###### Abstract  
The remolding toughness property of fine-grained soil has not been investigated that much, mainly because it has not lent easily to direct measurement, with soil toughness usually qualitatively described. In practical terms, as the plastic limit \(w_{\rm P}\) is approached, tougher soils require greater rolling effort during the \(w_{\rm P}\) test, such that plasticity and toughness properties can be used to distinguish those plastic soils having greater deformation resistance for various field applications. This state-of-the-art review paper presents a critical appraisal of soil remolding toughness determination and its limited use, to date, in the classification of fine-grained soils. The recent developments reviewed and critically assessed include mechanical thread rolling for nominal toughness measurement during the \(w_{\rm P}\) rolling-out procedure, various extrusion approaches, and proposed correlations between toughness and the plasticity index to liquid limit ratio. From statistical analysis of previously reported toughness-consistency limits data, some new correlations are introduced in the present paper. Soil classification using the traditional Casagrande plasticity chart is not entirely accurate for certain soil types in that one can observe soils that present high toughness (something typical of _day_) being incorrectly classified as _silt_ soil. From this perspective, a new toughness chart is introduced to augment (or for use instead of) the Casagrande plasticity chart in obtaining more reliable soil classification. This paper concludes with recommendations on future research efforts for routinely obtaining soil toughness measurements. extrusion; plastic limit; soil classification; soil plasticity; toughness; workability +
Footnote †: journal: Applied Sciences  
the hand pressure, and/or the initial and final thread diameter criteria) include the thread rolling device technique proposed by [PERSON] and [PERSON] [5]. Their method, which follows the same basic principles as the standard thread rolling (by hand) test, has been standardized and presented as an alternative method for \(w_{\rm P}\) determination in ASTM D4318 [3] and AASHTO T90 [6]. A comprehensive statistical analysis performed on a database of 60 diverse fine-grained soils demonstrated that these hand and device rolling methods produce essentially similar \(w_{\rm P}\) values for a given fine-grained soil tested [7]. For fine-grained soil (and mainly clays), remolding toughness (\(T\)) represents the mechanical resistance offered when its \(w_{\rm P}\) is about to be reached [8] for reducing water content in the plastic range. For instance, as the \(w_{\rm P}\) is approached, tougher soils require noticeably greater rolling effort for reducing the soil thread diameter from a starting 6 mm dia. down to approx. 3 mm dia. at the crumbling condition [3; 9]. Together, the plasticity and toughness properties are used to identify soils that provide greater resistance to deformation while retaining plastic behavior for use in various field applications. For instance, soil remolding toughness is important for the choice and performance of field compaction machinery in earthwork construction and it influences the integrity of earthwork components, e.g., the core of earthen dams and clay liners in water-retaining structures and landfills [8]. Greater plasticity and toughness arise from increases in the quantity of expanding lattice-type clay minerals with greater specific surface area (SSA) and cation exchange, and for decreases in the ion concentration of the pore fluid and the valency of exchangeable cations present [10]. Fundamentally, within the plastic range (of water contents), saturated clayey materials develop suction, such that active/high plasticity clays (characterized by the highest air-entry values) have the highest toughness.