## 8 Recommendations on Future Research Efforts for Soil Toughness Determination  
The authors' recommendations for future research work on the topic include investigations of soil remolding toughness employing (i) the MFI apparatus and testing approach [47; 48] and (ii) the moisture condition test and obtained MCV results [45]. As described earlier in the paper, the MFI tester measures the time period required for a known soil volume to flow through a die orifice under a constant deadweight load action. The  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method** & **Type** & **Advantage** & **Weakness** \\ \hline Manual thread rolling [3; 4; 41] & Direct & Meets original definition of toughness. & Qualitative (does not allow obtaining numerical values). Subjectivity; high operator dependence. \\ \hline [PERSON] thread rolling apparatus [8; 17; 18] & Direct & Meets original definition of toughness. & According to [50]: relatively complex measurement approach; slowness (many experimental points required); great skill on operator’s part to reach value of \(T_{\text{max}}\). \\ \hline Extrusion [12; 13; 46; 47; 48] & Indirect & \begin{tabular}{c} Objectivity (mechanically determined). \\ Good correlation between extrusion \\ resistance and toughness [12; 13; 46; 47]. \\ \end{tabular} & \begin{tabular}{c} Non-standardized (features of extruder device \\ and extrusion conditions may give different results). \\ Equipment cost. \\ \end{tabular} \\ \hline Estimation from consistency limits ([11], this paper) & Indirect & \
\begin{tabular}{c} \(w_{\text{L}}\) and \(w_{\text{P}}\) obtained using standardized and widely known methods. \\ Good correlation with [PERSON] values of \(T_{\text{max}}\). \\ Speed and low cost. \\ \end{tabular} &
\begin{tabular}{c} Of the parameters associated with consistency \\ limits, only the \(w_{\text{P}}\) test meets \\ toughness definition. \\ Based on estimates according to statistical \\ criteria, outliers may not fit estimated values. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Advantages and weaknesses of the main methods employed for soil toughness determination. Note: \(w_{\text{L}}\), liquid limit; \(w_{\text{P}}\), plastic limit; \(T_{\text{max}}\), maximum remolding toughness (mobilized at \(w_{\text{P}}\)).  
mobilized toughness for a particular fine-grained soil (and water content) is related to the flow time (soil extrusion rate), with tougher materials taking longer periods [47]. For the moisture condition test, tougher soils (and for lower water content) give higher MCVs, as more compaction effort is needed to remold the lumps of clay in performing the testing, with [PERSON] [8] envisaging that the MCV should correlate satisfactorily with the values of toughness. As such, increased research efforts should be directed toward investigating the MFI and/or moisture condition tester for soil toughness determination. Other ways of soil classification, based on remolding toughness parameters or incorporating the concept to complement existing plasticity-based classification systems, also merit investigation for obtaining more reliable soil classification when investigating some hitherto challenging fine-grained soil types.