Return to index: [Subject] [Thread] [Date] [Author]

Re: Foundation on Expansive Clay

[Subject Prev][Subject Next][Thread Prev][Thread Next]
James,

I have a serious difference of opinion with geotechs' recommendations 
concerning expansive soils.  First of all, I consider the "Expansion Index 
Test" as providing no valuable information to a Structural Engineer.  It 
gives an expansion index (1000 times the percent expansion of a sample) but 
doesn't give the force that the expansive soil exerts.  Therefore, I feel 
that a confinement pressure test is the minimum test that would provide 
valuable information to the Structural Engineer.  The confinement pressure is 
the pressure that is necessary to *prevent* the sample from expanding.  If 
the confinement pressure is 5 psf, I can handle that; it is no problem.  
However, if the confinement pressure is 5 tons per square foot, which I 
understand from literature is a valid pressure, I can't handle that.

With regard to the granular buffer between expansive soil and a slab, I would 
suggest visualizing an analogous situation:  Consider a deflated balloon in a 
container and have marbles on top of the balloon.  Now, start to inflate the 
balloon and the marbles will move.  The marbles represent the granular soil, 
of course, and the balloon represents the expansive soil.

An article in ASCE's Civil Engineering Magazine a number of years ago 
described a development near Denver, where expansive soil is a real problem.  
In preparing a hill for housing development, the developer cut off the top of 
the hill and pushed the spoil over the side.  Houses were constructed on both 
the cut area and the uncompacted fill.  FHA would not insure the houses on 
the uncompacted fill, but did insure the houses on the cut area.  Many of the 
houses on the cut experienced damage due to expansive soil, and virtually 
none of the house constructed on the uncompacted fill experienced damage.  I 
think that the reason is obvious --- the soil in the uncompacted fill had 
room to expand without lifting portions of the house, while the soil in the 
cut area could only expand upward, damaging the houses.

We, and geotechs, are brainwashed in that we think soil must be compacted, 
but it ain't necessarily so!  Sometimes the soil needs to be uncompacted.

Expansive soil will lift the lightest part of a structure first, which is 
usually the floor slab.  If bearing and exterior walls are virtually 
undamaged and concrete slabs-on-grade are cracked and non-bearing walls are 
damaged or are pushing into the ceiling, the problem is generally due to 
expansive soils.

One man's opinion.  I only ask that you think about it, not necessarily agree 
with me.

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

James Lane wrote:

--------------Begin Referenced Message Thread-------------

Mr. Conner and Mr. Turk,
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.
James

>From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
>Reply-To: seaint(--nospam--at)seaint.org
>To: seaint(--nospam--at)seaint.org
>Subject: Foundation on Expansive Clay
>Date: Wed, 4 Apr 2001 11:02:51 -0500
>
>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?

-------------End Referenced Message Thread------------