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Foundation on Expansive Clay

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Fountain,

I don't think that a 6-inch turn-down will make any difference in moisture 
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 the 
moisture.  (The soil under a rock is always wetter than the surrounding soil.)

To prevent expansive soil from moving, its moisture has to be kept constant, 
a virtually impossible task unless it is kept saturated at all times.  Slab 
turn-downs, if used, would have to be continued to a depth where the moisture 
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 sagging 
condition?

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Fountain Conner wrote:

. > 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?