That's a good question, and the book doesn't get into alternative foundation systems. Although I would really like to see an alternative to concrete and or some combination of concrete and rubble emerge that has wide spread acceptance and is code compliant, there aren’t many options at this time.
As the book project evolved, we considered including what might be called "guerrilla" building practices--there are many places in a construction where someone could substitute materials or methods that may have some foundation in local tradition, or are locally established and effective, but otherwise haven't been tested, and aren't in the building code. Bob even coined a term for structures carefully assembled from simple, local, common materials (e.g. pallets)--he calls them "feral" straw bale buildings. But we decided against.
Straw bale construction already enjoys widespread support in the natural building world, and where/when builders aren't required to follow codes, they might make those substitutions. We hope those builders find the book useful, if only to let them know what the building code requires as it pertains to the straw bale wall assembly, and more importantly, why.
We also want to promote the use of straw in the more mainstream building world, because of the approximately one-million new houses built in North America last year, probably less than 5% were "green" (energy efficient but not necessarily made with natural building materials), and a very tiny fraction of those used straw or any other "natural" material in them apart from wood.
If we want to make a bigger difference for both healthier housing and a healthier planet, we need more than a few thousand new straw bale houses each year; we need tens and hundreds of thousands more!
So the book's focus is on the straw bale wall assembly, not on challenging other building norms, much as they need to be challenged. Using straw as insulation and plaster as thermal mass is already a lot for most mainstream builders and would-be straw bale building homeowners to embrace. but we need them to embrace it because that's where 99.99% of construction is today!
I recently shared with colleagues at the California Straw Building Association my list of top "unknowns" where I'd like to see more research funding, and one of them is an alternative foundation system, or at least less concrete intensive foundations. Since most thick wall construction (straw bale, light-straw-clay, cob, et. al., are quite heavy compared to more conventional wall systems using far less natural materials (SIPS, fiberglass, foams of any kind), the code level foundations for these natural buildings tend to be somewhat larger—and require more concrete, which reduces the net positive impact of using as much locally grown (straw, wood, hemp) or gathered (stone, clay) materials as possible in the building. As I mentioned in other posts, the book was an entirely volunteer effort by all the contributors—book royalties are earmarked for research and development to further the use of straw as a building material.
So, back to your question. We don’t get into rubble trench, grade beam, charred timber, or sand bag foundations. My understanding is that where building permits and codes apply, when an alternative foundation system is used a structural engineer who is familiar with the alternative system is involved with the design. The building code officials evaluating the permit want to know that someone has “done the math” to make sure the foundation can handle whatever loads are imposed on it (gravity, soil movement, frost heave, seismic, etc.)