cob mix cracking....looking for advice! pictures included

hi y'all.
I made a mix to start building up the heigh of the foundation and provide a containment ring for the oven.

clay source: a brick company's clay soil. I was told it was close to 80% clay

I did a 2 parts sand to 1 part clay mix

didn't measure the amount of straw.

the result was really hard but also sort of easy to pull off of the stones.
not enough sand?
not enough straw?

thank you!!!

To much clay will make the cob shrink more in the drying process, and be more susceptible to cracks. If there is too much clay in your soil try adding more sand.

This research paper studied various mixes of Oregon cob, comparing strength of the mixes. From what I recall visiting the Cob Cottage Company a few years back, their preferred ratio was 3 parts sand to 1 part clay soil, and I don't recall if it was 1 part, 2 parts, or as much as possible for straw. The linked study found the more sand, the greater the strength, as well as straw/hay of at least 6 inches in length.

Other sites I've seen suggest 2 parts sand to 1 part clay/soil, and "enough" straw so every handful has fibers.