Retaining wall

[Eric D. Hart]

At 04:15 PM 11/18/96 -0800, Kat Morrow wrote:

>Also, I second Shannons opinion that cob walls could work in raised beds.
>I have seen a cob test wall (here in Tucson, a little different climate!)
>that had been exposed for about 2 years and had weathered but was still in
>good shape.  The rock covering would greatly increase the life of the wall
>as would allowing sufficient wall drying time before you add the garden
>soil.
        So, I'm seriously thinking about this raised bed idea.  Here are a
few of my questions:
1.  How long does the cob have to dry before its a good idea to add the
soil?  Its a bit tricky in Minnesota because the time to build such a raised
bed is short before you want to start planting.  About April 15th the snow
goes away and the ground starts to warm up.  That gives you about a month
before the majority of the plants have to go in.

2.  What conditions are optimal for the cob to dry?  In the time frame we
would build the wall, the temperature generally doesn't get below freezing
very often although the last frost is in the beginning of May usually.  High
temperatures are in the 50s usually.  It could rain or even snow at any
time.  How is that for drying the cob wall?  

3.  I was thinking that if I end up putting rocks over at least the inside
of the wall, why not do a rock wall with cob as the mortar?  I seem to
remember that cob needs to be unbroken by large rocks or other material so
cob mortar might not work.  If a cob wall is needed, do you just place the
rocks in the soft cob before it dries?

4.  Here's my latest thinking on a quick to build cob retaining wall.  Build
up the wall to full height and then add rocks to the top (to protect against
the wall).  Maybe even overhang the rocks a bit to protect the outside of
the wall from rain.  For the inside (where the soil is going) add metal
flashing to prevent the wall from getting too wet and from those pesky roots
that might get in and destroy the wall.  The metal can go underground a bit
and come to about one inch from the top (and be tied into the rock top).
Lining it with metal might make it possible to use the wall sooner.  I plan
on using salvaged metal  (from dumpsters or doantions from people's garages). 

5.  Construction sequencing:  Here's what I propose for a construction
schedule on this project.  My first take on this is that it will take 3
weeks and the raised bed will be at least 2 feet tall.  The first weekend
(around the third week in April) would be to construct the first part of the
wall (say 1 foot of the wall).  Two weeks later, the rest of the wall would
be built and the flashing and the top rocks put on.  One week after that,
the soil could be added and the gardeners could start planting in earnest.
People could get started sooner if only part of the soil was put in the bed
(away from the cob wall) and the rest of the soil added later.  

Eric D. Hart			
Community Eco-design Network	  
Minneapolis, MN  USA		
(612) 305-2899 
erichart at mtn.org			
http://www.tc.umn.edu/nlhome/m037/kurtdand/cen