Old tyres make for inexpensive building material. If you are building a retaining wall out of them, you'll avoid the use of concrete or wood. You only need the tyres and earth.
Work out how many courses (layers) of tyres you want - you'll need to allow 5-10 cm of space per course to get a staggered affect (this will improve stability). This will help you get your wall measurements.
Start by digging into the earth where you want your first course to go - you'll want to go about half the depth of the tyre so that half the tyre is in the dirt, and the other half is above.
Lay your first course of tyres out in the dug out earth. If your wall is going to be quite high, you may need to use another row of tyres rather than compacting the back with dirt (or even a row of half-tyres).
Fill your first course of tyres with earth, and compact with a sledgehammer. Make sure the dirt is level.
