## 2 Material and Methodology  
### Linear Cutting Machine (LCM) Test  
The full-scale cutting test is known as the linear cutting machine (LCM) test [29]. A large rock specimen is required to carry out the LCM test with real size disc cutters (usually 12-20 inches in diameter). Because a real disc cutter is used in the test, forces acting (i.e., normal, rolling, and side forces) on the disc cutter during excavation, as well as the optimum cutter spacing (s/p ratio), cutting efficiency, and other operational parameters, can be directly determined from the test. In Korea, the LCM system was built with 200 tons of loading capacity and consists of a three-directional servo-controlled loading system (Figure 1). The test data included in this study consisted of that of previously published studies [4; 30; 31] over the past decade involving full-scale LCM tests with Korean rock types. Table 2 shows the mechanical rock properties and brittleness indices of Korean rock types in these previous studies. We selected four brittleness indices among those listed in Table 1. Because B\({}_{4}\) and B\({}_{5}\) share the same mathematical expression (see Table 1), we omitted B\({}_{5}\) from further analysis and discussion. The database included eight igneous rocks, five metamorphic rocks, and two sedimentary rocks. The UCS of the rock types varied from 36.5 to 241.0 MPa, and the BTS varied from 4.7 to 25.2 MPa. Figure 1: The linear cutting machine system [4]: (**a**) photo; (**b**) schematic drawing. Table 3 also summarizes the previous LCM test results with Korean rock types; it should be noted that all cutting tests were carried out with a CCS (constant cross sectional) disc cutter with a diameter of 17 inches (432 mm) and a tip width of 14 mm. Additionally, the LCM results were obtained under the optimum cutting conditions for the statistical analysis. In these LCM tests, the optimum s/p ratio ranged between 7.5 and 18.0, and the cutter forces were measured as average values. The boreability index (in kN/mm) is defined as the required normal force to penetrate the unit depth (in mm) of rock; it was often referred to as the field penetration index (FPI) in a previous study [32]. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Rock Type** & \begin{tabular}{c} **UCS** \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} **BTS** \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} **B1** \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} **B2** \\ \end{tabular} & \
\begin{tabular}{c} **B3** \\ \end{tabular} &
\begin{tabular}{c} **B4** \\ \end{tabular} \\ \hline Granite \#1 & 209.0 & 9.2 & 22.71 & 0.915 & 109.1 & 961.4 \\ Granite \#2 & 91.3 & 10.1 & 9.06 & 0.801 & 50.7 & 461.1 \\ Granite \#3 & 107.6 & 7.4 & 14.48 & 0.871 & 57.5 & 398.1 \\ Granite \#4 & 135.3 & 6.8 & 19.93 & 0.904 & 71.1 & 460.0 \\ Granite \#5 & 36.5 & 4.7 & 7.71 & 0.770 & 20.6 & 85.8 \\ Granite \#6 & 145.5 & 7.8 & 18.65 & 0.898 & 76.7 & 567.5 \\ Diorite & 158.5 & 11.2 & 14.15 & 0.868 & 84.9 & 887.6 \\ Feliste & 145.5 & 9.5 & 15.31 & 0.877 & 77.5 & 691.1 \\ Gneiss \#1 & 167.5 & 10.6 & 15.80 & 0.881 & 89.1 & 887.8 \\ Gneiss \#2 & 91.5 & 15.2 & 6.03 & 0.715 & 53.4 & 695.4 \\ Gneiss \#3 & 123.8 & 11.2 & 11.02 & 0.833 & 67.5 & 693.3 \\ Gneiss \#4 & 241.0 & 13.3 & 18.05 & 0.895 & 127.2 & 1602.7 \\ Gneiss \#5 & 186.0 & 11.5 & 16.12 & 0.883 & 98.8 & 1069.5 \\ Tuff & 115.5 & 25.2 & 4.58 & 0.642 & 36.2 & 1455.3 \\ Limestone & 63.6 & 8.86 & 7.18 & 0.756 & 36.2 & 281.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties of rocks used in the linear cutting machine (LCM) tests. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Rock Type** & \begin{tabular}{c} **p 1** \\ **(mm)** \\ \end{tabular}  & \begin{tabular}{c} **S\({}_{\text{opt}}\) 1** \\ **(mm)** \\ \end{tabular}  & \begin{tabular}{c} **MNF 2** \\ **(kN)** \\ \end{tabular}  & \begin{tabular}{c} **MRF 2** \\ **(kN)** \\ \end{tabular}  & \ \begin{tabular}{c} **SE\({}_{\text{opt}}\) 1** \\ **(kWh/m3)** \\ \end{tabular}  & \begin{tabular}{c} **BI 1** \\ **(kN/mm)** \\ \end{tabular}