[Cob] Soil Survey

[Shannon Dealy]

On Tue, 28 Jul 2009, Janet Standeford wrote:

> Thank you all for the quick replies.
> Have any of you used the jar method for determining ratios? Do you really
> add salt?
>
> I will make some test bricks and a partial wall to see how it holds up to
> winter weather here on the mountain.

Hi Janet,

Yes, adding salt to the soil in the jar will cause the clay to settle out 
faster (depending on the clay it can sometimes take weeks otherwise). 
Unfortunately, while the jar test is interesting to look at, it tells you 
nothing useful, the total amount of clay is meaningless since how much 
clay you need depends on how sticky it is, and how much is to much depends 
on how much it swells and shrinks in the final mix.  Knowing how much silt 
is also useless since it only matters in the context of how well your clay 
is able to bind the mix.  These days I wet the soil and play with it (roll 
it up into a ball, roll it out into a long thin "worm" and see how well it 
holds together, then from this I decide what ratios to use in my test 
bricks.  Test bricks in my experience are the only test that matters as 
all others only give you (at best) a general idea of what might be 
possible, and at worst will tell you things that are incorrect.  Years 
ago I gave a talk about cob at a place where every test indicated the soil 
was unsuitable (insufficient clay) and wouldn't bind adequately, but since 
it was just a quick demonstration, I went ahead and demonstrated the 
technique using the local soil.  The next day before I left, I was 
surprised to find that the tiny demonstration wall was quite strong and 
solid.  Since that time I have never trusted anything but test bricks for 
my final assessment of a soil.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
Phone: (800) 467-5820 |          - Natural Building Instruction -
    or: (541) 929-4089 |                  www.deatech.com