permaculture

["Eugene F. Monaco" <EFMonaco@compuserve.com>]

Message text written by INTERNET:permaculture@listserv.oit.unc.edu
><< FWIW, Douglas Fir is used in planking wooden boats so it should not rot
too fast.  But untreated it will eventually rot.  You need redwood or
cypress or locust for long term ground contact. Heart pine from the South
is also very rot resistant but will burn even in the rain.  What is your
design life expectation? or is recycling downed logs more important? Mike
Miller>>

A point has been missed here.  Once the bank is stabilized, living plants
can
be established to take over the job.  
<

One more point.
It's a good idea to design the wall with the ability to creep.  To
understand this, visualize gravestones on a hillside, leaning; or many
retaining walls that are leaning over.  What happens is that the soil
particles on the slope of the hill behind the wall get uplifted by thermal
and moisture pressures, then they settle later, not just downward but also
some on the horizontal.  This zig-zag motion of the particles causes the
hillside to creep over time.  Classical civil engineering design calculates
the soil pressures behind the wall tending to overturn the wall, and seeks
to resist them by downward pressure on the base of an upsidedown "T"
section, or an "L".  If you visualize and upsidedown "T" that wants to
rotate, the part of the base of the "T" that rotates up is resisted by the
weight of the wall; the part of the base of the "T" that rotates down is
resisted by the soil below, provided the "T" keeps its structural
integrity, which it does by being made from reinforced concrete.  You can
estimate the forces expected by creep during the expected life of the wall;
they are larger than you'd think and are larger than just the weight of the
soil. 

One of the biggest pressures is the fluid pressure of the water behind the
wall.  Modern designs seek to minimize this by relieving pressure behind
the wall.  This is accomplished by putting weep holes in the wall at the
base; or if you want to get fancy, you can put a vertical layer of gravel
behind the wall with a vertical layer of geofabric between the gravel and
the soil benind it.  Then place a perforated pipe along the base of the
wall in the gravel (wrap the pipe with filter fabric to keep out the fines)
and drain the pipe downhill and out somewhere. This relieves fluid
pressure.

Even after all this, the wall can still lean after a hundred years due to
creep, and probably will.  So if you're doing a more indigenous design such
as Dan describes, just make sure the wall has some place to go when it
creeps.  In other words, don't build the foundation of your house right at
the base of the wall; better to hold back a few feet.

Gene Monaco