Having considered my earlier note on the design of a sousterrain for a month, I'm now going to rip it up and start again.
Reasons for not building in concrete
The first reason for not building in concrete is obvious. The embodied energy is huge. To present a largely-concrete sousterran as an energy efficient or 'green' building is hard to justify to me, to the planning authorities, or to anyone else. However, there are two other, pragmatically more compelling, reasons not to use concrete.
The first is that I have limited experience of using concrete, and that, once constructed, a concrete structure is very hard to modify.
The second is that to have any guarantees of the integrity of a concrete structure, the material has to be of a consistent mix and individual modules have to be poured and cured essentially in one operation – if part of a module is part-cured before another part is poured, you will have lines of weakness. That's not a problem if you use readimix. However, my proposed site is quite a long way from any road that a readymix truck could comfortably use, and I have no plans to construct such a road (nor can I afford to). And, even if I had such a road, it passes under electricity lines which do not in my opinion have safe clearance for something as high as a readymix truck.
Reasons for building in timber
Timber is in principle a sustainable resource. Its processing does not embody very high energy inputs. It locks up atmospheric carbon for the lifetime of the structure. As a building material, it's as green as it gets. Furthermore, it has other advantages. It is (relatively) light, strong, and easy to work. If you want to modify a wooden structure, it is generally easy to do so. Its disadvantage is a corollary of its advantages: it is bio-degradable. In the long run, it rots, losing structural integrity and releasing the captured carbon back to the atmosphere.
However, there is are ways of working around the rot problem. One with which I'm very familiar is epoxy encapsulation Done right, provided the encapsulation is not breeched and the epoxy is not exposed to sunlight, it prevents rot indefinitely. Of course epoxy is (at least at present) synthesised from fossil hydrocarbon. Furthermore it's at least conventional to use glass fibre as a reinforcing material in some parts of wood/epoxy structures, and in the case of the sousterran a layer of glass fibre cloth laid over the dome sections would help to prevent localised pressure points causing an encapsulation breech, while a tensile belt of glass fibre tape would prevent the dome spreading. However, the embodied energy represented by the epoxy and glass in a wood/epoxy composite structure is orders of magnitude less than that in concrete.
My original primary reason for not choosing timber is that if a structure is built underground and starts to rot, it seemed to me that it would be very hard to repair. But the truth is that this isn't going to be very far underground – a metre at most. In the event of problems, uncovering the problem area in order to repair it is not in fact a very big issue.
The original plan for the concrete structure was that it would be cast in components off site and then moved on-site for erection. In concrete this was somewhat implausible – individual components would weigh in tons, and would require specialist equipment to move. Boy Alex has such equipment, but even so it would require a substantial input of his time and equipment. But building in wood, it becomes much more practical. No component should weight more than at most 200Kg, meaning that it can be managed by a team of men and simple hoists. Building components off-site in the Void means that they can be built out of the weather, and consequently the encapsulated timber stays very dry.
The problem with building a lightweight timber sousterran is that actually you can't. If burying to an average depth of 1 metre, the overburden is about 1.7 tons per square metre, or 42 tons per dome. I'm finding it hard to believe that I can engineer a structure which can handle that amount of mass. So while a turf roof certainly is possible, and an overall finished shape that looks natural probably is possible.
One of the driving issues behind this note is this: before I have planning permission to build my permanent structure, I need to have somewhere to live. Furthermore, I'm now going to have very limited money – far too little to enable me to build my whole structure in one phase. I need a warm and weatherproof structure before next winter. I have considered buying a second-hand mobile home, or buying (or making) a yurt, or building a temporary cabin. Each of these would make a very considerable dent in my available funds. Why spend money on an essentially disposable structure, when I could spend it on part of my permanent structure?
The alternative is to build one dome of my permanent structure, and to add to it as funds become available. The problem is that it's going to take some time to get planning permission, that for a structure as radical as this anything which upsets the planners is going to make permission even harder to get, and that pre-empting planning permission is one well-known way to get planners' backs up.
However, if the structure is modular, and built of a (relatively) light material, it does not have to be initially erected in its final place. It could (with permission from my co-conspirators) even be erected temporarily inside the high-slat shed. Furthermore, building one dome early allows it to be treated as a prototype. In the process of building it, other, alternative ways of doing it better are bound to be learned.
A further advantage of having a prototype dome is that there is something physical to show to the planners and building regulations people, who are bound to be at least sceptical about the structure. Also, if they do insist on changes, only one dome needs to be changed.
Both this note and the previous one have essentially presented the concrete structure and the wooden structure as alternatives. It isn't necessarily so. Some degree of concrete foundation is likely to be necessary – even if it is only piles under the bases of the pillars. A pile-and-slab concrete foundation is also plausible.
Casting the pillars in concrete is relatively unproblematic – certainly very much easier than the flying buttresses or the domes. Building the walls in concrete blockwork is trivial (although it's easier to route services inside a wooden wall than inside a concrete one). Even the lintels joining the pillars are relatively simple to cast in concrete, whether they are cast in place or cast off-site.
So it would be entirely plausible to have a structure which was concrete with wooden domes, or concrete with wooden lintels and domes.
The flying buttresses in the original design then become an issue. With wooden domes having a tensile band, they aren't strictly necessary – the tensile band makes the dome self-supporting. But, sculpturally, I like them. They could be built in epoxy encapsulated timber; or they could be solid oak (which would be nice – a little expensive, but not dreadfully). Or they could be concrete. It's not a decision which needs to be made in a hurry.
Note that the hybrid structure doesn't work well with the idea with the idea of erecting the prototype dome off-site, and, for that reason, doesn't work well with the idea of getting a prototype dome up quickly to live in in the short term. However, if the pillars and walls of the prototype had to be sacrificed that is not necessarily a huge loss.
So, plan B is as follows.
- Start immediately (in March) to build one experimental dome in epoxy encapsulated timber with an intention to have it habitable by May
- Erect that in a suitable place 'off site' (and not earth sheltered) until planning permission has been obtained.
- When planning permission has been obtained, dig out the platform (the entire platform large enough for all four planned domes).
- Lay suitable foundations for one dome.
- Disassemble the prototype dome and re-erect it on-site.
- Back-fill over that dome only, leaving the remainder of the platform clear.
- Occupy that one dome, at least for winter 2011-2012; build other domes in a similar fashion at funds allow.