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close this book Handbook for building homes of earth
View the document Table of contents
View the document Foreword
View the document Chapter 1: Introduction - Types of earth houses
View the document Chapter 2: Soils - And what can be done with them
View the document Chapter 3: Stabilization of soils
View the document Chapter 4: Where to build
View the document Chapter 5: Foundations
View the document Chapter 6: Lightweight roofs
View the document Chapter 7: Preparing the soil
View the document Chapter 8: Making adobe blocks
View the document Chapter 9: Making pressed earth blocks
View the document Chapter 10: Making walls with earth blocks
View the document Chapter 11: Making rammed earth walls
View the document Chapter 12: Roofs for earth houses
View the document Chapter 13: Floors
View the document Chapter 14: Surface coatings
View the document Suggested references
Open this folder and view contents Appendix

Chapter 7: Preparing the soil

Once you have your source of soil and have experimented with it enough so that you know how to use it best, the next step is to work out a careful and orderly plan, step by step, of what you are going to do with it.

To build a house of earth blocks or rammed earth means that you will have to:

1. Move several tons of soil.

2. Have a source of water handy.

3. Prepare your soil mix (adding stabilizers if necessary).

4. Set up a block-making or ramming operation.

To do these things efficiently, you must have what you need where you need it. For example, if you plan to build a house of blocks, they should be completed and stacked for curing as close as possible to where they will be used in the walls of the house.

If the roof has been built beforehand, as suggested in Chapter 6, the block making operation should be set up under it. The roof will provide shade in sunny weather and will permit work to go on if the weather is bad.

In planning the location of each step, study the order shown in Figure 33.

DIGGING THE SOIL - It does not matter how the soil is dug and moved to the place where it will be used. There are, however, some points to remember whether the soil is dug by machinery or by men with picks and shovels

Machine dug soil usually is lumpy. It is important that all clods or lumps are completely broken up before mixing in the water and stabilizer.

If the soil is lumpy, you should provide a space to break it up so that it will not be mixed with the freshly dug earth. If the soil is dug with picks and shovels in the first place, you will find that very little breaking up will be needed.

Often, you will be mixing two or more soils together to make your best possible mix. If you do, bring your soils to your building site and keep the different kinds into different piles. When you blend them later, you can make the mixture you found to be the best from your earlier testing.

Often, the builder will find a situation like this: a layer of top soil about a foot or more thick; then a couple of feet of sand under that, and beneath the sand, a layer of clay or sandy clay. If the builder will first Bet rid of the top soil, he can make his building mixture as he goes along by digging down the sides of the hole through the layers. If he is able to do this, then, he does not have to spend a lot of time later on making his proper blend. He can do all this as he digs by paying attention to the amounts of the different kinds of soil he digs. Do it this way if you can, of course, instead of removing the soil by digging out each layer and stockpiling it separately.


Figure 33.

MOISTURE CONTENT - The moisture content of the soil as you dig might be very close to the right amount needed for best compaction. (See pg. 30 to determine whether you have the right amount of water.) If it is, you can put your mix directly into the block-making machine or in the forms for rammed earth, unless, of course, you are going to mix stabilizers in the soil.

If the soil type is just right, as it sometimes is, but the mixture is too dry, the pit can be sprinkled with water before the earth is dug out. A little experience will quickly tell the builder just how moist the soil should be and how deep the water will penetrate into the sides of the pit.

For adobe construction, it is better to add water while the digging is going on. In this case, the mixing is also done in the pit. But if dry or powdered stabilizers are being added, they must be added to fairly dry soil first because it is difficult to mix them evenly into wet or muddy soil.

PREPARING THE MIX - If your soil mix is made up of two or more separate soils, here are some rules to follow in order to get best results:

1. Get rid of undesirable material. Be sure to remove all roots, leaves, trash and any other organic material.

2. Break the soil down to proper size before blending.

3. Stones larger than 1½" should be gotten out.

4. After all dirt clods have been broken up and crushed, put the soil through a screen with openings between ¼ and ½ inch. See Figure 34 for screens that have worked out well. Anything that does not go through should be discarded or crushed more.


