Cob: rammed earth - bamboo

[Shannon C. Dealy]

On Sat, 17 Mar 2001, BOB LUITWEILER wrote:

> Though you have mentioned straw bale I was hoping someone would bring up
> rammed earth.  Many years ago I heard about a rammed earth building they
> wanted to dismantle in Washington, DC.  I understand they finally
> resorted to dynamite it was so tough.  Would not a good rammed earth
> building stand up even better then a cob construction, especially in an
> earthquake?.  Though you would do more work on the tamping end, the

I find it highly unlikely that a conventional rammed earth (either just 
dirt with an adequate clay content, or earth with a binder like
cement) wall would be stronger than the same size and shaped cob wall in
an earthquake.  Rammed earth is very much like a large brick, stress it
beyond a certain point and it will break - all the way through.  The fiber
in cob gives it better tensile strength than the rammed earth that I have
seen, so it is basically harder to break (just as adding rebar makes
concrete harder to break), and more importantly, even when it initially
cracks all the way through, the fiber in cob continues to hold the pieces
together.

> mixing might be easier and there would be no waiting for the cob to
> dry.  Forms could be moved around as parts of the wall were built and
> even curved forms would be possible.
[snip]

While curved forms may be possible, they are definitely not easy, rammed
earth forms are not like the forms you use for pouring a foundation or
making a slip-form stone wall.  The tamping of the earth in rammed earth
walls puts a tremendous amount of pressure on the forms, so they must be
very strong and solidly held in place.


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