| Summary: | Given its apparent limitations, various attempts have been made to develop alternative testing approaches to the standardized rolling-thread plastic limit (PL<sub>RT</sub>) method (for fine-grained soils), targeting higher degrees of repeatability and reproducibility. Among these, device-rolling techniques, including the method described in ASTM D4318/AASHTO T90 standards, based on original work by Bobrowski and Griekspoor (BG) and which follows the same basic principles as the standard thread-rolling (by hand) test, have been highly underrated by some researchers. To better understand the true potentials and/or limitations of the BG method for soil plasticity determination (i.e., PL<sub>BG</sub>), this paper presents a critical reappraisal of the PL<sub>RT</sub>–PL<sub>BG</sub> relationship using a comprehensive statistical analysis performed on a large and diverse database of 60 PL<sub>RT</sub>–PL<sub>BG</sub> test pairs. It is demonstrated that for a given fine-grained soil, the BG and RT methods produce essentially similar PL values. The 95% lower and upper (water content) statistical agreement limits between PL<sub>BG</sub> and PL<sub>RT</sub> were, respectively, obtained as −5.03% and +4.51%, and both deemed “statistically insignificant” when compared to the inductively-defined reference limit of ±8% (i.e., the highest possible difference in PL<sub>RT</sub> based on its repeatability, as reported in the literature). Furthermore, the likelihoods of PL<sub>BG</sub> underestimating and overestimating PL<sub>RT</sub> were 50% and 40%, respectively; debunking the notion presented by some researchers that the BG method generally tends to greatly underestimate PL<sub>RT</sub>. It is also shown that the degree of underestimation/overestimation does not systematically change with changes in basic soil properties; suggesting that the differences between PL<sub>BG</sub> and PL<sub>RT</sub> are most likely random in nature. Compared to PL<sub>RT</sub>, the likelihood of achieving consistent soil classifications employing PL<sub>BG</sub> (along with the liquid limit) was shown to be 98%, with the identified discrepancies being cases that plot relatively close to the A-Line. As such, PL<sub>BG</sub> can be used with confidence for soil classification purposes.
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