# Fuzzy Synthetic Evaluation of the Long-Term Health of Tunnel Structures
## 5 Conclusions  
Based on the application of the fuzzy synthetic method to the long-term health status evaluation of a selected tunnel structure and the associated analysis, the following conclusions can be made:  
1. A tunnel is a coupled system of the surrounding rock and the supporting structure. The safety status of the tunnel structure is a complex system that is influenced by various factors. In addition, these factors are coupled and interact with each other, which calls for a linguistic description of the tunnel safety level. Therefore, the fuzzy synthetic method should be adopted;
2. According to the investigation and analysis of the safety status classification of tunnels at home and abroad, and combined with the present situation of safety status classification of tunnel structures in China, the health status of a highway tunnel structure is divided into four levels: safe, basically safe, potentially unsafe, and unsafe. In this research, the fuzzy synthetic method is proposed to evaluate the tunnel safety level;
3. With the increase of service time of the tunnel, the primary support gradually deteriorates. The safety of the tunnel structure will be reflected in the safety of the secondary lining. Therefore, the stress of the secondary lining and the contact pressure between the primary support and the secondary lining are selected as a long-term monitoring project. Long-term monitoring sensors are embedded to acquire structural stress characteristics, providing data for structural safety evaluation. The sensors that monitor the stress of the secondary lining and the contact pressure are embedded in the lining of the Cang Ling Tunnel. From these sensors, the stress data for the safety evaluation of the tunnel structure are obtained;
4. Through analysis of the various factors affecting the health status of the tunnel structure, coupled with the long-term monitoring project carried out in the Cang Ling Tunnel, the two-level fuzzy synthetic evaluation model of the structural health status for the Cang Ling Tunnel is established. The fuzzy synthetic evaluation model mainly includes the stress characteristics of the tunnel structure, the engineering geological characteristics, the characteristics of the initial geostress, and the characteristics of the tunnel section. Moreover, the factors are refined into 14 specific indices;
5. As for the tunnel structural health, through the analysis of the distribution characteristics of the influencing indices, the membership functions of indices in the fuzzy synthetic evaluation model are established. The types of membership functions are mainly normal distribution and trapezoidal distribution. The corresponding relationship between each index in the fuzzy synthetic evaluation model and the health level of the tunnel structure and the determination criteria are established using model tests, field tests, and other methods. By using the AHP, the weight of each index in the safety evaluation system of tunnel structure is obtained. According to the membership functions, the determination criteria, and the weight values of the evaluation model, the subordinate degree is obtained using the method of fuzzy mathematics;
6. According to the principle of maximum membership degree, based on the contact pressure and the stress of the secondary lining acquired by the long-term health monitoring sensors of Cang Ling Tunnel, the health status of the main structure of Cang Ling Tunnel is evaluated using the proposed fuzzy synthetic evaluation. The results show that the current health status of most of Cang Ling Tunnel is at level I safety status. Only a small number of segments are at the basic safety status, which is level II. **Acknowledgments:** The authors gratefully acknowledge the support provided by the National Natural Science Foundation of China (No. 51378434, 51578456). The authors would like to thank [PERSON] of University of Houston for introducing the special issue to us. **Author Contributions:** [PERSON] and [PERSON] conceived and put forward the research ideas.