[Cob] strength of cob arches

[Predrag Cvetkovic]

My friend and me wanted to build a small round garden building about 3m in
diameter in his yard in our small town- primarily to gain some building
experience and then to have some space for sitting.
But later, to avoid possible problems with building codes, we decided to
modify the idea of building to be only with holes for doors and windows: we
will not build doors and windows in. Maybe during winter we could put some
protective covers on holes and because there will be a hearth, it could be
possible to sit inside.

A small model (1:10 ratio) pictures explain better then words, you can see
it at my friends link:
http://picasaweb.google.com/miodrag1963/Cob
We think of a stone foundation and stone arch for the front door. And a
gentle slope reciprotial live - green roof with a thin layer or earth (up to
5cm).

So, finally, our question is: are cob arches enough strong to bear the roof
in our garden house? Should we use wooden pillars as a support (from the
foundation to upper wooden beams)? We will put these upper beams on the
wall, to bear the roof, but in the case of round building shape it is more
difficult to do that then with rectangular one. Or maybe to put few wooden
posts inside the wall? We could build thick cob walls (about 30cm) but would
it be enough strong because in a big part of a building we will have only
cob arches and cob pillars instead of cob wall?
Hope that my question was clear enough. Any comment and suggestion are
welcome.
Predrag