|Bristol Green House||Car Tyre Retaining Wall|
How to ram a tyre wall...
Straw bale walls
Over 40 million tyres are disposed of every year in the UK. That's a lot of swings in chimp enclosures. Now that they can't be put in landfill something has to be done with them and we are demonstrating one jolly practical suggestion.
We have saved 180 tyres from being burned to power a cement factory (and saved your lungs from their toxic pong) and used them to build retaining walls. Since they won't biodegrade any time during then next few millennia, they make an excellent free building material.
The tyres get their strength from the process of being stuffed full of earth. This consolidates the material. In conventional building this is done by adding concrete which literally makes the material solid. Rammed earth tyres use human energy rather than hydrocarbon energy.
The Earthship Brighton (below) is built using 870 used tyres, so I invited the guys from the Low Carbon Network who built that to come and help me. They are the leading practitioners of tyre wall construction in the UK.
Our more modest garden workshop (below) is built on a hillside and so we must cut into the hill, dig out the earth and build retaining walls. Due to limits on space we have made the retaining walls part of the building itself. The design is an exciting hybrid of straw bale and earthship construction, the first of it's kind.
|I was very worried at the start that the physical effort required was too much and that we'd never get the tyres rammed. In the event I remember the tyre ramming weeks as the best time of the whole build. The weather was blazingly hot and the work was physically demanding, but there is something about the process of ramming tyres that made us ecstatic. We'd ram tyres all day, then drink and chat at the pub all evening. The team was led by George Clinton who is a true believer in rammed earth tyres and a great motivator of people. We had a large team of volunteers who day after day came back for more, despite, or perhaps because of the physical effort. It takes 40 minutes to an hour to ram a single tyre and when you think you've got it done, George would come along and make you ram it some more. I'm convinced that a machine can be invented to ram tyres, but George is dead against it. A machine would take away his fun.
Each course is set back an inch from the course below and the tyres are spaced rather like you'd lay bricks. At the end we hammered rebar through the wall plate at the top at a spacing of every third tyre.
Effectively wrapping around the tyres below, behind and above was a thick plastic membrane called Oldroyd XP. I also added thermal insulation so that the tyres would act as a thermal mass - storing heat from the summer for the winter.
The drainage of the soil behind the building is exceptionally important. Without good drainage the pressure of the water would push even this hefty wall over. See Foundations for details of this.
Above. Thermal insulation details.
|What I'd change for next time
This is a very new building technique pioneered by Mike Reynolds in New Mexico. UK conditions are very different to the desert conditions experienced there so designs for the UK have to be modified. I think we made some good innovations, but also some mistakes.
1. Thermal isolation.
In typical earthship design the entire building is underground with only a window exposed to the air on the south side. This means that the walls don't need to be thermally isolated especially if there is half a metre or so of soil on top of the building.
In our case, as we don't have a solid floor, the foundations of the building are exposed to cold air that circulates under the building (keeping it dry, we hope) And the tops of the walls are also exposed to the air where they meet the straw bales via a wooden wall plate. This means that the tyre wall should be thermally isolated from both the air and first half metre of soil (Ie above the frost line) and at the foundations. This is something I have realised in retrospect. Although I thermally isolated both tyre walls from the soil they are retaining and at the wall plate I did not thermally isolate from the gabion foundations. This means the walls now act as heat sinks rather than a thermal mass. This is a great shame as it will increase the cost of heating the building and reduce the benefit of passive solar heating from the large window.
To overcome this I could simply cover tyres as they are seen on the interior with a thermal insulation. Aesthetically I don't want to do this. Although, as I still haven't rendered the tyres on the interior I am considering using a paper mache which would have an insulating effect. I will report back on this issue after the winter 2008-9.
|Diagram above. Note the missing insulation between bottom tyre and foundations. In the event the thermal insulation we used between tyres and soil was polyfoam floorboard and the waterproof membrane was Oldroyd XV. The diagram below shows more accurately the layers of behind the wall - the pea gravel, Oldroyd, thermal insulation. Note that the chicken wire and leca were not used in the build.|
|2. If and when I work with tyre walls again I will approach the build in a more systematic way. We installed the huge sheet of Oldroyd XV waterproof membrane first then started filling tyres on top of this. There was a constant risk of damage to this membrane. Also stuffing the backfill against the thermal insulation and DPM was very difficult. In future I would build a temporary wooden structure to the full height of the wall which would hold in place the Terram (between pea gravel and soil), the DPM and thermal insulation. In this way the DPM would be protected.