People have been using earth to build for thousands of years. Not only were homes commonly made from the earth, but so were castles as well as other large structures. Some buildings have remained largely intact for over a thousand years. Earth construction is still relevant. An estimated 1.7 billion people live in houses made from earth. (1) Some methods like adobe are enshrined in modern building codes.
Depending on the building method, the advantages are vast. Earth building may produce structures that are:
- Fireproof
- Soundproof
- Bugproof
- Bulletproof
- Breathable
- Mold proof
- Non-toxic
- Regulated humidity
- Regulated temperature
- Very cheap to operate
- Very cheap to maintain
- Structurally sound
- Earthquake/hurricane/tornado resistant
- Low in embodied energy
- 1000-year durability
- Sustainable
- Recyclable
- Locally available materials worldwide
- Cheap as dirt to make
(2)
There are many different methods of building with the earth. Some dig into the earth and cover the building like berm, atrium, or PSP. Some hold the raw earth in the desired form with bags or other materials. Some compress the earth into rock like rammed earth or Compressed Stabilized Earth Block. Some are dried into bricks like adobe, and some are molded into shape like cob.
Soil
When looking for soil to use for building, rather than disrupting a natural ecosystem, you can use the landscaping to your advantage. You can use the hole left from digging for a basement, cistern, pool, irrigation, or wastewater treatment area. Rather than leaving a deep hole, you could shape the land for gardening or another purpose. Topsoil is a valuable material that can be taken off and set aside for use in gardening or landscaping.
Soil is made up of a spectrum of different sizes of material.
Pebbles – 200 – 20 mm (Too large for construction and should be sifted out.)
Gravel – 20 – 2 mm
Sand – 2 – .06 mm
Silt – .06 – .002 mm
Clay – .002 – 0 mm
You can do some simple field tests to help you determine the approximate percentages of the different sizes. One test is the mason jar test, where you fill the jar half full with dirt, then fill with water, and add something to break up the particles like dish soap or salt. Then shake for several minutes until the particles are thoroughly broken up and mixed with the water. Let it sit until the water becomes clear again. This may take more than a day. The heavier particles will settle first, so that you will be able to see the layers of the different sizes. You can measure how much of each material there is.
Another field test is demonstrated in this video, and measures the amount of sand, silt, and clay based on the texture of the soil when moist.
The larger particles help with strength, and the smaller particles help to bind the soil together. The amount of clay is important to the behavior of the soil. Some clay is necessary as a binder. However, a large percentage of clay will cause the soil to shrink while drying and may cause cracks. This is less of a problem with pre-cured bricks. Sand creates a porous surface for water to seep through while clay does a better job of blocking it. This is good if you want to promote drainage around a buried house, but not good if you don’t want water to seep through your earthen walls. Some types of soil in arid areas may have too much salt that dissolves in water, which makes the soil not suitable for building. Otherwise, the trick is about finding a good ratio of small to large sized material for the type of building you want to do, or choosing a method of building based on the soil you have to work with.
Stabilizers
Soil can be prepared by sifting and adding in larger or smaller grain sizes to adjust the percentages. When your construction method compresses the soil, like Compressed Stabilized Earth Blocks or rammed earth walls, you will want a larger percentage of larger grain sizes. Whereas if you are doing adobe or cob which is more freeform, you will want more clay.
Soil on its own if subjected to water will break down over time. Stabilizers can be added to preserve the material. The two main stabilizer options are cement and lime. Cement works by creating opposing shapes in the soil to gridlock it. It is better for soils with a higher percentage of silt, sand, and gravel. Lime works to chemically bond the soil together, and is best on clay-rich soil types. Cement stabilized blocks take four weeks to cure, whereas lime does not need curing. The range for the percentage of cement is 6-12% and more is recommended for wet mixes. For lime the range is 2-10% with 6% being the average. Flyash also works like cement in that it stabilizes by creating a stiff matrix. There are many kinds of stabilizers and stabilizer combinations.
To add tensile strength, stabilizers like straw, fur, and other natural and synthetic fibers can be used to for adobe and cob. The longer strands are thought to help bind these mixes which use a lot of water and clay and are more malleable.
Plasters and paints can also protect the soil from water, however if water finds its way in and doesn’t have a way to evaporate, it could break down the structure.
