Hi, that's an interesting idea! I have a lot of
experience being around the construction of rammed earth buildings, and then living in them for decades. I'll throw out a few ideas and reactions here, some of which might not be relevant.
The ancient rammed earth in the Himalayas where I live was not very hard rammed. We have resurrected this building method in our region, as well as introducing European style rammed earth.
The ancient Himalayan style of rammed earth is a wetter mix, with a slightly higher ratio of clay, and not as vigorously rammed as the European style. We generally make frames no more than 2 feet high and wait a a day or two before removing the frame, and another few days before building another layer above the old one. They are stamped (tamped?) by hand with a wooden or iron stamper as hard as possible. These walls take months to dry out fully, though they are damn strong from a week or so after construction. It requires plastering for aesthetics if you want to live in it, and I love living in it. Thermally it's amazing, and the acoustic and humidity-regulating qualities are nice too. Our buildings are cool in summer, warm in winter. There are ruins standing in my region that have not been inhabited or maintained for centuries and they are eroding very slowly, even though the wooden roof and lintels were poached ages ago. In my region this style of building has been supplanted by adobe bricks for a few centuries, and the old rammed earth style only used for garden walls and stuff like that. But I'm very fond of it and chose it for my personal house.
The European style of rammed earth is much denser, a slightly higher ratio of sand, a drier mix going in, and rammed to a much higher density (so less insulating, more conductive, almost like
concrete or stone). They are stronger and thus can be thinner, but then you lose some of those lovely thermal qualities (as well as some of the nice acoustic and humidity-regulating qualities). They can be made with plywood formwork and then if carefully planned, can be beautiful without any plaster. Wiring, plumbing, and everything else have to be planned before starting to avoid plaster patching, and I would recommend using an architect. Modern ones often incorporate decorative sedimentary-looking layers of different colored media.
There's also the Earthship style, which I have no experience of. It rams the mix into stacked tires, which stay in place, and then are plastered over. Like your idea, the formwork remains.
Ramming the earth in, in either style that I'm familiar with, puts pressure outward on the formwork. So your wattle will probably bulge outward, which might be beautiful if perfectly planned and executed with high quality sticks, but might look messy and uneven with broken sticks sticking out if not. On the other hand,
cob isn't rammed and it seems to be strong too, so if you're gently filling the wattle frame with cob, maybe it won't bulge much.
Leaving the framework in place may further delay the drying period, which for our 1.5 foot (45 cm) walls already tends to take at least half a year. (We always end up rushing to finish in autumn before the building season ends, so our walls usually stay damp through the first winter).
Rammed earth has great structural strength (mostly compressive), so the wattle framework is not structurally necessary once the earth dries. Keeping the wattle in place will use a huge amount of sticks. In my region those are too valuable. Do you have a huge supply of good sticks free or affordable?
What would be your tactic for covering the gaps between the wattle so the medium doesn't squish out like
toothpaste? Would you first daub and dry a layer on the inside of the wattle, and later fill it? Will the wall be wide
enough to let you in to do that?
My understanding of wattle and daub is that it is not stamped or rammed at all. It's a wattle structure, plastered with mud as thick as you like. The structure is the wattle, which may have some bend and sway, which may let the earth covering crack over time.
In the image you give, I don't think those red lines will ever be that straight with woven wattle. It's also huge, like fortifications, so it could have both surfaces made of wattle, and daubed or plastered comfortably from inside before filling with earth. That seems like it would require a huge team of slave labor, and/or big machinery that could haul the fill in, mix it, and then lift and dump it from above. If on the other hand you want your wall to be only 1 to 2 feet thick, how would you get inside to plaster the inside of the second side?
I'm such a big fan of earth buildings as is, and I live in a region where biomass and good straight sticks are in short supply, so I don't see value in keeping the wattle or other formwork in place. However every location has different opportunities and constraints, as well as different end uses, so it might be good for your situation.
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