COMPRESSED EARTH BLOCKS
Strong, dimensionally stable building blocks can be made by compressing slightly moistened earth under moderately high pressure using a device made specifically for this purpose, commonly known as a compressed earth block press. This concept was developed in Columbia during the mid-1950s to address the need for low-cost housing. The original "CINVA RAM" was a portable, manually operated machine that could produce building blocks made from soil available at the building site. Since its inception, the basic design has changed very little. In recent years, hydraulics and compressed air have been used to generate a compaction force far greater than that attainable by human power alone. Despite these innovations, manually produced blocks are virtually indistinguishable from those produced by power-assisted machines.

Adobe vs. Cement Blocks: Test Results

BUILDING WITH COMPRESSED EARTH BLOCKS
Compressed earth blocks are similar to adobes, with the main differences being they are not fully saturated with water, are more dense than adobes, and are usually significantly more uniform. Because of their uniformity, compressed earth blocks need little mortar, and can even be dry-stacked. This uniformity also speeds up the laying process and results in straighter walls.

While the manufacture of compressed earth blocks is labor intensive, they can significantly reduce the costs involved in building a house, outbuildings, fences, garden walls, and so on. If soil consisting of clay, caliche and silt is available at the building site, admixtures (such as cement and lime) can be omitted, thus the only cost involved in producing compressed earth blocks is the block press and the time invested by the builder.

Advantages of using compressed earth blocks:

• They can be removed from the machine immediately after pressing and stacked for curing which requires generally no more than about a week (slightly longer if cement and/or lime is added to the mixture).
• Having uniform dimensions, sharp edges and smooth surfaces, they are easy to lay.
• They can be produced at a fraction of the cost of conventional cement blocks.
• They have structural qualities superior to kilned brick and other masonry materials.

MAKING COMPRESSED EARTH BLOCKS
Most soils, when reasonably free from organic matter, will make good compressed earth blocks and tiles. Best results are obtained by first screening the soil through 1/8" wire mesh to remove any pebbles, rocks, chunks of compacted soil, etc. Portland cement and/or lime can be added to the soil to increase strength and resistance to erosion, but this will increase the curing time. When using Portland cement in the soil, getting the formula right is critical. Too little cement produces a block that will crumble when handled; too much cement greatly increases the cost with no additional benefit. The amount of moisture in the soil or soil/cement mixture is also critical; a too-wet or too-dry mixture weakens the block; experimentation is required to determine the proper amount of moisture.

Once the proper soil mixture has been determined, making the blocks is straightforward. Place a measured amount of soil into the block press and close the lid. The soil mix is then compacted, either manually or via the machine's air/hydraulic jack. Once compressed, the block is carefully removed from the press and stacked in an area protected from the elements where they can cure.

FOUNDATIONS FOR EARTHEN STRUCTURES
A bit of engineering knowledge is required to determine the size of the foundation needed to support an earthen structure. Compressed earth blocks are heavy, typically weighing from 25 to 50 pounds depending on their size and content. Using conservative figures, this translates to roughly 1200 pounds per square foot, not including the weight of the roof and loading factors. Doubling this value is usually safe, so the foundation for a typical earthen structure must be able to support and distribute a load of 2400 pounds per square foot.

Alternative foundations can be constructed using rammed-earth tires, "earth bags" and many other environmentally friendly techniques. You'll find plenty of information about these alternatives on the Internet.

PROTECTION FROM THE ELEMENTS
There are several admixtures that can make your compressed earth blocks stronger and more water-resistant. Any admixture will increase construction costs, but the end result is usually a better, more durable product.

Hydrated lime is often the admixture of choice: the lime and clay in the soil react to form a binder, resulting in a stronger, more water-resistant block. Portland cement can also be used, but it's tricky business -- finding just the right proportions of cement to soil requires a great deal of experimentation. Another common admixture is asphalt emulsion, an oil-based product used in road construction. It acts as a binder and is highly effective at repelling water, but many environmentally-conscious earth builders find it objectionable (and plaster doesn't adhere well to it, either). Finally, there are enzyme products, again used in road construction, that are safe and non-toxic, and their binding abilities are quite amazing. While a bit expensive, around $120 per gallon, it's highly concentrated and a little goes a long way -- one gallon treats 500 gallons of water. If you need to build or improve a road leading to your home site, enzymes may be an ideal choice. Learn more about enzymes.

Regardless of the admixtures in your compressed earth blocks, protection from the elements -- especially water -- is essential. Fortunately, this is neither difficult nor expensive. Providing a long roof overhang goes a long way in keeping water away from the walls and foundation of your earthen home, and further helps shade the walls and reduce the amount of heat they collect on hot summer days. A simple stone or brick facade around the bottom perimeter of the structure also helps keep water away.

In recent years, a great debate has arisen as to how earthen walls should be protected. Conventional builders (and in fact, many building codes) insist that stucco be applied to the exterior surfaces of earthen walls. Experienced earth builders disagree with this notion, stating that stucco only serves to trap moisture in the wall which eventually causes damage and could possibly lead to catastrophic failure.

Close examination of old earthen structures -- many of which are still standing today -- reveals that multiple layers of lime-bearing mud plaster was used to protect them. Each thin layer contains a slightly greater amount of lime, the top-most of which is almost a pure lime plaster. Despite the fact that lime absorbs moisture, it will not retain it. A wall protected with a lime-based plaster can "breathe", allowing any moisture it may contain to eventually escape. On the other hand, stucco and cement-based products generally act as a barrier that actually traps moisture.

The general consensus among earth builders is that the exterior plaster should be made of the same or similar ingredients as the wall itself. If you've added cement and/or lime to your soil, you should do the same with your plaster.