## 5 Conclusions  
1. Microwave irradiation causes some damage to both the surface and interior of the basalt samples. Under a fixed microwave power input, the wear loss of the reduced-scale TBM cutter is decreased, while the amount of rock removed increases upon prolonged exposure to microwave irradiation. This indicates that microwave irradiation can reduce wear, prolong service life, and increase the amount and rate of rock removal of the cutter.
2. After microwave irradiation of the rock, the cutting force of the reduced-scale TBM cutter decreases linearly with increasing exposure to microwave irradiation, and the average driving force and the average rolling force separately reduce by 33% and 32% (at most). This suggests that microwave irradiation can reduce the thrust and torque on the cutter head in the TBM tunnelling process.
3. The work done by the driving force and that done by the rolling force decrease linearly with increasing exposure to microwave irradiation, implying that microwave irradiation can decrease the energy consumption of the TBM cutter. The energy associated with the work done by the driving force is 0.08% of that associated with the work done by the rolling force, suggesting that the energy consumption of the TBM cutter is mainly attributed to the rolling force.
4. After microwave irradiation of rock, the specific energy of the reduced-scale TBM cutter to reach the same penetration depth constantly decreases with increasing exposure to microwave irradiation. The rock-breaking specific energy decreases with  
Figure 16: Relationship between the specific energy of the rolling force and the microwave exposure time.  
increasing exposure to microwave irradiation in an exponential manner. This indicates that microwave irradiation can reduce the specific energy consumption of the TBM cutter when breaking rock, thus also improving the efficiency of the process.  
Conceptualization, G.L. and J.Z.; methodology, G.L. and C.D.; experiment, C.D. and C.L.; formal analysis, H.L.; investigation, G.L. and H.L.; writing--original draft preparation, C.D.; writing--review and editing, C.L.; supervision, J.Z.; funding acquisition, G.L., J.Z. and H.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant number 42002281), the Natural Science Foundation of Henan Province (grant number 212300410325), the Science and Technology Research and Development Plan of China Railway Group Limited (grant number 2020-Zhongda-06), and the Science and Technology Innovation Plan of China Railway Tunnel Group (grant number Suiyanhe 2020-11). The APC was funded by the Science and Technology Innovation Plan of China Railway Tunnel Group.  
Not applicable.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
The datasets used during the current study are available from the corresponding author upon reasonable request.  
Thanks to the North China University of Water Resources and Electric Power for its human, experimental, and technical support.  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.