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  • From: Jeffrey Newman <newman@bcinternet.net>
  • To: permaculture@listserv.oit.unc.edu
  • Subject: Re: Retaining wall design
  • Date: Tue, 29 Sep 1998 15:28:56 -0700

I'm glad my embankment has sparked such input. It's certainly helping and
enriching my
little project. My embankment is pretty close to the house. The soil is
fairly dry and
stable but the top layer is disturbed - back fill. So, when I watered above
and back from
the embankment area, some water emerged from the (unstabilised as yet) bank
from between
the backfill and original soil level (about two thirds up the bank. There are
three
forty-year old firs growing above that bank, close enough for some of their
lesser roots
to protrude. These trees establish deep roots and have another 100 years or
so of life
ahead of them (I'm told). Nearby is a stump of a felled tree. It is about
four times the
diameter of my living trees.

I have enough space to angle the embankment at about 60 degrees froom
horizontal.

Jeffrey

Elfpermacl@aol.com wrote:

> 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



--

*******************************

Jeffrey Newman
My personal web site is at http://www.geocities.com/Heartland/Flats/5759/

My ICQ number is: 9879310





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