UniSA tyre wall study passes structural integrity test

Well, a study by the University of South Australia (UniSA) has shown that a wall made with tyres has the structural integrity comparable to traditional materials. The technology has been used in Australia's first council-approved 'Earthship' buildings made with upcycled materials (such as old car tyres) and lots of earth.

While earth-packed tyre walls have been used in niche construction scenarios for decades, there has previously been little strong empirical data available to support their use, which has limited their wider uptake by architects and engineers.

Supported by Tyre Stewardship Australia, a UniSA team consisting of PhD candidate Yachong Xu, and supervisors Yan Zhuge, Martin Freney, Reza Hassanli, Mizanur Rahman and Rajibul Karim, has assessed the structural integrity of a test tyre wall to examine how the structure performed under various stressors.

According to Dr Freney, scientific analysis and testing proved the wall was as structurally sound as conventional walls used in residential applications.

"While that structural integrity has been observed for many years in applications such as the retaining walls in earth-sheltered, Earthship homes, the lack of supporting data has prevented wider uptake of tyre walls by engineers and architects. We are hoping this study will change that and help mainstream tyre walls and Earthship homes," he said.

Testament to this is Dr Freney's bed and breakfast Earthship Ironbank, which derives its name from the Adelaide suburb it's located in. Opened in November 2016, the building is Australia's first council-approved Earthship.

The main walls of the Earthship are made from roughly 700 waste car tyres which were sourced locally at no cost. Thousands of tons of earth surround the "tyre walls" providing protection from bushfires and extreme hot and cold temperatures (zero to 43 degrees centigrade). A large "greenhouse" sunroom on the north side of the building traps winter sunshine and it contains an indoor garden which is irrigated with greywater from the shower and hand basin.

The slow combustion heater is only needed occasionally in very overcast cold winter conditions. The Earthship sails through heat waves easily with a maximum indoor temperature of 25 degrees celsius.

Underground pipes called earthtubes deliver 22 degrees celsius air on a 40 degrees celsius day, without the need for fans or electricity. The roof is earth-covered and a final protective layer of non-flammable gravel allows rain water to be collected in underground rainwater tanks; another idea to increase the bushfire resilience of the building.

The ship reference in the building's name references how you interact with the building - windows and other apertures need to be opened and closed according to weather conditions to help regulate temperature and air flow, much like a ship's sails need adjustment to capture the wind. Another similarity to a space ship or sailing ship is that the building is autonomous - it has systems to provide power, water and sewage treatment without the need for "the grid" - which is increasingly costly and environmentally damaging.

Power is from a 1kW solar array and 14kWh battery system and water is collected from the roof and stored in water tanks (60kl). Small swap-and-go gas bottles provide energy for cooking, however a long term goal is to retrofit a bio-gas system which produces natural gas from food scraps.

Reflecting on the comments made in the guest book, Dr Freney said the bed and breakfast guests appreciate the comfortable indoor temperature, the calm, quiet environment and the abundance of indoor plants.

"I think the curved walls and ceiling, and the earth renders and earth floor also contribute to the serenity of the space," Dr Freney said.

"In terms of economic benefits, the off-grid systems for power, water and sewage mean that the quarterly utility bills are a thing of the past. Future maintenance costs of the off-grid systems components (especially batteries) is likely to reduce in cost as the technology improves and an economy of scale is reached."

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An important finding of the research is that tyre walls are expected to be ideal in earthquake zones due to the flexible nature of the tyres. "The wall bounces back rather than being permanently deformed like conventional masonry walls" Dr Freney said.

The research also investigated the effect of different fill materials inside the tyres with a view to being able to use waste materials such as recycled concrete rubble or crushed bricks, two materials that would also offer excellent drainage.

While the study only tested one real world wall as part of the project, UniSA PhD candidate Yachong Xu developed software models that allow the data obtained to be applied to other designs, making the results applicable to a wide range of scenarios.

The team is looking for an industry partner to develop a range of real-world applications for the technology.

The article Experimental study on the structural performance of full-scale tyre wall for residential housing applications was published in Engineering Structures, Volume 259, May 15, 2022.

For more information about Earthship homes in Australia visit www.earthshipironbank.com.au and www.earthshipecohomes.com.au