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Re: Expansive Soil

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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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