Thank you for everyone's contributions on this thread, which I'm finding very useful for consolidating my knowledge of foundations. Foundations used to bother me as a natural builder because most rubble trench foundations I found on the Internet seemed to need a reinforced concrete beam that floats on top of the rubble, and I wanted to make foundations without any cement, and without the ubiquitous underfloor plastic sheeting too! In particular, I wanted to build a couple of small natural cabins on my land here in Guatemala.
We have high rainfall for half the year and I'd seen too many poorly built local houses literally sucking up all the water from the ground around them and causing all sorts of moisture-related problems like black mould, rusted rebar, blistering paint and rotting wood, and indeed health issues for their residents - asthma and chronic respiratory conditions in particular.
So as a largely self-taught builder, I found out about french drains and acquired a simplistic understanding of the basic principle that moisture isn't very well transmitted through air, at least not well enough to be a problem for buildings. Rocks do a pretty good job of stopping moisture transfer too, or at least the non-porous ones do.
I live in a valley that is made up of mostly debris from landslides, which means lots of rocks, gravel and sand of all sizes. "Use what you have" being my first principle for starting designing, I had lots of big rocks and my pet monster: a Milwaukee rotary hammer drill. So I figured I could put each of the four columns on top of a large rock and tie it in with a piece of 3/8" rebar set into the rock with a negligible amount of concrete mix (less than half a soda can of it). I shaped the top of the rebar into a hook that hides inside the bamboo and then drive a long 1/2" bolt through the bamboo and the hook, so as to stop the whole cabin from lifting off in high winds. Many folks fill the first couple of internodes of the bamboo with concrete but I really don't find it necessary or even desirable for such small constructions.
It took a while to dig the holes and get the big foundation stones level (each one weighs around 200-400lbs), and yet longer to dig out all the dirt from around and under the house and then replace it with small rocks covered by a layer of gravel, but once it was done I realised I'd come up with a very low-tech and effective way of insulating my house from the very damp earth beneath it. It didn't take too long - nor a huge leap in humility - to realise that many others before me had probably come up with exactly the same solution.
This was a couple of years ago now, yet I didn't come across some concrete examples (pun definitely intended) of this kind of foundation until reading Jay's first post on this thread, which came as a big vindication of my reinvention. So a big thank you for that Jay, not to mention all the excellent kanji image searches you provide.
Nonetheless, I have some further questions about these foundations, in particular: we have a LOT of hills and mountains here and I'd like to know how to build this kind of natural foundation on a steepish slope. One idea I have is to excavate a large hole, dump a load of gravel followed by a big thick, flat rock, pop the wooden post on top of that and then fill in the rest of the hole with a thick surrounding layer of gravel - is that how it's done? Does anyone have any other ideas? Previously I would just do it with a big concrete footing, but I'm determined to cure myself of this addiction! (Much as I love it…)
Furthermore, how long do these rock and gravel foundations last? Obviously that depends on rainfall and soils, but won't it eventually silt up? Does the soil creep into the gaps, eventually touching the building's other components and compromising their dryness? Is it worth putting geotextile down? I'm broadly against using synthetic materials but accept that sometimes they prolong the life of a building so effectively that its carbon footprint might even be halved.
One more difficulty I have with foundations is that I can find very few pictures and people's descriptions are often ambiguous and confusing. I would love to see more cross-sections of natural foundations, so here for starters is a diagram of the cabin foundations that I built (with the help of several others!), and a photo of what it looked like a few days before we built on top of it. You can just make out the four corners. The big rock in the middle was removed and the dirt replaced with gravel.
Regarding the discussion on modernity, I am at times a wilderness-loving hermit and at others a technology-marvelling geek. I lived the best part of my life in the heart of London in a century-old brick house with lime and rock foundations, which developed problems because of all the concrete applied around and in them in later additions and modifications - a good example of poor application of modern innovations. I've also watched the extraordinary spectacle of whole tower blocks built in the 1950's being dynamited and disappearing into dust in a second (only to make way for yet more tower blocks!) So I like to think we can still build relatively naturally and well in urban settings, and it's more a question of challenging ourselves to do so.
One further line of enquiry that I'm longing to pursue is reinventing Roman cement - their cement was much longer lasting than Portland cement and yet we continue to use Portland cement in vast quantities instead of developing a more stable version of it. This has been posted elsewhere but it seems relevant to post it here again: http://phys.org/news/2013-06-roman-seawater-concrete-secret-carbon.html
That said, I am looking forward to building lots more concrete-free foundations and learning ever better ways in which to do so. Thanks for your help with this.
With best regards,