Cob: Re: Foundations, tree roots

[John Fordice]

Kerry,
Not being an expert on trees, I'll speculate to say that if you build so
as not to compromise an existing root system, whatever growth the tree
does after your building is there will be adapted to the then existing
soil situation.  If the tree's growth future is limited by the fact that
your building is where it is, then the tree will not grow in a manner
that will cause damage to itself.  This is different from an act of
construction damaging an already existing root system & hence causing
damage to the tree depending on those roots.

Re your wondering about a foundation on the surface of the ground in
lieu of an excavated foundation.  The reason for excavated foundations
is to assure that the weight of the building ( which is considerable in
the case of a cob building ) sits on " competent " soils.  It is quite
possible that the upper portion ( i.e. topsoil ) has some give to it &
will subside some when subjected to the weight of a building.  In
general, this gets to be less the case the deeper you go into the
ground.  The building code assumes that at 12" of depth the minimum soil
bearing capacity is 1000 pounds per square inch.  At a depth less than
this, it is generally not wise, in the absence of other proof, to assume
the soil can successfully support your building.  If you calculate the
weight of your cob walls at 150 pounds per cubic foot for the whole
height of the walls including your foundations and set your footing
width accordingly so as not to exceed 1000 psi at the bottom of a 12"
deep footing. the buildings walls will ( according to the building code
at least ) be adequately supported by the soil upon which they sits. 
This does not take into account the loads of any associated floors or
roofs of course.  If the wall is carrying any other such loads, they too
should be taken into consideration when deciding the necessary
foundation width.

john fordice

Kerry S Tebbetts wrote:
> 
> Well, the problem is that the drip line will get bigger once some of the
> smaller trees are thinned out to make room for the house.
> 
> On Sat, 27 Apr 2002 11:42:51 -0700 John Fordice <otherfish at attbi.com>
> writes:
> > An arborist once told me that the roots of a tree will be a bout the
> > same area as the branch spread.  The limit of concern so as to not
> > compromise the root system is don't excavate or build or put
> > impervious
> > paving anywhere inside the drip line ( outer circumference  ) of
> > the
> > branches.  This was in discussion about protecting some huge oak
> > trees
> > in the Napa Valley.
> > john fordice
> >
> > Rick Fisk wrote:
> >
> >