Every time I have encountered clay the geotech has had us undercut (remove
the soil)for a specified number of feet from the bottom of the footing and
replace with compacted structural fill. This as I understand it acts as a
buffer between the clays and the bottom of the footing. The geotech also
uses similar fill adacent to basement or retaining walls, so as to reduce
the equivalent fluid pressure. This can mean decreasing the fluid pressure
from as much as 125pcf to around 60pcf. I suspect the same concept applies
for expansion of the clay on the bottom of the slab.
Geotechs have also told me that a deep turn down does help keep the soil at
a more constant moisture level under the slab.
The Slab on Grade Design Book is a good source for practical reading on
designing in expansive clays. Build the slab stiff is a good rule so when it
does move it moves as one.
From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Subject: Foundation on Expansive Clay
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
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. Slab
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
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
. > 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
. > 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?
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