Got together my native soils and made some test bricks. They all did really well with the drop tests. They all also had a little bit of shrinkage. I didn't think it was enough to worry about. So I built a test wall...it has cracks. It dried very fast so that didn't help im sure but still more cracking than I would like(good thing it was the small wall)
In light of this, I did the soil in jars test(I know I shoulda started with that) The pictures are after only sitting for a half hour. Don't think I have much clay at all. The red one looks like all silt. Here are my questions... do either of these soils look suitable for cob construction? They both stick together very well. Loaf up good etc. However my mix of 75% tan and 25% red is definitely wrong. The wall seems very solid, just shrunk and cracked alot. I can only assume that gets worse over time.
Is the wall a loss? Better to redo entirely? Can it be patched with a better mixture?
Neither are much good.
You need to have a graduation of particle sizes starting with big at the bottom and grading upwards.
The one on the right has some graduation but not a lot.
Clay cracks in part because its expansive sometimes.
Do the snake test by rolling a small amount in the palm of you hand and see if when dried it breaks just on the shelf.
If you rub some between you fingers you want a bit of slime left and feel a bit of fine sand or gravel.
It is rare that soils need to be mixed to create something to build a wall. Usually if you dig around you will find a naturally occurring mix somewhere.
Off the cuff it looks like not enough straw is getting used in your mix. Only the last photo did I really see any sticking out of the cob, and really the wall should look like it has long hair all over it with the straw dangling out. I think a higher ratio of straw would really help with the cracking.
What about shrinkage? It shrunk a quarter inch in spots. Dry slower? More larger pieces of sand? If I add some lime would it cut down on the elasticity? The wall is super solid(i punched it a few times) Here's another poc of the soil jars after sitting overnight.
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