Rammed earth construction at a future ecocenter

One of the reasons I do volunteer work is getting hands-on experience with different building techniques. It’s one thing to read how to build something and quite another to try to actually do it, solving all the unexpected problems that appear along the way. One technique that I recently got first-hand experience with is rammed earth, a very old technique that has been making a comeback in recent years as a sustainable building technique.

The site where I volunteered is located in the village of Rojas de Cuauhtémoc, close to Oaxaca city in southern Mexico. The land is owned by a local non-profit organisation called Puente, and there are three people working on developing the site with the help of volunteers. The plan is to turn the site into a type of ecocenter or demonstration site that promotes permaculture and ecological construction. The project started just five months before my visit, and the progress made in this short time is impressive. The focus during my stay was on the rammed earth construction, but I also got to participate in other tasks, from planting chickpeas to constructing a water tank platform.

The beginnings of an ecocenter

The project at Puente Rojas started in June 2016. The first task was making the off-grid site livable for my hosts and volunteers. After clearing the bush, my hosts built a composting toilet and a house that will be the future office. The site had the foundation and columns of an unfinished building from 20 years ago, and this foundation was used to build the office. The main construction material was brick, because it was quick and would work well with the existing concrete structure, but some recycled bottles and natural materials were also used.

The site is meant to promote permaculture, and a large part of the land has been devoted to gardens. This area of Oaxaca has a semi-arid climate with a rainy season between May and October. Because the project was started in the beginning of the rainy season, it was important to work in the garden, planting different crops and seeing how they would grow. One of the planted crops was amaranth, a traditional cereal of the region that Puente is promoting because of its high nutritional value, drought resistance and cultural importance. From this first trial they discovered that the main challenges were heat and leaf-cutter ants that caused many crops to fail.

The goal is to turn Puente Rojas into a demonstration site that offers affordable courses on sustainable living, including permaculture and natural construction. These courses would likely attract mostly visitors from outside the community, but working with the local village community is also important for my hosts. Indeed, my hosts have been trying to get the local community involved already, but this has proven very difficult. Community projects are always extra challenging, but hopefully interest in the site will grow as my hosts and the project become more known in Rojas.

Construction of rammed earth guest cabin

Now that my hosts and volunteers can live on the site and the seasonal rains have stopped, the focus has turned to building guest housing for the ecocenter’s future students and other visitors. The plan is to build five small cabins on the site, and the idea is to use different sustainable building techniques for each cabin. In this way the cabins become part of the site’s educational aspect.

For the first cabin, rammed earth was chosen as the main building material, and this is the cabin I helped build during my stay. Rammed earth is a technique where earth walls are directly built up in large pieces using a formwork that is moved along the wall when each piece is finished. A mixture of earth, clay, sand and water is poured into the formwork in layers of approximately 10 cm, and the earth is then compacted with a rammer. More layers are added and compacted until the formwork is filled. Because the strength of the walls comes from the compacting, very little water is needed and the formwork can be removed right after the piece is finished. The walls need to be thick (in this case 40 cm) in order to be structurally stable, and thanks to this rammed earth houses have a high thermal mass which helps create a comfortable indoor climate in places like Oaxaca where the outdoor temperature varies greatly between day and night.

It is best to use a concrete foundation so that moisture from the ground doesn’t destroy the earth walls. When I arrived the concrete foundation had already been finished, and we started the wall construction by preparing the earth. The earth came from a quarry just outside the site, and we had to first filter out the coarse gravel. We then mixed the filtered earth with sand in a 1:1 ratio, added water and let the mixture settle for at least a day before construction.

Earth, clay and sand are the main components of any earth construction, but other substances can be added to improve strength and durability. My hosts had already done some test and we did some more test with different mixtures that contained small amounts of cement, lime and manure. In this case the best result were achieved when the earth was stabilized with a small amount (approximately 7%) of hydraulic lime. The local earth has a high clay content, and because of this lime is a better stabilizer than cement, which in turn works better for sandy earth. Lime also has a much smaller carbon footprint than cement, because lime construction absorbs carbon dioxide from the air as it hardens and turns back into limestone.

One of the things that made a huge difference was the amount of water used. Rammed earth requires very little water and too much water can result in shrinking. However, we noticed that we were getting better results when we added a bit more water than advised, probably because the lime needs water in order to harden. But with more water the ramming became more difficult as the mixture kept getting stuck on the bottom of the rammer. Instead of adding more water to the mixture, we started spraying a layer of water on top of each finished layer, and later we also started wiping the finished block with a wet rag. We also covered each finished piece with a tarp so that it wouldn’t dry too fast in the heat. The joints between the blocks were made stronger by making a groove that attached the blocks together like legos, and we also added a very thin layer of lime mortar between the first and second layer of blocks.

During the construction we also faced many other challenges like cracks and problems with the formwork. Improvising and solving these challenges was a process that taught us all a lot about rammed earth. Even though the ecocenter is far from finished, the use of sustainable building techniques already makes Puente Rojas an educational site for volunteers and other visitors.