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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 2: Soils - And what can be done with them

Not all soils can he used successfully for earth houses. A few of them will be good in nearly any type of climate. Some of them will be good only in dry climates. Many soils can be made more suitable with "stabilizers," substances that hold them together or make them water resistant. The various kinds of soils, how to tell them apart, and how to find. out what they will do, will be discussed in this chapter.

Kinds of Soils

Broadly speaking, there are five kinds of soils: gravels, sands, silts, clays, and organic soils.

Gravel consists of coarse pieces of rock varying in size from ¼" across to 3". (Anything larger than 3 inches is called a boulder.) Gravel can be any shape - round, flat or angular - and it can be any type of rock - granite, limestone. marble, etc. If it falls apart or even gets soft after being under water for 24 hours, it is not gravel.

Gravel is found in the beds of fast-flowing streams, in areas once covered by glaciers and around mountains.

Sand consists of fine grains of various rocks, mostly quartz. It varies in size from ¼ inch to about the smallest grain you can see with the naked eye. Separate grains too small to see are either silt or clay.

Sand is found in the beds of most streams. except slowly flowing ones, on beaches, deserts, and in areas once covered by glaciers.

Silt is rock ground up so fine you cannot see individual grains with the naked eye. Silt will tend to hold together when wet and compressed. Too much water ma! make it sponge, but it does not get very sticky.

Silt may be found nearly any place: in the deposits of slowly flowing streams, in the "milky" colored streams coming from glaciers or mountains or where crust blown by winds has settled.

Clay is a natural, earthy material that is sticky when wet but hard when dry. Separate grains are too fine to be seen with the unaided eye. There are many different kinds of clay; some of them will shrink and swell greatly with drying and wetting, while others will not.

Clays can be found in the valleys where slow-moving streams and rivers flow, in coastal plains, in the fan-shaped deposits at the bottoms of mountains.

Organic soils have a spongy, or strings appearance. The organic matter may be fibrous, rotted or partially rotted vegetation. such as peat. Organic soils are very spongy when moist anti have an odor of wet, decaying wood. In nature, they will nearly always contain a lot of water. They are dark-colored, ranging from light brown to black.

Organic soils are usually found where water has been standing for long periods, for example, in swamp areas. The dark-colored topsoil found in many areas owes its color to organic matter.

The five types of soils are seldom found separately. Instead, you will find mixtures of them, such as a mixture of sand and silt, or silt and clay, and so forth. By combining the names of different soil types you can describe most of the properties of a soil mixture. For example, a soil with mostly sand and a little silt would be called a "silty sand." If it is mostly silt with a small amount of sand, it would be a "sandy silt." Some common examples are: sandy clay, clayey gravel, silty clay, sandy gravel, etc.

What Type of Soil Is Best for Earth Houses

The type of earth house you build, or whether you build an earth house at all, will be affected by the type of soil available and by the climate.

Gravels by themselves are not very good for earth houses because the particles will not pack down and hold together. Gravelly soils can be used if the rocks are not too large and if there is something to hold the rocks together such as a little clay. Clayey gravels often work out well.

Sands are about the same as gravels. Since they will not hold together by themselves, something else such as clay must be added. In fact, some sandy clays and clayey sands make the best earth houses. In the absence of good clay to mix with the sand, portland cement makes an excellent stabilizer.

Silts by themselves are not good for walls of earth houses. Although they will hold together, they are not very strong soils. They are difficult to compact and should not be used for rammed earth or pressed block walls. Silts also lose strength and become soft when they get wet. In wet, freezing weather they swell and lose their strength.

Silty soils can be stabilized to make a fairly good building material. Portland cement is good for sandy silts and lime works on clayey silts. Asphalt emulsion or any chemicals that waterproof such soils will do just as well.

Clays will pack down well if they have the right amount of water in them. In dry weather, though, they will shrink and crack and in wet weather they will absorb water causing swelling and loss of length. They would work well in extremely dry climates because they are very strong when kept dry; but, usually, clays are not found in very dry climates.

A few kinds of clays like the red iron- and aluminum" bearing clays found in the tropics (sometimes called laterites) are very stable clays. It is common practice in these areas to cut blocks of clay right out of the ground and stack them up to make earth walls. Experience of one's neighbors with this method will tell whether it is suitable in your area.

Many other clays can be made suitable with stabilizers. One of the best stabilizers for clay is dime. There are some clays that should never be used in earth houses. They just will not last.

Organic soils cannot be used to make a good earth wall. For one thing, they are too spongy. Soil that contains decomposing plant life continues to decompose and thus will never "set" right or hold together over a long period of time. A good rule to follow is this: if the soil is good for growing things, it will not be good for building.

Remember that the best natural soil you can use for making earth walls is a sandy clay or a clayey sand. If you happen to have such a soil, you have as good a natural building material as can be found. Without the addition of anything more than water, some kinds of sandy clays or clayey sands can be made into walls that will last a lifetime - or even longer.

If you do not have this kind of soil, you might be able to make it. If you happen to have mostly sand, maybe you can find some clay to mix with it, or if you have clay, you might find sand to mix with it.

WHERE TO LOOK - Often you will find a situation like this: beneath the organic topsoil, you will find a layer of sand. Below this- is often found a layer of clay. By mixing the sand and clay together you might make a good sandy clay. Also, remember that on the top of rolling hills (not mountains) or ridges you are more likely to find clays, and sands will be most common at the bottom. Probably just what you need, a mixture of both, can be discovered somewhere between.

If you are fortunate and have (or can make) a good sandy clay mix, a wise choice may be to build your house of pressed blocks which will last as long as any of the other wall types and may be easier to do. However, with a good supply of sandy clay available, both rammed earth and adobe can also be built very satisfactorily.

