COMMENT Discussion: Determining the plasticity properties of high plastic clays: a new empirical approach [Arab J Geosci (2020) 13(11), 394] Brendan C. O’Kelly 1 & Amin Soltani 2 Received: 27 November 2020 /Accepted: 17 February 2021 # Saudi Society for Geosciences 2021 Abstract This article presents a discussion of the Arabian Journal of Geosciences, 394, Vol. 13, No. 11, which (among other things) proposed a new empirical correlation for the prediction of the plasticity index magnitude (and hence by association the plastic limit value) of fine-grained soils sampled from different depths in the Bakırköy District of Istanbul City. As inputs, the proposed correlation employs the activity index, along with defining parameters of the semi-logarithmic flow curve obtained from the Casagrande (percussion-cup) liquid limit test. This article presents the following: (i) a discussion of the relevant literature that was not covered in the original paper; (ii) clarifications on, and observations regarding, the general approach adopted in the original investigation; (iii) concerns over whether the proposed new empirical correlation can be meaningfully recalibrated for particular clay soil types specific to other regions around the world; and (iv) suggestions for improvement of the new empirical correlation by adopting a different set of input parameters. Keywords Plastic limit . Plasticity index . Percussion-cup flow curve . Flow index . Clay mineralogy Introduction In his paper (Öser 2020), the author proposed a new empirical correlation (given by Eq. 13) for the determination of the plas- ticity index (PI) magnitude from the liquid limit (LL) test per- formed using the Casagrande percussion-cup (PC) method (i.e., LL PC ), in accordance with ASTM D4318 (2008). The proposed correlation collocates the Z and I f parameters, which respective- ly define (i) the water content w corresponding to a PC blow count of N = 1 and (ii) the absolute slope magnitude of the PC flow curve in the w:log 10 N space (i.e., I f = -Δw/Δlog 10 N), and employs both of them, along with the activity index A c , as inputs in order to account for the clay mineralogy effect on soil plasticity. As a starting point, the author makes use of the reported Eq. 5, attributed to Çinicioğlu et al. (2002) in the paper under discussion, but in fact originally proposed by Sridharan et al. (1999), to devise an empirical correlation between the PI and I f parameters, thereby allowing the indirect determination of the plastic limit (PL) by subtracting the estimated PI from the measured LL PC . In this discussion article, the relevant literature on soil plasticity determination, which was not provided in the original paper, will be first presented and discussed in detail. The discussers will then present some clarifications on, and make a number of observations regarding, the general approach adopted in the Öser (2020) investigation, after which they give their viewpoint on whether the author’s presented approach (and specifically Eq. 13 of the original paper) can be meaningfully recalibrated for particular clay soil types spe- cific to other regions around the world for the purposes of developing various new empirical correlations for PI (and hence PL) estimations. Finally, the discussers offer suggestions for improvement of the author’s new empirical correlation by adopting a different set of input parameters. Standardized rolling-thread PL method Contrary to one of the author’s opening remarks in the Abstract of the paper under discussion that stated “the plastic Responsible Editor: Zeynal Abiddin Erguler * Amin Soltani a.soltani@federation.edu.au Brendan C. O’Kelly bokelly@tcd.ie 1 Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin D02 PN40, Ireland 2 School of Engineering, IT and Physical Sciences, Federation University, Churchill, Victoria 3842, Australia Arabian Journal of Geosciences (2021) 14:715 https://doi.org/10.1007/s12517-021-06757-5