Hydro-thermal Effect
Earth has a unique ability to stay cool in hot weather and warm in cool weather because of how it interacts with moisture in the air. When it is hot outside, moisture in the wall evaporates, leaving the walls cooler. When it is cool outside, moisture condenses, adding heat to the walls. The hydro-thermal capacity of the wall is affected by the soil, stabilizer, and amount of compression. Especially when the soil is compressed it loses some of this ability. (4)
Methods
Underground Houses
Whether this conjures up ideas of an underground bunker or a hobbit hole, living underground has unique advantages and challenges.
| Pros | Cons |
| Tends to stay the temperature of the earth Little maintenance Blends into the landscape Saves space Can be built into hillsides Can use the dirt as building material Can be built by hand | When hiring builders, higher upfront cost Must take into consideration light, ventilation, moisture, and stability in design Should test for radon gas and build to address it if it is there |
One requirement for building is ground that has a higher percentage of sand and gravel. This will add to the strength and stability of the sides of the building as well as allow rain to pass through more easily.
The structure itself will need to be strong enough to endure the pressure of the sides and dirt cover. Common materials for this purpose are concrete and steel or wood. Concrete can be poured in place or concrete blocks reinforced with cement can be used. Steel will need to be protected from moisture to prevent corrosion. One method of using wood is PSP, or post, shoring, polyethylene. The posts are embedded into the ground floor while plywood is placed on the outside against the dirt walls to hold it back, and plastic is added to the outside for waterproofing.
Water is something that will have to be carefully managed. First, the house should be built so that rain is diverted away from it and does not pool around the roof or walls. A waterproof membrane may be buried that envelopes the house and directs the water away. Common materials are rubberized asphalt, plastic sheets, vulcanized sheets, and bentonite clay sheets. Drainage may be added, and the house should not be built where it is likely to flood.
Insulation is important since the house will tend to remain the subterranean 54° F. You will not need as much insulation as an above ground house would need. Insulation should be placed between the walls and the earth. (5)
Ventilation and natural light are especially important factors in an underground house. A heat exchange can keep fresh air coming into the house. Large south facing windows in the Northern Hemisphere can heat the house naturally, and smaller windows on the opposing side can balance the light creating a more comfortable ambiance.
These houses can be very simple or complex. A man in Idaho built himself a home for only $50. You can purchase a hobbit-like prefabricated home from https://greenmagichomes.com/
Bermed Houses
Bermed houses are built at ground level and buried on all sides except the opening and perhaps the roof. They have many of the advantages of the underground house, but don’t have the same issues with strength requirements. They are simple to build. They are basically just houses with dirt piled on three sides. They are cheaper since there is less need for doors and windows on those sides. A layer of sod on the roof can add thermal insulation.
Earthbag
This is a very cheap, very easy, and effective method. It can cost as little as a few hundred dollars to create a structure. This method was refined and called Superadobe by the creators at calearth.org. They use a long bag that moist soil is poured into and coiled into the layers of a dome, being compressed along the way. Barbed wire is placed between the rows to reinforce and add stability. The compressed soil hardens as it dries. A home can be built by one person with a little training. The soil requirements are not very picky, and the tools are basic. The cost for an eight foot wide dome could be as low as $300. These domes have been used in refugee camps or after disasters, and have been made into satisfactory regular dwellings. The bags do not have to be filled with dirt. They could be filled with an insulator like volcanic rock. Another great resource to learn about earthbags is earthbagbuilding.com.
Rammed Earth
Rammed earth is a method of building where a soil mixture is compressed into rock in the form of walls. The soil mixture is more particular. Soil must be high in sand and gravel, 60-75%, and low in clay to prevent cracking while it dries. Rounded gravel is better than flat sharp gravel since it lends itself better to compaction. When soil tends to clump or holds its shape when dug out, that is a good indication that the soil may be suitable.
This process also requires careful control over the moisture content of the soil within +/- 2% while it is being compressed. There is a certain amount of water for a given mixture of soil that will allow for the optimal strength after compaction. The amount of water is not standard since soil samples are so varied. Optimal moisture content of a mix can be tested by trying to break a clump. If it is crumbly, it needs more water. If it deforms but does not break, it needs less.
Forms are erected on the foundation and a humid sandy soil mixture is poured in a few inches at a time. The soil is tampered down until the strike makes a clear sharp sound and no indentation marks are left. One a form is filled, it is removed and moved to the next section. Some build the layers vertically and some build vertical sections side to side. The forms are similar to concrete forms. They may be wooden plywood braced with lumber, 2 feet high and 8-16 feet long.
Rammed earth walls can be produced relatively quickly. Two laborers can prepare the mixture while one pours the soil into the form and one compresses the soil in the form. It can be done by hand, but the use of machines will increase efficiency.
Rammed earth buildings have a solid, quiet feeling inside. The walls are smooth and there is a pattern of layered sand of varying shades.
