Cob Kids don't try this at home:

[goshawk at gnat.net]

Well I've been purposely pushing the limits of these sandbag vaults 
and today I succeeded in pushing past it's limits. 
After three...almost four days of a steady but not too  heavy  rain, 
I had a vault collapse. NO it wasn't the big one thats on the web 
site.  I went through it today examining every piece of evidence. 
I realize this is more superadobe technique that this is  cob news 
list,  but I don't think there is much data on this type of  earth 
building technique and I hope even my failures help add to the 
knowledge of this technique.  Plus I didn't know 
who else to report the information to
anyway ...  This is what I found. 

1.) If  I would have kept the form in and competed all the 
buttressing then dropped the form it would be up today.  (but what 
fun would have that have been!) I was trying to save wood and time in 
building as few forms as could.  I plan on building a few more now 
but still incorporate repeated usage for a single vault. 

2.) Build up equally on both sides of a vault. I had the back side of 
the vault buttress  a bit higher  and caused the  pressure to be more 
on one side than the other.  What happened was it got late and I got 
excited about pulling  out the form without checking the weather.  
Right after that the rains started.  

3.) Had it not rained it would still be standing. 


4.) I ran out of  four pointed 1/2 inch barbed wire. A main point of 
failure in the wall was because I had ran out of the four point 1/2 
barbed wire and had some old two point 1/4 inch barbed wire that was 
even a bit rusty. The section of the wall that had the four point 1/2 
barbed held even after it hit the ground. The section with the 
smaller barb slipped off  by the presure exerted from the uneven 
buttress. 

4a) Had I used the 1/2 new barbed wire rather than the old rusty 1/4 
inch barbed wire it would still be standing.

5.) The big vault you see in web site photo has a   first + coat of 
stabilizied earth and  stayed up even after sitting in the rain for 
several days (exposed) . Some water did seep through right a where 
the arch starts  on both sided.  It even has one uneven side as well. 
My guess is that the plaster adds more strength than I initially 
thought. 

6.) the vaults  weakest point is where the base of the arch in the 
vault. 

7.) I estimated 16 hours worth of work to rebuild what fell but I 
realized today  that I forgot to add in the clean up task that now 
exist. 

8)  Because the earth in the bags that fell never had a chance to set 
(dry), and it has rained for since  Wednesday, I can just refill the 
bags with the same earth. 

Changes I will now incorporate are:

1.) I will build the vault with the form in place. 
2. lay chicken wire over the vault.
3. build an arch over the front and back of the vault (I'll have 
photos of this in a month or so) these archs will sit down on the 
chicken wire and hold it in place.
4.) drop the form and move to the next vault. 
 
  Now go to the next vault.  My goal (quest you might say) is to 
build a completed s

I also played with some walls where I drove rebar and electric 
conduit (aluminum) down through multiple bags.  This must be done 
before the multiple bags are dried. (as in one session). 
I am also playing with some designs of weaving the barbed wire 
between layers to assist more in the stuctural integrity.  Even 
with a massive amount of weight, a stand of barbed wire dug a only a 
few inches into another bag and did not break. It really is some 
strong stuff.

Well I'm glad I learned what I did, but now that I have a single 
piece vault design which I think will work I will plan on being a bit 
more conservative in my building to avoid  these situations. I guess 
just  needed  to push it past it's limit one to say I did it and then 
settle down. 

Night you all. 
Pat
Mauk Ga
http://www.gnat.net/~goshawk
"In the beginner's mind there are many possibilities, 
 but in the expert's there are few."
                                    Zen Mind, Beginner's Mind