If the only material you can find is very clayey, probably you should build an adobe house. The clay causes the soil to shrink when it dries but since you let adobe blocks dry in the sun before you lay them in the walls, the shrinkage will not bother you too much. The next best thing to use is pressed blocks, since they also are dried before they are used. Rammed earth dries after it has been compacted in the wall and the shrinkage caused by too much clay will make the walls crack.

If your material has too much clay in it and not enough sand is available the only thing to do is add stabilizers.

If your soil is very sandy, with only a little clay in it you will not be able to build any type of earth house without adding some sort of stabilizer to it. You can probably get by with the least amount of stabilizer by making pressed blocks. Next would be rammed earth.

Probably the most difficult type of house to select a soil for is rammed earth. If the soil has a little too much clay in it this will cause shrinking and cracking of the rammed earth walls when they dry. If it has a little too much sand in it the walls might not hold up even during construction because the shocks from ramming might cause it to crumble. If you do find a soil that will be good for rammed earth it will also be good for pressed blocks or adobe. Then you can choose the type of construction you want based on which method seems to be easiest for you and gives the best looking house.

No matter what kind of soil you have it is well to bear in mind that the drier the climate year around the more satisfactory the building will be and the easier it will be to build well.

In areas where weather is subject to big changes in the course of a year such as hot weather followed by freezes which occur in much of the Temperate Zones, or areas that have definite wet anti dry seasons such as are common in the Tropic Zone, only the very best soils can Ix used without stabilizers.

All this does not mean, however that good earth houses cannot Ix built in wet climates or where great changes in temperature occur; it just means that under these conditions more care must Ix used in choosing the "raw materials" and greater attention must Ix given to the use of stabilizers and surface coatings.

FINDING OUT ABOUT YOUR SOIL - This is one of the most important jobs you have to do. If you make a mistake now it will cause you trouble later. [or example if you decide you have a good supply of sandy clay and it later turns out to Ix mostly sand, you will have to spend extra money for stabilizers that you had not planned on.

You will probably want your soil to come from a place as close as possible to your house.

THE FIRST THING TO DO IS TO GET SOME SAMPLES OF THE SOILS IN YOUR AREA - Here is the equipment that will help you do this. (See Figure S.)

1. A dirt auger to drill holes in the ground is ideal. Post hole diggers are also good, especially if you do not plan to go very deep.

2. Pipe extensions for the dirt auger. These are necessary only if you want to look at the soil at a depth greater than 4 or 5 feet.

3. Two pipe wrenches. Use these for adding the extensions to the dirt auger.

4. Shovels. If you don't have a dirt auger you can use an ordinary shovel.

5. Pick-axes or mattocks.

6. A supply of small bags (cloth, if possible) that will hold 10-30 pounds of soil.

7. A ball of twine.

8. A 6-ft. ruler.

9. Paper and pencils.

10. One or more large pieces of canvas about (6'x6') for soil samples.

The depth to which you are going to examine your soil will depend a lot on how you are going to dig the soil for your house later. If you are going to dig by hand, you probably will not want to dig more than 3 to 5 feet deep. If your soil will be dug by machine, you will want to examine the soil as deep as the machine will dig, perhaps 8, 10 or more feet deep.

First, dig out and toss aside the organic topsoil. In desert areas, there will be little or no topsoil as such. In wet, tropical areas, the top soil may be several feet thick. Once you are through the topsoil, start collecting the soil. The soil may change several different times, even at shallow depths. For this reason, you should separate each type of soil by putting it in a different pile.

Figure 6.

Usually, but not always, a change in color of the soil will mean a change in soil type. The best way to tell whether you are changing soil types is to use the simple tests described in the next section. These tests require no equipment and can be done as you dig the soil.

Here is a typical situation. Below the topsoil you might run into a layer of sand. Save all of this sand and put it in a single pile. Then you come to a layer of clay. Put all of the clay into another pile, and so on. When you are finished, you may have several piles of different soils. Figure 6 shows how this is done.

As you dig, write down the thickness of each layer, the color and type of soil, and an accurate description of the location of the hole.

Soils can vary widely even within a small area. For this reason, do not be satisfied with what you find in a single hole. Instead, dig several holes in an area that is big enough to supply all of the soil you want. If all of the holes produce the same kinds of soil. combine the same types into separate piles, such as all the sand samples together, and all the clay samples together. After making some quick tests, you may decide that a mixture of what you have should work out well. Since you have saved all of the soil from the holes, you might find you have the right mixture simply by mixing the sand and clay into one pile. But at the beginning, separate all the different kinds first until you are sure of what you have.

When you are satisfied that you have examined an area completely, put each soil type in separate bags. Label each bag with the hole (or holes) and the depth that it came from. These bags of soil will be used for the tests that will decide the type of soil you have and the type of house you should build.

How to Identify Soils

Here are some simple tests that will tell you what kind of soil you have. Do all of them on all your samples. Be sure that the samples that you test accurately represent the soils you will use in building.

If you are testing sands or gravels, first dry the soil by heating or spreading a sample in the sun. Make it into a cone-shaped pile, and carefully divide it into four equal samples. Combine two opposite portions into one sample and set aside the other two. You should end up with about a shovelful of soil. If there is too much soil after one such separation, repeat the process of dividing and discarding until a suitable size soil sample remains.

VISUAL TESTS - The appearance of a soil can tell you some important things about it. First spread the dried soil out in a thin layer on a flat surface. Then roughly separate the sand and gravel sizes by hand.

