Sedimentary rocks are formed over thousands of years, as layers of soil are carried to a settlement bed by water and compacted by each subsequent layer of sediment. Building a rammed earth home is like creating sedimentary rock but very, very quickly. The number of rammed earth construction crews is growing, but a house can also be completed by an owner without professional help.
In the days of building the Great Wall of China, people just rammed some earth and called it a day. Walls either stood the test of time, or they didn't. These days, we know more about the properties of soils. Rammed earth requires a cross-grade of soils, but the mix shouldn't have too much clay; excess clay will cause the walls to shrink and crack.
Can you go outside and just start digging? Well, you might want to create a test sample and take it to a geotechnical lab first. You can find a geotechnical lab by contacting agriculture departments at local universities or by searching for geotechnical engineering services in the phone book. The lab can test the sample's compressive strength. The sample should be able to withstand pressure of 300 pounds per square inch (PSI); this is the standard in earthen material building codes [source: Branch].
Rammed earth builder Quentin Branch estimates that he uses natural soil from the building site about 50 percent of the time [source: Branch]. Rock quarries also sell reject fill for a few dollars per ton. The mixture should also contain water, but no more than about 10 percent [source: McMeekin].
While earthen mixtures can be mixed by hand, it's more common to use a tractor or a tiller. Once the earth is mixed, however, it needs to go straight into the walls before it hardens and dries. For this reason, rammed earth construction is done on-site. Before mixing, builders need to assemble a set of wooden forms. These are like the buckets of sand castle construction, except they're rectangular plywood forms rather than plastic buckets. Two end pieces are the width of the wall, two other pieces are the length of each wall segment, and everything is held together by pipe clamps that can be loosened or tightened as the form is moved.
Next, about 4 inches to 6 inches (10 cm to 15 cm) of soil is shoveled into the form at a time [source: Easton]. Ramming can either be done by hand or by machine. A hand rammer should be about 15 pounds (6.8 kg), and it's usually a piece of steel attached to a pole [source: Easton]. Professional rammed earth builders use machines that are hooked up to air compressors. The air pressure does the work of tamping down the earth. When you first start ramming dirt, it makes a dull sound, but when it's adequately tamped, it will change to a ringing sound. Once the earth is ringing, then you can add the next 4 to 6 inches of soil. It is possible to ram too much, which will cause stress on the wooden form.
Once a section of wall has been constructed, the form is dismantled and moved to the next section, and the process begins again. Windows and doors are installed by ramming earth around separate forms that serve as placeholders. Once the earth has been cured (or set) around the placeholder, the form is removed and the earth will hold without it.
On the next page, we'll look at some of the benefits of building this way.