Building and Construction - Pier Foundation for clay soil, high water table, 4 foot frost

Author

Pier Foundation for clay soil, high water table, 4 foot frost

Big Papa G

2006-07-18, 8:25 pm

Greetings,

I have a 20' x 32' cabin in NY state that was built on grade. Frost
heave is tearing the drywall apart and shuffling the cabin down the
mountainside.

I will be putting a pier foundation under it this summer. A test hole
(protected from runoff) shows that the water table varies between 4
inches and 1 foot below grade, with a pure clay soil. I would like to
use sonotubes for the pier forms, but the adfreezing/heaving problem
might call for truncated pyramidal plywood-formed concrete piers. I am
told that the pyramid shape would allow the wet freezing clay to detach
from the piers as the soil expands and heaves.

I have these questions:

1. I would use sonotubes instead of plywood forms if I could find a
foam product with which I could wrap the sonotubes so that the freezing
clay would be unable to get a grip on the piers. The foam would have to
be food grade as the well is just a few feet away. Is there such a
thing?

2. Would the foam need to be in contact with the sonotube, or would it
be enough to build a blue construction foam box around the sonotube
pier to absorb the expansion?

3. Is blue foam elastic enough to return to its original thinckness
once compressed?

4. Given that there is near zero drainage through the soil, is there
any benefit to backfilling with granular fill instead of the clay from
the excavation?

Thanks,

Big Papa G

Italian

2006-07-23, 1:25 pm

I hope you have hired an engineer for this project. If not get one.

1. I would use sonotubes instead of plywood forms if I could find a
> foam product with which I could wrap the sonotubes so that the freezing
> clay would be unable to get a grip on the piers. The foam would have to
> be food grade as the well is just a few feet away. Is there such a
> thing?
Why not use the tubes and then at gade or slightly below form the
triangle you spoke of this way you get the best of both. Im not sure of
the whole soil griping the piers thing Ive never heard of such concern.
If this is your personal concern do the the triangled concrete or just
put a layer of crushed rock around the posts.
>
> 2. Would the foam need to be in contact with the sonotube, or would it
> be enough to build a blue construction foam box around the sonotube
> pier to absorb the expansion?

The concrete is not going to be the exspansion problem the ground is.

> 3. Is blue foam elastic enough to return to its original thinckness
> once compressed?

> NO

> 4. Given that there is near zero drainage through the soil, is there
> any benefit to backfilling with granular fill instead of the clay from
> the excavation?

Why would you want water to permiate the ground under the house if it
is not alreay doing so? The runnoff going straight under the house is
an ideal situation. If the footings for the piers are at proper depth
for the soil conditions the water is going to flow right past them.

A final note: If you have a engineer on the project this is not out of
his scope of work ti adress these issues. In fact the should be address
on the detail page of the plans.

Big Papa G wrote:
> Greetings,
>
> I have a 20' x 32' cabin in NY state that was built on grade. Frost
> heave is tearing the drywall apart and shuffling the cabin down the
> mountainside.
>
> I will be putting a pier foundation under it this summer. A test hole
> (protected from runoff) shows that the water table varies between 4
> inches and 1 foot below grade, with a pure clay soil. I would like to
> use sonotubes for the pier forms, but the adfreezing/heaving problem
> might call for truncated pyramidal plywood-formed concrete piers. I am
> told that the pyramid shape would allow the wet freezing clay to detach
> from the piers as the soil expands and heaves.
>
> I have these questions:
>
> 1. I would use sonotubes instead of plywood forms if I could find a
> foam product with which I could wrap the sonotubes so that the freezing
> clay would be unable to get a grip on the piers. The foam would have to
> be food grade as the well is just a few feet away. Is there such a
> thing?
>
> 2. Would the foam need to be in contact with the sonotube, or would it
> be enough to build a blue construction foam box around the sonotube
> pier to absorb the expansion?
>
> 3. Is blue foam elastic enough to return to its original thinckness
> once compressed?
>
> 4. Given that there is near zero drainage through the soil, is there
> any benefit to backfilling with granular fill instead of the clay from
> the excavation?
>
> Thanks,
>
> Big Papa G

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