Do this by putting all of the particles from the largest down to the smallest that you can see with the unaided eye in one pile. This will be the semis and gravels. What is left (normally this will be very fine powder-like materials) will be the silts and clays. If the silt and clay pile is large; than the sand-gravel pile, call the soil silt-clay for now and remember this. Other tests, described later, will tell you which it is.

If the sand and gravel piles together are bigger, you have a sand or a gravel. Decide which it is by putting all of the particles larger than ¼" (gravels) in one pile and all of the smaller particles (sands) in another pile. The soil is gravelly if the gravel pile is biggest and sandy if the sand pile is biggest. Remember which it is.

Here is what you do if you have a sandy or gravelly soil:

Take a small handful of the entire sample (not just the sand and gravel), get it moist but not soupy, squeeze it into a ball, and let it dry in sun. If it falls apart as it dries, call it "clean." Clean sands and gravels are not suitable for earth houses unless they are mixed with other materials.

Here is what you do if you have a silt-clay soil or a sand or gravel that is not clean:

Take the entire sample and collect all of the soil that is smaller than medium sand (1/64") by sifting through a very fine screen or a piece of coarse cloth. The tests described below should be made with this fine material.

WET SHAKING TESTS - Take enough of the soil to form a ball the size of a small hen's egg and moisten it with water. The ball should have just enough water in it so that it will hold together but not stick to your fingers. Flatten the ball slightly in your palm and shake the ball vigorously. This is done by jarring the hand against some firm object or against the other hand until the shaking brings water to the surface of the sample. The soil may have a smooth, shiny or "livery" appearance when this happens. (What you are looking for is to see how fast the water comes to the surface and gives the livery appearance.) Then, squeeze the sample between your thumb and forefinger to see whether or not the water disappears.

The following are terms used in describing the speed of the above reaction (See Figure 8):

1. Rapid - When it takes only five to ten taps to bring water to the surface, this is called a rapid reaction. Squeezing the sample should cause the water to disappear immediately so the surface looks dull. Opening the hand quickly should accomplish the same result. Continued pressure causes the sample to crack and finally crumble. This type of reaction is typical of very fine sands and coarse silts. Even a little bit of clay will keep the reaction from being rapid.

2. Sluggish (or Slow) Reaction - When it takes 20 to 30 taps to bring the water to the surface, you have a sluggish reaction. Squeezing the sample after it has been shaken will not cause it to crack and crumble. Instead, it will flatten out like a ball of putty. This shows that the soil has some clay in it.

3. Very Slow or No Reaction - Some soils will not show any reaction to the shaking test, no matter how long you shake them. The longer it takes to show a reaction, the more clay the soil contains. These sails will require the other tests described below before you can tell much about them.

THREAD TEST - To a lump of soil about the size of an olive, mix just enough wiser so the lump can be easily molded in your hands, but is not sticky. Next, on a flat clean surface roll out the soil into a thread. Use the palm of your hand or fingers and exert just enough pressure to make the soil thread get continually smaller. If it breaks before you roll it out to a 1/8'' diameter thread, it is too dry and you need to add some more water to it. When the soil is at the right moisture content, the thread will begin to crumble into several small pieces just when you get it to a diameter of 1/8"- If the thread does not crumble and break at 1/8", lump it together again, knead it into one lump, and repeat the rolling process until the thread crumbles at 1/8" diameter. (The thread will eventually crumble because it dries as you keep rolling it out.)

As soon as the thread crumbles, re-mold the sample into a ball and see how much pressure it takes to squeeze the ball between your thumb and forefinger.

This test gives an idea of how much clay is in a soil and also what type of clay it is. If the soil crumbles easily and you cannot roll the soil into a thread at any moisture content, it means that the soil does not have any clay in it. Here are some of the other reactions you can expect:

1. Tough Thread - If the remolded ball can be deformed only with a lot of effort and it does not crack or crumble when you do it, your soil has a lot of clay in it. It probably will not be good for earth walls unless you use a stabilizer.

2. Medium Strength Thread - This kind of soil can be remolded into a ball, but when the ball is squeezed between the fingers, it will crack and easily crumble. This soil may be good but may require some stabilization for certain areas. Check Table I to be sure.

3. Weak Thread - When the soil has a lot of silt or sand and very little clay, you will find that the threads cannot be lumped together in a ball without completely breaking up or crumbling. This soil may be good for earth walls; check Table I to be sure.

4. Soft, Spongy Thread - Sometimes you will find that the threads and the ball that you make with them will be spongy and soft. You can squeeze the ball between your fingers, but it acts like a sponge and bounces back. When this happens, the soil is organic and it's not suitable for building earth houses.

RIBBON TEST - This test gives about the same kind of information that the thread test gives. It helps to do both tests. One checks out the other.

Take enough soil to form a roll about the size of a cigar. The roll should not be sticky, but wet enough to permit being rolled into a 1/8" diameter thread without crumbling, as in the thread test. Put the roll in the palm of your hand and, starting at one end, flatten the roll by squeezing it between the thumb and forefinger to form a ribbon between 1/8" and ¼" thick. Handle the soil very carefully to form the maximum length of ribbon that the soil will support. See how long the ribbon will hold together without breaking. The reactions you can expect-are described below.

1. Long Ribbons - With some soils the ribbon will hold together for a length of 8 to 10 inches without breaking. This means that the soil has a lot of clay in it. Soils of this type will make long-lasting earth walls only if they are stabilized.

2. Short Ribbons - If you can - with some difficulty - ribbon the soil into short lengths of about 2 to 4 inches, the soil has a medium to small amount of clay in it. It will be about the same as the soils that give a medium or weak thread in the thread test. This soil will make good walls in many cases but to be sure check Table 1.

