[Cob] foundation question

[Shannon Dealy]

On Mon, 25 Aug 2008, Bob Smolen wrote:

> Time and again I read that a foundation must extend below the frost 
> line to avoid frost heave. I also read that a floating slab is suitable 
> for building a structure. These slabs dont always extend past the frost 
> line. The posts in the old barn on my place was supported by piles of 
> rocks placed on the surface of the earth . The barn  failed after 100 
> years because the roof was not maintained.
[snip]
> that rubble foundations are acceptable for buildings.Do rubble 
> foundations have to extend below the frost line? Can someone explain how 
[snip]
> frost?  Have studies actually been done to prove that extending below 
> the frost line prevents heave more than a foundation above the frost 
> line. Why was my barn able to survive 100 years sitting on loosely piled 
> field rocks? Why are earth bags suitable for foundations when placed at 
> very shallow depths? Is it all dependent on location? It is all a bit 
> confusing. Is the advice based on science ?

Hi Bob,

Frost heave occurs when moisture trapped under the support for a structure 
freezes and causes the building to shift.  When water freezes, it expands 
and can destroy just about anything which traps it and can't move.  This 
one way that rocks are often broken down by nature over time, moisture 
(from rain, humidity, flooding, etc.) gets trapped in a tiny (even 
microscopic crack) in the rock, then when the weather gets cold it 
freezes, expanding the water which forces the crack open wider. 
Eventually with enough cycles of moisture filling the crack and freezing, 
the crack is pushed wider and wider until the rock breaks.  In the same 
manner, moisture trapped in the soil under a building can freeze and 
expand, lifting up the building.  Simply lifting the building would not 
necessarily be a problem, however, it is not uniform, some parts of the 
area supporting the structure may not have sufficient moisture trapped 
under it to cause expansion when it freezes, and different areas may 
freeze at different times or some areas may not freeze at all.  This 
results in different forces acting on different areas of the building, so 
one part of your foundation may be holding up a much larger section of the 
structure than it was designed for, while another one next to it may not 
be supporting the structure at all.

The maximum frost depth for an area is the worst case depth at which the 
ground freezes, so if your foundation extends below that level, the soil 
it is resting on will not freeze and expand, so frost heave won't occur.
With regard to the rubble trench, the rubble is compacted and a building 
is sitting on top of it, so the rubble is no more free to move than a 
solid foundation, and it still needs to extend below maximum frost depth.
Speculating here: I would expect that were frost heave to occur in a 
rubble trench, the rock would be able to shift laterally somewhat and 
better distribute the load so that the effect might not be as severe as 
with a cement and steel foundation.

With regard to your barn, it is important to remember that the freezing of 
the ground comes from air temperatures dropping, and a building over an 
area is going to heavily insulate the ground underneath it and will also 
tend to keep the ground dryer (since it's not getting rained on), so posts 
set into the ground inside a barn well away from the walls are unlikely to 
experience frost heave in all but the coldest climates, the primary 
concern is at the perimeter of the building where the ground will freeze, 
so generally the foundation will extend down to maximum frost depth at the 
perimeter, possibly with insulation used around the edges to keep freezing 
cold from penetrating beyound the perimeter to the core.

Getting back to why your barn lasted so long, aside from the inner 
supports probably not experiencing heave, old barns generally have ALOT of 
give to them, so they can shift quite a bit without much in the way of 
structural issues.  A modern house is much more rigid and most people 
wouldn't find cracks in their interior wall surfaces acceptable, where a 
barn doesn't even have finished interior walls.

As far as earthbags being acceptable at very shallow depths, I'm not aware 
that they are, though when used as foundations, they are filled with rock 
not earth, and are therefore, more like a rubble trench, so they may as I 
speculated above shift somewhat laterally when heave occurs, lessening the 
effect.

With regard to floating slab foundations, if they are in fact not 
extending down below frost depth at the perimeter, I would speculate that 
they simply build the slab strong enough that it won't crack when the 
effects of frost heave cause one part of the foundation to take up more of 
the load than other areas.  I don't know since I haven't worked with a 
"floating slab", but that would be my guess.

And yes, frost heave is a well documented and researched problem.

One thing to keep in mind if you see something new / unusual being done 
like earth bags at shallow depth, it may be somewhat experimental in 
nature, and may or may not have problems down the road.  Also, the fact 
that something is being done does not necessarily mean it is "acceptable" 
since many areas don't have codes, and even in those that do, many 
alternative buildings get built without review or inspections by local 
building departments.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
Phone: (800) 467-5820 |          - Natural Building Instruction -
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