[Cob] cob greenhouse question

[Malcolm Schluenderfritz]

 I am building a small greenhouse, 16 feet by 8 feet, and 8 feet tall. The
North and West walls will be cob, and about 18 inches thick. I will be
inserting some pieces of concrete as we go, in the center of the wall. The
walls are straight, not curved.
Do I need to add buttresses to this wall to keep it up, or is it thick
enough? The North and West walls already form a corner, so they provide at
least a little Lateral support to each other.