However
the alternative, given that we have reasonably steep slopes, is to
get down out of the wind. And if you do that you also lessen the
landscape impact of the dwelling dramatically.
We
all need structures which are low cost and simple to erect (since
we're likely to be using mostly our own labour).
My
suggested response to this set of problems is to dig into the
hillside, build structure, and backfill over it. The modules I'm
suggesting are hexagonal, simply because I like the shapes
sculpturally – cross vaults might be more practical, because you'd
get a rectangular floor plan, but you'd also end up with straight
lines on the hillside which would be more visually intrusive. One
advantage of the hexagonal module is that you can put the modules
together to follow the contour in a more flexible way. The
approximate size of the module is 2.4 metres per side, or 4.8 metres
diameter. This gives a floor area of 15 square metres (about 150
square feet) per module.
Note
that I'm guessing that this structure would be low cost – I haven't
had it costed by anyone remotely qualified to do so.
Sustainable?
This
proposal is mainly concrete. I would have preferred timber but am not
confident I can design in timber in a way that would be durable and
maintainable under ground. As such there's a substantial energy cost
in making the cement. There's also embodied energy in vermiculite
used as filler. Overall, though, it isn't hugely much concrete –
probably not as much per living unit as in a conventional block of
flats. Because there's no wind exposure, and all the exposed sides
are oriented towards the sun, it should have adequate passive heating
even in winter; any supplementary space heating needed, water heating
and cooking can be provided by a stove burning fuelwood grown on
site. Large opening windows allow for cooling in summer.
Visual impact
One
of the main objectives of this design is to minimise visual impact on
the landscape. Because the terrace retaining wall is half the height
of exposed sides of the structure, and because the slope above the
structure matches the slope below the structure, from upslope or
downslope there will be very little to see until you're very close.
From
further away there will be a horizontal visible feature in the slope,
but because all the curves are organic it shouldn't look too visually
intrusive or artificial.
Elements
The
building is constructed of four different concrete elements, as
follows:
Dome segment
Obviously,
six dome segments are needed for each dome. These will be the
heaviest elements, and most difficult to move; it would be preferable
to cast them in place, by erecting the pillars first and then
erecting the former where the dome segment is to be located. Even if
the segments are cast in place, however, they probably need to be
separate segments rather than a single monolithic dome because of
expansion issues. The cross section of the dome is probably a
catenary, rather than an arc.
Pillar
One
pillar at each corner of each dome, obviously. Hollow pillars –
even if they do not contain drains – will be lighter and easier to
install without being substantially weaker, so the core of the pillar
element might be a spun concrete pipe section. The pillars need only
be two metres tall because the natural shape of the edge of the dome
will in any case form a slight arch.
Flying Buttress
Domes
have thrust: they want to fall down, and in doing so they push out
radially at their base. This thrust has to be resolved. In the
unexposed edges of the dome (which, when building into the hillside,
is most of them) that thrust is resolved simply by backfilling the
soil. On the exposed edges – which we want, because we want light
an air into the dwelling – the nature of hexagonal cells is that
some corners are convex, and some are concave. How many of which you
have depends on the shape of the site. On each convex corner you need
something to take the thrust of the dome, and my choice it to have a
flying buttress. This would span a terrace in front of the building
and transfer load to its retaining wall.
Eve
The
eve element is essentially just cosmetic; it joins adjacent flying
buttress or eve elements into a continuous organic curve, again
helping to reduce straight lines in the landscape. The eve units are
designed so that although they show a fair curve on the outside they
have a horizontal on the inside, so that ordinary standard patio door
double glazed units can be used in all the exposed sides.
Building method
Again,
this needs to be run past people with more experience than I have.
Preventing damp ingress into the dwelling needs to be thought about
carefully.
Dig back into the hillside
Strip
off the turf and retain it. With a mechanical digger, dig back into
the hillside; obviously, choose a site with contours which minimise
the amount of digging that needs to be done. Reserve the soil
removed, we'll need it later. Reserve topsoil separately from
subsoil. The rock on site is part of the Kirkcudbright Shales and is
probably soft enough to break up with a digger, is actual rock needs
to be removed.
Lay drains
Lay
drains under the site from uphill to downhill; these will later be
joined to lateral collectors on the uphill side to reduce water
pressure uphill. Rainwater drains can simply discharge downslope –
they're only carrying water which would naturally drain down the
slope anyway. A separate soil drain needs to lead to some suitable
treatment.
Level platform
Level
roughly with hardcore; finish with sand.
Pour slab
Taking
one hexagonal module at a time, lay a waterproof membrane on the sand
with at least 2.5 metres overlap on the uphill side, shutter the
hexagon, and pour a self-levelling concrete. I need advice on the
thickness of the slab. It almost certainly needs to be thicker under
the pillars.
Erect pillars
It's
possible that the pillars should be hollow with a drain down the
centre of each one, linked into the under-slab drains, to move water
from over the structure to under it. However, there's an issue about
how to achieve a watertight seal at the top of these, so unless
someone else can solve it that isn't part of the plan. In any case
Pete suggests that the pillars should be founded on concrete piles
going down to bedrock.
Erect uphill walls
Between
each pair of pillars on sides which will be backfilled, erect a
breeze block wall.
Erect terrace retaining wall
In
front of the structure along edges that are to be exposed, erect a
dry-stane dyke about a metre high and about two to three metres away
from the structure, with footings in the right places for the flying
buttresses. The exact width of the terrace at any point should be
chosen so that when the structure is complete and backfilled, the
slope above the building should continue as a fair curve in the slope
below the building.
