I get comments from people wondering how clay-straw mix called 'cob' hold's-up to rain during construction (prior to receiving a lime based plaster on the exterior walls). The answer is quite well. We had about 8-10" of monsoon rains here in southern Arizona this summer and their was minimal erosion to the walls.
The clay on my land contains almost no sand and I don't add any to my mix. I just add more straw to account for the expansive properties of the clay. Cob is a fantastic building material.