High Water Table

[doxford jim]

I used 12" sona tubes when I built my car port with I think, 4000lb concrete mix in 60lb bags.. I had a 42" long 12"dia auger with a 12" extension allowing me to go down to 54" depth. A few holes I was able to go down a bit deeper using a hand held post hole digger with handle extensions (pipe). I also carried the tubes (footings) about 15" above ground.

I put these down three years ago and so far everything seems fine. No noticeable up lifts yet from the cold winter temperatures though knowing my luck, that will probably change at some time.

I am attaching a few photo's so you can see what I did - don't be too critical as they aren't perfect.
I will be installing the metal roof in a couple of weeks time - waiting on material. The tarps did the job for two winters - they actually only last about one year before they break down due to UV and other causes. They sport many patches and have done the job and will now be replaced with sheet metal (5 rib type) in a few weeks.

Anyhow, we get down to around -40C in the winter on occasions, so our weather is probably similar to yours.

 

[doxford jim]

Forgot to mention, the carport actually measures 26ft long and 28ft wide - the roof trusses are 24ft between supports with 2ft overhang on each side. I had 100lb snow load for the trusses but should have gone bigger I think - maybe to 200lb. Normal house design snow load is 50lbs I believe for this area. Just something to think about when you do yours.

 

[doxford jim]

And, finally (duh!!!), I used three pieces of rebar 5/8" dia. and one foot longer than the tubes, to stiffen the concrete in each tube. The extra foot length was driven into the soil at the bottom of the tube so the other end of the bar was just below the top of the tube. The post connector has a six or nine inch long piece of rebar attached to the bottom. When full with concrete (took around 10 x 60lb bags per tube) the whole lot is pretty solid.
I think I am finished now - ????

 

[skyhook]

Come on Jim, we all know it doesn't snow in BC...lol
Looks good.:thumbsup:

 

[Cord]

I'd just pour the tubes using a tremie and then not worry about the water.

 

[EddieWalker]

If I hit table water, I would find another place to build. When water freezes, it expands and this force is what cracks foundations all the time.

Why do any of you put soni tubes inside a hole that you drilled and fill them with concrete? I only use them for above grade because I want the concrete to be in contact with the undisturbed soil on the sides of the hole. Most of the strength from concrete in a hole is from the sidewalls, so when you put a tube in there, all of the weight is now resting on the bottom of the hole and you get nothing from the sidewalls. It is impossible to fill and compact the space between the cardboard tube and the undisturbed soil to any measurable level. Add to that the fact that you have a layer of cardboard in the ground and it becomes even more impossible to get any strength from the sidewalls.

Eddie

 

[dave1949]

Photos of tough sonotube footing forms on the job - how to use footing forms

I think I would consider these for a pole footing. They have a bigfoot 'cap' for use in wet areas that is supposed to keep the water and concrete separated while pouring.

In ground freezing climates, you really don't want a tight coupling between a sonotube and the dirt around it. That will just help the ground heave get a grip on your piling. Better yet for preventing heave lift is a pyramid shaped form that narrows as it rises.

I picture a sonotube as something rot-proof that transfers the vertical load down to stable undisturbed soil below the frost depth, plus its weight provides an anchor for the post.

 

[EddieWalker]

In ground freezing climates, you really don't want a tight coupling between a sonotube and the dirt around it. That will just help the ground heave get a grip on your piling. Better yet for preventing heave lift is a pyramid shaped form that narrows as it rises.

I don't understand what this means. Are you saying that it's better for the sonotube to be loose in the hole? How does this keep water from entering the hole in the fill soil around the sonotube?

Eddie

 

[dave1949]

I don't understand what this means. Are you saying that it's better for the sonotube to be loose in the hole? How does this keep water from entering the hole in the fill soil around the sonotube?

Eddie

The tube is not going to be loose in the hole. I don't think it could be kept loose over time with settling, even if that was wanted. It means it should be smooth so that when the frost heaves the ground it doesn't have very much friction around the tube.

The friction would be caused by a hole with irregular sides, pockets, etc. in the wall of the hole. If a hole is bored and filled with concrete, the concrete will fill every little rough spot in the walls of the hole, that provides the friction.

One advantage of the 'bigfoot' foundation I linked is that flare at the bottom. Any frost lifting of the piling within the frost depth would have to pull that flared section up through the soil. That generally is not going to happen.

With decent overhangs, good surface drainage slope, and a reasonably well drained site, there shouldn't be that much water available around the sonotubes. There are areas where some of that is very difficult to achieve--flat as a pancake and a high water table in heavy clay soils--but people do build in those places by necessity.

 

[EddieWalker]

I agree that you can build anywhere if you have enough money and knowledge. But I disagree about your view on the tube not being loose in the hole. The biggest disadvantage to the bigfoot design is the amount of material that has to be removed to get it into the ground. There are augers that open up to do the same thing once you get down to depth that this product is trying to copy without the problems of trying to fill up the hole again after you pour the concrete. I think the bigfoot product is a waste of money and it fails to accomplish anything.

The problem with filling dirt back into a hole is that you are creating a sponge that holds water. It is impossible to get the fill material to compact anywhere close to the undisturbed soil, so now you lose the advantage of all the strength of the sidewalls, and you now introduce a place for water to collect. As you mentioned, frost heave requires water in the hole. Using a sonotube provides that source of water.

Why not just pour concrete in the hole and not use the sonotube at all unless you need to go above grade? Then just cut it to length and put it on top of the poured concrete in the hole?

How much more could it cost for concrete compared to the cost of the sonotube?

Eddie