All of our earthbag projects have been built using the same road base material. It has worked exceedingly well. After tamping, the earthbags are solid and turn into very hard earthen blocks. Here I am testing the hardness of bags on our roundhouse.
But from time to time I would see areas with less than optimal bonding on the various samples I’ve made. Usually you can’t see the results inside the earthbag, but the samples enabled closer examination. Some of the samples looked a bit granular (lacking in clay) and so I got to wondering if the mix could be improved. After all, we’re not buying an engineered mix for highway construction, just local material right out of the ground. In most cases that’s fine, but what if you want to achieve maximum compaction and strength? (Which reminds me, keep your soil covered if possible because rain will force the clay to the bottom and throw off the mix.)
You could pay extra for an engineered mix, of course, and be certain of an ideal ratio. But for us it’s cheaper to just add a little extra clay and then test the results, so that’s what I did. I made a sample earthbag with 5% extra clay to see how it would turn out. You could do the same thing with whatever type of soil you have. Make a few samples and then compare them after they dry.
Here’s the basic procedure. We typically use five 2-gallon buckets of soil per bag. This means each bucket is equal to 20%. I wanted to add 5% additional clay, so I prepared ¼ bucket of sifted clay and mixed approximately the same amount with each bucket of soil. The mix appears stickier and looks like it will produce a stronger earthbag.
Text for video available at my Naturalhouse YouTube channel.