Repeatability and Agreement Between Methods for Determining the Atterberg Limits of Fine-Grained Soils

The Atterberg limits (plastic limit [PL] and liquid limit [LL]) of fine-grained soils are important for civil engineering and agronomic applications. Several methodologies exist to determine the PI, and LL, each with associated merits and shortcomings. The reproducibility and uncertainties associated with the various methods have not been evaluated for different samples that differ in terms of clay mineralogy and geologic origin. The objectives were to (i) assess the repeatability of two methods each for determining the PL (thread rolling and rolling device) and LL (Casagrande cup and drop cone penetrometer), and compare the subsequent plasticity indexes for 30 samples from 10 countries; and (ii) evaluate the effect of particle size (2 mm and 425 mu m) on the plastic and liquid limits for 28 Venezuelan samples. For the PL, the repeatability (estimated as the repeatability coefficient, RC, among replications) for the thread rolling and rolling device methods was similar (2.28 and 2.39%, respectively). For the LL, the drop cone method was 16% more repeatable than the Casagrande cup method. There was a strong correlation between the two respective methods for PL (r(2) = 0.98) and LL (r(2) = 0.99). For samples that have LL < 45, the LL from the Casagrande cup was slightly higher than LL from the drop cone, and vice versa for samples with LL > 45%. No significant differences were observed between PL measured on <2-mm or <425-mu m samples; for samples with LL < 35% the two particle sizes gave similar values. There were however larger discrepancies between the two particle sizes for samples with LL > 36%.