3. Will Not Ribbon - Some soils cannot be formed into ribbons at all. This means that they contain either a very small amount of clay or none at all. Such soils with a little clay may make good rammed earth walls. If the soil is all sand it is not suitable unless stabilized heavily with portland cement; to be sure, check Table 1.

DRY STRENGTH TEST - This is another simple test that will help you determine how much clay you have in the soil. Prepare two or three wet pats of the soil about ½" thick and 1" to 2" wide. Use enough water to make the soil quite soft but still strong enough to hold its shape when you form it into pats. Then allow the pats to dry in the sun or in an oven until they are dry all the way through. Break the soil pat and then try to powder it between your thumb and forefinger. Here is what you are looking for:

1. High Dry Strength - If the sample has high dry strength it will be very difficult to break. When it does break it will snap sharply, like a crisp cookie. You will not be able to powder the soil between your thumb and forefinger. You may be able to crumble it a bit with your fingers, but don't confuse this with powdering the soil. Soils with this reaction have a lot of clay in them, and they will be satisfactory only if stabilized.

2. Median' Dry Strength - When a soil has a medium dry strength, it will not be too hard to break the soil pat. With a little effort you will be able to powder the soil down to its separate grain sizes between your thumb and forefinger. This soil is good but may require a stabilizer to reduce shrinkage; check Table 1.

3. Low Dry' Strength - A pat with very little clay will break without any trouble. It will powder easily. Pats of very sandy soils will crumble in your hand before you have a chance to powder them. Before a final decision on the use of this soil, check Table 1.

The four tests described above are the most important ones and it will pay you to use them all in finding out about your soil. There are some other simple tests that will also aid you. Use them if you need to. They are given below.

ODOR TEST - Organic soils have a musty odor, especially when freshly dug. You get the same odor for dry organic soils by wetting and then heating them. Don't use these soils in earth walls.

BITE TESTS - This is a quick and useful way of identifying sand, silt, or clay. Take a small pinch of the soil and grind it lightly between your teeth. Identify the soils as follows:

1. Sandy Soils - The sharp, hard particles of sand will grate between the teeth and will create an objectionable feeling. Even very fine sands will do this.

2. Silly Soils - Silt grains are much smaller than sand particles and although they will still grate between the teeth, they are not particularly objectionable. They feel a lot smoother than sands.




If the silt-clay pile was larger than the sand and gravel piles together, then use the Table below to determine what kind of soil it is.

Names of Soil

Reaction to Wet Shaking Test

Dry Strength Test

Thread Test

Ribbon Test

Additional Tests

Suitability for Earth Homes



Very fine sands, silty fine sands, clayey fine sands, clayey silts

May be rapid to sluggish, but never very slow

Low to none; usually none

Weak thread to no strength in thread

Short ribbons; may not ribbon at all

Washes off hands easily. Will not stain hands

Usually suitable for all types, particularly adobe if stabilized

Portland cement most suitable. Asphalt emulsions also work as do most waterproofers

May be affected by frost

Silts, very

May be anything from sluggish to none

May he low to medium

Weak to medium strength thread

Short ribbons


Should not be used if possible. Stabilize heavily if necessary to use

Portland cement, asphalt emulsions if soil is not too sticky

Will usually require surface coatings in addition to stabilizers

Gravelly clay, sandy clay, silty clay

May be very slow to none

May be medium to high

Medium strength thread

Short to lone ribbons

Will usually require stabilizers most suitable for rammed earth and pressed blocks


Lime Sand Gravel

Can be very good if amount of sand or gravel is high

Clays, fat clays


High to very high

Touch thread

Long ribbons

Very sticky when wet, difficult to wash off of hands

Should never be used for earth houses


Organic silts, organic silty clays


Low to medium

Weak thread and feels spongy

Short ribbons or may not ribbon at all. Spongy feel

A Pat of moist soil has a mushy odor when heated

Should never be used for earth houses


Organic silts, organic clays

Maybe very slow to none

Medium to high

Weak to medium. Threads feel spongy

Short ribbons, spongy feel

A pat of moist soil has a mushy odor when heated

Should never be used for earth houses




If the gravel pile was larger than the sand pile, then use the

Table below to decide what kind of gravel it is.

Silty gravels, sand-silt-gravel mixtures


Low to none; usually none

No strength of thread

Will not ribbon

Fine material washes off easily. Will not stain hands

Usually suitable if it is first stabilized. If almost a "clean" gravel it may be necessary to first add more fines

Portland cement most suitable. Asphalt emulsions may also work.

May be affected by frost

Clayey gravels, gravel-sand-clay mixtures

Sluggish to very slow


Medium strength thread

Short ribbons, may be long

Finer material not easily washed off of hands

May be very suitable for all types of earth houses. If almost clean, it may be necessary to add some fines

Lime most suitable. Portland cement may work if soil mixes easily


Clean gravel

Not necessary to run these tests on clean gravels


Not suitable for earth houses. Can be mixed with fines(silt or clay) to make suitable soils for earth houses


If well graded, will be very good for aggregate in concrete for foundations



If the sand pile was larger than the gravel pile, then use the

Table below to decide what kind of sand it is.

