Stabilized Earth Construction
 
OVERVIEW
 
Soil is a universal building material and is one of the oldest known to humanity.
Simple soils (without additives), or soils improved by adding stabilizing materials
such as bitumen or cement, are suitable for homes, schools, roads, and other construction.
 
For construction purposes, soil is usually formed into blocks. Two general types of
blocks are described here: adobe block and stabilized earth block formed under
great pressure. Adobe blocks are made from moistened soil that may be mixed
with straw or other stabilizers. They are formed without pressure and usually
cured in the sun. Stabilized earth blocks (sometimes called rammed earth blocks)
are made from soil mixed with stabilizing material such as Portland cement,
formed into blocks under high pressure, and cured in the shade.
 
Low cost is a primary advantage of soil block construction. An overall cost
reduction of about 50 percent over conventional construction can be realized.
Other advantages are that building materials are usually readily available and
little skill and training are required for their use. The material is culturally
acceptable in nearly all countries, including the United States.
 
SOIL CHARACTERISTICS
 
The composition of soil varies from one region to another, and with soil depth. In
any one area, it may be desirable to mix soils from several locations or depths to
obtain a composition more suitable for construction.
 
The primary components of soil that are of importance in construction are sand,
clay, and silt. (Organic materials are also found in surface soil. These tend to
reduce the quality of the blocks.) The fraction of clay in the soil is important
because it acts to bind the larger soil particles together but the clay content
should not exceed one-third. Above that, deep cracks and weakening of the dried
blocks are likely to occur. Silt, which is usually found mixed with the sand,
should not exceed one third because silt is vulnerable to erosion from wind and
rain.
 
Proportions of sand, silt, and clay vary widely. One of the few soil block
standards that exist is California's Uniform Building Code Specification, which
recommends 55 to 75 percent sand, and 25 to 45 percent day and silt. A good
mixture for most blocks might be:
 
    sand .... 65 percent
    clay .... 20 percent
    silt .... 15 percent
 
To assure that the composition to be used is suitable for construction, several
test blocks should be produced using various mixtures. After curing, the test
blocks should be hard and resist a scratch or prick from a knife. Striking two
compressed/stabilized blocks together should produce a click sound. The blocks
should sustain a drop of two feet (.6 meter) without breaking. If the block
crumbles or breaks, the sand or organic content is probably too high, and clay
should be added to the mix. On the other hand, if large cracks appear during
curing, the clay content is probably too high and sand should be added to the
mix.
 
TESTING THE SOIL
 
Soil tests should be made before any block production is started. If the testing is
not done first, a great deal of time and money may be wasted in the production
of unusable blocks. The agricultural departments of most countries can provide
laboratory tests at modest costs. If field tests must be made instead, some simple
methods to determine the soil's suitability can be tried.
 
Composition Test
 
o  Pass the soil through a 1/4" (6mm) screen to remove stones and other
   large particles.
 
o  Pour the screened soil into a wide mouth jar until it is half full.
 
o  Fill the jar with water. (You may add two tablespoons of salt to make
   the soil settle faster)
 
o  Cover the jar tightly, and shake vigorously for two minutes.
 
o  Let settle for at least 30 minutes.
 
The small gravel and sand will settle rapidly to the bottom of the jar. The clay
and silt will settle more slowly. After 30 minutes, the jar should look like the
drawing in Figure 1c. Hold a scale vertically on the side of the jar to measure

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the amounts of sand, silt, and clay. Record the sample number and the amounts.
Then convert the amounts to percentages.
 
Compaction Test
 
In addition to the soil composition test, a compaction test should be done to
determine the packing quality of the clay, which depends on the percentage of
clay in the sample and the quality of the clay itself. A simple field test can be
done as follows:
 
o  Take a handful of dry, screened earth and add some water to it until it
   is damp enough to form a ball when squeezed in the hand, but not so
   damp that it leaves more than a slight trace of water in the hand when
   squeezed.
 
o  Drop the ball from a height of about 3 feet (1 m) onto hard ground. If
   the ball breaks into a few small pieces, the packing quality is good to
   fair. If it disintegrates the quality is poor and a soil mix with more
   clay should be prepared and tested.
 
