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.
Lightweight structure
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.
Overburden
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.
Temporary structures
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.
Hybrid structure
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.
Plan B
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.
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