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

[Speireag Alden]

Sgrìobh Robert Tom:

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

     nb:  and since some moisture is always present, in order to heave 
it has to be enough moisture that when it expands on freezing, it 
over-fills any available voids.

>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.

     This is the part I've always been skeptical of.  I've no doubt 
that an RTF will move about a tiny bit, but I've always suspected 
that it's not enough to make any difference in practice.  I have 
read, though not from primary sources, that Frank Lloyd Wright, who 
used rubble trench foundations, tried to use them without driving 
them all the way to frost depth, relying on this movement to prevent 
heave damage, and got heave damage anyway.

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

     I agree.

     In addition, any house necessarily raises the frost line around 
it, even if there's no intentional heat input.

     So, unless you leave the windows and doors open all winter, the 
house acts to retard frost penetration into the ground under and 
around the house.  There is still some small amount of solar gain, 
and once the temperature of the house gets below the heat of the 
ground under it, the house itself acts as an insulation buffer for 
the ground it sits on.

     In my area, during a cold winter, frost can penetrate down as far 
as seven feet under plowed roads, but the frost line for building 
purposes is five feet.

-Speireag.

-- 
A wise man will make haste to forgive, because he knows the true 
value of time, and will not suffer it to pass away in unnecessary 
pain.
--Samuel Johnson, lexicographer (1709-1784)