Silty- sands


Low to none, usually none

No strength of thread

Will not ribbon

Fine material washes off easily. Will not stain hands

Usually suitable if stabilized. If almost a "clean" sand it may be necessary to add more fines

Portland cement is best. Asphalt emulsions may work clayey fines

May be affected by frost

Clayey sands

Sluggish to very slow



Short ribbons but may be long

Fine material not easily washed off of hands

Usually very suitable for all types of earth houses If almost clean, may add some clayey fines

Lime is best. Portland cement will work if soil mixes easily


Clean sands

Not necessary to run these tests on clean sands

Not suitable for earth houses unless mixed with fines

Clayey fines

If well-graded will be good for aggregate in concrete for foundations


3. Clayey Soils - The clay grains are not gritty at all. Instead, they feel smooth and powdery like flour between the teeth. You will find that a dry pat of soil with a lot of clay in it will tend to stick when lightly touched to your tongue.

SHINE TEST - Take a pat of either dry or moist soil and rub it with your fingernail or the flat side of a knife blade. If the soil contains silt or sand - even with the remainder being clay - the surface will remain dull. A soil that has a lot of clay in it will become quite shiny.

TRY WASHING YOUR HANDS - You can tell a lot about a soil in the way it washes off of your hands. Wet clayey soils feel soapy or slick, and they are hard to wash off. Silty soils feel powdery like flour, but they are not too difficult to wash off. Sandy soils rinse off easily.

Color is important in classifying soils. Olive-greenish and light brown to black colors may mean organic soils. Red and dark brown colors may come from iron in the soil. Soils with a lot of coral, limerock, gypsum, and caliche may be white or some shade of gray.

After you have done all of the tests given above and have decided what the reactions to them are, you are ready to use Table 1. It will tell you exactly what kind of soil you have and what kind of house you can build with your soil.

Here is the way to use Table 1: Suppose you found that your soil was a gravelly soil. This means that the sand and gravel piles together were larger than the silt-clay pile, and the gravel pile was larger than the sand pile. Use the gravel chart in Table 1 - this is for the gravels. Suppose the tests you did on the part that passed the fly screen showed your soil reacted rapidly to the shaking test, had weak soil threads and very low dry strength. Then your soil would be a silty gravel. It would not be suitable for earth houses without stabilization.

Getting More Exact

Of course, the tests just described are pretty crude according to the standards of a soils engineer. But once you have performed them a few times and "get the feel" of your soil they will give you the information you need.

However, in order that you might know what a soils engineer would do, following is a list of tests that he would perform (or you could do yourself if you had the equipment). If you can do these tests yourself, or have someone do them for you, tables - similar to Table 1 can be used to determine more accurately the type of soil you have and what can be done with it. The tests are described in detail in Appendix A.

1. Gradation tests will tell you more exactly about the size of soil particles. There are two techniques of doing this, a simple method which uses little equipment, and a more complicated method involving special equipment.

2. Lineal shrinkage tests are a fairly accurate and simple way of telling how much clay your soil contains and how the clay will act as far as shrinking and swelling is concerned.

3. Atterberg limits give you much the same information that the lineal shrinkage test does, but more accurately.

Tests on Blocks

The simple field tests you have done tell you much more than you knew about your soil before. But these tests alone can't tell you everything you need to know about your soil. For this you must do some more tests. These tests will require you to make some actual blocks of the type of construction recommended for your soil in Table 1. It takes about a month to make, cure and test the samples, but it is worthwhile. Your house, well made, will last a lifetime.

It is best to use actual size blocks as test samples, but if you are testing many different soils, or one soil with several stabilizers, this may require a large quantity of soil, Then, you can make smaller test samples roughly this size: 6x3x2 inches. You will need 7 test blocks of each soil. This will take about 4 shovelfuls of soil for these blocks.

If Table I shows that your soil might work with more than one type of earth construction, then the best thing to do is to make 7 test samples of each type recommended, test the samples, and then decide on the type of construction to use.

When you have done all the tests and finally decided on your soil and type of construction you will use, it is a good idea to make a few actual size blocks (if you used 6x3x2-inch blocks in your evaluation tests) and test them just to make sure they act like the smaller blocks.

Here is what you do for the different types of earth construction.

ADOBE - First, see how water mixes with your soil. If it doesn't mix easily into a smooth mud, but instead sticks to everything including your mixing tools, it won't make a satisfactory adobe house. (It contains too much clay.)

If you still would like to use adobe construction anyway, you will have to add a stabilizer. There are several, as you will see from Chapter 3, but let's suppose you've decided to use lime.

For one test block mixture, add one part of lime to 50 parts of soil; for another, add one part lime in 25 parts soil, and for another, one part lime in 17 parts soil. Make enough

Of each of these mixtures to make 7 blocks. Mix the soil and stabilizers together until you get a uniform color. Mixing is very important; so, do it well.

Then - whether your soil has stabilizer in it or not - gradually add water until you have a thick mud. You can tell when en it is right by running a pointed stick through it. If the bottom of the groove barely closes due to its own weight, it is right.

Place your wet soil mix in a form box. Figure 11 shows a form box for small samples that has enough space for eight blocks, 6x3x2 inches.

You can be sure the forms are properly filled by working the mud around a bit with your hands until there are no more air pockets. Scrape the excess mixture from the top of the form with a board or the edge of your shovel.

Let the form set about fifteen minutes so that it can be lifted off the blocks without the blocks losing their shape very much. If the blocks slump or settle, you have added too much water and you should begin again.

After a few days turn the blocks on edge and let them cure. Let unstabilized blocks cure in the sun for four weeks.

If the test blocks contain a stabilizer they should be sprinkled for at least the first week or else kept fully covered to keep them moist. Blocks should be protected from rains with anything that will keep the water off them. At all times however air should be able to get to them.

While the blocks are curing if large open cracks appear you can tell without waiting for four weeks that they contain too much clay. Full size adobe blocks should not have more than 2 or 3 narrow cracks and these should not go completely through the block. The small 6x3x2-inch blocks should not have any cracks at all. If the blocks can be crumbled easily after a week or so the soil is too sandy.

