig a hole to a depth well below the frost line - five feet in my case - fill the hole with cement - jab the sonotube into the wet cement and adjust the height to be on grade. Push a round wire cage inside the sonotube and half way into the wet cement. On a 12" diameter sonotube - use a 6" diameter wire cage about two feet long. One foot pushed into the wet cement - one foot out of the cement and centered inside the sonotube. Fill the sonotube with cement - check that top of sonotube is on established grade and adjust as necessary.

These sonotubes on the cabin have stood tall, strong and proud now for 42+ years.

 

[marchanna]

I used to put them in will footings also, but none of the building codes here require it.

With the metal reinforcement mentioned above and a thick wide footing, you can use a much smaller diameter sonotube, but it is a little more work.

 

[bdog]

We don’t have frost heaving here to worry about and no problems with expansive soils. I have set hundreds of fence posts around my place in concrete and never had one move.

I did however try to relocate a fence post once and it didn’t work. Had a post set maybe 3.5’ in the ground in concrete and pulled it out, concrete and all with the loader. Augered a new hole for the post with a larger auger and placed it in there and packed dirt around it the best I could. It was not stable and if you grabbed the top of the post you could move it around. Seems like these sonotubes would be the same way if they were shoved all the way to the bottom of the hole even if you packed dirt around the exterior of them.

 

[EddieWalker]

I watched the video without the sound on, so I might have missed something. In my opinion and experience, compacted soil around a post is always softer then undisturbed soil. When you have a really heavy rain, that soil will wash out. So putting the sonotube inside the hole and then filing in around it with loose soil and trying to compact it will never give you as much strength as you would get if you just filled the hole up with concrete and let the undisturbed soil contact the concrete.

If frost heave is a big issue, digging out the bottom of the hole bigger and then lowering a sonotube into the hole part way has merit. You create an anchor that way, and a larger footprint at the bottom. It's not a concern for me where I live, so it's not something I'm ever going to do.

The only time I use sonotubes is when I want to extend the footing above grade. Usually for setting a post on. I decide how high above grade my deck or pad will be, and then add an inch. Then when I pour my pad, it's still an inch high so water cannot get to the bottom of the post.

The other thing they did that really confuses me is putting gravel in the bottom of the hole. If you just left the hole empty, it would act as a bowl and hold water long after the rain is gone. The undisturbed soil will absorb some of the water, the rest will run off. What is absorbed will go sideways to where the hole is and keep it filled for longer. Adding rock just creates voids for the water to remain in the bottom of the hole longer, and actually draws in more water then if there wasn't any rock in there.

Keep it simple. Dig your hole as deep as it has to be for your area, fill it up with concrete and only use a sonotube if you need the concrete to go up higher then the surrounding grade.

 

[bdog]

In my opinion and experience, compacted soil around a post is always softer then undisturbed soil. When you have a really heavy rain, that soil will wash out. So putting the sonotube inside the hole and then filing in around it with loose soil and trying to compact it will never give you as much strength as you would get if you just filled the hole up with concrete and let the undisturbed soil contact the concrete.

This is exactly what I was try to say is the problem I see with putting the tube all the way in. If you dig a hole and put a post in it and fill with dirt it will never be stable. However if you drive the post in which many people are doing here now it seems to work fine.

 

[marchanna]

It all depends on your climate and soil type. Frost is the biggest factor. Around here, driven in posts don’t work at all they heave so bad that I have seen wood fences collapse after just a couple years. What I see a lot of fence installers do is auger the hole and pour the concrete and set the post right in the hole. It seems to hold up well for many years and it is quick and efficient.

 

[Cord]

They're setting them all the way down so they don't have to support the tube and it'll use less concrete. Depending on your soil type, the tube can help to restrain the soil and help save the pier if it should slump. Personally, I'd prefer to have the pier be tight to the hole wall because the earth will help stabilize the pier. Unconsolidated backfill will contribute no lateral stability. The frost upheave on the side of a pier is minimal and is typically not considered. It's more important the bottom of the pier be extended below frost and the bottom of the hole be clean of any loose spoil. You should add at least one piece of rebar to provide some tensile strength to the pier.

I will say that Waxahachie Texas was the second worst project that I had for expansive soils. The worst site was southwest of Denver.

 

[MossRoad]

I think those tubes are meant for piers, not fence posts. There's a big difference. Piers support weight and let you use posts not sunk into the ground. Great for wood deck posts and preventing rot due to ground/water contact. I don't think you'd want to use tube forms for fence posts.

 

[npalen]

Read thru the posts fairly fast but didn't notice that anyone mentioned what happens if/when the tube itself deteriorates. Not proclaiming a problem with that but just something that probably happens over time.

 

[MossRoad]

The tube will rot away. They can be removed, if desired, by making a slit on two sides, then around at the depth you want to remove it, then peel it off. It's waxy coated inside for easy removal.