Figure 35.

Of course, if rocks up to 1½" are being used in rammed earth or blocks, then, the size of the screen openings should be big enough to allow the size you want to use to go through. In this case, extra care must be taken to see to it that clods of dirt are completely broken up. If large clods are in the mix, water may not penetrate them. Later, when the wall becomes soaked by rain, the clods may swell and ruin the wall, or they could wash out and leave a hole that weakens the whole structure.

When two or more soils are to be blended, for example, when they come from different pits, it is best to sieve them separately and keep them in separate piles. Then, when it is time to mix them together, the correct blend can be measured out, such as "one pail of this soil to two pails of that soil," etc. This operation is shown in Figure 3a. Weighing the separate soils does a more accurate job, but it also takes much longer. A little practice will teach the builder to make his blends accurately enough so that weighing is not necessary.

During the blending operation, be sure the right mixture is kept the same. If it is a 2-1 mix, be sure it is always two to one.

MIXING THE SOIL - This is one of the most important steps in the whole operation. Mixing with the right amount of water is absolutely essential. The quality of a finished wall depends on good mixing and the right moisture content at the time the soil is being pressed into blocks or into a wall.

There are three methods for mixing the soil you can use:

1. A motor-powered mixer.

2. A manual or animal-powered mixer.

3. Or a mixing board and shovels.

If available, a motor-powered mixer can save you a lot of time on a large project but you must have the right kind Of mixer. Mixers which have paddles attached to the drum so that drum and paddles rotate as one piece (such as in an ordinary concrete mixer) will not be satisfactory for mixing soil for pressed blocks or rammed earth unless the soil is very sandy. The best type to use is one that has paddles or teeth that rotate in a stationary drum or container. If you are making adobe, a concrete mixer will work fine because an adobe mix is much wetter.

Several commercial mixers are available but a homemade mechanically powered mixer can be built if some type of motor is available. For small mixers, a gasoline engine of approximately 5 horsepower works well. Tractors or automobiles may be modified so that a belt or chain drive take-off can be used.


Figure 37.

A 55-gallon oil drum makes a good container for mixing. Figure 36 shows a successful mixer that you can make yourself.

A 25 or 30-gallon oil drum can be wed to make a smaller hand or animal powered mixer.

Be sure you have mixer large enough for the job.

For a single house project, the cheapest and easiest method for mixing soils is by hand. All you need is a flat surface and a shovel or a hoe. The floor of the house, if concrete, makes an excellent surface if you have built it beforehand.

To make sure that all batches of mix are the same, soil, water and stabilizer (if you are using them) must be measured accurately. The best way to do this i. to lay out a long row of soil, about 18 inches wide. Then use a template as shown in Figure 37 to level off the row to the right height, maybe six inches or a foot. Fill in the low spots and level again with the board. For every batch of mix, be sure that the rows are the same length and all are of the same width and thickness. If you are using stabilizers, sprinkle the right amount on top of the row. Pour a little water at a time and we the shovel to turn the material over and work it toward the center of the row. An ordinary garden sprinkling can is ideal for adding the water. But be sure to add the same amount of water and stabilizer (when used) to each batch. If you make each row the same way every time and use the same amount of water for each row, each batch of mix will be the same.

Do not mix more soil than you can use at one time. There is one exception, however. In cases where you are using lime with soil containing a lot of clay, thoroughly mix about half of the lime required and then add the water and allow it to "season'. for a day or two. While it is seasoning, cover the mix with wet sacks or leaves so the soil will not dry out. During this seasoning time, the lime will react with the soil to break down any clods or lumps that clay has. After the mix has seasoned, work it again carefully with shovels, add the rest of the lime, mix and add a little more water, if needed. Then, it is ready to be made into blocks or rammed into a wall.