Above: Terrace formed by re-used tyres that are filled, stacked, and bound together by stainless steel screws.
Above: A mature and completed terrace. The Eucalyptus trees have grown to the adequate size for espalier posts and have been ring barked to stop growth. The berry bushes and fruit trees have matured and now cover and beautify the tyre wall.
Pioneer Permaculture Studios 
Hi, how big are the nuts and bolts for joining the tyres, and how much do they cost per metre of length when stacked 4 high like that? Another question: have you cut and dried eucalyptus to make posts, or have you grown them on site and just stripped them? Are all kinds of eucalypts o.k?
No bolts, use screws and if tension is a concern, use one screw in each direction, or screw into a stiffer material. Sometimes I cut a thick part of a sidewall, or I use pressure treated wood, when inside the walls in this example, so the treatment copper, arsenic and chromium are not exposed to the soil. The cost of stainless steel is worthwhile -and don’t skip using washers, too.