## 3 Results
### Surrounding Rock Excavatability Classification Based on Rock-Breaking Data
#### 3.1.4 Validation of Surrounding Rock Excavatability Classification Criteria  
KS Tunnel was used as the research object, with some TBM sections selected for analysis. The average values of TBM tunneling parameters of the corresponding construction sections and the TOPSIS closeness of SRE were calculated, as shown in Table 6.  
To verify the classification results in Table 6, the surrounding rock classification used for KS Tunnel was used as a benchmark for evaluating the actual SRE. The actual surrounding rock types of individual samples were compared with the calculated SRE classification, as shown in Figure 22.  
Figure 22 indicates that the SRE grades of samples 1, 3, and 4 were classified as L-1\(\sim\)L-2, showing that the excavatability was \"very good-good\". The actual excavation shows that these three samples belonged to Class V. The actual excavation of KS Tunnel Class-V surrounding rock (Figure 23a) shows that this kind of surrounding rock had low integrity, hardness, and stability, requiring complicated and strict initial support. However, Class V surrounding rock was easy for rolling-disc-cutter rock breaking and had the best excavatability. This is consistent with the calculated SRE grade. The SRE grades of samples 2, 5, 6, and 7 lie within L2-L-3, and the corresponding surrounding rock type was Class IV.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **Mileage** & **Rock Types** & \
\begin{tabular}{c} _SER/_ \\ **(m\({}^{3}\)-rev\({}^{-1}\)-Cutter-kN\({}^{-1}\))** \\ \end{tabular} &
\begin{tabular}{c} _PR/(m/h)_ \\ \end{tabular} & **Closeness** & **Grade** \\ \hline
1 & 64 + 781.46\(-\)64 + 786.52 & \begin{tabular}{c} Tuffaceous sandstone \\ 68 + 415.99\(-\)68 + 694.9 \\ \end{tabular} & \begin{tabular}{c} Tuffaceous sandstone \\ 64 + 499.9\(-\)64 + 506.1 \\ \end{tabular} & \begin{tabular}{c} 11.0707 \\ 2.1633 \\ \end{tabular} & \begin{tabular}{c} 1.9747 \\ 4.3026 \\ \end{tabular} & \
\begin{tabular}{c} 0.6352 \\ 0.4212 \\ \end{tabular} &
\begin{tabular}{c} L-1 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ -2 \\ \end{tabular} \\
2 & 68 + 585.69\(-\)68 + 602.26 & \
\begin{tabular}{c} Tuffaceous sandstone \\ 64 + 571.03 \\ \end{tabular} &
\begin{tabular}{c} 2.1633 \\ -1.2231 \\ -1.0423 Figure 23b shows that rock stability, integrity, and hardness were only better than those of Class V surrounding rock, i.e., lower excavatability than those of Class V surrounding rock. This indicates that the classification results calculated for samples 2, 5, 6, and 7 were accurate. The actual surrounding rock of samples 8-10 belonged to IIIb and had higher integrity and hardness than those of Class IV and V rock types shown in Figure 23c. Therefore, Class IIIb surrounding rock had lower excavatability than Classes IV and V rocks. The SRE classification results in Figure 22 show that samples 8-10 were consistent with the actual situation. From Figure 23d,e, the integrity and stability of IIIa surrounding rock were only second to Class II surrounding rock. This indicates that among the five surrounding rock types, Class IIIa and II surrounding rocks had lower excavatability, and particularly, Class II surrounding rock had the lowest excavatability. Figure 22 shows that the actual rock types of samples 11-12 were Class IIIa, with an excavatability grade of L-5. The actual surrounding rock type of samples 13-15 was Class II, with an excavatability grade of L-6. In summary, the classification results of all 15 samples were consistent with the actual situation, indicating that the adopted SRE criteria were accurate.