What about using tires for foundations? In my opinion, earthbags are superior for foundations and walls. If you’re not convinced of this, tamp one tire (the way it’s supposed to be done, which takes 20 minutes or so). Then tamp the equivalent (in cubic inches) in earthbags and see for yourself which is easier. But it gets better, because lower courses can be filled with gravel (double bagged for durability). I could fill and stack about one-half to one whole course of earthbags on a small dome in the time it takes to tamp one tire. That’s a 10 to 20-fold improvement in speed! Swinging a sledge hammer is gut busting hard work. Tamping earthbags isn’t exactly “easy” but it’s way easier and faster than doing rammed earth tires. And with earthbags you can use insulated fill material such as scoria or perlite to create an insulated foundation.
I love the look of earthships and admire all the wonderful eco features, but I’ve learned how the same things (roofwater collection, solar hot water, etc.) can be integrated into a well designed earthbag home. With earthbags you can have these same features at a fraction of the labor and time.