Compressed Stabilized Earth Block (CSEB)
Compressed Stabilized Earth Blocks are a newer invention. They have the advantage of the strength of rammed earth walls, but unlike rammed earth walls there isn’t the same concern that the clay content will cause the wall to crack from shrinkage after it has been formed. Compared to adobe blocks, they are stronger, more dense, and more water resistant. However, they are also less insulative because of their increased density and have a lower hydro-thermal effect.
Gravel larger than ¾ inch should be removed, and the percentage of clay should be low—at least less than 20% but preferably below 10%. Optimal moisture content is the same as rammed earth. It is tested by trying to break a clump. If it crumbles it is too dry, and if it doesn’t break it is too wet.
The process of laying CSEB is similar to laying bricks. The mortar can be made with lime or cement. A protective surface coating may be advised in very harsh climates.
There are a number of different presses for CSEB. Some can be made with steel fabrication tools like a torch table and welding equipment.
This press was developed to reduce the cost of manufacturing CSEB. It is an automated press that can produce ten blocks per minute, and the open source plans can be made with a CNC torch table.
Here are DIY instructions for a simple CINVA press.
There are several commercial presses on the market.
| Name | Manufacturer | Blocks per day | Automated or Handpowered | PSI | Molds | Notes |
| Landcrete | Messrs, Soutth Aftica | 500 | Automated | |||
| Winget | Messrs, England | 100-150 | Automated pressure | 1000-1200 | Rotating table with three positions for three workers | |
| Ellson Blockmaster | Ellson Equipment, South Africa | 900-1100 | Semi | 9” x 12” x 4” 6” x 12” x 4” Interlocking hollow | Weights 485 lb Makes two bricks at a time | |
| CINVA-Ram | Richmond Engineering Co, United States | manual | 3 ½” x 5 ½” x 11 ½” 1 ½” thickness available for floor or roof tiles | Low maintenance, light weight |
Adobe
Abobe can be constructed with either heavy machinery or by hand. This is a more labor-intensive method as the bricks are handled three times while making, curing, and building with the blocks. This also requires a lot of space to lay the blocks and time to let them cure. However, the blocks can be prepared on a flexible schedule before constructing the walls. The soil needs to be mixed with any stabilizers into a loose mud and poured into forms. It should have 15-30% clay content. A high clay content will cause the clay blocks to shrink and crack, and a low clay content will cause it to fragment. To have the best stability, check the moisture content of the mix by cutting a V-shaped groove. If the edges are smooth and stable it is correct. If they are rough it needs more water. If they close it needs less.
The forms usually create blocks that weight around 50 pounds for ease of handling. The typical dimensions may be 5″ x 10″ x 20″ or 4″ x 12″ x 18″. The length of the block is the depth of the wall. Forms usually accommodate four blocks at a time.
The mixture is poured, and the form is lifted to create more bricks, leaving the bricks to dry. If the blocks are not stabilized, they will be turned on edge after hardening 2-4 days. If they have stabilizers like lime or cement, they are covered and left for seven days before turning and being stacked to save space. Blocks should be covered with plastic or otherwise protected from the rain until they are finished curing, which is after four weeks total.
The mortar for the blocks should be weaker than the blocks themselves. It can be made out of a 1:2:8-9 part mixture of portland cement, hydrated lime, and sand. If there is no lime, you can use the same soil mixture that was used to make the blocks.
Cob
Cob is a very simple and very inexpensive method that can be done with one person and a flattening tool, but it is labor and time intensive. The soil mixture is the same as with adobe, with 15-30% clay. More straw is customarily added to cob, which increases its insulative value, but reduces its load bearing capacity. Soil is mixed in a pile and treaded. It is mixed with water into a stiff mud and football sized balls are clumped onto the wall. They are pounded in place by hand. Each row is allowed to dry for two to three days before the next one is started on top. The walls are shaped with a knife or similar implement and worked until smooth. As the clay dries it will crack, but the cracks are usually small and dispersed. They do weaken the wall, but do not usually form a large damaging crack and are good for bonding to the surface coating.
These are some of the more popular methods of building with the earth. They show a lot of promise of providing quality affordable housing to more people.
References:
- http://earth-auroville.com/
- https://www.youtube.com/watch?v=IuQB3x4ZNeA
- https://static.tti.tamu.edu/tti.tamu.edu/documents/TTI-1962-2.pdf
- “Earth Construction, a comprehensive guide” – CRATerre, Hugo Houben and Hubert Guillaud
- https://www.energy.gov/energysaver/types-homes/efficient-earth-sheltered-homes
Disclaimer: please follow any local building laws and regulations and conduct your own research before building.
S. A. Clark 