Glenn Herbert

If the dirt/duff layer is on top of a membrane, I would guess that the dirt is expected to be non-pokey, while the duff may contain twigs or other material that could puncture a membrane. Of course, that presumes the "dirt" in the location does not have sharp stones or gravel in it.

If driving equipment over the structure to place material, duff would easily shift and possibly tear up whatever is beneath it, while dirt would stay in place unless bulldozed.

You can probably eliminate half of the counties simply for being too urban or other considerations. Look at the rest and see whether they have positive characteristics that you might like. Then start calling the ones you feel best about, and expand your inquiries as needed.

Using a shorty core this way certainly makes the oven floor better. However, I think it degrades the bell performance. With the core riser tight to the right and back walls, and wood storage below the core, there is no way for most of the ISA to get heat beside conduction or radiation from the core shell. If you move the core 2 or 3 inches left, make a slot at the right edge of the oven floor above this gap, and lower the wood storage to allow airflow below the core, you will get free flow in all directions. There will still be plenty of space to the left of the core.

I see, a cottage rocket type of unit.

The burn tunnel doesn't need to be built smaller than everything else, just not bigger. Ash will naturally decrease the height a bit during use.

Unless you have a dry climate, fir or other softwood partially embedded in the ground will probably start rotting in a few years, so I would plan for easily replaceable parts. Fir pins, while an attractive idea, will rot even faster. Do you have any black locust you could use for stakes to hold the risers? Osage orange and some other species are also supposed to be rot resistant.

To drive wooden pins or stakes, unless the soil is soft and rock-free, you really need to make pilot holes first. A digging bar (around 4-5' long and heavy enough to drop and drive a hole with its own weight), wiggled sideways at each drop to make the hole open enough to continue progress, will give a path that a stake can be driven into. If you have to work while the soil is dry and hard, water poured into the hole and allowed to sit for a bit will make it much easier to drive a good hole. For a stake projecting around 8" from the ground to hold a 10" riser, I would recommend setting the stake no less than a foot deep; 16" would be better. Making the hole deeper than this lets the stake drive easier.

Titebond III is good for damp or wet locations... something that should never occur in an interior finished floor. Titebond II is excellent and should work fine for the long term in your application. It has held up for decades in some exterior structures I have built. I can't speak about other glues in this application.

If you are cutting all the pieces into 3/4" end grain sections, I can't see expansion and contraction being a serious issue - the wide pieces will just crack internally if they need to shrink a lot. There will be lots of closely spaced joints, so no place for stresses to build up without relief points. Let them all acclimate to the space before gluing, as for any flooring. I presume you will be gluing the pieces to plywood subfloor; wood planking would definitely have movement issues.  

If the hot water pipe goes down after exiting the collector, air locks are possible at that point. You would need an air vent that would release any trapped air. The same goes for any high point in the circuit.

But if the hot pipe drops between collector and storage, you will lose much of the thermosiphon effect. You might be okay if you have large, highly insulated piping and a strong total gradient, but it would be much safer and more reliable if the pipe only rises to the storage tank. You would also need to overcome the drop bottleneck before the loop heats up, with a powerful rising effect out of the collector. Is it not possible to have the collector mounted on or next to the house? Failing that, a bridge beam that carries the insulated hot pipe overhead to the house would work.

The design appears to be a black oven, so the oven ceiling is the bell ceiling.

Even if the oven floor were brick, it would only absorb heat at startup and soon become saturated, no longer counting as ISA. As a kiln shelf or similar, it will have even less mass and be essentially useless as ISA.

I think it would be beneficial for heat flow generally to have at least one of the shelf wings next to the riser open, so heat could easily circulate to the bell side next to the riser.

It is common to have the feed tube about twice as high as the system dimension (in US terms, 8" system and 16" feed, floor to top), as this is the standard firewood length. Other countries may have different, even multiple, standards for firewood. Starting from a widely available wood size will make operation easier. You want the firewood to fit completely inside the feed tube so you can cover it and smother the flames in case of emergency. Also, double the burn tunnel height gives a vertical intake draft, which then has a sharp 90 degree bend to introduce beneficial turbulence and gas mixing.

15cm is about equal to 6". I would make the feed height at least 30cm, which is still very short. Do you have a strict limit on how tall the riser can be? I have found that 1:1.5:3 works fine in my 8" system. The actual dimensions of my feed and burn tunnel are 7" square, per first-generation RMH advice to keep cross section exactly equal, before it was widely known that a square is functionally equivalent to a circle of the same diameter for gas flow. A 30cm feed and a 90cm riser would still be very compact; even 40 and 120 is not very tall.

I think a chimney cap damper would depend on the situation. For a larger flue like 8 x 8 or more that a fireplace would have, it could be very useful even if the bottom was closed off. For a 6" diameter flue, air might not be able to circulate within the height of it so a top damper might not make a difference. An exterior chimney would get cold full height whether or not it had a damper on top.