So why are tires used, and what is this tire pounding thing I keep talking about? Let’s start with some conceptual stuff.
In a conventional house, when you’re building a wall that will support the structure, you need to put a footing under it. Usually this is a concrete pour that is about twice the width of the wall. So if your wall is 8 inches (20.25 cm) thick, the footing for that would be 16 inches (40.5 cm). This is to spread the load as the roof bears down on the wall.
Now, in an Earthship, we use automobile tires pounded tight with dirt to build the walls. Each tire is about 32 inches (81.25 cm) in diameter after it has been rammed with dirt. As you can see, this is already way thicker than the 16 inches of the conventional house. As a result, the tire wall can support greater loads and is its own footing as well. Here is a diagram showing the differences.
So, we have tires, and we want to fill them with dirt. How do we do that? First of all, we sort the tires into sizes. Ideal sizes are:
If you don’t know what those numbers mean, google it. Tire sizing is pretty weird. The R number is the rim size in inches, though, so we’re looking at 15 and 16 inch tires. For our purposes, we’ll start with 235s at the bottom and over the course of working our way up, end up with 205s on the top course. There is some mixing and matching to make it all work.
How do you pound the dirt into a tire? Well, let me show you. First you start with an empty tire and put a bunch of cardboard in it. Like this:
The cardboard is used to prevent the dirt from coming out the bottom as you move up the courses. You don’t need cardboard for the first course of tires. Once you have your tire in place, you start pouring dirt into it. At first you just work with your hands to push the dirt into the sidewall. Then once you’ve filled it as much as you can with your hands, you start to pound it with a sledge hammer. Thus the term pounding a tire.
It works best if you have at least two people to work on pounding a tire. One to do the pounding, the other to be what is affectionately called, the Dirt Bitch. This is the person who is there keep adding dirt to the tire as it is being compacted, keeping the one doing the pounding going. It’s pretty labor intensive, but if you have a partner where you can switch between being the dirt bitch and the pounder, it works out pretty well. After you have pounded the side walls pretty much as compacted as they can get, you take the time to level the tire, then you just pound in the rest of the dirt into the middle. The end result looks like this:
Here is a picture of me pounding a tire, just in case you thought I was making all of this up 😉