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-   -   Concrete Pillars Raising Deck (http://www.diychatroom.com/f19/concrete-pillars-raising-deck-53446/)

Ladybear 09-20-2009 04:14 PM

Concrete Pillars Raising Deck
 
My deck posts are resting on concrete pillars that are four feet in the ground. My deck raised up the last two years and I have it leveled and resting on jacks. Should I remove the pillars and place the posts on slabs above ground or as someone suggested to me, use "Elephant Feet" for a new footing.

Gary in WA 09-20-2009 06:01 PM

Here is some hind-sight: http://books.google.com/books?id=1gg...num=3#PPA31,M1
Notice the use of gravel and plastic round the pillars with a square footing at the bottom to prevent frost heave. If you back fill with dirt and have dirt under it, the freeze can still form ice-lenses to pop the pillars up. Added insurance would be some 2" thick extr. foam boards buried around each, just below the top soil to keep the freeze from penetrating to pop them up. http://rockproducts.com/mag/rock_agg...prevent_frost/ http://www.waltersforensic.com/artic.../vol1-no11.htm
Be safe, Gary

Ladybear 09-23-2009 08:07 AM

Thank You

concretemasonry 09-23-2009 09:30 AM

The raising could be from several sources.

1. The concrete "posts" were not deep enough for your frost exposure. Where are you located?. Usually the required values force people to be well below the frost line, but in some cases, exposure (north facing), lack of snow cover can produce a greater frost depth.

2. If the "posts" were hand dug and the holes was filled with concrete, the resulting surface can be rough or scallop ed and the soil can freeze to the rough face and raise the concrete as the ground freezes downward an created ice lenses starting at the surface and progressing down to the final frost depth when there is sufficient moisture if the soil is frost susceptable.

The second is rarely an issue if Sonotubes (waxed forms/molds) are used because the surface is not likely to be pulled up by the surrounding soil. There are the usual enlarged bases that can be used with Sonotubes. These are usually used to create a larger footprint to increase the bearing area is the soil is bad. the ability to resist the frost heaving is over-rated if the bottom is not deep enough since the entire ground above the frost level goes up as the frost proceeds deeper.

Putting the deck on a surface pad will not stop the heaving, since the heaving is measured as the difference between the house (or other piers) and where you measure the heaving. If everything goes up the same amount, there is no noticeable heaving. - That is why roads can heave, but if the difference between two places cannot be seen, it is not though of as being heaved.

If you do replace the pires, make sure there are deeper and you do what you can to keep the water draining away from the area.

Dick

Ladybear 09-23-2009 10:02 AM

Concrete Pillars
 
The holes were dug 4 ft. below ground and the concrete was free poured and does have a rough and scalloped surface as you mentioned. When the holes were dug, there still was some water at that depth. Someone suggested using "elephant feet" which I'm guessing is some sort of pad for the bottom of the hole.

Thank You for your input

concretemasonry 09-23-2009 10:27 AM

The "elephant feet" or whatever they are called are usually used with a tube form like Sonotubes.

When you backfill, do not use just "dirt", but sand gravel and rock.

Free pouring or just filling the holes use a lot of concrete and it leaves a rough surface that will attach to the soil as it freezes.

Dick

jomama45 09-23-2009 11:54 AM

Quote:

Originally Posted by concretemasonry (Post 331337)
The raising could be from several sources.

1. The concrete "posts" were not deep enough for your frost exposure. Where are you located?. Usually the required values force people to be well below the frost line, but in some cases, exposure (north facing), lack of snow cover can produce a greater frost depth.

2. If the "posts" were hand dug and the holes was filled with concrete, the resulting surface can be rough or scallop ed and the soil can freeze to the rough face and raise the concrete as the ground freezes downward an created ice lenses starting at the surface and progressing down to the final frost depth when there is sufficient moisture if the soil is frost susceptable.

The second is rarely an issue if Sonotubes (waxed forms/molds) are used because the surface is not likely to be pulled up by the surrounding soil. There are the usual enlarged bases that can be used with Sonotubes. These are usually used to create a larger footprint to increase the bearing area is the soil is bad. the ability to resist the frost heaving is over-rated if the bottom is not deep enough since the entire ground above the frost level goes up as the frost proceeds deeper.

Putting the deck on a surface pad will not stop the heaving, since the heaving is measured as the difference between the house (or other piers) and where you measure the heaving. If everything goes up the same amount, there is no noticeable heaving. - That is why roads can heave, but if the difference between two places cannot be seen, it is not though of as being heaved.

If you do replace the pires, make sure there are deeper and you do what you can to keep the water draining away from the area.

Dick

These were my exact thoughts also Dick when I read this post last night. I didn't want to comment, as once before when I suggested on here that frost can "pick" at the sides of a rough surface & heave it, I was shut down by others. I was told by a few others on here that sonotubes were merely used to save concrete. My first hand experience with frost & foundations definately tells me otherwise, though. :whistling2:

Gary in WA 09-23-2009 03:31 PM

jomama45, don't take it so personally. The ones answering on the forum have far different levels of knowledge and experience. Some have a lot of posts to their credit yet have no experience other than building one addition or remodeling one house. Yet with so many numbers, you'd think they really knew what they were talking about. But as they parrot something they heard once, they soon put their foot in their mouth! I encourage you to keep posting as that's the best way to learn and it's a lot more fun with more people involved!! We are never too old to learn.
As in my previous post above, the Google book article mentions plastic on the sides of the sono's to create a slip surface, connecting a square footing to the sono's, and back-filling with gravel to the sides and bottom of the footing. I forgot this, using foam: http://www.oikos.com/esb/43/foundations.html
Be safe, Gary


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