When the adobe blocks are fully cured they are ready to be tested.

PRESSED BLOCKS AND RAMMED EARTH - One of the differences between making adobe block and pressed block or rammed earth lies in the amount of water used in preparing the soil. Adobe, we will call wet; the others should only be "moist." It is Important to get the right amount of moisture in the soil. Proper anti complete mixing is also essential.

To check the moisture content take a handful of moist soil and make a ball with your hands about the size of a small orange. Press it together as firmly as you can. Then drop it onto a hard surface from shoulder height. If it shatters into pieces so that it is about the way it was before you molded it the moisture content is right. If it breaks into a few large pieces or flattens out it is too wet. If it is difficult to press into a ball that holds together or if you can crumble it easily between your fingers it is too dry. This test will apply whether the soil is stabilized or not.

Once the moisture content is correct you are ready to make trial pressed blocks or rammed earth.

Of course if you have a machine for making full size blocks. use it. If you do not you will need a mold such as shown in Figure 12 and some means of applying pressure to compact the soils. You should apply about 300 pounds for every square inch of block surface so the molds will have to be able to withstand a lot of pressure. Make each of the trial blocks exactly the same way.

Remove each block from the mold and allow it to cure the same way as the adobe for four weeks. Look for cracks in the blocks during the curing period. If full size blocks contain more than one narrow crack in them, they will not be suitable.

Rammed Earth

Make a form as shown in Figure 13, about one foot by one foot by eight inches deep inside. It should be made of seasoned lumber that will not shrink, and it should be coated with oil before using it. Then make a tamper for ramming the earth. A simple tamper can be made by threading a heavy, flat-faced piece of metal on to a piece of pipe.

Fill tile form (not including the collar) about ¾ full of loose well mixed soil and ram it 50 times. Then put the same amount in the form again, and ram it 50 times. You should end up with a block - made up of 2 layers - slightly more than inches thick. Use a knife or flat piece of steel to smooth the top of the block. Be sure that you ram each of the trial blocks the same way as all the others.

Remove the form from all blocks except the last one made and cure the blocks the same as adobe. The last block should be carefully cured in the form. If the soil shrinks away from the form as it dries, it will not be suitable for rammed earth.

TESTS ON BLOCKS - After all test blocks have cured for at least four weeks, the test described below can begin.

ABSORPTION TEST- This test tells you how fast your blocks will soak up water and whether the water will cause them to swell. It should be done on every soil you intend to use in earth walls, regardless of the type of construction used. If you have several soils available and are trying to decide which one to use, this test can help you decide.

Equipment and supplies needed:

1. Two of your seven blocks of each soil mixture.

2. A shallow pan that will hold water at least I" deep, and large enough to hold several blocks at once. Use heavy wire grating or mesh to fit in the pan for the samples to rest on. The grating should be placed in the pan so the samples will be in 1/8" Of water. Support the wire grating in several places so it will not collapse when several samples are placed on it. Make a hole in the side of the pan at the correct level so that the water will not get higher than 1/8" on the samples. Then, by letting a small amount of water drip in the pan and run out of the hole you can be sure that the samples will always be exactly 1/8" deep in water. Figure 14 shows how one of these pans looks.

Figure 15. Simple lever test for determining strength of blacks (see Table 2).

3. Fine wire screen (like fly screen) to be placed between the blocks and the wire grating to keep weak samples from falling through the wire grating.

4. A left. ruler divided to 1/16ths inch with at least I inch divided into 1/32nds inch.

5. Wax paper or plastic sacks if available. Get the right size to fit loosely over the samples. These sacks are not absolutely necessary hut they will make your test more accurate because they keep water from evaporating from the samples. They are most useful when the test is performed in hot dry weather.

6. A supply of clean water.

7. A clock or watch.

8. A form such as shown on Figure 17.

The test starts as soon as the block touches the water. Blocks are stood in exactly 1/8" of water. As they soak up the water you will see a wet line extending around them. After 5 minutes, with your ruler measure the height of the water line above the bottom of the block. Often this line will not be straight and level. Measure the best average height you can get to the nearest 1 16". Measure again at the following times: 1, 2, 4, 8, 24 hours and once each day thereafter or until the water reaches the top of the block. The heights that you measure should be written down on your form. Also, write down the time when the water rises to the top of the block and all of the block is wet.

If you have a scale, weighing the blocks each time you measure heights will give a better idea of when the block stops absorbing water. There is a space for the weights on the form.

To check whether the block swells, measure the longest side of the block to the nearest 1/32" before the test starts. At the end of the test measure the block again at exactly the same place. There is also a place for these measurements on the form.

A good time to test your blocks for strength is at the end of the absorption test, because they are in their weakest condition then. Test them as soon as the water line reaches the top of the block and call this the "wet" strength of the block.

STRENGTH TEST - The strength of soils is determined by c crushing (compressive strength) rather than by pulling apart (tensile strength). This test is very important for earth houses and should be done with a great deal of care.

Equipment and supplies needed:

1. Two of your dried, cured blocks plus the two absorption test blocks. The size and shape of the blocks is very important when testing a soil to determine its compressive strength. They should be roughly twice as long as they are wide. Your 6x3x2-inch blocks are ideal, but the rammed earth blocks should be trimmed to size first. Do this carefully so the blocks are not damaged.

Figure 17.

2. A way to apply and measure the load to crush the blocks. In commercial laboratories they use a machine. Another way is to use a hydraulic jack with a gage on it that will accurately measure the crushing load. You can also make an attachment for the CINVA-Ram block-making machine as shown in Figure 17 that will break the blocks. Note, that in this figure the block is not in the correct position for a compression test. You can also make a simple, lever type machine such as the one shown in Figure 15.

