Retaining wall

[Shannon C. Dealy]

On Sun, 24 Nov 1996, Eric D. Hart wrote:

> 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.

For this application, it doesn't need to be totally dry, but you will want
to wait until the wall is dry enough that it doesn't give when you
sit/stand on it, otherwise the weight of the earth inside the bed may
slowly push the walls out.  Depending on the weather and the thickness of
your walls, this can be as little as a few hours after you are done to a
week or more.  I have no experience with the weather in your area, but at
that time of year in the area of Oregon I live in I would expect it to
take a week if a tarp is suspended above the wall so that air can
circulate but the rain will be kept off.  If you get some direct sun, take
the tarp down to give the wall as much exposure as possible since sun can 
really speed up drying time.

> 
> 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?  

For fast drying, you want high temperature and low humidity, though this
can result in the cob drying to fast which will cause cracking.  Under the
conditions you describe, it should be adequately dry within a week if you
get a fair amount of sun during the day.

> 
> 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?

The problem with using cob as a mortar is that it does not bind to the
surface of your rocks the way that a conventional cement mortar does.
Putting rock into a cob wall will work, but the rock diminishes the
tensile strength of the wall since the rock basically just takes up space
inside the wall.  The real tensile strength of the cob comes from the
straw fibers which tie it together.  For compressive strength, the rock is
fine.  For a raised bed, using rock in the cob should be alright.  I think
however that if I were doing it, I would build a plain cob wall and
when I started putting the soil into the middle, I would start adding
rock/gravel around the edges so that when the bed is full there would be a
4"+ layer of rock (without cob or earth in it) between the cob walls and
the dirt filling the bed.  This would keep the moisture from the bed away
from the walls allowing them to last much longer.

> 
> 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). 

You should probably put some rocks (or some other moisture break) on the
ground under the wall to reduce wicking of moisture into the wall.

> 
> 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.  

For some reason I was thinking of taller walls, at two feet high and given
the above schedule, I wouldn't worry to much about drying conditions as
long as there is a moisture break between the wall and the soil,
and the wall is reasonably thick, I wouldn't expect two feet of soil to
exert enough pressure against the walls to be much of a problem.


Shannon Dealy
dealy at deatech.com