Cob: sandy soil

[Shannon C. Dealy]

On Sun, 30 Sep 2001, Sbay wrote:

[snip]
> I'd love to go all cob, but codes and stuff will force us to build
> either conventional or log.  Anyway, our soil is extremely
> sandy.   Actually it is sand, all sand, nothing but sand.  I had some
> clay brought in but it is not the best quality.  Nothing like
> the clay I used this spring from a lakeshore to build little test
> structures with my kids.  This stuff is not very sticky, really light
> tan and kinda sandy or something.  So, do I need to bring in some better
> clay?  Or just use more than the advised amount of our sandy, not so
> sticky clay????  I've made test bricks with 50/50 and they weren't as
solid as i'd hoped.

What do you mean by "advised amount"?  Normally, you would just add clay
until you have a mix you are happy with.  If the clay is of very poor
quality but it works, I know of no reason not to use it, but I wouldn't
pay to have any more of the poor quality clay delivered.  You need alot
less good quality clay, so it will most likely be much cheaper to get some
good clay delivered.  In some cases, with a really good quality clay, you
could get by with 10% or less, and with most clays of reasonable quality
you should need no more than 20%.

> Also I'm not going to build a foundation, so do I need to put the cob a
> little below ground or just slap it right down on the grass?

I don't know what your climate/water table are like, but I've been
tinkering a bit with no foundation cob and would recommend the following
as a minimum if you have any concerns about water contact with the base
of the wall:

   1 - Build it on a high point and/or contour the surface of the ground
       so that it slopes away from the structure in all directions
       (don't start it below ground as you mention above).
   2 - Put a thin layer of rock, gravel, and/or tar paper under the base
       of the exterior walls to serve as a moisture break in order to
       block/slow moisture wicking up from the ground.
   3 - If water is a major concern, you may want to put a drainage trench
       around the exterior of the wall.

> And what should I do for a bare bones floor?  Just move the
> grass and tamp the earth some?

That should work.


Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 451-5177 |                  www.deatech.com