Soil as a Structural Material

Lucia R

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Earth construction is one of the oldest building methods around. Now that architects around the world are more concerned than ever with climate change and green alternatives, choosing this eco-friendly and cheap source is looking very appealing- but still very unknown which is why not everyone thinks of it as a solution. Among other benefits, earth structures have good insulation properties which makes them an even better choice if you're thinking of going green and save some air conditioning energy in the process.

But, using earth as a structure is not as simple as it used to be as our needs have changed through the ages: we aim for durable structures capable of sustaining things like earthquakes and the trials of weather. There are different methods, each suitable for different needs; if you're thinking of using soil as a material in one of your future projects, here are a couple of methods to help you choose the right one for your needs.

"Adobe" Method:
This method consists of building with a mud and hay mixture. Hay provides the mix with a firmer internal structure preventing breakage. Most of the time, the mix is made with soil found in-situ, as the soil needs to be muddy at the time of putting it on the structure. That means it's made right at the time it's needed. Below are different ways to approach the adobe method:

Simple adobe blocks: This method consists on making large bricks with the adobe mixture still fresh, using more mud to glue them together. This structural method is the less stable one; it doesn't stand well on earthquakes, or sustains its own weight efficiently. Eventually, these simple adobe structures may crack or collapse onto themselves. Nonetheless, there's a way to avoid this, or at least delay it: make the walls thicker than usual- think about three times thicker than a concrete wall- about 3ft 3,37 inches (1 meter). Thicker walls also mean naturally better insulation properties for the building you're projecting which could be a plus.

Confined Adobe blocks: This method consists of creating an armored concrete structure to go around the adobe, confining the weaker adobe blocks and stabilizing the overall structure. It uses the same principle of confined masonry methods, only you use adobe blocks instead of regular bricks.

Armored Adobe: This method is more akin to concrete than masonry, and it's the most stable of the three adobe methods. First, you'll need to create a structural skeleton before placing the mix of mud and hay to shape the structure; this mix is a lot thicker than concrete, which means you can shape it like you would shape a clay sculpture, achieving a wider range of plastic shapes than you would with concrete, and faster. You can help stabilize all three methods a little more by using a wire mesh net as an external supportive structure; after the mesh is on the surface of the finished wall, you'll need to add another layer of adobe to make sure the mesh joins with the wall structurally.

Cooked Clay
This method is newer and experimental, but still an interesting method if you're looking for an almost surreal structure. It uses the same principle of clay artifacts as sculptures use, only at a much bigger scale. To achieve this method, you have to cook the clay in-situ as you build, heating it piece by piece. The best example of this method is the "Terracotta House", located in Colombia, built by the architect Octavio Mendoza.

Packed Earth
The packed earth method is a lot like the cast concrete method. It's achieved by pouring a mixture of mud, pebbles, sand and clay into a mold or a cast; then, the mixture is packed or "rammed" into the cast, to destroy any bubbles of air inside the mix. The casts are removed after the mixture is dry enough- not completely dry; the mix could take up to two years to dry completely on the inside- also, the drier it gets, the stronger it gets, which means the structure shouldn't be put under too much strain during its first years. Just like with adobe, you can make blocks out of packed earth, then glue them together using a thinner mixture of mud.


Some considerations:
Earth, on its own is not very durable: when using adobe or packed earth, make sure to add an extra layer on the outside to protect the structure from the elements. Rain and excess humidity can damage it, reverting it into its mud-like state and collapsing it. You can use a layer of plaster or concrete, for instance, and seal it against moisture. This problem is not particularly worrying for baked clay structures, as the baking hardens the mud and changes its physical properties.

Not all soils are created equal: Make sure to test different types of soil; sometimes the soil available in-situ is not the best for what you're trying to achieve, which means you'll have to bring it from somewhere else (therefore, adding to your building's carbon print).

Do you know of other methods to build using earth? Would you be willing to try one of the ones mentioned above? Leave your thoughts.

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Answers (2)

Vivien C, Designer • Dec 2

The Windhover Contemplative Center (https://www.modlar.com/inspiration/rammed-earth-architecture/#windhover-contemplative-center) is technically out west in Palo Alto, California outside of a desert setting. It seems to be doing well! Granted, its lowest temperature doesn't reach freezing...

Mark H, Architect • Dec 2

Has anyone tried this outside of the desert southwest? It seems that dampness and freeze/thaw cycles would not be kind to even a reinforced soil structure.

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