Ditch Concrete Blocks: How To Build A Retaining Wall From Rammed Earth

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Concrete is almost a miracle substance; it's tough enough to use in construction, even for mighty dams. But the "almost" part of that sentence is a significant con to using the cement-based material. Concrete consumes a perplexing amount of energy to produce and relies on fossil fuels. While it'll leave you with a reliably solid retaining wall, its environmental impact is concerning. (Plus, concrete isn't particularly attractive.) To all this, we say it's time to say, "Goodbye, concrete," and hello to rammed earth, an eco-friendly alternative to highly polluting concrete.

Rammed earth is an ancient technique that compresses clay soil, gravel, sand, and water into one rock-hard substance, one that's arguably better looking than standard gray concrete. A retaining wall made from rammed earth will do the job as well as concrete blocks while 1) being nicer to look at, and 2), more importantly, much lighter on the planet.

The ingredients to create a rammed earth wall are comparatively affordable, and the process isn't highly complicated (although it is work-intensive). It's possible to construct your own rammed earth walls with little more than soil, water, a reinforced wooden form, and a few other tools. However, it's not fair to oversimplify the process. While the steps are somewhat basic, there are a lot of them.

Before you go all in on building your new rammed earth retaining wall, there are some important considerations to know about, starting with cement. After digesting its ecological ills, you may want to opt out of cement for good; however, a small amount will help your work withstand the elements. Rammed earth also needs about at least one month to cure fully before it reaches its peak strength. This long cure time may require you to add supports such as rebar.

If possible, a short consultation with a professional could save you a lot of headache (as well as backache). They can also advise on how to prepare where the wall will sit so that it will be securely anchored in place. You can use dirt from your own yard, but you will likely need to bring in additional earth from elsewhere. It's also probably wise to plan your build for the dryer time of year, given rammed earth's long cure time.

Here's how to mix up this old school building material: Mix together 3 parts clay soil, 2 parts course sand, and 1 part small rocks. Pour in enough dry cement to make up 3% to 7% of the mixture. (Also, wear protective gear while working with the dry cement.) Combine the dry ingredients before slowly stirring in water. Add water by sight and feel; the mixture should be damp with still some dry spots.

As demonstrated by Natural Buildings on YouTube, the rammed earth is ready when you squeeze a handful into a ball, and it holds its shape. To test it, drop the ball from shoulder height onto a hard surface. The pile should peak at its center while drier crumbles spray outward. If your mix doesn't pass the tests, mix in water until it does.

You will create a trough-like structure with plywood boards used for the length, and 8-inch boards used for the shorter ends (width). Above on the left, you can see a scaled-down version of a formwork to give you an idea of how one needs to be constructed.  On the right is a full-size formwork made from metal. But you can construct one from plywood, two-by-fours, pipes, and pipe clamps.

Formworks are reusable. Once ready, the boards will be pulled away, revealing the rammed earth piece. You could also use waler brackets, if you have access to any. Used in construction, they're not typically available from home improvement stores. Instead, use pipes to reinforce the wood, then apply the pipe clamps to keep the entire formwork together.

This formwork will need to be strong enough to withstand immense pressure. As for the tamping, you'll use an 8 inch by 8 inch tamper, like this one from BlumeTrec ($43.99 at Amazon). If you end up doing more rammed earth projects, you might consider getting a pneumatic tamper.

Now you're ready to start building your rammed earth wall. To begin, scoop about 2 inches of earth mixture into the form, smooth the top with a pallet knife, and pack down the surface evenly with the tamping tool; you should lose about 1 inch of height after tamping. Every time you tamp, you will lose half. It will be super compact, which is why the pipe clamps are needed. Otherwise, the form will blow apart.

Continue this process, on up until the height you want: 2 feet, 3 feet, or 4 feet. The tamping will take a few hours, as you will need to go through this process at least 24 times, on up to 48. Once you have your wall at the proper height, let it dry and cure in the formwork for a day. 

After 24 hours, undo the formwork and pull the boards away. How long this should take is another question for your professional, as it may vary according to your area's climate.