[Cob] Foundation depth for garden walls?

[Thomas Gorman]

Lynn-
My reading of the post from Toronto that you refer to is that the  
gravel in the trench came UP to a point 6" below grade and doesn't  
mention how deep the trench actually is.  Conventional wisdom is that  
you want your rubble trench to be dug down below the frost line for  
your area.  The landscape fabric is to keep dirt and silt from  
infiltrating the gravel and inhibiting it's ability to drain water.

I just did a workshop where we built a garden wall.  The foundation  
and stem wall was prepared by the owner of the property on which the  
wall was being built and unfortunately was only about a foot deep and  
the stone stem wall was short and not very sturdily laid.   The whole  
wall would rock back and forth in an unsettling manner when  
vigorously laying the cob as it's height increased.  Sadly, I'd be  
surprised if the wall lasts more than a year or so- it was located at  
the bottom of a hill where water is going to collect, soak the ground  
under the foundation and freeze, which will unevenly heave the wall  
leading to failure.   If you don't want to worry about frost heave  
cracking and maybe even tumbling your wall, dig a proper foundation  
for it!  It might even be more critical to get well below the frost  
line in a garden wall than in a house foundation- a lived-in house  
will be heated, essentially raising the frost line depth a bit, but a  
garden wall foundation will be surrounded by the cold.  You want any  
water out from under there!

Tom