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RE: Expansive Soil

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A lot of this is related to the actual mineral structure of the clay.

Snethen is an excellent source.  However, I expect most of his research,
since He's up the street in Stillwater, was done on Oklahoma Clays.  Need to
ask him the next time I'm in Stillwater (Yep, still around, don't know him
personally, but have the greatest respect for him and his students).  The
other prevalent method is AASHTO T284 "Potential Vertical Rise" based on
some Texas research.

The USACOE has an excellent tome here
http://www.usace.army.mil/inet/usace-docs/armytm/tm5-818-7/basdoc.pdf that
can be used as a reference.  They discuss the various empirical methods for
estimating heave.

LL, PL, and PI is not necessarily the best method to estimate swell.  Clays
with the same or higher liquid limits, may have , less potential swell than
other clays due to mineralogy.  Saw some of this in the Franklin Mountains
has really high PI clays in the mountains, weathered from the dolomite with
low swell potential.  Go just a few miles east into a near by bolson, and
you get clays with similar plasticity index and swelling pressures up to
9000 psf with potential swell on the order of 6% or more. Since this is an
semi-arid (DRY, DRY, DRY, no grass to cut:))) region the clays are
desiccated and can be overlooked if the layers are thin enough.

I quite agree that this is not a good idea in the code and that a swell test
is important.  Unfortunately, this practical, but time consuming and
sometimes costly test is overlooked due to the rapid schedules and tight
(Buffalo Screams) budgets for geotechnical work.  Instead, we get (at least
in Oklahoma) some very conservative recommendations that can cost more time
and money than doing a more through geotechnical investigation.  Standard of
care don't you know.

Arvel

-----Original Message-----
From: L. Pack [mailto:Lloyd(--nospam--at)pecid.com]
Sent: Monday, April 11, 2005 2:20 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Expansive Soil


On 9 Apr 2005, at 8:40, GSKWY(--nospam--at)aol.com wrote:

> Per IBC Section 1802.3.2.
>
> Soils are considered expansive if:
>
> · They have a Plasticity Index (PI) of 15 or greater, determined in
> accordance with ASTM D 4318.
> · More than 10 percent of the soil particles pass a No. 200 (75 µ) sieve,
> determined in accordance with ASTM D 422.
> · More than 10 percent of the soil particles are less than 5 micrometers
in
> size, determined in accordance with ASTM D 422.
>
>
> These last two requirements don't seem to make sense.  If more than 10% of
> the particles are smaller than 5 micrometers (5 microns),  obviously more
than
> 10% are smaller than the No. 200 sieve (75 microns).
>
> It is different tests - the test for the 5 micron size is done with a
> hydrometer,  but it still doesn't seem to make sense.

I agree, this doesn't make any sense.  I pulled out my '97 UBC and my '00
IBC and the UBC doesn't have this definition in it.  The '97 uses the swell
index of 20 as the guide (the fourth option in the IBC)
>
> Is there more to this?  I would assume it came from the UBC,  since a lot
of
> the soils stuff in the IBC seems to have come from the UBC.
>
> Gail Kelley
>

I pulled out Braja Das's Principles of Foundation Engineering, 4th ed.
and on p. 748 there is a table of various criteria for identifying swell
potential.  The method used by the U.S. Army Waterways Experiment
Station was established by Snethen, et al, and is claimed to be the
most widely used in the U.S.A.

Snethen uses:  LL>60, PI>35, tau nat.>4 and SP>1.5 for High potential.

LL between 30 & 60, PI between 25 & 35, tau between 1.5 & 4, and
SP between 0.5 & 1.5 for medium potential.

LL<30, PI<25, tau<1.5, and SP<0.5 for low potential.

I would be hesitant to classify a soil as expansive with a PI of 15 on just
the PI alone.  A swell test would be the best test to establish whether
or not the soil is expansive.

I didn't find any criteria in Das that matched those in the IBC so I don't
know where they come from.  I don't like this type of thing in the code
books.  I hope that it gets modified soon.


Take care,
Lloyd Pack, P.E.


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