Daniel: that is wonderful news!! Have you noticed any structural or cosmetic issues with the gravel (crumbling/excessive cracking)? What was the approximate ratio of sand and gravel? Were there any surfacing issues with the large gravel sticking out after trimming/shaping the surface, or did they push in easily? (or did you mix this separately as infill for the middle of the bench only?)
I did 4 buckets of clayey soil and 1 bucket of gravel. The clayey soil is 20% clay 80% sandy soil. I didn't think it would be the greatest for finishing anything so I only used it for the "inside" of the bench where it was surrounded by normal cob. It is completely dry and doesn't show any issues. I only did it because I had a small pile of gravel getting in my way, but I always bury garbage material in my cob. My bathroom wall has a broken waffle iron somewhere inside.
You could try out a few cobs using different proportions of your available materials, let them dry, and see how you like them. Kick them, rub them, pour water on them, see if they crack while drying (though a drier mix may help), etc. That's generally a good way to prepare for earth building on a new site.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
Concrete is made with gravel for a reason - aside from economizing on the processed cement component, the aggregate makes the structure stronger. My native clay (glacier-deposited) is well over half sand and gravel and rocks; I just pick out the stones that are too big, depending on the intended use, and the rest is perfect cob. Getting a finished surface is the issue I have, and I do find that it is easy to push the stones that are too large into the mass while the cob is still pliable.If you are adding a harder finishing surface like lime plaster, having stones sticking out a bit (less than about half the desired finish layer thickness) gives good tooth to connect the finish layer to the base.
Thanks Glenn, John, for your input; we seem to be building consensus here, and your experiences are very encouraging - when I'm ready to start attempting construction in a few years(small shed that doesn't need a permit) I'll make some test blocks with the DG just to confirm, but it looks like it should work well!
I currently live among the loess deposits on either side of the Missouri River basin. Has anyone had experience making cob with loess subsoils? I understand that along the Yangtze River basin houses and other structures are traditionally excavated out of loess soils.
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