Saturday, October 6, 2012

Compressed Earth Block (CEB) Soil Preparation


Without question, the most important issue relating to sound CEB construction is getting your soil prepared and moist for pressing. It is meaningless to rush off to buy a press without first of all solving the more important problem of how to prepare your particular soil for the job.

What NOT to Do:

Concurrent with building a durable and consistent block press, we have also been constructing and testing soil sifters and stabilization processes. Here it must be pointed out that if you should (mistakenly) search the internet for how to do anything with soil, you will find thousands of opinions and examples—all of them suspect! To make sense out of all the downloaded junk on the internet, you must have already understood the solution because you will not know how to sort the data tsunami you find unless you know more than anyone posting their opinions.

Proper Resources:

The most reliable resources and knowledge for preparing and stabilizing your site soils will come from textbooks and published material on the subject. These will be peer reviewed and have a track record of competence in the industry. CEB and rammed earth (mass wall construction) is not new since even the Egyptians left records, and there are hundreds of modern texts that virtually all converge on the solution. In other words, they all pretty much agree on how to do it. That’s not what a glance at the internet would suggest!

Soil Texture—particle size:

The first thing you must do is get a reliable (sealed, stamped engineering report) written analysis of your sample soil material regarding its texture. Texture is simply a cover term for the distribution of various sized particles through the sample. Clay is the finest, and it is the most important type of soil. Next is silt, which is very fine but not as fine as clay and distinguishes itself from clay for having no electrical charge on it. In other words, it isn’t sticky. Finally the largest sized particles are called sand.

USDA Texture Triangle:

Most soils will have a fairly even distribution of these three textural materials with sandy loam or clay loams, for example, consisting of 30-35% of each of the three. What is vital to get right is exactly what the distribution is in your particular sample. There is no short-cut, notwithstanding the popularity of a vast range of field tests from jars to hand washes. Trust none of them! Get an engineered analysis with a USDA textural class triangle marked with your exact distribution and then a written statement of the analysis clearly stating that clay is X%, silt Y% and sand Z%. You can see an example of what both look like below from the study of our own soil stock-pile.


Soil Sifting

Once you have understood exactly what kind of soil you have and you have at least the 30% clay fraction that CEB blocks require, then you can begin to prepare the soil for pressing. There are two more stages to get the soil in perfect condition for compaction.

The first step in soil preparation is sifting out the rocks and debris. Let’s call it soil sifting. In Africa, where labor was cheap, sifting the rocks and grass out of the freshly dug material was done by hand. Laborers shoveled the dirt onto a screen and separated the rocks and clods leaving only small particles to pass through.

When we need to sift or screen hundreds of tons when manual labor is short and expensive, a machine must be fabricated to do this quickly. There are many soil sifting and conditioning machines on the market. All of them cost more than the CEB press!

In Kenya I had worked to make local cement and had seen a drum screen for sorting limestone. I designed a similar machine with a changeable screen for an initial sorting of stone and clods to make the blocks. But we will also need huge quantities of slurry, which require extremely fine soil. A 1/8th inch diamond wire additional screen can be added to the drum screen to sift this very fine material very quickly.

The picture below shows the drum screen at work. It took 30 seconds to sift one bucket load on the tractor which is equivalent to 10 x 5 gal. buckets. The second picture shows the hatch open for filling and a side hatch opens to remove rocks and clods when they take up too much space. 



With this drum screen we can process freshly dug soil with great speed and thus increase the average number of blocks produced per hour. The drum screen now sits over a 5 ft. deep pit lined with plywood to prevent debris falling on fresh soil as well as making it easier to pick it up and move it to the cement mixing pit and then to the block machine.


1 comment:

Zach Dwiel said...

Thanks for your post, there is such little information out there about CEB, its helpful to see what others are doing.

Do you think that a tiller, either push behind or tractor mounted, would work to help break the soil up small enough, or is the screening pretty critical?

I know FeF has done some work with tilers, but I'm not really sure what their results were.