## 5 Conclusions  
Based on theoretical analysis and field tests, this paper studies the optimization of rock mass cracking parameters and the application of high-pressure gas expansion rock-cracking technology in hard rock tunnels. The main research results are as follows:  
1. The rock mass cracking effect of the cutting mode of \"central vertical empty hole + double wedge cutting hole\" is better than that of the cutting mode of \"central vertical empty hole + single wedge cutting hole\". The stripping rock volume in the cutting excavation can reach 6.7 m\({}^{3}\) once with the cutting mode of \"central vertical empty hole + double wedge cutting hole\". The rock fragmentation is relatively uniform and suitable, and the distribution of the rock blocks is relatively concentrated. It is proved that the high-pressure gas expansion rock-cracking technology can achieve ideal cutting rock mass cracking effect in hard rock tunnel with the proper cutting mode. 2. The rock mass cracking effect of the auxiliary hole network parameter of \"0.8 m \(\times\) 0.7 m\" is better than that of the auxiliary hole network parameter of \"1.0 m \(\times\) 0.9 m\". The stripping rock volume in the upper step excavation can reach 39 m\({}^{3}\) when the cutting mode of \"central vertical empty hole + double wedge cutting hole\" is used to match  
Figure 16: Site photos of vibration monitoring. (a) Monitoring point 1; (b) Monitoring point 2. Figure 15: Arrangement of monitoring. points. the auxiliary hole network parameter of \"0.8 m \(\times\) 0.7 m\". The rock fragmentation is relatively uniform, and the distribution area of rock blocks is mainly within 3 m in front of the tunnel face. It is proved that the high-pressure gas expansion rock-cracking technology can achieve an ideal excavation effect in hard rock tunnel with a suitable cutting mode and hole network parameters. 3. The blast vibration logger with the trigger lever of 0.1 cm/s was not triggered, which was at 60 m away from the rock mass cracking point. It indicates that the vibration of monitoring point is less than 0.10 cm/s, which meets the safe allowable vibration speed of general ancient buildings and historic sites. It is proved that the high-pressure gas expansion rock-cracking technology can realize the safe and environmentally friendly excavation of hard rock tunnel under the Great Wall. As long as there is the high quality of hole plugging and no punching, the high-pressure gas expansion rock-cracking technology has the advantages of little vibration, low noise and less flying rocks. 4. High-pressure gas expansion rock-cracking technology can realize the safe and efficient excavation of hard rock tunnel, and the vibration generated by its rock-cracking process does not affect the safety of the Great Wall. This is the first application for the continuous non-explosion excavation of hard rock tunnel near historic sites. It brings a new idea of rock cracking for hard-rock projects located in the city center, near important buildings or in other complex construction environments. In this paper, the excavation plan of the hard rock tunnel near the historic site is formed around high-pressure gas expansion rock-cracking technology. However, the plugging technology needs to be further improved to raise the efficiency and quality of plugging, so as to provide a high guarantee for the rock-cracking effect of high-pressure gas expansion rock-cracking technology. Conceptualization, D.L. ; methodology, D.L. ; software, C.W. ; validation, C.W., Y.T. and H.C.; data curation, H.C.; writing-original draft preparation, D.L. and C.W. ; writing-review and editing, D.L. and C.W. ; visualization, H.C.; supervision, Y.T. ; project administration, Y.T. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Data Availability StatementData will be made freely available on request.