3. A left. ruler divided to 1/16th inch.

4. A copy of the form shown on TEST FORM.

The methods discussed here will be easy ones which require little or no equipment. If you have any of the better equipment mentioned above, the test procedure will be about the same. The results will be more accurate, of course.

The blocks should be tested by loading them in the direction of their longest dimension. Make sure that the top and bottom are square so the block does not tilt during loading. The exact area of the block is important. To get it, measure the exact dimensions of the crushing face, and multiply them together. Write this down on your form.



Soil Information

Location of Soil___________________________________________________________________


Type of Teat Block (Circle One) Adobe Pressed Block Rammed Earth Amount and Type of Stabilizer________________________________________________________________________

Absorption Test


Height of Water Line

Weight of Block


No. 1

No. 2

No. 1

No. 2



5 mins.


1 hr.


2 hr.


4 hr.


8 hr.


24 hr.


2 days


3 days


4 days


5 days


6 days


7 days


Time for water to reach top of block_______________________

Length of block before start of test________________________

Length of block at end of test____________________________


Strength Teat


Dry Blocks

Wet Blocks


Block 1

Block 2

Block 1

Block 2

Area of block, square inches


Crushing distance, ft.


Crushing strength



"Simple" Strength Test

Block 1 (circle one) - Very soft, soft, medium, stiff, very stiff, hard

Block 2 (circle one) - Very soft, soft, medium, stiff, very stiff, hard

Spray Teat

Depth of Pits; Block 1, Block 2


Using the lever arrangement shown in Figure 15, place the block the lever and apply the load. Figure 15 shows a 130-pound man sitting on the lever, but you could also hang a bucket of stones or water (or anything weighing 130 pounds) from the lever. Start with the man or weight close to the block and move slowly outward to the end of the lever until the block breaks. Then measure the distance from the end (or chain) to the weight. Call this the Breaking Distance. Form Table 2, you can find the crushing strength of the block. For in-between dimensions not shown in the Table, you can estimate with good accuracy.

Even if you cannot make the lever machine shown in Figure 1 S. you can still estimate the wet strength of blocks following the absorption test. (All dry blocks will be hard, and it would be difficult to even estimate the difference in strength between several blocks without some sort of equipment.) For this, use the "simple" strength below:

"Simple" STRENGTH TEST - The reactions that you can expect to this test are as follows:

Very Soft - The block can be easily pinched apart with only the thumb and forefinger or it may even slump under its own weight.

Soft - If the block can be easily penetrated several inches with the thumb, call it "soft."

Medium - If the thumb will penetrate a block about I inch with moderate effort, it is of medium strength.

Stiff - Soils which are stiff can be indented with the thumb, but only with great effort.

Very Stiff - The sail cannot be penetrated at all with the thumb, but it can be penetrated with the thumbnail.

Hard - Very difficult to dent with the thumbnail.



Area of Block in Square Inches

Dist. "B"

"CRUSHING DISTANCE" (Length From End of Lever To Weight For Blocks of Strength Shown)


100 psi

125 psi

150 psi

250 psi

300 psi





49'- 7"












































































Where * appears, use 2 men (or 260# weight) on lever. Measure distance from point "A" of lever to center of the weight.


The only blocks which will be normally suitable for earth houses are those which are called "hard." "Very stiff" ones may be suitable for dry areas.

Remember to do the strength test on both the dried, cured blocks (call this the dry strength) and on wet blocks after the absorption test (call this the wet strength). Always use at least two of your blocks to determine their strength. Use the average strength of the two blocks.

SPRAY TEST - This test tells you how your block will hold up in a hard, driving rain. Most accurate results require laboratory equipment, but there is also another way that is satisfactory.

Equipment and supplies needed:

1. Two of your seven blocks of each mixture.

2. A spray nozzle that can produce a hard spray all over a block. A four-inch diameter shower head is usually used.

3. Some wire mesh covered with fly screen, such as used in the absorption test, to place your blocks on.

4. A water supply that will deliver a fairly constant pressure for two or more hours. The water pressure usually used is 20 pounds per square inch.

5. An accurate gage for measuring water pressure. Mount the gage in the pipe supplying water to the spray nozzles at a point near the nozzles.

6. A copy of the form shown on TEST FORM.

Place the wire mesh on bricks or wood blocks so it is suspended a few inches off the ground. Then put the test blocks on the mesh with their largest face square to the spray nozzles and exactly 7 inches from the nozzles. Start the water spray, keeping the water pressure as close as possible to 20 pounds per square inch.

After two hours of spraying remove the blocks and examine them closely. Measure the depth of pitting or surface erosion. Also write down the time required for any blocks to completely fall apart or get washed away by the spray.

Be sure and write down the results of the spray test on your form with the results of the other tests so you will have a permanent record of all of the tests.

Now that you have done these tests, you must look at the results and decide whether your blocks are suitable for making earth houses. Remember that soils vary a lot. It will be hard - even with these fine tests to tell exactly how your soil will act. But if you use the tests wisely, and benefit from your neighbor's experience with earth houses, you should be able to build a safe house.

Start by looking at the results of the spray test. If you live in an arid (dry) area - one where the rainfall is less than 20 inches per year - then your bricks are satisfactory if they have some pitting, say ¼" to ½" deep

If you live in an area where the annual rainfall is between 20 and 50 inches per year, then the blocks should have only minor pits, less than ¼" deep.

If you live in an area where the annual rainfall is greater than 50 inches per year, then your blocks should have no pitting at all, but slight roughening of the surface is to be expected.

