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Expansive Soil

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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.
 
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