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Re: GSBN:Cemento (combined responses)

Hi, Martin,

Thanks for your summation, with which I agree.  I would like to hear
more on one point.  You say:

Re: less bearing capacity in a rubble trench foundation, I estimate
it has 70-85% the capacity of a solid concrete foundation. (With the
same foundation widths and soil bearing capacities.)

I am comfortable with your estimate, but I request that you say more
about the "same foundation widths" part.  In my limited experience,
foundation widths don't tend to be equal.  Soil bearing capacity is
almost always sufficient for a foundation width much narrower than the
bales.  So the architect and builder are faced with a dilemma.  A
concrete foundation as wide as the bales increases materials costs.  A
narrow foundation requires some additional detail to support the full
width of the bales.  I think the latter is the more common choice.

Widening a rubble trench foundation to the width of the bale will
usually incur a lower cost penalty.  Most of the rubble trench
foundations that I have seen for SB houses were made at the bale width
or wider.

My conclusion is that most rubble trench foundations for SB are wider
than concrete foundations for SB.  I wonder if your experience is
similar.  Using your 70-85% estimate, and the greater width of the
rubble trench, I'm thinking that the bearing capacities would be
similar.  Again, I welcome your comments.


--On Monday, July 9, 2007 10:01 AM -0700 Martin Hammer
mfhammer@... wrote:

Rikki, did you ever imagine you would get such mileage out of your
Cemento subject? . . . .

Derk Roff wrote:
A cement grade beam on a rubble trench saves a lot of concrete, with
potentially superior performance.

Mark Piepkorn wrote:
         To be fair, I guess we also have to say that a full-depth
concrete foundation offers potentially superior performance to a
rubble trench with a grade beam, too. It just depends.

I see pros/cons for rubble trench foundations compared to concrete
foundations as follows:

A. Advantages
1. Less cement used
2. Self draining (especially with an integral perforated pipe)
3. No additional excavation and materials needed for drainage
4. Potential use of salvaged material such as broken concrete, or of
local or site available rock
5. Less expensive (materials and labor) (depending on where rubble
comes from?)
6. Allows some flexible movement in an earthquake (?)

B. Disadvantages
1. Less bearing capacity (The loads are delivered to the ground
through the contact area between the rubble and the earth.  For a
given foundation width, there is less such area compared to a solid
concrete foundation, because of the voids in a rubble trench
2. Allows too much movement in an earthquake (?)
3. Can't be used for piers in a pier and grade beam foundation (?)

Re: less bearing capacity in a rubble trench foundation, I estimate
it has 70-85% the capacity of a solid concrete foundation. (With the
same foundation widths and soil bearing capacities.)  This is an
educated guess without rigorous analysis or calculation.  I've yet to
see a published number for this.  Has anyone else?  In most cases for
SB buildings (and other modestly sized buildings) I don't think the
diminished capacity is a problem, or you can always make the footing
a little wider if needed.

Re: earthquakes, I list potential movement as an advantage and
disadvantage. Controlled or limited movement in an earthquake is good
thing, to absorb some of the energy with little or no damage.  But
too much movement can, of course, be a bad thing.  I wrestle with
this important question.  I think it depends on loads and the details
of the wall, stem wall, and rubble trench.

Martin Hammer

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Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek@...