Several attempts have been made to devise alternate plastic limit (PL) determination methods, targeting higher degrees of repeatability and reproducibility. Among these, empirical-type correlations linking the plasticity index (PI) to the flow index (FI)—the slope magnitude of the flow curve—seem to be gaining increased attention, particularly for the fall-cone (FC) approach, and hence demand further examination. To better understand the true potentials and limitations of this emerging practice for soil plasticity determination, this study presents a critical statistical appraisal of FI-based correlations—using a large and diverse database of 230 FC tests (for the 80 g–30° cone)—in estimating the PI (and hence the PL). It is demonstrated that the so-called “strong” correlation between the PI and FI reported in some literature, favoring the use of FI as a PI estimator, is an overlooked “statistical pitfall” originating from an over-reliance on the coefficient of determination (R2) statistic. Employing appropriate error-related statistics, it is shown that the PI predictions made by FI-based correlations are associated with high average errors of 22–33 %. Hence, such correlations, at best, can only provide a rough approximation of the actual PI (and hence PL). An attempt is also made to assess the validity of FI-based correlations in the context of soil classification using the Casagrande-style plasticity chart. The agreement level between the conventional classification approach and that performed using PI deduced from FI-based correlations was 75–80 %. This analysis, however, did not account for errors in the rolling-thread plastic limit data, maintaining a strong possibility that FI-based correlations may be suitable for routine soil classification purposes.
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