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Re: Foundation on Expansive Clay

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Thanx, Roger,

The structural design is not a problem.  I'll have enough reinforcing in it
to handle the load any way it turns.

But differential settlement is a challenge.  If one side of the foundation
moves 1/2", relative to the other, the mast will be nearly 1 1/2 inches out
of plumb.  I can tolerate that much movement, but the owner would begin to
get uneasy with twice that  ;-)

You have, however, eased my mind somewhat.  With such a small foundation, I
may not have a problem.  I would think that 1/2 inch differential in 8 feet
would be pretty dramatic.


> From: Roger Turk <73527.1356(--nospam--at)>
> To: seaint(--nospam--at)
> Subject: Foundation on Expansive Clay
> Date: Wednesday, April 04, 2001 11:02 AM
> Fountain,
> I don't think that a 6-inch turn-down will make any difference in
> migrating under the slab, nor do I believe that capillary action is the 
> culprit.
> Moisture will migrate naturally from wetter soil to drier soil.  When the

> drier soil is under a slab (or even a rock in the middle of the driest 
> desert), it will be captive as the slab/rock will restrict evaporation of
> moisture.  (The soil under a rock is always wetter than the surrounding
> To prevent expansive soil from moving, its moisture has to be kept
> a virtually impossible task unless it is kept saturated at all times. 
> turn-downs, if used, would have to be continued to a depth where the
> content of the soil is constant at all times, and the turn-down would
have to 
> be waterproofed similar to a basement wall.
> Since the slab is small and lightly loaded (and 3-feet thick), have you 
> considered designing it like a ship; for a hogging condition and a
> condition?
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
> Fountain Conner wrote:
> . > I need to put some lightly-loaded foundations on clay "with the
> . > 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. 
> . > 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
> . > 6 inches.  If this clay is expansive, it shouldn't be given to
> . > action, and will provide a seal, preventing the soil beneath the 
> . > foundation from seeing the varying moisture content.
> . > What do you think?