Part-backfill the uphill side
Bring
the excess waterproof membrane laid down when pouring the slab up the
sides of the newly erected uphill walls. Backfill, installing lateral
drains as you go. At this stage, backfill to about 50cm below the
tops of the walls.
Erect flying buttresses, eves and (possibly) lintel arches
In
order to resolve the thrust on the domes, the flying buttresses and
eves must be in place before the domes are cast – otherwise they'll
just fall down as soon as the armature is removed. However, not all
the pillars are supported against thrust in all the necessary
directions until all the domes are up – to some extent, once
completed the domes will support one another. One solution to this
would be to cast lintels between each pair of adjacent pillars. If
this solution is chosen, they must be installed at this stage.
Alternative solutions are
- Embed a tensile steel reinforcing belt as low as possible in each dome;
- Construct separate armatures for each dome and leave them in place until all domes are cast and cured.
Erect domes
For
each dome that is erected, a wooden armature needs to be erected to
support the segments until all are in place. This armature needs to
be supported on wedges so that when the dome is complete, the wedges
can be driven out and the armature disassembled and moved to the next
dome. Before the armature is removed, the flying buttress and eve
elements which support the exposed edges of the structure must be
installed.
Because
of the steepness of the lower slopes of the dome, some though is
needed on how to prevent the concrete from slumping before it is
cured. Some upper surface shuttering may be required, and thought
needs to be given to how to secure this. Also, it will probably be
necessary to erect some sort of gantry or scaffold in order to apply
and smooth concrete over the centres of the dome – because
certainly that will be out of reach from the edges while the concrete
is not set.
Fit chimney outer
Cut
hole(s) for the chimney(s) in the appropriate place(s) in the domes
(or, preferably, leave them when casting). Install vertical concrete
pipe over the hole to the final ground level.
Fill valleys between the domes
On
a settled, dry day, or better still on the first day of a run of
settled dry days, fill the valleys between the domes with a
lightweight inorganic filler material such as vermiculite. The object
of this exercise is simply so that from every point on the structure,
when the upper membrane is in place water will naturally drain to one
edge and will not pool; the valleys do not have to be completely
filled by any means. This doesn't need to be done if a watertight
means of joining the upper membrane to drains running down internal
pillars can be found.
Lay upper membrane
Lay
a waterproof membrane over the whole structure, overlapping the
uphill sides by half a metre. Fold this overlap down over the overlap
of the lower membrane. Sealing around the chimney and any internal
pillar drains is an issue I haven't solved, but I imagine suitable
mastics may be available.
Backfill
Backfill
over the whole structure, first with subsoil, then with topsoil.
Contour the backfill to form a natural slope. Turf over. Similarly
backfill against the retaining wall on the downslope side.
Install glazing
Ordinary
standard double glazed 'patio door' units are installed into the
exposed sides.
Fit out interior
To
taste.
Rough costings
The
following costings are preliminary, and are based on concrete costs
only, with no allowance for levelling the platform, fitting drains,
services, labour and so on; it's mainly to cost the bits of the
structure I don't have a good feel for. This looks startlingly low –
as if I've got something wrong by a factor of ten. I hope not!
| Sousterran: quantities and mass | ||||||||||||
| Concrete option | Cladding | |||||||||||
| Constants | Unit | Item | Unit | Price | per sq m | |||||||
| Mass of 1 cubic metre of concrete | 2400 | Kg | Bituthene 3000 | 20 metre roll | £163 | £9.03 | ||||||
| Mass of 1 cubic metre of soil | 1700 | Kg | Styrodur 3035CS, 30mm | 14 sheets at 1250x600mm | £70 | £6.67 | ||||||
| Price of 1 cubic metre of concrete | 100 | Pounds sterling | Styrodur 3035CS, 60mm | 7 sheets at 1250x600mm | £70 | £13.33 | ||||||
| Price of 1 8x4 sheet 15mm exterior ply | 40 | Pounds sterling | Total, with 30mm | £15.69 | ||||||||
| Price of 1 sq metre concrete blockwork | 10 | Pounds sterling | Total, with 60mm | £22.36 | ||||||||
| Dome radius (long axis) | 2.4 | metres | ||||||||||
| Area of dome floor | 14.98 | square metres | ||||||||||
| Area of dome surface | 36.19 | square metres | ||||||||||
| Number of wall panels | 11 | at | 4.8 | sq metres | ||||||||
| Elements | Area sq m | Thickness m | Volume cu m | Mass tons | Concrete Cost | Cladding cost | Number of | Total cost | Total mass | Shuttering sheets | Shuttering cost | |
| Floor | 104.83 | 0.1 | 10.48 | 25.16 | £1,048 | £399 | 1 | £1,448 | 25.16 | 1 | £40 | |
| Dome | 36.19 | 0.1 | 3.62 | 8.69 | £362 | £809 | 4 | £4,685 | 34.74 | 42 | £1,680 | |
| Dome segment | 6.03 | 0.1 | 0.6 | 1.45 | £60 | 24 | 34.74 | 7 | ||||
| Pillar | 0.5 | 1.2 | £50 | 17 | £850 | 20.4 | 6 | £240 | ||||
| Eve | 1 | 2.4 | £100 | 5 | £500 | 12 | 4 | £160 | ||||
| Flying buttress | 2 | 4.8 | £200 | 4 | £800 | 19.2 | 6 | £240 | ||||
| Wall | 4.8 | 0.1 | 0.48 | 1.15 | £48 | £75 | 11 | £1,357 | 12.67 | 0 | ||
| Overburden | 104.83 | 0.2 | 20.97 | 35.64 | £0 | 1 | £0 | 35.64 | 0 | £0 | ||
| Subtotals | 50 | £9,639 | 194.56 | £2,360 | ||||||||
| Total, cost | £11,999 |
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