Shrinkage Test
 
If stabilizing material such as Portland cement is to be added to the soil, a
shrinkage test of the soil should also be made. This test will indicate the
suitability of the soil and also the best cement-to-soil ratio to use. It measures
the shrinkage of soil that contains no stabilizer. As shown in Figure 2, the box

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should have these inside measurements: 24" x 1-1/2" (4 cm x 4 cm x 60 cm).
 
To test soil with this method:
 
o  Oil or grease the inside surface of the box thoroughly.
 
o  Pack the box well with moist soil (previously passed through a 6mm to
   10mm (1/4" to 3/8") mesh screen. The soil should be moistened to pack
   well, but it should not be muddy.
 
o  Tamp, especially at the comers.
 
o  Smooth off the surface with a stick.
o  Place the box in the sun for three days or in the shade for seven days.
   It should be protected from rain.
 
Measure the contraction (shrinkage) by pushing the dried sample to one end of
the box.
 
Shrinkage                       Cement to Soil Ratio
 
Not over 1/2" (15 mm)                         1 part to 18 parts
 
Between 1/2" and 1" (15 mm - 30 mm)           1 part to 16 parts
 
Between 1" and 1-1/2" (30 mm - 45 mm)         1 part to 14 parts
 
Between 1-1/2" and 2" (45 mm - 60 mm)         1 part to 12 parts
 
When lime is used instead of cement use double the amount. Do not use the soil
if it has many cracks (not just three or four); if it has arched up out of the box;
or if it has shrunk more than 2" (60 mm).
 
MAKING ADOBE BLOCKS
 
To make adobe blocks, add water to the soil mix until it is plastic enough to
mold. Water content should be between 16 and 20 percent of the soil by weight.
The water and soil must be throughly mixed. Since all except the dryest soils will
already contain some water, it is advisable to test the sample for water content
first. Do this by weighing a soil sample, drying it, and then reweighing it to
calculate water content.
 
Even the best adobe blocks may develop some cracks. To reduce the number of
cracks, and also to make the blocks more weatherproof, stabilizing materials are
often added to the mix. When stabilizers are used they must be thoroughly mixed
with the soil or much of their benefits will be lost. The most widely used
stabilizers are straw, rice husks, asphalt emulsion, Portland cement, and lime.
 
Asphalt emulsion can improve the waterproofing quality of the blocks, and also
their elasticity and toughness, so that they are less likely to break during
handling. Add asphalt emulsion between 5 and 15 percent by weight to the dry
soil mix For soil mixes with high sand content (55 to 75 percent sand) the
asphalt emulsion should be nearer the 5 percent figure.
 
Portland cement stabilizers improve the bonding properties and add strength to
the blocks. Only 5 to 6 percent cement by weight is needed for soil mixes with
high sand content, but up to 20 percent by weight may be required for soils high
in clay and silt. If the soil requires a large percentage of cement, it can be
combined with an equal amount of lime, which costs less.
 
Equipment required for making adobe blocks is shown in Figure 3. The number of

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shovels, molds, etc, will depend on the size of the job. Using this equipment, and
supplied with the mixed adobe, a team of two molders can produce about 1,000
blocks (10 x 4 x 14") per day.
 
Select a large level area for mixing, molding, and curing the adobe. Mixing can be
done in a hopper, or by making a shallow mixing pit in the ground. If possible,
make the blocks near the construction site. If the mix is lumpy even after
repeated working, let it soak overnight.
 
Block molds can be made in various sizes to fit the needs of the construction.
But adobe blocks should not be larger than 81cm (32") around the outside. A gang
form that will mold eight blocks of .009 cu. meter (one 1/3 cubic foot) can be
operated by one worker. Before starting work, the mold should be thoroughly
soaked with water to prevent the adobe mud from sticking to it.
 
Production steps are as follows:
 
o  Rake or drag a large ground area level.
 
o  Place mold on level area, on a piece of building paper if available, and
   dump the mud from a wheel barrow or hopper into the mold. Work the
   mud firmly into all corners of the mold.
 
o  Scrape off excess mud from top of mold to leave a smooth, flat
   surface.
 
o  Remove the mold by lifting it slowly and evenly up from the ground
   level. Move the mold to the next adjacent level area and repeat the
   process.
 
