[Strawbale] Frost Heave (was re: Too Late to Plaster?)

[Robert Tom]

On Tue, 13 Nov 2007 07:08:02 -0500, Speireag Alden <speireag at gmail.com>  
wrote:

> Hank Carr wrote:
>
>> With rubble trench and slab foundations the building should be heated
>> in the winter whether you are living in it or not to keep the ground  
>> under the building thawed and to prevent heaving and cracking.
>

> If the trench of rubble goes deeper than the frost line, then you
> should be all set even with an unheated structure.


In order for frost heave to occur, two conditions must be present:
(i) moisture in the substrate
(ii) freezing of the substrate

Eliminate one of the above and frost heave will not occur.

The drain for properly-constructed rubble trench foundations (RTF) should  
extend to below the frost line and as such, since the drain is not subject  
to freezing, will function to carry away water, even in winter, assuming  
that the drain tile and the trench were sloped properly and that an outlet  
for the drain was provided.  No standing water + no freezing of the soil  
supporting the foundation = no frost heave.

Second, the interstitial voids between the rubble allow for drainage and  
provide space for any residual moisture to expand, thereby eliminating  
expansion stresses within the rubble at any level.

Third, unlike rigid masonry or monlithic concrete structures, RTFs are  
"flexible" and able to withstand lateral expansion stresses that would be  
imparted to the material in the trench from adjacent soil that may be  
subject to frost expansion.

All of the above obviously eliminate one or more of the conditions  
necessary for frost heave to occur with properly designed/constructed  
RTFs, even in unheated buildings.

So if one has to heat a building with an RTF in order to prevent frost  
heave, then the RTF was designed/built improperly.


As for slab foundations, in 2007, it is simply unacceptable to design and  
construct an uninsulated residential slab (ie one that would allow  
substantial heat loss to the substrate beneath the slab to occur) in a  
cold climate region and to construct a slab foundation (insulated or not)  
on a poorly-drained substrate that would be subject to frost action  
demonstrates [insert your choice of descriptors for "poor judgement and/or  
incompetence" here], not to mention being a contravention of the Building  
Code in Code-regulated jurisdictions.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
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