## 7 Conclusions  
The damage and wear of a slurry discharge pipeline during slurry shield tunneling in a pebble layer and rock ground were investigated over a period of slurry shield tunneling for a water supply project in Beijing, China. The following conclusions can be drawn:  
1. Controlled by the movement of the transported rock particles, damage and wear mainly occurred at the bottom wall of the pipeline. Compared with straight pipes, elbows/bends were more easily worn out. Inclined straight pipes had a higher wear rate than horizontal straight pipes. Pipes installed along curved tunnels suffered more wear than those in straight tunnels. The reduction in slurry velocity reduced pipe wear, while sharp particle shapes increased wear, and the shape of the particles had a greater impact.
2. The measured results showed that the wear rates of the straight pipe, pipe along the curved tunnel, 60\({}^{\circ}\) inclined pipe, and 90\({}^{\circ}\) elbow pipe in pebble ground were approximately 0.71 mm/100 rings, 1 mm/100 rings, 2.14 mm/100 rings, and 4 mm/100 rings, respectively. After segment ring 705, the machine drove into rock ground, and the wear rates increased by one to two times, which could be attributed to the rock particles of the pebble layer being polished and the rock fragmentations in the rock ground having sharp edges.
3. The arrangement of the pipeline should favor pipeline inspection and maintenance. Because of the insufficient space to repair the worn-out bottom walls of the pipeline, the slurry discharge pipeline should not be mounted on the tunnel floor. Additionally, pipelines should be tightly fixed to reduce their movement.
4. To fight pipeline wear and leaks and maintain sustained shield tunneling, countermeasures should be planned and prepared beforehand. First, the layout of pipelines in an existing tunnel should favor pipeline maintenance and repair. Second, for long tunnel projects, preparing a slurry discharge pipeline system ready for use is suggested, especially at the rear half of the tunnel. Third, wear-resistant elbows/bends should be used, and reinforcement plates should be welded to them in advance to prevent the occurrence of leaks. Other measures, such as rotating a worn pipe 180 degrees for reuse and exchanging slurry feed lines and slurry discharge lines, are also proposed.  
X.L. ([PERSON]): Conceptualization, Methodology, Investigation, and Writing. Y.F.: Methodology, Investigation, and Validation. Y.G.: Investigation and Validation. X.L. ([PERSON]): Conceptualization and Methodology. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The authors declare the following financial interests/personal relationships that may be considered potential competing interests: [PERSON] reports that financial support was provided by the National Natural Science Foundation of China. Support from the Chinese National Natural Science Foundation (No. 52278386) is acknowledged.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data used to support the findings of this study are available from the corresponding author upon request.  
**Acknowledgments:** The authors would like to acknowledge the National Science Foundation of China under Grant No. 52,278,386 for supporting this research and [PERSON] for providing engineering information.  
**Conflicts of Interest:** The authors declare no conflict of interest.