You supposition that they may not understand is possible. More likely
a "and" or "or" was eliminated somewhere in the editorial cycle and, since this
conservative, left alone.
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
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