Author Topic: foundation for wet soil  (Read 6093 times)

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Offline nantiinaq

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foundation for wet soil
« on: December 27, 2007, 04:29:50 PM »
I'm brand new to this and have a question about the starting point... the foundation.  I'm planning to build the 20 x 34 two story.  I live in Homer, AK which doesn't stay too cold but is quite moist.  My soil is rather waterlogged.  Water is only about 4-5 feet down and doesn't drain fast.  I was originally thinking about concrete piers as I could probably do that myself... but am concerned about how far they would need to be buried.  I don't think I can dig too deep without getting water.  One option would be to have metal piers driven in, but that gets expensive fast!  I've been hearing a lot about ICF's too.  Any suggestions? 

Offline peternap

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Re: foundation for wet soil
« Reply #1 on: December 27, 2007, 04:45:12 PM »
You really only need to get below the frost line. Here, it's 18".

You do need a Bottom though. That's solid soil. Without seeing your soil, I can only generalize. Bore a hole to the water and pay attention to the soil type. Look for solid clay or marl (Grey stuff)...AKA Blue Marl.

If you have anything like that, fill the hole with crusher run gravel to the frost line. Compact it well. When you finish, compact it again twice as well. Put your pole in there.

PS....you don't want to break through this solid layer. Boring a hole through the solid stuff into running sand or something like that, will cause you problems in the future.
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Offline bobtheengineer

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Re: foundation for wet soil
« Reply #2 on: December 27, 2007, 04:58:16 PM »
I would check out what some of the locals are using.  Your frost depth there might be 5-6'.  If that is below the water table, there isn't much you can do about it.  A couple of options I came up with: 1)  Frost free shallow foundation, you basically pour a slab or footing on grade, and keep the whole thing shallow, say 16" deep, then insulate the outside of the slab/footing, to keep the frost from coming below the footing.  They use this type in northern europe, and its been tested out in Alaska, because of the very reason you are running into.  2)  Another option would be a drilled concrete pier.  You might have to dewater the holes (a small sump pump would do the trick), to get them poured, but once they are poured they would hold up well. 

I would stay away from putting any gravel with a post on/or in it.  The gravel, with its low permability, will give the water a great place to travel to.  You definately do not want the posts to be in wet service, they won't last too long that way.

You'll find lots of references to the frost free foundation on the net.  There is one listed here: http://www.toolbase.org/Design-Construction-Guides/Foundations/Design-Guide-Frost-Protected-Shallow-Foundation

Just my 2 cents worth, a wood post and beam foundation is not a fit all for all situations.

Offline MountainDon

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Re: foundation for wet soil
« Reply #3 on: December 27, 2007, 05:24:36 PM »
I believe the best answer to this will come from surveying local builders, asking at your local building permit/inspection department. They will know what has worked and what has not.

I've also read somewhere that the shallow frost free system is used all over the northern European countries, Sweden, Norway, Finland. But you need to check locally.
Just because something has been done and has not failed, doesn’t mean it is good design.

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Offline nantiinaq

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Re: foundation for wet soil
« Reply #4 on: December 29, 2007, 11:42:01 AM »
well, thanks for the ideas so far.  I was trying to avoid a slab foundation as I would worry about not being able to access parts, and was looking to have a crawlspace for water equipment and storage.  But, if it ends up needing to be a slab, then so be it.  As far as slabs go, does anyone have experience with solar slabs?  I have been reading as much as possible and found the solar slab idea in the book, "the passive solar house".  Just curious if anyone had any experience with that and found it worthwhile.  I don't know if it would work with the FFSF anyways.

Thanks!

Offline peternap

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Re: foundation for wet soil
« Reply #5 on: December 29, 2007, 03:37:49 PM »
The idea of the gravel is to allow drainage. Here's a picture of the type pole footing I'm talking about. In fact, he used a tube in the ground to pour the gravel and pole.

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Offline Homegrown Tomatoes

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Re: foundation for wet soil
« Reply #6 on: December 29, 2007, 06:49:38 PM »
Nantiki, is this you?  I am Homegrowntomatoes from TMEN forums.  How are you?

Offline bobtheengineer

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Re: foundation for wet soil
« Reply #7 on: December 29, 2007, 07:40:38 PM »
As far as that post in gravel, thats fine if you are dealing with no groundwater and a well drained soil.  In soil that isn't well drained, or high ground water table, the gravel gives the water a great place to run to.  If you drill a hole into a place where you have a high water table, you might not have water right away, but over a period of hours, or days after the hole is drilled, you'll see the hold slowly fill with water.  In some cases, if the groundwater is being restrained by a layer of clay, the water table can actually be forced upward in the hole, even when it is filled with gravel.  A hole filled with crushed rock, or gravel, is an excellent place for water to hole up, pun intended.

I did some more thinking on this, and I would put in drilled concrete piers.  drill em and fill em up right away, without giving the water any chance to fill in the hole.   Drill a 12 or 18" hole, then fill it in with concrete right away, you wouldn't have to fill the whole thing at once, just fill it in above the water table, then you can come back in later and finish it up at your leisure.

Offline peternap

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Re: foundation for wet soil
« Reply #8 on: December 29, 2007, 08:34:04 PM »
Bob, his groundwater is 4 to 5 feet, not inches.
I doubt very seriously he has an aquifer at that depth so the water is very likely in running sand. If you put in a concrete plug as you suggested, it will simply pump and be worse than frost heaving.

But rather than argue about soil conditions neither are sure about, it would be in his best interest to have a soil analysis done or at least do a bore hole himself to see what.s there,
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Offline John Raabe

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Re: foundation for wet soil
« Reply #9 on: December 29, 2007, 08:59:39 PM »
The advice about getting a true picture of your soil conditions and what others find is working is right on. For an accurate picture you need to understand all the seasonal variations (if any).

For marginal bearing soils that are waterlogged and soggy with differential freeze/thaw cycles - pretty much the worst case - there are not many long-term good solutions that are also inexpensive and easy to build.

Floating insulated structural slabs can pretty much ride on top of the muck but boy they eat up concrete and steel. Post and pier may work if you can get solid bearing but they may still require periodic shimming to keep things square and perhaps post replacement if you are using wood in wet soil.

Even if you have a foundation below frost depth you can get adhesive ice lenses that can lift posts and break concrete in wet clay soils. Very nasty, and another reason to backfill with drain rock and get that water away from the foundation.

But, maybe your soils are not as bad as these horror stories from Northern Canada describe. :D
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Offline lonelytree

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Re: foundation for wet soil
« Reply #10 on: December 30, 2007, 10:25:44 AM »
If it is an area that is low lying and full of vegetative matter you may have to either use piles driven in or excavate to solid ground and backfill. You may also have to put in some sort of french drains to funnel the water away. Either way could be a bit expensive.


Offline bobtheengineer

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Re: foundation for wet soil
« Reply #11 on: December 30, 2007, 06:01:57 PM »
Just stating what I've seen in the past.  The foundation will have to go below frost depth, which the local area, may well be 5 to 6' deep.  If that gets into groundwater, then what ever you put in that deep, will have to deal with the groundwater.  Concrete will work in that case, a treated post may last for a while, but not forever.  Another note, an aquifer by definition is water in groundwater, it may be a shallow aquifer, but its still an aquifer.

Offline ScottA

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Re: foundation for wet soil
« Reply #12 on: December 30, 2007, 06:28:06 PM »
This may be a dumb idea but what about simply accepting that the water is there and build the house on skids resting on a gravel bed so you can re-level it easily as needed. Make all the utility connections flexable so they won't break if the ground heaves.