Soil
I did these tests:
1. Mason jar or sedimentation test.
2. Cigar test.
3. Biscuit test.
Soil Test : Mason jar / sedimentation test
After 10 minutes. |
This picture was taken 10 minutes after shaking the jar. You can see a clear water layer at the top and soil suspended in water at the bottom. Clear water layer indicates there are no fine particles at all. But the soil looked pretty fine. I expected some fine particles to remain suspended in water. But this result does not show fine particles. I thought - may be I did a mistake and did not take right sample or did not shake the jar well when mixing soil sample with water. Therefore I repeated the test with another sample from the same pile of soil.
After 24 hours |
Even then I waited till 24 hours to see whether more particles settle. Nothing happened - it did not show any changes after 24 hours.
Cigar test |
Remove pebbles / stone pieces from the soil. Don't sieve it, just hand pick pebbles from some soil; couple of fist full is enough. Mix enough water to make plastic dough. Take a medium lemon size dough ball and roll it between your palms to make cigars. When you push it out of your palm, it bends, hangs and ultimately breaks. It should break between 2-6 inch length. It is considered too sandy if it breaks below 2 inches (too fragile/brittle). It is considered too clayey if it breaks above 6 inches (too sticky).
Biscuit test |
First dry them in shade for couple of days and later couple of days in sun so that they dry completely. Keeping them in sun first may lead to cracks since the soil dries too quickly. After the biscuits dry completely, take a biscuit and snap it in 2 pieces with your fingers. Soil is too sandy if it shatters or breaks into too small pieces or crumbles into powder. Soil is too clayey if it is hard to break it. Soil is considered good if it snaps into 2-3 pieces easily.
Soil Composition
Particle
ratio of 40% sand + gravel, 40% silt and 20% clay is well suited (all
ratios approximate) for rammed earth. Soil is a complex material.
Science (science of soil is called Soil Mechanics; you Civil Engineers - remember 2nd/3rd year?) has not been able to understand soil properly because soil does not follow precise scientific rules
and formulas. Formulas and rules mentioned in soil mechanics are
empirical. Therefore any rule (including the soil composition rule) is
only indicative, not deterministic. Therefore soil without having one or
more types of these particles or particle types in very different
ratios can also make good/acceptable rammed earth walls; especially with some
stabilizers like lime or OPC (ordinary portland cement). Lime is used
with clayey soils. OPC does not play well with clay because clay is soft
and expands when wet. Cement is hard and brittle after setting.
Therefore OPC should not be used for stabilizing clayey soil. OPC is
best suited for sandy and/or gravely soils. OPC with sandy and/or
gravely soil results in soilcrete (like concrete - but with soil instead
of gravel and sand [called coarse aggregate and fine aggregate in
structural engineering] in concrete).
Proof of the pudding ... rather proof of the wall!
Soil suitability for rammed earth construction can be verified by making a sample rammed
earth block and testing it. I made a small box of 12 inch x 5 inch x 5
inch for ramming a test block. Don't jump on me for the unorthodox size -
I ended up with this size because I used some wooden boards lying
around.
Test block |
I was too lazy to cut the boards to regular brick size. Box sides are held with screws so that I can disassemble the box to take the block out after ramming. Well, that turned out to be a bad decision in hind sight because pressure of ramming caused the screws to pull out and the box disassembled itself! After that I made a tourniquet setup to hold the boards of the box tightly in place.
I rammed first block without any stabilizer. Block came out looking well - in the sense there was no shrinkage and no cracks. Probably I ended up using less water than needed while ramming. Therefore this block did not come out that strong. Rammed layers of soil separated and broke when I dropped it on the ground from shoulder height. Conclusion: 1st attempt is a failure. No worries, failures are stepping stones of success!
I stabilized second block with 5% cement. Block looked better than the previous block. Probably I used less water for this block too. I allowed it to cure for 7 days and then dropped it from shoulder height. It fared far better than the block without stabilizer. Rammed earth layers did not separate out when dropped from shoulder height. But the block broke into 2 pieces. Rammed earth layers separated when one of those pieces was dropped from 15 ft. height. Conclusion: 2nd attempt is a moderate success. I told you - first attempt was the stepping stone ;-)
I stabilized 3rd block with 5% cement . I used more water than earlier blocks. After 3 days of curing it is looking really good. I did a drop test after 7 days of curing. Block passed drop tests from shoulder height and 15 ft. height without major damages. What I mean by that is - block was intact, rammed layers did not separate, only the corners got damaged when dropped; which is expected.
Later I had put the third block in the farm under irrigation sprinkler for testing the effect of water on the block. This irrigation sprinkler sprays water every day for 1 hour. I did not find major erosion of the block after keeping it under the sprinkler for 15 days. Looks good and I feel confident of using 5% cement stabilization with little higher water content than usually recommended in rammed earth literature. Slightly more water content is better than less water content. Lower water content does not allow us to compact the soil to needed extent. Higher water content also affects compaction, but not as much as lower water content. Therefore it is ok to have slightly higher water content than lower water content.
Note: After having rammed the whole building, I know slightly higher water content is better for sure. There are instances where my helpers put in little more water. After ramming such soil mixture we found it "sweating". We saw sweating on the inside of the form work when we removed the form work and also on the surface of the rammed wall. It is extra water oozing out of the rammed soil. Later, after curing, I found such walls to be dense and nice. On the other hand, walls with lower water content did not turn up that well. I found them to be less dense and porous. Therefore slightly higher water content is better than lower water content.