A Note on the Determination of Plastic Limit of Fine-Grained Soils

Abstract

Plastic limit of fine-grained soils is conventionally determined in the laboratory by the soil thread rolling method. Many adverse comments have been recorded in the geotechnical engineering literature on the method about its reproducibility and operator dependency. The present experimental study, which is based on a well-planned and meticulously executed experimental program, critically evaluates the effect of size of the rolled soil thread on the plastic limit of fine-grained soils and the operator dependency of the results. The results have shown that if the plastic limit tests are performed by a trained operator, then consistent results can be obtained and that the effect of size of the rolled soil thread on plastic limit is negligibly small.

Review of Recent Developments and Understanding of Atterberg Limits Determinations

“…As originally described by Atterberg [1,2], the PL HR is determined using the handrolling method, being standardized worldwide (e.g., BS 1377-2 [7], ASTM D4318-17e1 [8] and EN ISO 17892-12 [9]). Note, it has been shown that the thread diameter requirement for the crumbling condition-specified as approximately 3.0 mm [7] or 3.2 mm [8]-is not critical, with no statistically significant trend of varying PL HR water content with the soil thread diameter (2-6 mm range investigated) for the crumbling condition [10,38].…”

“…While it has been often argued that the method is overly dependent on operator performance and judgement [17,18,21,[39][40][41], with an operator error as great as 10-15% when testing the same fine-grained soil [17,40,41], this would point to the absolute necessity for the PL HR experiment to be performed only by experienced and attentive personnel, with strict adherence to the standard procedure [38,42].…”