Bristol Green House
Gabion Foundations

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Foundations
Tyre walls
Straw bale walls
I-beam roof
Living roof
Clay/Lime render
Hempcrete
Insulation
Earthbags

Map
The ruins of a green house dating from the early 20th century on the site of the new 'green house'. These bricks will be reused in the foundations.

Gabion foundations.

This small building is very heavy. The rammed earth wall and living roof will weigh over 50 tonnes between them. Solid foundations are therefore essential.

Almost all buildings built since the early 20th century have concrete foundations. In the history of building this is a very short time. The making, transportation and mixing of concrete is highly energy dependent, even when it's made from recycled materials. It makes me mad when people on Grand Designs proclaim they are building green, then immediately create a slab of concrete covering the entire footprint of the building. Reducing the energy required to make this building was an underlying design parameter.

Stop. Is there another way of doing your foundations?

Left. The rubble excavated from the site of the build. This was reused in the foundations and later in landscaping. And I still have rubble left over!

Many straw bale buildings use car tyre pier foundations, a very simple method, but for this build, because it is cut into the hillside, that method was not appropriate. Instead I opted for steel cages, or gabions, filled with the bricks from some derelict brick greenhouses on the site. You can't get more locally sourced than that. Using what you have to hand is naturally the soundest method for eco-build. People tend to want off the peg solutions, but adapting to local conditions is the best eco-policy.


I sourced them from Intermesh in Cheltenham. The life span is roughly a hundred years.
I lined the cages with filter fabric so that any smaller pieces wouldn't fall out, but I certainly don't think this is essential as you are only supposed to use rubble or stones larger than the mesh.
The cages tend to swell and bow as they are loaded. Filling neatly caused less buckling of the cages than roughly chucking chunks in. They are fairly imprecise building blocks. I used a thin layer of concrete on top of the gabions to even out the height differences so that the wall plate would be horizontal.

Drainage

I launched myself into this build with no previous building experience and my first major problem arrived almost immediately - drainage. I had barely considered it, but quickly realised that if you get the drainage wrong your building won't be sound. I didn't do a very good job of the drains initially due to lack of experience and the imperative of pressing on with the tyres. Over the first winter the foundations became very wet and I retro fitted further drainage in the spring.

The original drainage took the form of a perforated pipe run along the outside bottom edge of the gabions. This was then covered in a narrow strip of pea gravel reaching all the way to ground level. This is separated from the soil by a filter fabric. My mistake, I think was not to pay attention to the sloping of the drains and not to make them lower than the bottom of the gabions. I added an extra drain, about 6 inches lower than the bottom of the gabions on the interior of the building across the back wall, leading away to a soak away.

The foundations seem to be dry now.


Above. This diagram shows how we did it. There were a few minoar alterations. We didn't use chicken wire under the floor but breather membrane (roofing paper)and we insulated with rockwool (very cheap at the moment as it is subsidised by the electricity companies) Originally I insulated underfloor with loose straw held in place by plastic. Bad idea. It started to rot so I replaced it with the rockwool and breather membrane. I have put heavy protection at the vents to the under floor area to prevent foxes and rodents getting under the building. The top tyre is not filled with leca (not structurally strong enough).

There are a couple of things I would change. Firstly, lower the drain below the bottom of the foundation, moving it slightly away to prevent weakening the foundations. Secondly I would add thermal insulation between gabion and tyre wall to prevent this acting as a heat sink. If it were insulated below it would act as a thermal mass. Several other ideas in this diagram changed in practice.