the plasticity index, or PI is an engineering terms that gives an indication of how expansive a soil is; expansive soils will usually have a PI of 20 (some engineers consider a soil to be expansive if it has a PI of 10 or more); in the Greater Houston Area PIs in the 40s, 50s and even 60s are not uncommon south of I-10; a soil with a PI of 40 to 60 is considered to be highly expansive; any stated plasticity index should be understood as an approximation since it is calculated from the liquid limit and the plastic limit and these values always rest is part of the subjective engineering judgment of the engineer making the tests

(engineering) the water content percentage between the liquid limit and the plastic limit.