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Shallow foundation in a cold climate i.e., MONTANA  RSS feed

 
Danette Cross
Posts: 73
Location: St. Ignatius, Montana, zone 5b
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OK, it's been 3 years and THIS year is the root cellar year, by God!  I have two good possible locations, each with their own challenges, but both are the side of a hill facing North.  So I go studying (again) on how to start with the foundation.  Where I live, the ground is loaded with rocks.  Seems every time I garden, I get a bumper crop! It's daunting.  Just to put anchor pins in to keep my hay shed from blowing away took an impact wrench and 24" carbide steel rod, which actually STOPPED at 18". No, that's not low flying aircraft, it's me, whining!

So I know my frost line sits at about 3' deep.  The thought of attempting to dig a 3' deep x 24" wide x 56'(total length) trench-works in an area that no backhoe can access, well, I just want to start  .  So me thinks, I will look until I finally read someplace, anyplace that says I don't have to do that and I will go with what they said! (Who's with me?!)  And sure enough, I found it on Earthbagbuilding.com/faqs/foundation.htm:

Would it work if I just dig about 1.5, 2 feet rubble trench and start just below grade instead of 4 feet (frost grade) with first 3, 4 rows of stabilized earth hydro-isolated on the outside, and an isolation skirt around the perimeter of the house to prevent freezing?

Yes, frost protected shallow foundations have been proven to be quite effective in eliminating the need for deep foundations and providing better thermal protection for the house.


So dig (well, pry rocks out of) a trench about 1.5 feet deep and I am GOLDEN!

Then the paranoid evil twin takes over my brain and says "You, my dear, are an idiot."

Thoughts?


 
Dale Walker
Posts: 19
Location: Starksboro, Vermont
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Some good info this doc:
http://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf

I know it deals with more "conventional" materials... but may provide some good insights.
 
Danette Cross
Posts: 73
Location: St. Ignatius, Montana, zone 5b
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Dale Walker wrote:Some good info this doc:
http://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf

I know it deals with more "conventional" materials... but may provide some good insights.


Great resource. Thank you!
 
Hans Quistorff
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Location: Longbranch, WA
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Remember frost heave comes from the expansion of water freezing. if it is dry it will not heave. So as you pry and dig consider whether water will get under the foundation and your plan to prevent it.
The worst frost heave I ever saw was on the military road south of Fort Kent Main which had a spring under it and it was kept plowed at all times. ye nearby were houses built on ledge rock close to the surface that never moved.
 
Danette Cross
Posts: 73
Location: St. Ignatius, Montana, zone 5b
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Hans Quistorff wrote:Remember frost heave comes from the expansion of water freezing. if it is dry it will not heave. So as you pry and dig consider whether water will get under the foundation and your plan to prevent it.
The worst frost heave I ever saw was on the military road south of Fort Kent Main which had a spring under it and it was kept plowed at all times. ye nearby were houses built on ledge rock close to the surface that never moved.


That's interesting to think about.  Both my house and my big metal building 36'x48' are built on 6" monolithic slabs. All they have under them is about 4" of crushed stone I believe, and they have been solid for over 20 years. I have wondered if trenching would actually disturb all that rock and make heaving more of a problem than it is from rocks that have settled and are packed tight naturally.  I would think a moisture barrier under and around the foundation would keep the water from getting in and under. Not sure how insulation would help anything in the case of a root cellar. I'm not heating the space.
 
John Schinnerer
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First off, a clarifying question...with a bit of a preamble:
You write this is for a root cellar. When I visualize root cellar, I visualize a dig into a hillside (or down into the ground, in flat country) that is intended to use the very stable earth temps below frost-line to keep a "root cellar" temperature year round. E.g. 55 deg. F or something in that range, maybe a bit cooler in your location, but probably not below 50 deg. F. The structure I visualize is only enough (height) to cover this excavation, assuming the hillside isn't steep enough for it to just be a cave with a Hobbit style door or hillside mine-shaft type entrance into it.

So, question is, am I misunderstanding what you are trying to do in terms of "root cellar?"

The "shallow foundation" doesn't seem to fit the named use.

If you build an unheated structure in St. Ignatius MT that does not go below frost line inside (by pure depth of excavation and/or earth-berm to raise frost line around it), I am pretty sure you will have a freezer in the winter, not a root cellar...regardless of whether your foundation heaves...

Or, perhaps, if you insulated a big enough area around the structure well enough, AND highly insulated the structure itself, you might create a PAHS type bubble and get sub-frost-line temps to persist inside. So there's a trade-off to consider, between excavating deep in your rocky backhoe-inaccessible ground, or buying a lot of petrol-based insulation for what would be IMO more of an experiment than a certainty. A third option, supporting shallower excavation, would be earth-berming around the structure to raise frost line (possibly with some PAHS type insulation around as well). Is it easier to move earth to berm with with than to dig deep, on your site?

Also of interest might be, how did the "old time" euro-american colonizers of that area build their root cellars, if any did, or anyone still remembers?
They didn't have backhoes or petroleum-based insulation.