Blocks must be allowed to cure for about 14 days. After several days, the
partially cured blocks may be carefully turned on edge so they dry more evenly.
On very hot days, in direct sunlight the blocks may dry too rapidly and crack. To
prevent this, cover the blocks with paper, leaves, or straw. Since rain will
destroy unstabilized blocks, waterproof tarps may be needed.
 
To store the blocks after they are cured, stack them on edge. If left stacked flat,
they will break of their own weight.
 
MAKING COMPRESSED EARTH BLOCKS AND TILES
 
Compressed earth blocks can be made by ramming the earth in forms, or by using
a block making machine, such as the CINVA-Ram Block Press. Blocks made by
machine are less costly and have superior uniformity.
 
Some machine made blocks tested by the U.S. National Bureau of Standards had
compressive strengths up to 800 pounds per square inch (56 Kg/cm), with 300 to
500 psi strength as the average. (This is three to eight times the compressive
strength of adobe blocks). These test blocks contained 50 percent sand, and 50
percent clay and silt, mixed with 8 percent cement by weight.
 
Although one worker can make blocks with the CINVA-Ram, the process is best
as a team effort with two to four workers each performing one task. (It is good
to rotate tasks among the workers on an hourly or daily basis.) The CINVA-Ram
is portable and can easily be moved about the work site to reduce carrying raw
materials or finished blocks.
 
Floor tiles can also be made with the machine, using inserts to adjust for the
thinner tiles. The mixture for floor tiles is two parts fine sand to one part
cement. Mineral coloring can be added to produce colored tiles.
 
Average production rates and cement required are:
 
o  Average number cement blocks or tiles
   (made by two workers per day)                      300-500
 
o  Average no. blocks for a two room house               2500
 
o  Typical block size: 9x14x29cm (3-1/2"x5-1/2"x11-1/2") which lay up to:
   10x15x30cm (4x6x12 inches).
 
o  Average number blocks per 100 lbs cement:              150
 
Stacking the blocks for curing requires care. The blocks should be stacked on
edge on clean planks. If planks are not available, stack on flat ground that has
been covered with paper or leaves. The blocks should be covered with plastic or
old cement bags that have been cut open. Stacks should not be greater than five
blocks high, and some air space should be left between the blocks. For the first
four days, sprinkle the blocks lightly with water to prevent them from drying too
quickly. The total curing time is about 14 days, depending on the weather.
 
BUILDING WITH STABILIZED EARTH BLOCKS
 
A firm, flat, water-resistant foundation should be built first using blocks with a
higher percentage of cement and lime. Blocks should be joined by mortar about
one half inch (1.25 cm) thick. The recommended mortar mix (by weight) is:
 
o  one part cement
o  two parts lime
o  nine parts soil (used to make the blocks)
 
Let the applied mortar dry for about a week; then paint the mortar joints with a
thin, milk-like mix of cement and water. Stir this mixture often. After a day, the
finished walls can be coated (3 coats recommended) with this same mixture, or
with a coat of lime. Or, a waterproofing coat of silicone based wash may be
added.
 
Sources:
 
Alfred Bush, Chris Ahrens, Balla Sidibe, VITA volunteers
 
References:
 
Making Building Blocks with the CINVA-Ram Block Press. Arlington, Virginia:
VITA, 1977.
 
Bush, Alfred. Understanding Stabilized Earth Construction. Arlington, Virginia:
Volunteers in Technical Assistance, 1984
 
"Building Materials and Structures Report BMS 78", Gaithersburg, Maryland: US
National Bureau of Standards
 
Sidibe Balla. Understanding Adobe. Arlington, Virginia: Volunteers in Technical
Assistance, 1985
 
U.S. Agency for International Development, "Handbook for Building Homes of
Earth", Action Pamphlet No. 4200.36, Wolfkill, Dunlop, Callaway, Washington, DC,
Peace Corps, 1979.
 
Ferm, Richard. Stabilized Earth Construction: An Instructional Manual. Washington,
D.C.: The International Foundation for Earth Construction.
 
Manufacturer:
 
The CINVA-Ram Block Press is manufactured in Bogota, Colombia, by METALIBEC,
S.A. The press may also be purchased in the USA for $400 (1987) from Schvader
Bellows Inc., 200 West Exchange Street, Akvon, Ohio 44309-0631. Telephone: (216)
375-5202. Similar, locally manufactured presses can often be found in other
developing countries.