Not really...it's more of an "undefined" plastic index, since the requirement is to form a 1/8" or so "worm" of soil, from which the moisture content is taken just prior to the crumbling/fissuring state. If no ribbon or worm can be rolled into the required 1/8" diameter, then there is no way of determining the moisture content at the prescribed point...therefore, undefined.
I would say, yes. While Ron is correct, Atterberg's method requires some thread be made to show the Plastic limit, in theory a non plastic soil would never behave in a plastic state, so below the liquid limit there'd be no plastic behavoir, which defines a PI of zero.
According to ASTM D4318-05, para 11.4, "If, after several trials at successively higher water contents, the soil pat continues to slide in the cup or if the number of blows required to close the groove is always less than 25, record that the liquid limit could not be determined and report the soil as nonplastic without performing the plastic limit test."
And according to the required reporting specified in para 18.1.1, "If either the liquid limit or plastic limit could not be determined, or if the plastic limit is equal to or greater than the liquid limit, report the soil as nonplastic, NP." The liquid limit test cannot be performed when it behaves in manner described in the paragraph above. The plastic limit test cannot be performed when a 1/8" thread of the material cannot be rolled in less than two minutes.
So, non-plastic soils do not have a PI value of zero. They have no PI and they are reported as nonplastic.
