## 7 Advantages/Limitations of the Existing Methods for Toughness Determination  
In the preceding sections, four different approaches were highlighted as the fundamental ones for soil toughness assessment/determination: i.e., manual rolling of soil threads [3; 4; 40; 41], Barnes' thread rolling apparatus [8; 17; 18], extrusion approaches [12; 13; 46; 47; 48; 49], and via various correlations deduced in the present paper and reported in [11] between the consistency limits and values of Barnes' \(T_{\rm max}\). Their advantages and disadvantages are summarized in Table 2. Manual and mechanical thread rolling are classed as direct methods as they conform to the definition of toughness and the original protocol for its determination.  
Figure 7: Toughness-based classification of fine-grained soil: (**a**) classification chart of Moreno-Maroto and Alonso-Azčrate [11]; (**b**) chart for estimating maximum remolding toughness, \(T_{\rm max}\), from consistency limits measurements (present study). Note: CH, clay of high plasticity; CL, clay of low plasticity; MH, silt of high plasticity; ML, silt of low plasticity; Ip, plasticity index; \(w_{\rm L}\), liquid limit water content; \(w_{\rm LFC}\), fall-cone liquid limit water content.  
Considering their various advantages and weaknesses, as listed in Table 2, it is not a simple task to determine which of these four approaches may prove to be the more effective for toughness determination. When the aim is to get accurate results, the use of relatively complex measurement devices, such as Barnes' thread rolling and various extrusion apparatuses, seems to be a good alternative, particularly the Barnes' approach, as it is a direct method. However, along with associated equipment costs and complexity, these mechanized methods can present certain operational disadvantages during the testing, as well as a long test duration with associated time/cost implications. On the other hand, manual thread rolling would be the simplest and fastest of all four approaches and the one that most closely approximates the original definition of toughness. However, being qualitative, subjective, and hence highly dependent on the operator's performance [25; 33], the manual thread rolling approach is deemed not suitable when trying to get quantitative and precise results. Therefore, the statistical approximations obtained via consistency limits results (e.g., [11]) do seem to find a balance with what has been said previously, with the obtained toughness predictions conforming satisfactorily to the actual \(T_{\text{max}}\) values measured for dozens of different fine-grained soils investigated by [PERSON] [8; 17; 18]. Given that the consistency limits are basic index tests performed on fine-grained soils, the use of their parameters for obtaining \(T_{\text{max}}\) estimations does not entail any additional testing cost or time (i.e., from the authors' perspective, the purpose of soil toughness determination is not to replace the ubiquitous consistency limits testing used to obtain \(w_{\text{L}}\) and \(w_{\text{P}}\)). As it is based on statistical criteria, probably one weakness of this approach is that it could potentially give erroneous \(T_{\text{max}}\) predictions for soils with atypical behavior. In this regard, the authors consider that the observations of an experienced operator in judging the relative force and effort required to form and roll out uniform soil cylinders for the thread rolling test method would serve as quality control of the mathematical \(T_{\text{max}}\) estimation. Therefore, continued use of standard consistency limits testing along with the estimation of \(T_{\text{max}}\) via correlations with \(I_{\text{P}}/w_{\text{L-FC}}\) or \(w_{\text{P}}/w_{\text{L-FC}}\) (i.e., using Equations (3) and (4)) is judged as the most satisfactory option.