Rammed Earth Homebuilding

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Rather, various combinations of particle sizes blend together over time to create soil mixtures: silty sand, clayish gravel, sandy gravel, etc. Which soil to use in constructing a rammed earth wall is a matter of availability, economics, and climate. Some soils, of course, are far more suitable than others.

The logical point to begin our research into which soils are best suited for building new rammed earth walls is to look at old walls. What is the typical composition of the soil that was used to construct walls that have survived for a century or more? It turns out that most of the world's oldest rammed earth walls were constructed with basically the same soil composition — roughly 70 percent sand and 30 percent clay. Need we look further? The answer, unfortunately, is yes we do. Most of the world's soil is not composed of 70 percent sand and 30 percent clay. In fact, soil varies enormously from region to region, even from one side of the valley to the other, and from the top of the hill to the bottom. Finding a suitable soil right beneath the spot on which you have chosen to build your house is like finding money buried there.

The reality is most of the soils we have encountered in our work are too high in clay content to be used as they naturally occur. Hence the need to understand the range of suitable soils and the means to amend those soils that are less than ideal. Prior to commencing a project of any significant size, test samples of the proposed mix design must be evaluated either by a laboratory or by your own field methods. The results of this preconstruction testing will determine the percentages of stabilizer and amendments to be used on the job. When both water and cement are being added to a soil, thorough mixing of the ingredients before they are compacted in the forms becomes critical.

A wall is, in fact, not much stronger than its weakest sections, and, when a soil is inadequately mixed, the advantages of adding cement are significantly lessened. Based on numerous observations in the field as well as laboratory testing, it appears that mixing, even beyond the point of visual homogeneity, plays a crucial role in obtaining the maximum strength attainable for any given soil formula. The fine particles of cement need time and agitation to incorporate themselves within the other fine-particle matrix of the clay-based soil. We have found that perfectly adequate and affordable mixing can be accomplished by using a rototiller mounted on the back of a tractor, affordable, that is, assuming that a tractor is already part of your wallbuilding equipment. When either borrowing or renting a tractor isn't in the budget, rammed earth can be built using wheelbarrows and five-gallon buckets, and a garden tiller can substitute for the tractormounted type.

Just remember, it will take a lot longer. For speed and efficiency, the tractor is an important part of the overall success of the project. During the mixing process, in order to maintain uniformity of color throughout a wall panel, it is advisable (though not essential) to make batches of mix that are big enough to fill the wall form you have set up. In other words, if you are building a panel that is 8 feet long, 7 feet high, and 18 inches thick, the total volume inside the form is a little over 3 cubic yards (8 x 7 x 1.5 / 27). Since most soils can be compressed to about 60 percent of their uncompacted volume, you'll need about 5 cubic yards of loose mix to fill this particular form. Different soils have different "bulking" factors. A bulking factor is basically the amount of air that is trapped inside a pile before it has been compacted to maximum density. Sands and gravels, soils which have low expansion/contraction ratios, will compact less than a "fluffy" clay-rich soil.

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