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RE: Foundation on Expansive Clay

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Design for expansive clays is very dependent on the local climatic
conditions. The key is getting the support down deep enought that you are
below the depth of seaonal change in m/c. If the foundation is three feet
thick, in many areas you are below a large percentage of the movement
already (assuming the surface of the slab is at ground level). OTOH, here in
Houston the depth to constant m/c can be deeper than 12 feet (near trees).
You might want to consider piers, although this only solves the settlement
problem, unless you are willing to have an elevated foundation, and at three
feet thick I do not think this a probably a good idea.

eric green

-----Original Message-----
From: Fountain Conner [mailto:fconner(--nospam--at)]
Sent: Wednesday, April 04, 2001 6:05 AM
To: seaint(--nospam--at)
Subject: Foundation on Expansive Clay

I need to put some lightly-loaded foundations on clay "with the potential
for expansion".

These foundations will carry a bark blowline in a paper mill.

If all were normal these foundations would be 7 feet long x 8 feet wide x 3
feet thick.  They each would have a vertical pipe 20 feet tall, carrying a
horizontal blowline.  Maximum soil pressure (max load and wind) would be
about 2000 psf.  Static maximum load is about 650 psf.  The geotechnical
report allows at least 2500 psf, plus 25 percent for short-term live load
(like wind).

Plain vanilla...  Until I factor in the potentially expansive soil.  

My thinking is to turn down the outside edge of the footing an additional 6
inches.  If this clay is expansive, it shouldn't be given to capillary
action, and will provide a seal, preventing the soil beneath the foundation
from seeing the varying moisture content.

What do you think?

Item 2:

I had intended to place each "mast" in a sleeve, plumb it and grout it
into position -- cheap, but effective.  I *can* put a mast on a large,
baseplate, and perch it on nuts below the base plate, with double nuts
above.  If I do this, I can allow enough room below the lower nuts to
adjust each mast plumb if there is some movement.  I'm hoping you guys can
convince me that this step is overkill.



Fountain E. Conner, P.E.
Gulf Breeze, Fl. 32561
   Phone 850-932-5547
   Fax      850-934-1916