For the requirements above, it is assumed that adequate protection from splash exists. This means that either the foundation wall is high enough so the lowest soil layer or blocks do not get splashed, or that the blocks-in the splash region at the bottom are protected.

If your blocks do not meet the requirements above, then you must do something to them if you want a long lasting house. Here are some things that you can try:

1. Change the amount of sand in your soil mix. Sometimes more sand will help. It's worth a try.

2. Try a surface coating. (See Chapter 14.) One of the main reasons for the use of surface coatings is to reduce the amount of erosion due to the feathering simulated by the spray test. When you test a block with the surface coating of your choice, the entire block must be coated even though you will coat only the outside face in the actual building.

3. Try adding one of the stabilizers discussed in the next chapter. Even small amounts of lime, cement or asphalt will often increase the resistance of blocks to spray tests. In fact, you might try anything that you have in the way of waste products. But you should remember that the addition of stabilizers will also change the way the soil acts as far as strength and absorption. For example, small amounts of lime and cement may decrease the strength of the soil slightly. So when you try a stabilizer, you must also start all over again with the other tests such as the strength and absorption.

The results of the absorption test are much harder to analyze than those from the spray test. All soil blocks and even burnt clay bricks will absorb some water. (In fact, during recent tests high quality burnt clay bricks absorbed as much water as pressed earth blocks.) For earth houses, you would like to have a soil that will absorb little water, and which will not swell or lose strength during absorption. Unfortunately, this will occur with only a few unstabilized soils. However, by looking at the absorption test on earth blocks in connection with the strength and length changes of the blocks, you will at least be able to tell much more than from the absorption test alone.

Blocks made of clayey soils will take longer to absorb moisture than sandy blocks, but walls made from clay blocks will absorb more moisture over a long period and the moisture will creep higher in the wall. In dry areas - less than 20 inches of rainfall per year - the absorption of blocks can be high and they will still be satisfactory if they are strong enough when wet. On the other hand, blocks which absorb a lot of water will not be suitable in very wet areas even if they are strong. The inside of the house will be much too damp and wet to be comfortable.

Stabilizers can reduce the absorption. Asphalt emulsions do well on sandy blocks. Lime works on clayey blocks and will also increase the wet strength and reduce the swelling.

Length changes during absorption on small test blocks (those less than 6 inches long) should be less than 1/32 inch. On large or full size blocks, allow no more than 1/32 inch for a block one foot long. If this amount is exceeded, it can be reduced by adding stabilizers but again this means that other properties such as strength and absorption must be checked for the new mixture. Adding Sand to clayey blocks will help, and lime and cement also do a good job of reducing the swelling. Unlike blocks which show too much spray loss, those which swell too much cannot be protected by surface coatings.

The water will eventually get through the coating and coating will crack when the blocks start to swell.

Strength of your earth blocks is an important factor. Adobe and rammed earth should have a minimum dry strength of 250 pounds per square inch. Pressed earth blocks should have a minimum dry strength of 300 psi because they are used in thinner walls. Most soils will be this strong when dry unless they are very sandy or have a lot of organic matter in them. But the wet strength of the blocks after absorption is even more important than the dry strength. Tests have shown that the wet strengths should be at least one-half of the dry strengths. This means that adobe and rammed earth should have a minimum wet strength of 125 pounds per square inch and pressed blocks should have a minimum strength of 150 pounds per square inch.

In dry climates - less than 20 inches of rain per year - you can get by with 100 pounds per square inch for adobe and rammed earth if you use a good surface coating and if the dry strength of your test blocks was high enough.

In wet climates - more than 50 inches of rain per year - you should try to get wet strengths which are close to the ones given above for dry strengths. This means 250 pounds per square inch for adobe and rammed earth and 300 pounds per square inch for pressed blocks.

When the tests are finished, you will have one block left from the original seven. It is a spare in case one of the other blocks breaks. But you can also use it for some special tests.




Lees than 20 inches rainfall per year

Between 20 and 50 inches rainfall per year

Greater than 50 inches rainfall per year

Spray Test

Pits less than ½-inch deep.

Pits less than ¼-inch deep.

No pitting of surface, alight roughening allowable.

Dry Compressive Strength

Minimum of 250 psi for adobe and rammed earth. Minimum of 300 psi for pressed blocks.

Minimum of 250 psi for adobe and rammed earth. Minimum of 300 psi for pressed blocks.

Minimum of 260 psi for adobe and rammed earth. Minimum of 300 psi for pressed blocks.

Wet Compressive Strength

Minimum of 100 psi for adobe and rammed earth with good surface coatings, 175 psi without surface coatings. Minimum of 150 psi for pressed blocks.

Minimum of 125 psi for adobe and rammed earth. Minimum of 160 psi for pressed blocks.

Beat soils will meet requirements for dry compressive strength. Can be somewhat less.

Length Changes

Maximum of 1/32" for 1 foot block.

Maximum of 1/32" for 1 foot block.

Maximum of 1/32" for 1 foot block.




If you live in an area where it freezes a lot, try this test:

Place your block on the absorption pan for 24 hours. Then remove it and freeze it for 24 hours. Let it thaw on the absorption pan for 24 hours and repeat the process as many times as possible.

If you live in an area where it rains almost daily, try this:

Immerse the block in water halfway up its side for 5 hours. Then let it dry in the sun or in a warm oven. Continue the soaking and drying steps as many times as possible.

Neither of these tests will tell you exactly how long your earth block will last, but they will help you in deciding between several soils that you are thinking of using.

The test results that have been discussed are briefly summarized in Table 3.