###### Abstract
[32] system requires only fall cone \(w_{\rm L}\) test results to achieve the classification of fine-grained soils to an acceptable degree of accuracy, i.e., the approach does not require \(w_{\rm P}\) measurement, removing the dependence on the thread rolling \(w_{\rm P}\) test that can have high operator variability [25; 33]. There is also greater awareness of the limitations of standard/conventional consistency limits test methods and fine-grained soil classification systems for investigations on unconventional soils, including (fibrous) peats [34] and diatomaceous soil [35]. In other words, the measured consistency limits of these soils do not provide reliable information on the soils' likely geotechnical properties as they are conventionally expected to do. On the subject topic of soil toughness, [PERSON] [36] introduced an index of toughness at the \(w_{\rm P}\) (denoted herein as \(I_{\rm T(C)}\)), defined as the logarithm of the undrained shear strength (\(s_{\rm u}\)) ratio determined for the \(w_{\rm P}\)-to-\(w_{\rm L}\) water contents. [PERSON] [36] also proposed that the value of \(I_{\rm T(C)}\) could be determined as the ratio of \(I_{\rm P}\) to the flow index (\(I_{\rm F}\)), with \(I_{\rm F}\) determined as the gradient of the \(w\) versus Log\({}_{10}\)\(N\) plot obtained from percussion cup test data for \(w_{\rm L}\) determination. However, this approach for \(I_{\rm T(C)}\) determination is not reliable as it assumes a linear Log\({}_{10}\)\(s_{\rm u}\) versus \(w\) correlation applies over the full plastic range of fine-grained soil, which is generally not the case in practice, instead often exhibiting highly nonlinear behavior [19; 37; 38; 39]. Since the early work of [PERSON] [36], the property of soil toughness has not been investigated that much in geotechnical engineering, mostly because it does not lend itself easily to direct measurement. In a follow-up paper, [PERSON] [14] gave a qualitative classification of soil toughness considering the \"cohesiveness\" near the \(w_{\rm P}\), using the descriptive terms of \"very weak\", \"weak\", \"firm\", \"medium tough\", \"tough\", and \"very tough\". The ASTM visual-tactile field tests [40] include subjective assessments of the finger pressure required in rolling out uniform soil threads and the stiffness of a soil lump formed from amassing those threads, assigning descriptive terms of \"non-plastic\", and low, medium, and high toughness accordingly. The US Department of Agriculture Soil Survey Manual [41] considers the relative force required to form 3 mm dia. soil threads at a water content near or at the \(w_{\rm P}\). In other words, soils are classed as having low, medium, and high toughness for the exertion of <8 N, 8-20 N, or >20 N of force applied by the operator's finger pressure during the repeated traverses [41] because when testing tougher soils, such as those with a high montmorillonite content, greater force is required to roll out and cause a reduction in diameter of the soil threads prepared at the \(w_{\rm P}\) water content [9; 13]. Various attempts have also been presented for soil toughness classification employing the Casagrande plasticity chart, with different zones or classifications of toughness at the \(w_{\rm P}\) identified in the chart. For instance, the Naval Facilities Engineering Command [42] uses the qualitative terms \"slight\", \"slight to medium\", \"medium\", and \"high\" when identifying toughness zones for soils plotting above and below the A-line boundary marked in the Casagrande plasticity chart. Some quasi-toughness tests have been developed specifically for the ceramics industry, with various investigations performed to assess soil remolding toughness. These include a novel proposal by [PERSON] et al. [43], who defined soil toughness considering the amount of energy absorbed by the test specimen during one cycle of a cyclic torsion test and which was quantified as the area within the experimental stress-strain hysteresis loop.