From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Wed, 4 Apr 2001 11:02:51 -0500
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
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
A. Roger Turk, P.E.(Structural)
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?