## 5 Conclusions  
To explore the quantitative evaluation method of tunnel face stability, this paper realized a quantitative description of a tunnel face under pipe roofing support, bolting support, grouting support and reserved core soil. The tunnel face stability discrimination and support optimization system was developed, and supporting effects were quantitatively evaluated based on a buried tunnel in Chongqing. The following conclusions can be drawn:  
1. The quantitative analysis model of tunnel face stability is established based on the silo theory. The tunnel face stability discrimination and support optimization system is developed, which can rapidly determine tunnel face stability and then provide a quantitative evaluation method for the assessment of tunnel face safety.
2. Based on the quantitative analysis model of tunnel face stability, the strengthening effect of different support schemes of the tunnel face is systematically analyzed. It can be found that the grouting support has the most significant effect on improving the tunnel face stability, followed by the reinforcement effect of an anchor bolt and reserved core soil, and the reinforcement effect of pipe roofing support has the worst effect. The reserved core soil is preferred for use as a support if the initial stability coefficient of the tunnel face cannot meet the stability requirements. Then, the reinforcement through the tunnel face grouting is more effective than other measures when the stability requirements are still not met.
3. The tunnel surface stability of an underground rail transit tunnel passing through suburban expressway in Chongqing was analyzed based on the tunnel face stability discrimination and support optimization system. The optimization analysis of support measures successfully predicts the over-limit settlement of an auxiliary road of the left line. Then, the optimization scheme of pipe roofing support + reserved core soil + grouting support is finally formed considering the effect of support measures and construction difficulty. On-site monitoring shows that the farther from the tunnel face, the slower the cumulative displacement increases, and the smaller the maximum cumulative displacement. The settlement displacement after optimization is reduced to 15.50 mm, which is about 44.93% before the optimization of the reinforcement scheme, ensuring the safe construction of the suburban expressway.  
Conceptualization, G.-N.Z.; methodology, G.-N.Z. and T.-T.Y.; software, T.-T.Y. and H.L.; validation, T.-T.Y.; formal analysis, G.-N.Z. and T.-T.Y.; investigation, Z.S., Y.C. and J.-J.H.; resources, G.-N.Z. and T.-T.Y.; data curation, Z.S. and H.L.; writing--original draft preparation, Z.S.; writing--review and editing, Y.C. and Z.-P.S.; visualization, Z.-P.S.; supervision, T.-T.Y.; project administration, G.-N.Z.; funding acquisition, G.-N.Z., T.-T.Y. and Z.-P.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Science and Technology Innovation Project of China Railway Construction Bridge Engineering Bureau Group Co., Ltd. (No. DQJ-2020-B07).  
Not applicable.  
**Acknowledgments:** The authors would like to thank the editor and the reviewers for their contributions.  
**Conflicts of Interest:** The authors declare no conflict of interest.