I'll stop there, let me know if I'm totally missing your point/question or not, any other details that would clarify...thanks.
 
Danette Cross
Posts: 73
Location: St. Ignatius, Montana, zone 5b
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John Schinnerer: yes, I agree, a root cellar needs the earth to have a stabilized temperature., so my thought is, since the ground is almost a solid layer of rocks, hard packed, that if I go deep into the hill (where there are less rocks and more soil), then have the structure bermed on 3 sides completely with a deep soil roof, I should be able to "recreate" the in earth environment - I hope. I still plan on trying to dig as deep as possible, so some testing of that will come first.  It's more like using a large pry bar than digging.  But man, won't I have awesome rocks for the front exterior! So yes, my thought is to berm and use 2" rigid foam insulation around the foundation.

A note on the hill:  it is really more of a cliff about 15-18' feet in vertical height.  From the path, if I go straight into that hill, there is at least 7' of soil before you hit the surface if I dig a cellar 8' high.  There is at least 10' of soil on either side of the 10' area I will dig out.
 
Rez Zircon
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Location: Brendansport, Sagitta IV
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I'm near Billings. My front garden is a strip just below the raised front yard, against a wall of rocks and and dirt about 3 feet high. Apparently having the yard looming over that garden strip is sufficient to keep it from freezing hard (and there are always "chimneys" in the snow with warm air wafting up from between the rocks). If that little protection does that well, seems to me your idea is a good one and the hill should provide plenty of thermal mass.
 
G. Karl Marcus
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Danette,

Any relation to Wayne, from Camus Prairie?  Old friend from a previous life.

As I see the situation, you're planning to tunnel into alluvium.  Sounds like a mining venture.  You might want to consider how to stabilize the seven feet of overburden during construction.  As for protecting foundation walls from frost heave, the two side walls and rear wall should be sufficiently buried to prevent any disturbance from frost.  Only the base of the front wall, and perhaps a short return on the side walls will be close enough to the surface to be vulnerable.  If you go up Valley Creek a short ways and walk up the steep slope to the north, you'll find an old homestead.  Look for the lilacs.  Northwest of the house foundation you'll find a root cellar with a barrel-vault concrete roof.  It's pretty cool.  Are the rocks on your property predominately red?
 
Mark Clipsham
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It may not seem green but in this case a little bit of 2" rigid insulation goes a long way especially since you only have one face to protect - theoretically. We do FPFs a lot with my grain bin designs - also with the evil petroleum based pour foam - it is THE best product for this - we're not using gas in our Hummer to cruise the mall but to get the best building with the highest performance for the least labor which saves the most energy. I've done some "digging" such as you describe in Arkansas/lower Missouri - what a lesson in frustration. Maybe put an oversized overhang on your roof to get the water away from the foundation and a swale uphill to divert the water. In Kansas I have seen barrel vault cellars (storm/root) made of limestone - truly magical spaces especially in a hot summer. This is a labor of love - get started and do a little - do a little more - decide when you've had enough - think of it as therapy. You will get to the desired depth without the need for any band aides. You could use a section of a bulk bin as your vault and lay your stone on top of that (would recommend mortar if the stones are not a good shape for laying up) and then cover with your excavation material - it should be there for a long time. If you use the bulk bin for your form load it evenly from both sides and some bracing is a very good idea or it will become your final resting place. Be careful. You could build the vault with the bag method - see Dream Green Homes. Best wishes.
 
Danette Cross
Posts: 73
Location: St. Ignatius, Montana, zone 5b
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Mark Clipsham wrote:It may not seem green but in this case a little bit of 2" rigid insulation goes a long way especially since you only have one face to protect - theoretically. We do FPFs a lot with my grain bin designs - also with the evil petroleum based pour foam - it is THE best product for this - we're not using gas in our Hummer to cruise the mall but to get the best building with the highest performance for the least labor which saves the most energy. I've done some "digging" such as you describe in Arkansas/lower Missouri - what a lesson in frustration. Maybe put an oversized overhang on your roof to get the water away from the foundation and a swale uphill to divert the water. In Kansas I have seen barrel vault cellars (storm/root) made of limestone - truly magical spaces especially in a hot summer. This is a labor of love - get started and do a little - do a little more - decide when you've had enough - think of it as therapy. You will get to the desired depth without the need for any band aides. You could use a section of a bulk bin as your vault and lay your stone on top of that (would recommend mortar if the stones are not a good shape for laying up) and then cover with your excavation material - it should be there for a long time. If you use the bulk bin for your form load it evenly from both sides and some bracing is a very good idea or it will become your final resting place. Be careful. You could build the vault with the bag method - see Dream Green Homes. Best wishes.


I have some good instructions tucked away on how to frame a vault, so those will come in handy.  I also saw this product -Concrete Cloth!  Not "green" but when it comes to some types of construction that can be dangerous, I tend to give a little.  I wonder if I could Lay it over the vault frame.  But I think I may want to insulate the roof as well, besides the several feet of soil that will be replaced on top.  I plan on digging out the entire space, then replace the soil, no cave digging for me!!
 
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