What about weight? I have a basement, so I'd be building the heater over the basement on a floor suspended by basement rafters. Has anyone done this?
Also, I called a local excavator and tried to explain about cobb and asked about clayish soil. He said what they term as "Berm" soil would probably fill the ticket. My land is rock. I can hardly plant a petunia, so I gotta get it from someone.
Weight is definitely an issue. It can be possible to build a RMH over a wood-framed floor, but it needs to be assessed by someone knowledgeable to be safe.If you can run the bench mass crosswise to the joists so that there are a dozen or more joists sharing the load, that helps. Running the bench along a foundation wall line so the load is near one end of the joists instead of the middle also helps. You may need to add some posts and possibly a beam beneath the joists to get enough stability. Code requires continuous masonry support, no wood, so if that concerns you, you may need to cut out part of the floor framing, resupport and add a footing and piers.
What you actually need to do depends on your particular floor structure and the size and shape of the RMH you want to build. How strong is your floor (does it bounce if you jump on it)?