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[Cob] foundation question

Shannon Dealy dealy at
Fri Aug 29 00:31:52 CDT 2008

On Fri, 29 Aug 2008, Bob Smolen wrote:

> Shannon,
> Thanks for the detailed explanation. I assume you think using  earth 
> filled bags as a foundation is not a good idea in a cold  climate?
> Bob

Not sure if you meant this quite the way it came out, but to clarify:

I am not aware of anyone using "earth filled" bags as a foundation, some 
people use the "earth bag" technique, but fill it with rock for the first 
few courses in order to provide a foundation and moisture break.

With regard to using it as a foundation in a cold climate, if it is filled 
with clean rock (no earth/fines) and set into a trench that goes below the 
frost line, I would have no problem with using it for one of my projects. 
If it is used as the foundation for a garden wall or other non-structural 
use, I would have no problem with using it above the frost line, even just 
set right on the surface of the ground.  The wall might crack a bit, but 
so what, you will have to patch the plaster about as often as the cracks 
are likely to appear any way and as the wall isn't structural, it doesn't 
really matter.  As far as using "earth bags" filled with rock under a cob 
wall but above the frost line, I would view that as experimental and as 
such would only use it for a building I was prepared to experiment on, 
fully recognizing the risks of damage to the structure over time.  This 
wouldn't bother me since I often experiment, but most people aren't as 
crazy as I am and are unlikely to put up a cob building expecting to throw 
it away "just to see what will happen" :-)

It might work out without problems, particularly if the ground you are 
building on is not to hard and the rock in the bags is not to large, if 
frost heave occurs, the rock in the bags if it isn't to large may shift 
somewhat and redistribute the load, and if the soil below is somewhat 
soft, it may give a bit evening out the load, I find both of the 
possibilities particularly likely for a cob or other mass structure given 
the weight of the walls, where a lighter structure might be more likely to 
shift and twist when frost heave occurs.

NOTE: It is not always necessary to go below the frost line to 
build a foundation, there are approaches using insulation and other 
techniques to prevent frost heave, there is no reason some of these 
couldn't be applied to an earth bag type approach.  The main thing 
is that one of the follow three things must must be true to prevent 

   - The ground under your foundation can't freeze

   - The ground under your foundation must be dry enough relative to
     the granularity of the soil that it won't expand if it freezes

   - Your foundation/structure must be designed to compensate for the
     ground expanding when it freezes

If one of these things is true, then you don't have a problem.  Someone 
posted to the list some information about a google search or links on 
shallow/frost protected foundations the other day which you may want to 


Shannon C. Dealy      |               DeaTech Research Inc.
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