## 2 Soil Remolding Toughness Measurement  
### [PERSON] Thread Rolling Apparatus and Method  
[PERSON] [8; 17; 18] developed an apparatus (see Figure 1) and test method for the rolling of a uniform soil cylinder (thread) between two configured plates. For each traverse, the applied downward force is controlled and recorded, and the actual tread diameter is recorded, such that the graphs of nominal acting stress against the diametrical strain can be computed for a range of investigated water contents (Figure 2a) that reduce in value to the \(w_{\text{P}}\). Based on the areas beneath the produced experimental stress-strain plots, nominal toughness measurements can then be obtained, and are plotted against water content (Figure 2c). In quantifying remolding toughness, the measurement units of toughness (i.e., work/unit volume) must take into account the number of traverses undergone by the plastic soil thread during the mechanical rolling-out procedure. [PERSON] [8; 17; 18] decided that the value of toughness \(T\) assigned to the test soil would be based on the cumulative work undertaken per unit soil volume per 100 reversals (i.e., giving units of kJ/m\({}^{3}\)/100 r) in reducing the soil thread diameter from initially 6 mm down to 4 mm because, over this range of diameters, the soil threads were found to undergo plastic straining in a steady fashion. As described by [PERSON] [8; 17; 18], from investigation of the plastic test soil prepared at a range of different water contents, various toughness-related properties can be derived from the obtained experimental \(T\)-\(w\) plot. For reducing water content, these include the toughness limit \(w_{\text{T}}\) (i.e., the water content at zero toughness), the stiffness transition \(w_{\text{S}}\) (i.e., the water content below which the remolding toughness increases at a greater rate for many soils), and the maximum remolding toughness \(T_{\text{max}}\), which is mobilized at the \(w_{\text{P}}\) and represents the main toughness indicator. Figure 2 shows the experimental sequence carried out by [PERSON] [8; 17; 18] in determining the value of \(T_{\text{max}}\) (i.e., the value of \(T\) corresponding to the measured \(w_{\text{P}}\) water content). Figure 1: Elevation drawing of the [PERSON] mechanical thread rolling apparatus [17]. Figure 2: Determination of \(T_{\rm max}\) according to [PERSON] method [8; 17; 18]: (**a**) graph of nominal stress against cumulative diametrical strain obtained for clay cylinders (threads) rolled out using [PERSON] apparatus; (**b**) cumulative work per unit soil volume plotted against thread diameter, with the former obtained as the product of nominal stress and diametrical strain for 100 reversals; (**c**) produced relationship between remolding toughness (\(T\)) and water content, where \(T\) is calculated as the cumulative work per unit volume required to reduce the thread diameter from initially 6 mm down to 4 mm. The presented plots have been extracted and edited from [PERSON] [18]. Note: \(w\), water content; \(w_{\rm P}\), plastic limit \(w_{\rm S}\), stiffness transition water content; \(w_{\rm T}\), toughness limit; \(T_{\rm S}\), toughness mobilized at \(w_{\rm S}\). Based on the deduced bi-linear \(T\)-\(w\) relationship (Figure 2c), the plastic range of a given test soil can be subdivided into three distinct regions, namely a workable stiff plastic region for \(w_{\rm P}<w<w_{\rm S}\), a workable soft plastic region for \(w_{\rm S}<w<w_{\rm T}\), and an adhesive plastic region for \(w_{\rm T}<w<w_{\rm L}\)[8; 18]. In other words, at the \(w_{\rm T}\), fine-grained soil behavior transforms from workable soft plastic to almost non-workable adhesive plastic. Moreover, for \(w>w_{\rm T}\), the soil would not only be difficult to form (roll) into a thread to display plasticity, but it would also be very sticky.