Cob Earth berming cob buildings?

[Don Stephens]

On Mon, 4 May 1998, Jud Malone wrote: "I'm going to build a house into a
south-sloping mountainside in WVA this summer, and I would like to know if
it is possible to berm cob walls.  Assuming that I would have good
drainage away from the house, would the cob walls absorb too much moisture
from the surrounding dirt?  How about an earth-bermed rock wall faced on
the inside with cob?" 

Don Stephens, Spokane WA USA replies: By way of background, I've been
designing, building, writing about and teaching residential eco-earth-
sheltering/underground construction since 1960, almost exclusively with
moisture-sensitive natural materials.  I'm still low on the learning curve
with cob but have worked with bermed and, indeed, earth-covered strawbale
walled (and roofed)homes for several years with success and straw in bales
is certainly as vulnerable to wicking and other wetness issues as cob!

Success with what you propose is definitely workable, given the right
techniques, water barriers and a "friendly" site.   First, the site - What
is the general slope at and above the building zone?  What kind of plant
life does it support?  have you done a test excavation to see what the
soil stratigraphy (layering) looks like where you propose to put your back
wall?  Are there springs or freshets (post-winter temperary creeklets) 
above the site or wet layers in your excavation?  Are there signs of slope
instability - generally out-sloping trees or those who's butts slope out
before turning vertical, errosion gullies, exposed break-aways, slides or
sloughs - which would require addressment for any kind of excavation or
construction? 

Assuming all the geo-signs above are promising, it should just be a matter
of making sure that the capilarity with the soil below is well broken,
that you have moisture barriers between the cob wall and the hill behind
it, that you address lateral pressures exerted by the earth, that you
provide positive drainage (preferably backed up by a sub-grade waterproof
diversion sheet - 4 mil CROSSLAMINATED polysheet like Tu-Tuf by Sto-Cote
or equal) and that you adiquately intercept downslope surface flow from
above (with drainage swails, trench-type gravel diversion "dikes" and
tiling.) 

One good way to deal with all of these but the up-hill response to down-
slope surface flow is with what I call a vertical crawlspace, which has
proven a multi-helpful element of a number of my residental projects.  To
do this (Gee it would be handy to be able to sketch on the email screen!) 
means to set the back (cob) wall of your home out several feet away from
the stablized earth slope behind it and extend the roof overhang out over
the intervening space to a footing on the undisturbed slope. Depending on
soil type, there is a natural angle of downward load transfer from that
outrigger footing; oftenen this is assumed at 45% unless the soil is
extremely sandy or filled with organics.  This back earth wall can be
stabilized at the established angle with a series of back-sloping rock
terrace walls which can also provide handy storage shelves within the
resultant "vertical crawlspace" (some prefer to stucco the rock and soil
for a more "sanitary" finished look.

Besides the relatively tempurature-stabile storage area this provides, it
offers a number of additional benefits - no lateral pressure on the back
house wall, a place to run, modify and service plumbing, electrical, etc.,
a great place for a composting toilet, a heat plenium to incorporate in an
annualized passive solar heating system (using the resultant dry soil as
thermal storage/flywheel mass and a place for movable insulation on that
wall of the house, to receive or resist heat transfer from that soil mass.

I've gone on long enough on peripheral elements on a site to be restricted
to cob, but if others are interested in these ideas related to earth-
contacted cob and other moisture-sensitive materials and/or techniques of
annualized/isolated-gain passive solar, I'd enjoy hearing from you on the
side.  Jud, I hope some of this is of interest and help in your project.
I'll finish by responding to you request for help by quoting somebody's
(?) caution to "beware of all advice, including this!"