Cob wouldn't be my first thought, no. I would look at something whose material properties are better known under tension and compression.
You have the clay road road base mix. Why not make up a dry rammed earth mix and ram that down into the forms? If you add about 5% portland cement or equivalent for stability, it should do exactly what you're looking for.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
I would recommend the same as Chris. This is very similar to what I used. Built foundation with 2 wide ICF blocks, put up plywood forms to create 400mm thick wall. then did a rammed earth on that. Used a 3 to 5 % dry cement mixed with clay and sand. Slightly damped. Ended up with a 4 ' high rammed earth wall. This was all below grade. Then built aprox. 3' high cob on top of that. I did use vertical rebar between the rammed earth and cob.
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