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Posts Tagged ‘clay’


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.

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If you’ve been watching our videos, you’ve seen us making various earthen blocks. This one here is the best one so far. It has clay and rice hulls. Another very good block was the clay with vetiver grass. So I had the idea to combine the best – the rice hulls and the vetiver. That’s what we’re testing today. This block back here is adobe, just the traditional adobe mix. We’re using this wood form that’s 10”x14”x6” high. That’s a typical size earthen block. You could put these same materials in earthbags, but it’s a lot of extra work. It would only be worthwhile probably if you wanted something special like a lightweight insulated earthbag.

We’re using the same basic ingredients – rice hulls, chopped vetiver for fiber to hold the block together, sifted sand and clay soil. This is not pure clay, this is clay soil. You have to experiment with your own soil and ingredients to get the right mix, but this is what we’ve been using approximately 2:1 — two parts clay soil to one part sand, one part rice hulls, one part vetiver, and enough water to make a stiff mix, but no extra water. Add the clay first – the clay soil actually – with a little bit of water and let it soak. This saves a lot of mixing. Add a little at a time. And again, it would be easier to make this in large quantities in a pit or on a large tarp. We’re just making one small sample here. Add the ingredients in layers to reduce mixing. Add the fiber at the end to make mixing easier.

I like the vetiver grass, because it adds termite resistance and tensile strength. We made a second small batch in order to fill this mold. I think this is going to be our best block so far. We’re combining everything we’ve learned to make a block that’s stronger, lighter, insulating and insect resistant. Another thing we’re doing this time is we’re pouring it in the mold in place. This adobe block we made elsewhere and it cracked as I was carrying it over here to dry. This time we’ll make it right here and we won’t move it around. We should have a stronger block. In about two weeks or so we’re going to test all these blocks that we’ve made and see which ones are the strongest and the best. So stay tuned for our next video.

Naturalhouse’s YouTube channel now with 84 natural building videos.

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Road base is a mixture of clay and gravel used in road building throughout the world. As an engineered material designed to withstand extreme loads (trucks and cars), you are assured of good working properties for earthbag building. While site based soils are less expensive and require less transport, the working properties are unknown and so you will want to do a series of tests to determine the best clay to gravel ratio. With road base, no additional tests are necessary and you can begin building right away.

Moisten road base slightly before filling bags.

Moisten road base slightly before filling bags.

Road base is typically very inexpensive, largely because the mixture is just clay and gravel. With huge quantities needed for road building, many suppliers are available to meet demand. That often means a truckload can be delivered to your site in short order from a nearby gravel pit.

Road base is a uniform mixture without clumps of clay or large rocks. Using road base eliminates digging for soil on your land, thus preserving the natural beauty of your site and freeing up your time for building. And since it is delivered by truck, you can have it stockpiled around your house just where you need it to minimize labor. Road base excels in compression, and therefore is ideal for dome building. It dries almost rock hard, creating incredibly strong walls. You can use the same material under your house to raise the building site.

Note: road base is a generic term and the mixture may mean different things in different places. Ask for a clay/gravel mixture as used in road building (not just sand/gravel). You need the clay portion, which acts as the binder.

If you live in cold climates, then consider using lightweight fill material such as scoria, perlite or vermiculite for high R-value and ease of use. See Insulated Earthbag Houses for more information.

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