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Expansive Soils Part 2

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I should have probably included these excerpts as well:
When evaluating the expansion potential of a soil, the relative percentage of different particle sizes should be taken into consideration.  In particular, it should be noted that Atterberg Limits are done on the particles smaller than the #40 sieve; if these particles represent a very small percentage of the soil, the soil may not be expansive, even if it has a high PI value.  The IBC does not classify a soil as expansive unless at least 10% of the particles are smaller than 5 microns.  (The IBC appears to consider a particle smaller than 5 microns to be clay, which is an outdated definition of clay particles.  In 1935, the Department of Agriculture changed the definition of clay to particles smaller than 2 microns.)
Because of the sedimentary origin of fine-grained soil deposits, clay soils are seldom composed only of clay minerals.  So-called clay deposits are typically a mixture of clay and silt particles.  In addition, clays typically have a significant fraction of particles that are larger then than the No. 200 sieve (sand and gravel.)  Sands and gravels are by definition, non-plastic; they do not absorb water. Their engineering behavior is not significantly affected by surface electrical charges because of their comparatively low surface-to-mass ratio. 
Although soils referred to as silts typically have some amount of clay, silts are generally non-plastic.  Silts with a high plasticity index typically have a high organic content; it is actually the organic material that is absorbing the water.  If the organic material content of a clay or silt is high enough to affect the soil properties, it is referred to as an organic clay or silt.