A mathematical approach is presented herein to determine the plasticity index of soils on the basis of the percentage of particles smaller than 0.05 mm in size. The particle-size distribution can be determined easily and quickly, and then, with a few calculations, the plasticity index can be estimated with fair degree of precision. The laboratory determination of plasticity index requires more laborious work. The equation developed is exponential, as follows:
Plasticity index = —0.052394 + 3.1538e0.029284X, where e= 2.71828 and X is the percentage of particles smaller than 0.05 mm. in size.
The correlation coefficient obtained was on the order of 0.92. On the basis of the percentage of particles smaller than 0.05 mm., about 85 percent of the variability of the plasticity index values was explained. The plasticity index is one of the more useful engineering properties of soils.