permaculture

["Gene Monaco" <efmonaco@comcast.net>]

I have a little background in this, and I am also trying to fix a leaking
pond made by a predecessor.
The type of clay and how it is compacted is what it is all about.
Ideally, you can find clay that is a deposit from eroded volcanic ash.  This
is very very fine and highly expansive.  Around here it is known as
montmorillonite.  Bentonite is a type of montmorillonite, and is commonly
used in construction.  
Modern methods sandwich about 5 or 6 mm (1/4") of bentonite between two
sheets of something.  Earlier products used paper (like drywall) as the
binder, but now they use geofabrics as well for the pond liners, especially
for landfills.  They also sell bentonite pellets in 50 lb bags that you can
till into the insitu soil if it is already clayey and dry enough to till.  
The liners are very effective but are expensive.  They are however efficient
as far as the clay is concerned because the manufacturing process puts
precisely that 1/4" between the sandwich sheets.  If you try to till it in
by hand or spread it out, you could never get 1/4" uniformly spread out over
the surface; rather, you have to put about a 2" layer down and then till it
in as best you can, or cover it with 18" of packed cover.
The key is that the expansive clay has to be contained by pressure over it.
This means in practicality, that you have to over excavate the pond 18" or
so, lay down the sheet (or till in the 2" layer) and then cover it with 18"
of overburden, preferably clay.  That way when it gets wet and expands, it
will have nowhere to go and will form the seal you're trying to achieve.  
If you till it in to the top 6" without the overburden, you are taking your
chances that when it expands it will lock into the surrounding particles
instead of fluffing and allowing the water to run around it, rendering it
useless.
The principles described here are applicable to any clay site.  You need
something impervious enough and packed down enough under enough pressure so
that when it expands, it won't fluff so much as to allow the water to run
around (through) it.  So the type of clay and how it is installed is
everything.

One detail is at the top.  I've seen a number of ponds that held water to
the design level at the beginning, but then dropped a foot or two or more
after a few years.  This is because the clay at the edge is not under the
weight of the water and tends to fluff.  Also, animals climb in and out
right at the top edge and tear it up as well (especially true for EPDM
liners).  To deal with this, you need to run the clay seam back far enough
outside the edge of the pond and pile up an 18" or 2' berm around the edge,
plus add rocks for weight and protection from animals.


Gene 


Date: Sat, 2 May 2009 12:47:26 -0700 (PDT)
From: Rain Tenaqiya <raincascadia@yahoo.com>
Subject: [permaculture] suitablility of clay for building ponds
To: permaculture@lists.ibiblio.org
Message-ID: <655408.34901.qm@web38107.mail.mud.yahoo.com>
Content-Type: text/plain; charset=iso-8859-1

I'm building a two acre foot pond to irrigate our permaculture demonstration
site and I'm having difficulty identifying appropriate soil for building the
dam and lining the pond.? We have quite a mix of soils varying from
sandstone-derived loam to serpentine-derived clay.? Is the clay content the
most important variable?? We have some very high clay content soil that has
a crystal like structure when exposed.? It is very expansive, and when dry,
it gets blocky and is easily broken up.? I think it would be fine where it
will be under other layers or always wet.? Does anyone have any answers or
comments on this?
?
Thanks,
?
Rain