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RE: Expansive Soils

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That's slick and I like her attitude.  Who does she work for?
-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)]
Sent: Tuesday, April 12, 2005 3:08 PM
To: seaint(--nospam--at)
Subject: Re: Expansive Soils

I read it the same way you did.  I chalked it up to stupidity, and immediately ignored it, as I don't do the testing or make recommendations pertaining to the geotechnical aspects of the soil. Those come, like magic, from a geotech.

On the topic, my favorite local soil scientist laughs at every mention of using PI as a measure of expansive potential. She generally runs the physical tests for a preliminary fee, then will send the samples out for xray crystallography (hope I got that test right...once magic - remember?) to determine the actual composition for levels of  expansive clay types like montmorillanite (sp?) and bentonite.  That $300 additional test can save several thousand in construction costs, but she's one of the only ones around here that does it.  Since she actually did her graduate work on expansive soils, it may just be that no one else realizes that the test exists and/or can't decipher the results.

GSKWY(--nospam--at) wrote:
My question on IBC section 1802.3.2 actually has nothing to do with the plasticity index,  it has to do with the second and third items under the definition of an expansive soil. 
1.  They have a Plasticity Index (PI) of 15 or greater, determined in accordance with ASTM D 4318.
2.  More than 10 percent of the soil particles pass a No. 200 (75 µ) sieve, determined in accordance with ASTM D 422.
3.  More than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D 422.

Unless I am missing something,  item 2 is completely unnecessary.  If more than 10% of the soil sample is smaller than 5 microns,  obviously more than 10% is smaller then 75 microns.   As written both apply to the entire soil sample.
If I am not missing something,  I wonder whether the writer(s) of this section understand what they have written.  It seems like no one has questioned it,  since it is in both the 2000 and 2003 IBCs, which leads me to wonder how many people in Texas have actually read the IBC.
One issue with respect to using Atterberg limits to classify soils is that the actual test  is done on the fraction of the soil that is smaller than the #40 sieve.  A soil with a small percentage of minus #40 material could have a very high PI value but still not be expansive.
With respect to design of foundations on expansive soils, the IBC  allows the building official to approve foundations other than those designed by the PTI and WRI methods even when the soils are nominally classified as expansive. 
Gail Kelley

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