Rammed Earth Homebuilding

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You'll need to experiment to learn the ratio of loose volume to compacted volume for the soil being used on any given project. Calculating the amount of cement to add to the pile is based on the compacted volume of earth, not the loose volume. For example, the form panel we have set up has a volume of 84 cubic feet (8 x 7 x 1.5), or 3.1 cubic yards. Let's say the minimum adequate rate of stabilization determined from our preconstruction testing is 7.5 percent. Then by simple mathematics we know that the number of sacks that should be added to the mix pile is 6.3 (84 x 0.075 = 6.3). Each sack of cement is exactly one cubic foot. Regardless of how "bulky" the loose soil is, once it is fully compacted in the form it will have a volume of 84 cubic feet. Six and one-third sacks of cement will combine with that 84 cubic feet of soil, yielding the desired stabilization ratio of 7.5 percent.

Once you have determined the moisture content of the mix to be "just right," it is important to transport the stabilized earth to the formwork and accomplish the layering and compacting as quickly as possible. The cement in the mix will begin to hydrate as soon as water is added. It will start to absorb the moisture in the soil and, if not used in a relatively short period of time, this process will dry out the mix significantly enough to affect the compaction. How much time you actually have varies depending on the weather. If it's cool and cloudy, the mix will keep for a few hours. On the other hand, when it's hot and windy, the mix can become too dry after only one hour.

If this is the case, you may even need to sprinkle and turn the pile several times as the wall panel is being constructed. You'll just have to keep your eye on the mix as you're building panels. Soils vary in how quickly they give up their moisture to the cement.

Ramming the Earth

Think of rammed earth as a sort of "instant rock." The natural process of creating sedimentary rock occurs over a span of thousands and millions of years. An earth rammer, on the other hand, creates it in a matter of minutes.

The historically proven tool for compacting soil is the hand tamper, a heavy block of wood fitted with a pole for a handle. Slow and tedious, tamping by hand is nevertheless extremely effective. Millions of yards of earth, on six continents, have been rammed into stone under the repeated blows of a hand tamper. The average rate of production for a crew building rammed earth by hand, as documented in several older publications, is 1 1/2 cubic feet of wall per man per hour.

A good hand tamper can be made with a 4" x 4" x 5/16" steel plate welded to a sixfoot-long piece of one-inch steel pipe. This results in a tool with sufficient weight for achieving good compaction, while still light enough to operate for long periods of time without tiring the arms. The one-inch diameter of the pipe fits well into the hands.

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