howto cobstruction

[M J Epko]

At 02:47 AM 9/6/96 -0700, Shannon wrote about how set-up / dried-out the
previous day's work should be before adding more:

                [snips indicated by "..."]

>... the main concern is 
>that you be aware of how soft the wall is in a given area, so that you don't
>build it up to fast and cause it to squish out (that was a real helpful 
>description wasn't it).

        I think I understood it completely; and if *I* got it, then yeah, it
surely was a helpful description   ;)
        As long as the previous day's layer is self-supporting and not
shape-shifting (*poof* your cob wall turns into a chattering monkey and runs
off into the woods), it's safe to build.

>A fairly simple test for short sections of the
>wall is to stand on it and see how much of an impression you feet make
>and how much the sides bulge out.  Generally, you can always add some
>cob each day, as it doesn't matter how soft the cob is when adding to
>it, the point is that the weight of the new cob will cause the cob below
>it to bulge if it is too soft to support the weight of the layer you add,
>so if you start to see it bulge out while adding a new layer, don't build
>the new layer up any higher until it has had some time to dry ...

        But - but - but - how can I over-analyze the process when it's so
simple? Are you suggesting that I should follow my instincts and enjoy
myself? *shudder*     ;)

>Basically what I'm trying to say is that if a section of the wall is soft
>when you start the day, only add a couple inches of cob, if it's hard,
>you may be able to add a foot.  How you determine how much cob to add in a
>given day requires only a few days of experience and careful observation 
>of the wall as you build it ...

>A cob wall that is too dry will basically feel like rock or the ground 
>beneath your feet in dry weather.  A wall that is not too dry you will be
>able to press your thumb all the way into, though it may require your 
>full body weight if the wall has been drying a few days (or even one day
>for those of you in hot dry climates).

        Thank you. I appreciate your helpfulness (and patience.) 

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Shannon wrote:

>You don't need to screen the soil [used for the cob mix] for rocks, though 
>it's probably a good idea to remove any that are larger 1 inch if you mix 
>bare foot (which I do).  With regard to larger rocks to eliminate some of 
>the need for cob, the thing to remember is that while it can be done, the 
>cob encases the rock, but unlike conventional mortar, does not bind to it, 
>so this area will be weaker since its strength is based on the strength of 
>the interconnection of the cob surrounding the stone.  From the perspective 
>of compressive strength, I believe that you are correct, rocks would make 
>it stronger, but from a tensile strength perspective, it would be much 
>weaker. This would be fine if gravity was the only concern, but earthquakes 
>and other natural disasters tend to find this kind of weakness ...
>One additional note, since we have been discussing R-values in the SB list, 
>putting lots of rock into your walls will also decrease its R-value.

        Understood. I guess I was getting ahead of myself and dipping into
my other post from yesterday, where I started talking about interior-cob /
exterior-SB. The R concerns in that case would dropped wholly in favor of
masseffect insulated from seasonal swings, and bearing becomes less of an
issue (depending on other decisions.)

>My 2 bits for what it's worth.

        Worth far more'n two bits.

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M J