Teresa said: Im thinking a wide trench filled with 3/4" rock for the foundation and then rammed silty sand (my native soil) with about 5% cement (or putting this mixture into bags) about a foot high from grade level.
here is a similar great idea I saw in several cob houses at Emerald Earth community in N. Ca. www.emeraldearth.org
they did a rubble trench, and at the ground level they took long planks of local harvested wood from trees on their property. (This is redwood, so strong, and resistant to water, bugs, etc., soft wood, but that's OK).. and built a "form box' all around the perimeter.. a 2' wide box that sits directly on the stones under it, and on the ground. then they fill in more stones as the capillary break and have a 1 ft tall footer to build on. then start the cob on top, and have a very strong very cheap foundation.
they said when they replace a random plank that rots, ( every 20 years or so) that the stones roll out. I suggested using an earth bag first-- or lay a wire cage lining the form box FIRST before filling it with stones, they they dont come out on a repair. metal baskets made of wire are called "gabions" and are used under straw bale homes, I've worked around them, and unless they are custom made (ti fit under the bale exactly) you are banging/scraping your ankles against the metal edges while plastering the straw. you can buy 'off the shelf' but they cost money, ( hilfiker.com to see examples) and so the wood form is a cheaper way.
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Also your idea to add more cement to use PC as a load bearing column wont be safe, as real cement has a bundle of metal rods up the center for strength in holding a roof or second floor. so choose something appropriate,and let the pc do all the other good stuff it it can be used for.
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