permaculture

[Elfpermacl@aol.com]

In a message dated 9/28/98 8:23:14 PM, you wrote:

<<
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>>

Nice Additions, Gene:
        A crib wall rots away and will be gone long before 100 years.  It 
naturally
slopes back away from the base like any sensible slope.  If the slope exceeds
the resting angle of the soil, then it will have a tendency to slump.  I think
the concerns about hydraulic pressure and frost heave are particularly vaild
in clay or soils with high clay content, probably also peats.   Well drained
soils are not so much of a problem.  One assumes that permaculture is
practiced elsewhere besides the wall, in any case, with folks on this list,
and so the moisture will be managed with diversion drains, swales, etc.
        Since the crib wall is the first stage for revegetating with a set of 
species
with different root profiles, some shallow and mat rooted to hold surface soil
and some with deep tap roots to anchor the whole mess, the slope should trash
about rather less than any other vegetated slope of the same angle in the
vicinity.  I have trouble envisioining it leaning forward like a snow drift
and causing a earthen avalanche.  Like unstable people, unstable soils require
more caution in our dealings.
        Permaculturists are supposedly trained not to build below unstable 
slopes, on
ridge tops, in flood plains (particularly the outside curve of a river bend),
at the edges of volcanoes, and at ground zero in nuclear weapons testing
areas.  Avoiding such type one errors requires observation, a bit of research
(like talking with oldtimers in the area), and common sense. 
        The landscape is a continual dance to the music of the spheres, and 
one of
the steps is the pulse between leveling by entropy and uplifting from
geological forces.  Vegetation is a great moderating force from leveling, not
only slowing erosion but also in holding soil against subsidance.  I've got
pictures from widely separated parts of the world showing the slopes sliding
away after critical tree cover has been cut.  All the terracing and inert
walls in the world won't resist the sliding soil once it is no longer held in
place by trees.  So any measure that gets trees back in place, assuming proper
species selection, seems to me to be worth doing.

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since 1982),
Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and annual correspondence
courses via email.  Copyright, 1998, Dan & Cynthia Hemenway, P.O. Box 52,
Sparr FL 32192 USA  Internships. YankeePerm@aol.com  

We don't have time to rush.

A list by topic of all Yankee Permaculture titles may be found at
http://csf.colorado.edu/perma/ypc_catalog.html
Elfin Permaculture programs are listed at the Eastern Permaculture Teachers
assn home page: http://home.ptd.net/~artrod/epta/eptahmp.html