Applied Clay Science 200 (2021) 105955 Available online 29 December 2020 0169-1317/© 2020 Elsevier B.V. All rights reserved. Review article Review and critical examination of fne-grained soil classifcation systems based on plasticity Jos´ e Manuel Moreno-Maroto a, * , Jacinto Alonso-Azc´ arate b , Brendan C. O’Kelly c a Department of Chemical, Environmental and Materials Engineering, Higher Polytechnic School of Linares, University of Jaen, Scientifc and Technological Campus of Linares, 23700 Linares, Ja´ en, Spain b University of Castilla-La Mancha, Department of Physical Chemistry, Faculty of Environmental Sciences and Biochemistry, Avenida Carlos III, s/n, 45071 Toledo, Spain c Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin, Ireland A R T I C L E INFO Keywords: Soil classifcation Fine-grained soils Atterberg limits Plasticity Casagrande chart ABSTRACT Soil classifcation systems represent powerful tools not only to facilitate soil identifcation, but also to predict its possible behavior. While the use of arbitrary standards based on particle size distribution may be applicable to coarse-grained soils, those approaches based exclusively on textural principles are ineffective in classifying fne- grained soils, where clay content and its mineralogy dictate the general properties. In this sense, the measure- ment of plasticity represents a more appropriate parameter than particle size. This fact has led various authors and technical committees to develop fne-grained soil classifcation systems based on plasticity. However, the disparity of criteria makes it necessary to review them in order to glimpse the weaknesses and strengths of each of them. This paper includes the review of the six main existing proposals together with the possible variants arising from them: Casagrande (1947)-Unifed Soil Classifcation System (USCS), American Association of State Highway and Transportation Offcials (AASHTO), Federal Aviation Agency (FAA), Saito and Miki (1975), Poli- dori (2003, 2007, 2009) and Moreno-Maroto and Alonso-Azc´ arate (2017, 2018) classifcation charts, which are designed on the basis of Atterberg limits. After conducting a complete and thorough examination, it is shown that although the Casagrande (1947)-USCS approach is the most widely known, of all the proposals examined, only that of Moreno-Maroto and Alonso-Azc´ arate (2017, 2018) is based on well-founded criteria, presenting a strong predictive capacity, as well as being simple, precise and adaptable to needs. 1. Introduction Soil, whether as an environment (e.g. crop medium in agriculture), raw material (e.g. in the ceramic industry) or object of study in geotechnical engineering (e.g. in soil mechanics), is characterized by a high degree of complexity due to the interaction of solid, liquid and gas phases of different nature, which makes its behavior diffcult to predict. To address these diffculties, scientists, engineers and technicians from different disciplines have used various classifcation systems over the years for two main purposes (Liu, 1970): i) as a systematic means of studying soil more effectively; ii) as a common, simple and concise language enabling professionals to understand the general characteristics and likely behavior/proper- ties of the soil without the need for lengthy explanations and in- depth investigations. Therefore, a short and appropriate defnition may be that of Prakash and Sridharan (2012), which points out that “a soil classifcation is a systematic method of categorizing soils into various groups and subgroups according to their probable engineering behavior but without detailed description”. A correct classifcation can be essential from an engineering point of view. For example, an inadequate classifcation of a silty soil as if it were a plastic clay could lead to cost overruns associated with the stabiliza- tion or discarding of certain soils for use in civil–geotechnical works by incorrectly considering the soil as not meeting the necessary material specifcations, when in fact the opposite is the case. On the contrary, the reverse could also occur, considering an unsuitable soil as suitable fll material for use in civil engineering. For the ceramic industry, if an inadequate fne-grained soil classifcation system is used as raw material evaluation method, this could mistakenly lead to selecting, for example, silty soils as suitable for use, when they really are not, while for other * Corresponding author. E-mail addresses: jmmaroto@ujaen.es (J.M. Moreno-Maroto), jacinto.alonso@uclm.es (J. Alonso-Azc´ arate), bokelly@tcd.ie (B.C. O’Kelly). Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay https://doi.org/10.1016/j.clay.2020.105955 Received 7 August 2020; Received in revised form 11 December 2020; Accepted 14 December 2020